SOUND

the increase in our knowledge of the subject of acoustics (the science of sound) during recent years has been largely associated with the war conditions which pre- vailed from 1914 to 1918. as a consequence of the war the development of this science has been abnormal, and research has been directed towards the rapid realisation of practical acoustic devices and methods for immediate use in warfare, both on land and sea. some aspects of the investigations have received continued though less urgent attention since the war, with important results. a general survey of the work done shows that the advances consist of applications of well-established principles rather than the discovery of new phenomena. gener- ally, the observations made have proved to be in accordance with previous theorctical investigations, mainly due to the first lord rayleigh.!. the work falls naturally under two headings, viz.: (1) the detection and perception of direction of sounds in air, and (2) the detection and perception of direction of sounds in water. theoretically these two problems have much in com- mon, but practically there are important differences which make it desirable to treat them in separate sections. a special section (3) is devoted to the important advances in auditorium acoustics, and the remaining section (4) deals briefly with miscellaneous outstanding features of modern work on sound. 1lord rayleigh’s work is contained in his collected papers (no. 6, 1920). his contributions were numerous between 1911 and 19109, when he died. 587 i. detection and perception of direction of sounds in air? detection.—the human ear itself is a remarkably sensitive detector of the air vibrations which constitute sound. it is still much superior in this respect to any mechanical device which has yet been produced for recording the vibrations visu- ally. thus the perception of feeble sounds of necessity depends upon the limitations of audibility, either indirect listening, or with the ear aided by the intervention of an electrical device such as a microphone. the audibility of a feeble sound can be very largely augmented by making use of the principle of reso- nance, provided that the sound itself approximates to a pure tone. this can be secured, for example, by the use of a helm- holtz resonator applied to the ear in the case of direct listening, and in addition by tuning the diaphragm receiver when micro- phonic listening is adopted. it has happened fortuitously that some of the chief sounds in air which it is important to be able to detect, viz.: those emitted by aircraft, does contain predominant notes, apparently due to engine exhaust, which enable the appli- cation of resonance, as above indicated, to increase largely the range of audibility. the operation of the deppler effect, arising from the relative motion between the aircraft and the observer, prevents the possibility of the identification of the machine by means of the observed frequency, this being liable to change by as much as 20%, according to the speed and direction of flight. an inter- esting observation which has been constantly made is that the notes of low pitch continue to be heard at ranges where those of high pitch have ceased to be audible. this is in accordance with the theoretical expectation that damping increases with frequency. the determination of the direction whence a sound arrives is theoretically possible by a variety of methods dealt with below, several of which have been tried in aircraft localisation. (a) binaural listening.3—lord rayleigh’s experiments (collected papers, vol. 5, p. 347) have shown that low-pitched sounds are determined in direction by the observation of the phase difference between the vibrations arriving at the two ears. this principle has been applied in direction-finding, and the effect has been exaggerated by increasing the distance between the two points of reception. the sound is received by two equal trumpets or horns rigidly con- nected together and capable of rotation about axes perpendicular to the line joining them. separate and exactly equal tubes lead from the trumpets to the two ears respectively, and the apparatus is rotated until the sound under observation appears to come from directly in front. the line joining the sound receivers is then per- pendicular to the incident sound stream. <an alternative method which dispenses with the necessity of rotating the apparatus is to use a compensator or phase-measurer, which consists of tubes, adjustable in length, inserted between the sound receivers and the appropriate ears, so as to provide a path difference equal to that between the distant source of sound and the two receivers. adjust- ment of the tube lengths is made until the impression received 1s that the sound is neither to the right nor to the left, and the deter- mination of direction is then a matter of simple geometry. in prac- tice the compensator is graduated to give direct angular readings. the practice of binaural listening has verified theoretical conclu- sions in several important respects. it has been found that it ts easicr to perceive the direction of a mixed sound, or noise, than a pure note. apparently it is necessary that the wave train should contain more or less isolated spect characteristics whereby the sa aa sis ns seo ae mn sen ars de 2 el as lee ar se ee 2 the information contained in this section is largely drawn froma manual entitled dezelopment of sounds, kindly placed at the writer's disposal by the british munitions inventions department. 8 this method of perception of direction has been largely used also in a connection which scarcely justifies treatment in a separate section. the geophone is an instrument for direction-finding of sounds proceeding through the earth, and its particular use during the war was for localising the sounds of picks, etc., used in tunnelling and land mining. it consists of two hollow boxes connected by equal tubes to a stethoscope arranged so that the sounds proceed from the two boxes to separate ears. the boxes are laid upon the ground a few fect apart, and moved about until the sounds of the pick appear to come from straight ahead. it is then known that the sound source is on a line perpendicular to that joining the two geophone receivers, since the sounds arrive through the earth in synchronism. by combining several pairs of geophones separated by considerable distances, the actual position of the pick can be estimated, for st | lies at the intersection of the several perpendiculars above specified. 588 phase difference can be readily appreciated. in the regular sine wave corresponding to a pure tone each vibration is exactly like those which immediately precede and follow it, and the ears are unable to identify corresponding displacements. it is apparently also necessary for successful binaural listening that the two por- tions of the incident wave which enter the two receivers should be free from subsequent distortion; in particular, that the sound receivers should be as nearly as possible non-resonant for the vibra- tions in question. any amplification of the sound which depends upon resonance, therefore, such as the use of helmholtz resonators already referred to, is incompatible with efficient direction-finding by observations of phase difference. the method is subject to many errors, chiefly those arising from the motion of the sound source, refraction due to temperature in- equalities in the air, and the effect of winds. the necessary correc- tions are tabulated for use in practice. (b) sound mirrors.—some success has been attained in direction- finding by means of concave sound reflectors, the chief limita- tions have arisen from the question of size, and, consequently, of portability. in optics the size of mirrors commonly in use is very great in comparison with the wave-lengths of the light; in the cor- responding problem in acoustics it is almost impossible to make them so; and yet this is a necessary condition for the geometrica laws of reflection to apply with accuracy. in the largest sound mir- rors—perhaps 20 {t. in diameter—the size is at most only a few wave-lengths for the aircraft sounds under investigation with the result that the image of a distant sound obtained at the focus proves to be an area much larger than that corresponding to optical calcu- lations. there is therefore no advantage secured by making the mtr- ror paraboloidal instead of spherical, and considerable roughness of the surface is not detrimental. the mirrors were usually made of concrete, and listening was effected either by means of a small horn receiver placed in the focal plane and connected by a tube to the ears, or by means of a microphone placed in a similar position. if, as was more usual, the mirror was fixed, the direction of the sound source could be found by determining the position of maximum inten- sity in the focal plane. it may be noted that in this method of direc- tion-finding amplification is obtained on account of the area of the mirror, and that further augmentation is attainable by using resona- tors, to which the same objections do not apply as in binaural listen- ing. the accuracy of the determinations varies very much with fre- quency, being much greater for notes of high pitch than for low, as would be anticipated from considerations of wave-length. (c) interference and diffraction methods.—there have been many attempts to apply the principle of interference as a substitute for binaural listening, t.e., by ultimately mixing the sounds entering the two receivers, instead of leading them to different ears and adjust- ing the compensator until the total sound heard is as loud as possible. theoretically this will occur when there has been provided in the compensator a difference of path equal to the path difference out- side the receivers. the method has not proved very successful, for a variety of reasons, some of which are obscure. on the other hand, remarkable results have been obtained by the application to sound waves of a phenomenon well known in the diffraction of light. a small distant source of light gives in the mid- dle of the shadow of a small circular obstacle a luminous region, called the ‘‘ white spot,”’ arising from the diffraction of light round the edges of the obstacle. the same phenomenon is observable in sound under suitable conditions. thus a large horizontal disk, at least 20 ft. in diameter, and made of material which either refiects or absorbs sound, will give below itself a sound shadow of a sound source, such as an acroplane, above it. near the centre of the shadow, in a position depending on that of the source, there is a region where the sound heard on a suitable microphone placed there is com- paratively loud—in many cases much louder than it would be if the disk were absent. the relation between the direction of incidence of the sound and the position of maximum intensity has been calculated, and the method provides, perhaps, the most reliable means of per- ceiving the direction of air-borne sounds. (d) sound ranging.—this special military aspect of the localisa- tion of sound sources, viz., those arising from gun-fire and shieil bursts, is dealt with in the article sound ranging, il. detection and perception of direction of sounds in water! of all the methods practised for the detection of submarines, that depending on the sounds which they emit has been of the widest application. the question of detection has becn, of course, of nearly equal importance in the opposite sense, viz.: the hearing of surface ships by the crew of a submerged subma- rine. the sounds created in the sea by a screw-propelled ship are of a very complicated character, arising partly from the 1 the following publications should be consulted, although they form by no means adequate references: h. c. hayes, engineer, p. 491 (1920); c. v. drysdale (kelvin lecture), journ. i. e. e. (1920); w. h. bragg, ‘ submarine acoustics,” nature, july 1919; f. l. hopwood, ‘‘ submarine acoustics,’’ nafure, aug. 1919. sound interaction between the propeller and the water, and partly from the vibrations of the machinery which are transmitted through the walls of the ship into the sea. they vary greatly from ship to ship, even of the same class; and, in the later stages of the war, submarines had been constructed which, when cruising submerged at certain slow speeds, emitted practically no noise at all. in many ways the detection of submarines in the sea is more difficult than that of aircraft in air. normally, listening in air takes place at stations which are fixed; in submarine listening the stations were most frequently ships which, for tactical rea- sons connected with their safety, had to be constantly on the move. their own machinery noise and the acoustic disturb- ances arising from their motion through the water were very apt to drown the noises proceeding from more distant sources. the noise of the sea, too, even in weather not at all stormy, interfered greatly, and the range at which a submarine could be heard varied much from day to day. a scrious additional limitation was that recourse could not normally be had to the reflection of ordinary sounds (as is possible in air) chiefly by reason of the great size of the necessary reflectors. tor the speed of sound in sea water is more than four times that in air, so that the wave-lengths are larger in the same ratio. this necessitates a corresponding increase in the linear dimensions of the sound mirror, if equal efficiency is to be obtained. hydrophones.—hydrophones, or under-water sound detectors, were already in use before the war for signalling purposes, being car- ried by ships for listening to submarine bells operated by trinity ]louse as warnings in foggy weather. they consisted of small, metal, water-tight cases of which one face was a metallic diaphragm oper- ating an enclosed microphone. the electrical disturbances of the microphone caused by any vibration of the diaphragm arising from sound pressure waves in the sea were conveyed to telephone recciv- ers on the ship, where listening took place. it was usual to suspend the hydrophones in water-filled tanks attached inboard to the outer shell of the ship, which, owing to the fact that stecl.in water trans- mits sound almost completely, does not diminish appreciably the intensity. normally the hydrophone diaphragm was tuned so that its natural frequency in water? approximated to that of the sig- nalling bell, and so that increased range could be secured by depend- ing on resonance. the earlier hydrophones used for naval purposes were of much the same type, although the resonant diaphragm proved to be by no means an unmixed advantage. <all sounds containing a com- ponent corresponding to the diaphragm frequency were distorted in reproduction, and what was gained in sensitivity was liable to be lost in the difficulty of recognition, or, in other words, failure in discrimination between genuine noises due to a submarine and other noises inevitably present in the sea. appeal to resonance is only really advantageous when the sound under observation has a pre- dominant note, as in the case of an aeroplane; and submarines do not display this characteristic. ultimately hydrophones of a non- resonant character came to be preferred, and were frequently used in practice. these consisted most usually of enclosures made of rubber, sufficiently thick to withstand the pressure of the sea at the usual depth (about 15 ft.), and having natural frequencies below the limit of audition. an alternative type of hydrophone consisted merely of a hollow enclosure without a microphone, sometimes with a metallic dia- phragm, and sometimes simply a rubber tube, filled with air and connected by long tubes to stethoscopes applied to the ears. these are operated by the transference of the pressure vibrations from the sea to the air cavity and thence to the ears. electrical hydrophones have the advantage over non-electrical ones that their sensitivity can be readily augmented by various means, e.g., by the use of thermionic amplifiers (see amplifiers). in cases where the hydrophones had to be used by ships in motion, they were sometimes fitted into the hull of the ship; or themselves consisted of fish-shaped bodics towed at a considerable distance behind the ship. the former precaution, 7.e., making the shape stream-like, aimed at diminishing the vibrations created by the passage of the hydrophone through the water; the latter had in view the partial elimination of the disturbances arising from noises in the towing ship. even so, it frequently became necessary to stop the engines temporarily, and listen with the towed hydrophones while the momentum of the ship continued to carry it forward. this proved to be feasible only at comparatively slow specds. directional iiydrophones.—all the hydrophones so far described are of a non-directional character, z.e., the intensity of the sound heard in them is practically independent of the orientation of the ‘sensitive receiving diaphragm with respect to the position of the source of sound. the limitations of dimensions necessitated by con- 2 this is considerably lower than the natural frequency in air, on account of the additional loading by the water. sound siderations of portability, etc., are such as to render the instruments much too small to give an effective ‘ sound shadow.” the reason for this has already been mentioned, viz., the great wave-lengths corresponding to audible sounds. at a frequency of 500 per second, for example, the wave-length in water is nearly ro feet. differential itydrophones.—curiously enough, however, one type of directional hydrophone, here called for distinguishing purposes a differential hydrophone, did, in fact, depend upon the small differ- ences of pressure operating upon its two sides; and it met with con- siderable success. it was made in various forms, the simplest of which consisted of a circular metal diaphragm, bearing at its centre a water-tight box containing a microphone, and clamped round its rim to a heavy metal ring. when placed in the sea so that the plane of the diaphragm passed through the position of the sound source, the pressure variations on the two sides are the same both in ampli- tude and phase, with the result that the diaphragm, and therefore the microphone, has no motion imparted to it, and the sound heard isa minimum, if, however, the diaphragm faces the sound source, there are, apparently, differences in the pressures on the two sides (probably in both amplitude and phase), and a small! differential vibration takes place, with consequent sound in the receiving tele- phones, actually, as the hydrophone is rotated through 360° about a vertical axis, two maxima and two minima of sound intensity are observed, in this form, therefore, the instrument is what is called, bi-directional, t.e., it is unable to distinguish between sources in front and behind. the desirability of obtaining a t-directional instru- ment led to the introduction of the so-called baffle-plate, the behaviour of which has not yet been explained satisfactorily in terms of ortho- dox theory. the essential characteristics of a baffle appear to be that it should be made of non-resonant material and have air cavi- ties within it. such a plate, fixed at a small distance (which has to be determined by trial) from one face of a hi-directional hydro- phone, transforms it into a uni-directional instrument. <a single sound maximum is now obtained upon rotation, occurring when the sound source and the baffle plate are on opposite sides of the dia- phragm; and a single minimum—this when the baffle lics between the sound source and the diaphragm. the ‘ edge-on ”’ minima, observed when the baffle is absent, now disappear. binaural listening.—the principles underlying this method of direction-finding have been described already. in the present case the main difference is that the sound receivers have to be submerged hydrophones. it has been found to be equally necessary for success that these should be as completely as possible non-resonant. the simplest arrangement used in practice was two rubber cavities placed several feet apart horizontally, and joined by separate equal tubes to the two ears. the device could be rotated about a vertical axis, usually passing through the hull of the operating ship. as in the case of air listening, compensators were often used in order to avoid the necessity of rotating heavy apparatus. an arrangement much preferred was to tow two or more fish-shaped hydrophones in known positions (about 12 {t. apart) behind the ship. here elec- trical transference of the acoustic disturbances had to be adopted, and this necessitated great care in the choice of the microphones and telephone receivers so as to avoid selective resonance. (the behav- iour of microphones in this respect was often unsatisfactory, and telephone earpieces, or magnetophones, were frequently substituted for them. this results in diminished sensitivity, but the binaural effects are much improved.) the telephone receivers delivered the sound into the compensator, and the phase difference was measured in the usual way; allowing, of course, for the difference of speed of sound in the sea and in the air channels of the compensator. in using a compensator with two sound-receiving units only, there remains an ambiguity of estimated direction, 7.e., one cannot dis- tinguish between the angles @ and wr+e@. the introduction of a third unit, so that the three form a triangle of known dimensions — the units being used in pairs with the compensator—ensures the correct choice between the alternative angles. other methods of direction finding.—several other methods of per- ception of direction, not easy to classify, have found application in practice. one of these consisted in fitting in the shell of a ship, on opposite sides, two diaphragms with microphones attached, arranged so that the hydrophones thus formed were of as nearly as possible equal sensitivity, these were listened to alternately by means of a reversing switch. the ship, on account of its considerable size, was capable of giving a marked sound shadow. thus the starboard hy- drophone would give greater response than that on the port side if the sound source were to starboard, and vice versa. by steering the ship so that the responses were equal, it could be inferred that the course was directed towards the cause of the sound. the limita- tions of the method were mainly those arising from local noises; and the speed had to be small while listening took placc. in other cases large numbers of sound receivers, usually simple diaphragms whose function was to transfer the vibrations from the sea to the air inside, were inserted in the ship’s hull. good results were obtained by an arrangement of this kind, called the walser gear, in which the sound receivers were disposed at regular inter- vals on a large bulge, in the shape of a spherical segment, incor- porated in the hull on either side towards the bows. the system acted as a sound lens, a sound focus occurring at a point where the ‘sound paths’ by alternative routes were equal. application of 589 the laws of geometrical optics enabled the relation between the posi- tion of the focus and the direction of incidence of the sound wave on the ship’s side to be determined. sea sound ranging.—the methods hitherto mentioned for per- ceiving sound direction would all fail when the sounds are abrupt in character, because they all require an appreciable time for carry- ing out the necessary tests. to determine the position of an explod- ing submarine mine or torpedo we require, therefore, a different device. a suitable method is one identical in principle with that practised on land. the main variations in applying the device to the sea are that the microphones must be in submerged hydro- phones, and that a correct knowledge of the velocity of sound in sca water must form the basis of the calculations. ordinary non- directional hydrophones of the type first mentioned have proved to be quite sufficiently sensitive, but specially constructed hydro- phones, sensitive to shocks and irresponsive to general sea noises, are superior. indeed, the greatness of the distances at which explo- sions in the sea have given unmistakable impressions on the record- ing film has been surprising. small detonators serve at distances of several miles, while the explosion of 40 lb. of guncotton is operative more than 100 m. from the receiving hydrophones. many experi- ments have been carried out by the british admiralty with hydro- phones disposed in suitable positions on the east coast of britain, and the installations promise to be useful, not only for locating mine and torpedo explosions in circumstances of war, but also for naviga- tional and surveying purposes. a ship in fog, for example, could ascertain its position by exploding (with due notification by wireless to the sound ranging station) a small charge near itself in the sea; the station could within a few minutes inform the ship—again by wireless—of its position. (see surveying). an important additional method, the so-called r-a or radio- acoustic device for locating positions at sea, has been developed since the war by a. b. wood and ii. e. browne, working for the british admiralty (sce proc. phys, soc. lond., vol. 35, p. 183, 1923). it con- sists of measuring the difference of time of reception at shore stations of ordinary wireless signals (which travel practically instantaneously) and sound signals in the sea (explosions) sent out simultaneously from a ship. both incoming signals are recorded on the same moving photographic film, and such great precision of measurement has been attained that the method has served to check, and sometimes to correct, hydrographic surveys made in the orthodox manner, sea sound ranging has necessitated the accurate redetermination of the velocity of sound in sea water. this has been accomplished by the inverse of the processes just mentioned, making use of explo- sions in accurately known positions. this velocity and its variations due to temperature and salinity are now known with great precision. for example, the velocity in sea water at 15° c. containing 35 parts of salt per 1,000 is 4,935 ft. per second. ficho sounding.—the accurate knowledge of such velocities has led to another remarkable device being designed and used successfully by the british admiralty. this measures the depth of the sea at any point, and provides a means of survey of the sca bottom far more rapid than is possible by ordinary sounding lines. a description of the method has been given by f. e. smith, admiralty director of research, in a royal institution lecture on ‘‘ modern naviga- tional devices ”’ (proc, rov. inst., vol. 24, p. 342, 1924). in principle it consists of measuring the time taken for an abrupt sound to travel to the sea bottom and return as an echo after being reflected there. the sounds are made by an electrically operated hammer striking at regular intervals a diaphragm fixed in the hull of the ship; the sound receiver is a non-resonant hydrophone, also suitably immersed in the sea. the timing mechanism, which is of ingenious construction too elaborate to describe here, measures the time of the double sound journey in terms of the angular separation of two disks rotat- ing synchronously on the same shaft. this permits of calculating the sea depth, the velocity being known. an indicator in connection with the mechanism performs the necessary calculation automatical- ly so that the depth can be read off. a most important advantage of the method, in addition to its accuracy, is that the ship carrying out soundings need not stop to do so (see sounding). il, auditorium acoustics the acoustics of public buildings have been put on what approximates to an exact scientific basis largely as a result of the work of w. c. sabine (v/. frank. inst., 179, p. 1, 1915). for good hearing three conditions are necessary and sufficient. the sound heard must be loud enough; the simultaneous constituents of a mixed sound must preserve their relative intensities; and the successive sounds must remain distinct and in the correct order, and be free from extraneous noises. the extent to which these conditions are fulfilled depends on the construction of the auditorium, its shape, its dimensions and the materials of which it is composed. in already finished buildings radical alterations of the first two are not often feasible, but great improvements can be secured solely by suitable changes in the internal features. 590 the main difficulty arises from what has been called rever- beration, due to the multiple reflection of sound at different parts of the room. if the reverberation is prolonged, it means that the rate of absorption of the sound is slow. thus, in a lecture room at harvard, where these experiments were commenced, the rate of absorption was so small that a single spoken word continued to be audible for 5} seconds. successive syllables thus had to be heard and appreciated through a loud mixed sound due to the reverberation of many previous syllables, and the conditions of hearing were intolerable. great reduction of the multiple echoes constituting reverberation can be made by increasing the rate of absorption of sound. it is apparent that, in the space of several seconds, the sounds travelling at 1,100 ft. per sec. will have suffered many successive reflections, and will, therefore, have penetrated to practically all parts of the room. the sound will have become diffuse radiation, and absorbing material introduced almost anywhere will be equally effective in reducing reverberation. an open window proves to be a com- plete absorber, in the sense that it permits the egress of the maxi- mum possible quantity of sound radiation (cf. the properties of a small aperture in an isothermal enclosure in heat radiation). the introduction of cushions, carpets, wall hangings and people also largely diminishes reverberation, because of their consider- able absorbing powers. sabine has made a systematic study of the coefficients of sound absorption of various materials by the inverse method of measuring their effect in reducing the dura- tion of reverberation. measurements, for example, indicated that an audience gave an absorption equal to 44% of that due to an equal area of complete absorber, thus accounting for the improved hearing conditions that are very often found in well- filled buildings. the absorption of sound can thus be adjusted with precision, but it must not be carried too far, otherwise the sound intensity is too much diminished. the ear is able to disregard, or even to take advantage of, reverberation which is not too prolonged, and the extent of absorption has to be adjusted to the appro- priate amount. too many apertures such as open windows or doors must be avoided. sabine has also made examination of the exact manner in which sound is reflected in an auditorium by constructing scale models of the latter, and photographing the sound waves at various instants after creation, using the beautiful method due to toepler (axnalen der phystk, v. 127, p. 556) and elaborated by r. w. wood (physical optics, 2nd ed., 1911, p. 94). by this means the positions of the sound waves, both incident and reflected, are capable of observation at all instants and at all points in the model room, and they provide data upon which can be based correct architectural construction from the acoustic point of view. iv. miscellaneous advances absolute measurement of sound.—a, g. webster (nat. acad. sci., proc., §, pp. 173, 1919) has advanced to a considerable extent the methods of absolute measurement. for this purpose it is impossible to rely upon audition, handicapped as it is by the vagaries of the ear. what is required is a reliable mechanical device, the performance of which is constant, to record the sound vibrations with sufficient magnification. webster has made an exhaustive study of the properties of various materials, and has constructed from those most suitable for the purpose two instruments which he has called the phone and phonometer respectively. the “ phone” provides a means of creating a simple tone of intensity and frequency which are under control and capable of exact measurement. the phonometer is an instru- ment for measuring absolutely the vibrations received by it. it consists of the combination of a diaphragm and a resonator, both of which are adjustable in frequency. the motion of the diaphragm is observed by making it a reflector and part of a michelson interferometer, so that the amplitude is measured in terms of the wave-length of suitable monochromatic light. in practice the interference fringes are photographed on a moving film upon which they appear as wavy lines. against this instru- ment, which is regarded as a standard, other portable phonomce- ters can be calibrated, these depending on the simpler process of sound the deflection of a beam of light set into angular oscillation by the receiving diaphragm. with such instruments and also with d. c. miller’s phonodeik (referred to later), l. v. king has carried out an elaborate investigation on the propagation of sound in air and fog-signal efficiency (phil. trans., v. 218, p. 211, 191g) in the region near father point, quebec. king, in this paper, also describes a modification of the siren called the dia- phone, used as a standard source of sound in his research. analysis of sound.—webstcr’s phonometer described above is a resonant instrument, and, therefore, unsuitable for the anal- ysis of mixed sounds. much progress has been made, however, in the analysis of such sounds, using non-resonant recorders— for example, d. c. miller’s phonodeik. this instrument, which depends on the motion imparted to a tiny mirror by the opera- tion of a vibrating diaphragm, is described in miller’s science of musical sounds (1916), where also will be found the results of the analysis of various sounds. in such instruments the sound record is of the ordinary type, and consists of the trace on a moving photographic film of a spot of light vibrating at right angles to the motion of the film, thus forming a transverse wave. records of a different type have been obtained by a. o. rankine (proc. phys. soc. lond., v. 32, p. 78, tor9 and nature, v. 108, p. 276, 1921), in which the sounds are caused to vary the intensity of a narrow beam of light, which gives on a moving film a line image perpendicular to the motion. the record thus consists of a negative film of varying transparency along its length. it is not so suitable as transverse records for direct analysis of the component frequencies, but it has the advantage that it admits of reproduction of the sound by means of a selenium or other photoelectric cell, such as is used in phototelephony. this arrangement constitutes a novel type of phonograph oper- ated by light, first invented by ernst ruhmer in 1900, but hitherto little known. mainly owing to the demands of wireless broadcasting (q.v.), the question of the composition of sounds, particularly speech sounds, has become acute, and many investigations on this subject in relation to more perfect telephonic reproduction have been carried out since 1922. there has been besides a remarkable advance in out knowledge of the structure of speech due to the work of sir richard paget (proc. roy. soc., a., vv. 102, 1923, and 106, 1924), who has examined systematically by direct listening the component characteristic frequencies of vowel sounds in particular and, to some extent, consonants also (see voice sounps). the accuracy of these observations is established by ‘the fact that speech sounds have been imitated artificially by employing the appropriate frequencies, and a speaking machine, consisting of the hands, a reed and bellows, called the cheire- phone, has been evolved. speaking films.—vrogress has also been made in the applica- tion of photographic sound records, in conjunction with photo- electric cells, as mentioned above, to the synchronous reproduc- tion of pictures and sounds (see nuture, v. 108, p. 276, 1921). bergland in sweden, masolle, vogt and engl in germany and de forest in america appear to have been the principal experi- menters, and de forest phonofilms are now (in 1926) to be seen and heard in certain british picture theatres. in this particular phonofilm the sound record, which is of the varying transpar- ency type, occupies a strip about 75 in. wide on one side of the picture series. it is made by sounds acting microphonically on the luminosity of a special lamp to which the moving film is exposed through a slit. for reproduction a photoelectric cell, under the influence of the fluctuations of light caused by the developed film record, operates loud-speaking telephones, the necessary intensity being secured by sufficient valve amplifica- tion. the pictures and the corresponding sound record being on the same film, synchronism is perfect, even after the fracture and repair of the film which frequently occurs in practice. what one sees and hears are, of course, but imitations of the original appearances and associated sounds, but the double deception of the eye and ear is in fact almost, if not quite, complete. the phonofilm reproduction dives more nearly than the silent moving picture. sounding bibliography.— w. ht. bragg, the world of sound (1920); lord rayleigh, collected papers, vol. 4 (1920); dictionary of applied physics, ed. sir r.t. glazebrook, vol. 4 (1923); w.c. sabine, collected papers on acoustics (1923); h. lamb, the dynamical ig heory of sound, 2nd ed. (1925). (a. o, r.) sounding (sce 25.460).—from the year 1o1r until the com- mencement of the world war there was no improvement in the methods of deep-sea sounding; but certain advances were made in connection with sounding in shoal water, the best known british systems being called after their originators, ‘ douglas- schiffer”? and “ somerville’; these two appliances are now fitted in all british surveying vessels, and enable accurate and rapid soundings up to 30 fathoms to be obtained easily when the vessel is steaming at the rate of about four knots. both methods use a wire traveller carrying a lead between the stern of the vessel and a spar projecting from the bows, and entirely prevent the entry of the lead into the water until it is disengaged atits greatest distance forward, thus enabling a vertical sounding to be obtained as it passes the stern. effect of the war.—during the five years of the war original methods of sounding were not improved upon, this being largely due to the lack of time for experimental work in this direction, and also because after the war, and even now (1926), certain nations still treat certain war inventions, to some extent, as ~ confidential. perhaps the non-circulation of such information may be due to the aversion to publication of imperfect data, as well as to the necessity for safeguarding commercial rights to entirely novel appliances. although not directly applicable to the subject of ocean sounding, reference must now be made to subaqueous sound- ranging, a form of the utilisation of sound-waves which proved eventually to have such a remarkable effect upon the former. subaqueous sound-ranging is of distinctly higher accuracy than that through air, owing to the velocity of sound through water being considerably greater and comparatively unaffected by extraneous causes; in water, sound travels at about 4,900 ft. per sec., which is more than four times its rate through air. this method of obtaining a distance by sound was used largely in connection with the detection of submarines, and was invalu- able in accurately fixing a position which could not have been ascertained otherwise. sound waves were also utilised for locat- ing the explosion of mines and for fixing any positions at which a small explosive charge was fired intentionally. after such an explosion the sound travelled through the water to two or more sets of suitably placed hydrophones whose positions were ac- curately known and which were connected to organised shore stations; after the reception of the sound of the explosion by the hydrophones, not only was its approximate distance obtained, but, by mathematical calculation, a line of direction was also given. thus the combination of these distances and lines of direction from two or more hydrophone stations gave the position of the explosion with considerable accuracy. in practice when used in the north sea the results were obtained without difficulty up toabout 200 miles. the greatest distance for which this method was utilised was in the case of obtaining the position of vessels sunk during the battle of jutland, which was nearly 300 m. from the english coast. the value of this method in obtaining accurate and rapid positions for sounding purposes in connection with the numerous minefields tn the north sea cannot be overestimated, and its enormous utility is especially realised when the thick weather which is so prevalent there prevents the taking of all astronomical observations, and, further, when most of the positions were out of sight of land. utilisation of war methods for purposes of peace-—on the termination of the war, inventions which had been utilised during its progress were turned, when possibile, to the purposes of peace, and a large amount of material and knowledge gathered together in connection with subaqueous sound-ranging were gradually utilised in the direction of sounding, and in this special field the utilisation of acoustic sounding in substitution for the direct measurement of oceanic depth is its practical result. o91 echo sounding.—this new method is known as echo sounding, and depends upon the principle that any sound originating at or near the surface travels through the water at a known speed, and after striking any large object or the bottom of the sea is reflected back to the surface, the time between the emission of the sound and its reception back again being a matter for accurate measurement. while the value of 4,900 ft. per sec. may be taken as the aver- age velocity of sound in water, it is obvious that the question of an exact value of this measurement is a most important item in this connection. experiments are being carried out by several nations on this subject, which involves questions of depth, den- silty, temperature, salinity, etc., all of which play a vital part. in these investigations. the perfection of some means of obtaining an accurate measure- ment of the time from the moment when the sound is first produced until its echo is received back is therefore the practical problem to be solved satisfactorily, and if a form of apparatus carrying out the above requirement is produced, a succession of measurements, 7.c., soundings, can be obtained easily. experimental work. nations are known to have carried out much experimental work in the perfecting of such an apparatus as is required—france, the u.s.a., germany and great britain are those especially prominent in this direction, and each nation’s investigations have resulted in evolving its own special form of apparatus for carrying out echo sounding; in this article, no attempt has been made to describe the devices of the various nations; the whole subject is at this date in an experimental stage, and it appears more than doubtful if any of the existing appliances will survive the test of time in their present form. use of system by french—the french hydrographic service was the first known to have made practical use of echo sounding, and in april 1922 one of its surveying vessels ran a line of echo soundings across the mediterranean from marseilles to philippe- ville in algeria, these soundings being required in connection with a projected submarine cable. the existing recorded depths were found to be very similar to those obtained, and the result generally was most successful; the majority of the depths were slightly under 3,000 metres (about 1,000 fathoms). the acoustic apparatus employed by the french was invented by monsieur marti, a hydrographic engineer of the french navy. all the surveying vessels of the french navy are fitted with some form of sonic sounding apparatus, and the apparatus is also supplied to some of the larger vesscls of the navy itself. utilisation by u.s.a.—in june 1922 the u.s.s. “ stewart ” ran a line of echo soundings from american waters to gibraltar; the voyage occupied about nine days, during which about goo soundings were taken, the depths varying up to 3,200 fathoms, and the time between the soundings usually being about 20 min.; no difhculty whatsoever was experienced in taking these sound- ings, and the speed at no time was reduced for this purpose; the average speed was 15 knots. the apparatus used on this occasion is called the “‘ navy sonic depth finder,” and was developed by dr. hayes at the engineering experimental school at annapolis, maryland. at the latter end of 1922, two american vessels were fitted with a similar form of apparatus for echo sounding, and carried out an extensive bathymetrical survey off the west coast of north america, at the request of the carnegie institution of washington, about 5,000 soundings were obtained during the survey, and the resulting chart has proved of great value in connection with those disturbances which have been unfortunate- ly so prevalent on this coast. in those regions in which land changes occur frequently, their study will be immensely advanced by a greater knowledge of the off-lying ocean depths, now rendered feasible by this new form of sounding. in the year 1923 an american cruiser was fitted with the sonic depth finder and crossed the south atlantic to the cape, and thence proceeded up the east coast of africa. the apparatus was in constant use during the whole voyage, its results being found in close agreement with the existing soundings on the chart; and from a 592 navigational point of view it proved of great value, especially in the dangerous vicinity of cape guardafui, which was rounded by its means during thick weather which entirely prevented land being sighted. a sonic depth finder is now fitted in several ships of the american navy for navigational purposes, and in all surveying vessels. german ap puratus.—mention must now be made of a german apparatus, invented for the purpose of obtaining sonic depth soundings, by dr. behm. a stucdy of the question of submarine explosions and their reflection from the bottom of the sea was commenced for this purpose by him about the year 1912, but further improvements are required before the invention can be described as being a practical success. use by brilish navy.—the british navy have now perfected an apparatus which carries out all the necessary purposes of the sonic depth finder; it is comparatively simple in character; no calculations are required, the actual depth being read directly on a dial. this apparatus is produced in two patterns, the first being for use in comparatively shoal water and a second for ob- taining deep-water soundings; the first is quite sufficient for the purposes of ordinary navigation, while the second is suitable for hydrographic, oceanic investigation and for submarine cable requirements. all british surveying vessels in home and foreign waters are now fitted with the latter apparatus. reference must be made to the utilisation of ultra-sonic waves by professor langevin, of the paris university, in connection with deep-sea sounding. this method makes use of sounds in water which are comparable with the hertzian waves of the air. there appears to be a great deal to be said in favour of this apparatus, but more experiments with it are required before it can be said to be a complete success. in spite of the undoubted success of echo sounding in obtaining deep-sea measurements, the old apparatus for wire sounding will be required for oceano- graphic investigations, as specimens of the bottom tempera- tures and salinity will still be necessary, but the number of such observations will be very small compared with those required for deep-sea sounding. since 1874 the method of sounding by wire as described in vol. 25,461, has been in practice, and was, in its turn, an immense step over the former clumsy method of utilising hemp, but the advantage of echo sounding as compared with that by wire is so enormous that practically there is no comparison. in the present case the apparatus is unlikely to be damaged in any way, whereas under old conditions not only was it extremely likely that the apparatus itself might be injured, but the loss of both wire and lead was unfortunately not infrequent. the time necessary for obtaining a sounding is almost infinitesimal, the sounding in ex- treme depths taking little more time than one in the shallowest water. the speed of the vessel is maintained throughout the soundings, and the soundings themselves may be accurately taken at extremely short intervals. bad weather does not affect the work, the complete apparatus is of moderate cost, and if even this were gradually lessened could be fitted in all ocean going vessels, and become therefore as useful for general navigation purposes as in the special fields of hydrography and oceanic research. sec also surveying, nautical. ch) see the itydrographical review, vol. 1, 2 and 3, published by the international hydrographical bureau, monaco. sound ranging.—the method of locating hostile guns by the sound, or sounds, consequent on their discharge was intro- duced on the british front in france during 1916. it works as well at night or when, owing to fog, mist or smoke, the visibility is poor as on clear days; it can detect batteries so well hidden as to be invisible from the air or on air photographs; it 1s always ready when once the apparatus has been installed; and a location can be obtained, under favourable conditions, within a minute or two of the arrival of the report of the piece. the chief dis- advantages of the method as used in the world war were that the installation necessitated the laying and maintenance of sev- eral miles of wire; that the method would not work well during a heavy bombardment; and that certain weather conditions, to be specified later, rendered location almost impossible. sound ranging the two latter difficulties seem insuperable. the wiring trouble can be minimised in two ways: either by devising a method of wireless transmission of the signals from the instru- ments to the central station, which is quite feasible, whatever the practical difficulties in reducing the weight and cost of the apparatus to reasonable limits may be; or by modifying the method of time recording so that much shorter bases can be employed. considerable progress has been made in this direc- tion. the method has been elaborated to permit the directing of fire on a hostile piece by comparing the record of the sound of discharge of the piece with that of the burst of the shell di- rected against it. with 12-in. and g-2-in. howitzers destructive shots have been directed very successfully by sound ranging. principles —the method generally adopted in the british, french and american armies is to record the instant of the ar- rival of the sound made by the hostile piece at certain fixed and carefully surveyed posts, spaced at intervals varying from 1,000 to 2,000 yards. if it be assumed that the sound spreads out from the source with a known velocity, the same in all directions, then a known interval between the arrival of the sound at two fixed posts will determine a curve on which the source must lie. this curve is a hyperbola with two posts p; and ps. as foci, for the determining condition is that the difference of the radii vectors sp; and sp. be constant. if, in addition, the time of arrival at a third post be known, then the interval between this and the time of arrival at either p; or p, will fix a second hyper- bola on which the source must lie, and so determine the position of the source. in practice three posts are not sufficient, since any uncertainty caused by the recording of a spurious sound at a post would falsify the location. in general six posts are used, which, taken consecutively in pairs, give five lines which should all intersect at one point. any accidental selection of the record of a spurious sound at one or more of the posts is then at once detected by the non-intersection of the curves. records of the sound at five, or indeed four, of the posts are generally sufficient for the experienced sound ranger, even when several guns are being recorded at short intervals, so that the use of six posts allows for the sound not being successfully recorded at one or tio of the posts. instruments.—the instruments comprise: (a) detectors, placed at each of the six surveyed posts, which give an electrical response to the arrival of the sound, and (8) a recording instrument, placed at a central station and connected by a separate circuit to each detector, which registers the exact time at which each detector responds. (2) the method described assumes that the sound travels out from the source in all directions with the same velocity, which is true of the sound of discharge of the piece. the passage of a projectile through the air with a velocity greater than that of sound gives rise, however, to the so-called ‘‘ shell-wave,” which travels with a velocity above the normal in the forward direc- tion, the velocity being greatest along the line of fire. (this “ convection ” of sound by the shell, if it may be so named, was explained by mach and others as long ago as 1889.) the ear and the ordinary carbon microphone, if situated some distance in front of the piece, usually hear the shell-wave better than the sound of discharge. for sound-ranging a special type of micro- phone, particularly sensitive to the low frequency disturbance of the sound of discharge, was invented by maj. w. s. tucker. it depends on the principle utilised in the hot-wire anemom- eter, i.e., the change of electrical resistance consequent on the change of temperature of a heated wire which ensues when the air round it is set in motion. a very fine wire of platinum, whose resistance at atmospheric temperature approaches 100 ohms, is mounted in the form of a grid over a circular hole some seven mm. in diameter. it is included in one arm of a bridge circuit, and sufficient current passed through the network to heat the wire to a dull red. the bridge is balanced so that when the air round the wire is undisturbed no current passes through the galvanometer. motion of the air causes the resistance of the wire to decrease, upscts the balance, and so causes a current to pass through the galvanometer. sousa—south africa, union of the microphone wire is mounted in front of an air container of some 16 litres capacity. the instrument so completed is in- sensitive to all sounds of speech, musical sounds, traffic or even rifle fire. it responds readily, however, to gun sounds (which are low frequency disturbances) even when they are inaudible, and records also the shell-wave. its reaction is very rapid, and the small lag which does occur appears to be the same for all similarly constructed instruments. (b) the recording instrument used is the einthoven galva- nometer, in which the moving part is a very fine wire (through which flows the current to be detected) mounted in a magnetic field. the wire moves in a direction normal to its length and to the direction of the field. six wires, insulated from one another, and provided with separate terminals, can be mounted side by side in the field produced by a single small electromagnet. this provides in a small space what is essentially six independent galvanometers, one of which is included in the bridge belonging to each microphone. as the sound reaches successively different microphone posts the corresponding galvanometer wires move in rapid response. the instant at which each wire begins to move is registered on a moving photographic film or sensitive paper strip. the camera in which the film runs vertically is furnished with a horizontal slit, a cylindrical lens in front of the slit reducing its effective breadth. shadows of the perpendicular galvanometer wires, cast by means of an electric lamp and an optical system mounted in the pierced poles of the galvanometer magnet, fall on the slit, and are focused on the film, appearing there as six points of shadow on a horizontal line of light. as long as the wires are still each point leaves on the running film a straight line; the movement of a wire registers itself as a break in this trace. a special device, by which a tuning-fork effects a periodic interruption of the light beam, records a time scale on the film itself, so that the distance between parallel lines on the film corresponds toa given time interval whatever be the rate at which the film is running. originally the film was cut off after the required record had been taken, and developed in a small dark room adjacent to the instrument. later a method of automatic development was devised, by which the film passed successively through developer and fixer while running, and emerged ready for interpretation. influence of weather conditions.—the method in use demands that to every time interval shall correspond an exact distance, a standard velocity of sound being assumed which corresponds to some standard temperature, and still air. the intervals measured have, therefore, to be corrected for the prevailing temperature and wind; the velocity of sound is independent of the pressure. it is found that the temperature and wind which are concerned in these corrections are not those prevailing at ground level, but at a height of between 250 and 500 ft. up. owing to the refraction of sound by wind the record of a given sound at ground level is greatly influenced by the variations of wind velocity at different heights above the ground. this wind gradient determines largely whether the conditions are favour- able or unfavourable for the detection of sounds. in the case of a wind increasing in velocity with height, a following wind, be- sides increasing the velocity of the sound, tilts the wave front so that the sound converges on the listener or instrument on the ground, and is well heard. an opposing wind causes the sound to tend to pass upward and leave the ground. hence a wind blowing from the instruments toward the hostile piece often renders sound-ranging almost impossible if it be of any strength. the temperature gradient also affects the refraction of sound. work in the field.—it is not feasible to have the film running continuously during any period when records are expected. it should be started a second or two before the sound reaches the first microphone. in the field this is effected by having two for- ward observers in front of the line of microphones, so placed, one to each flank, that either the one or the other of them must hear the sound of the hostile piece a few seconds before it reaches any of the microphones. these observers are provided with keys, the depression of which starts the film running. 993 the microphones may be placed anywhere where the hearing is good: the only obstacles which se¢em to cast sufficient sound shadow to affect them are high hills just in front of them. they may be put in shallow depressions dug for them, and should be protected from splinters, and also from wind and draughts. canvas and hurdles may be used for this latter purpose without appreciably affecting the sensitiveness of the instruments. ranging on hostile pieces —a heavy burst near a hostile gun position will furnish a sound record of ifs position just as does the gun itself. fire may be directed on a piece which is in action by comparing the records which it supplies with those of the bursts of one’s own answering shell. since both gun and burst are located by the same method all uncertainties introduced in an ordinary location ly ignorance of the precise atmospheric conditions are climinated. a differential method is adopted, the difference of times of arrival of the sound of the hostile gun and of the friendly burst at each microphone being plotted as ordinates against a certain simple function of the relative positions of the microphones and the hostile gun as abscissae. a horizontal line then corresponds to a direct hit; a straight line sloping to left or to right to an error of line to one side or the other; a curve (approximately an elliptical arc) convex or concave downwards to an error of range. the magnitude of the corrections neccessary is easily estimated from curves previously prepared (see artillery). (e. n. pa c. a.) sousa, john philip (1854- ), american bandmaster and composer, was born at washington, d.c., nov. 6 1854. studying the violin from an early age, he appeared at the age of 11 as a solo violinist, and played in various orchestras, also acting as conductor. in 1880 he was appointed leader of the band of the u. s. marine corps, serving in this capacity till 1892, when he resigned in order to organise sousa’s band. under his directorship this band rapidly acquired a foremost position in the united states, and made several successful tours of the world. on may 31 1917 he was appointed lieutenant- commander in the u.s. naval reserve force and assigned as musical director to the naval training station, great lakes, illinois. as a composer of marches he acquired an international reputation, among his greatest successes being the washington post, stars and stripes forever, under the double eagle, thiundercr and king cotton. he also composed the music for a number of comic operas, including ei cupitan (1806), the charlatan (1897) and the bride elect (1898). in 1890 he published a compilation of the national patriotic aud typical atrs of all countries.