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Les diagrammoe suivants illustrent la mAthode. 1 2 3 1 2 3 4 5 6 MKaOCOPV nSOWTION TBT CHART (ANSI o"d ISO TEST CHART No. 2) A ^IPPLIED B V MGE he 1653 East Main %\f—\ HoctWilw, N«« York t4«09 USA (716) 48? - 0300 - Phon* (7t6) 2M-3S89-fa> FROM THE TRANSACTIONS OF THE ROYAL SOCIETY OF CANADA SECOND SERIES— igoi-igoa VOLUME VII SECTION :il MATHEMATICAL, PHYSICAL ANO CHEMICAL SCIENCES A Century of Progress in Acoustics PRESIDENTIAL ADDRESS TO SECTION By PRESIDENT T. LOUDON, LL.D. rOK 8ALB BY J. HOPE ft SONS, OTTAWA ; THE COPP-CLARK CO., TORONTO BERNARD QUARITCH, LONDON, ENGLAND igoi ii SicnoN III., 1001. [43] Tran-. R. S. C. VII. — A Century of Progress in Acoustics: — Presidential Address to Section. By Presidext J. Loldon, LL.D. (Head May SI. 1!M>1.» In soleoting.thc I'rogrcss of Acoustics, on its experimental side, as the .-subject for tins years I're^^idential A(!dre;.>, 1 am fully alive to the fact that this branch „f ^t. iice has been comparative'" -glected by iihysicists for many years, and that consequently I eanirt). 'lope lo arouse the interest. which the choice of a more popular subjtc migit command. It is, however, just because of this neglect of an important field of science that 1 conceive it to be my duly to direct jome atten- tion thereto. This duty 1 can be^t perform, it seems to me, by taking n -arvey of the work accomplished in this particular lield during the . ntury that has just closed. Such a survey will make it evident not only that the science of acoustics has made iinuunse progress during that time, but also that many of the exiierimental n}elliiHl> in use in other branches of jiliysical science were invented and first iinploycd in the course of acoustical research. This latter fact, though not gui- erally recognized, furnishes an iihl^tralion of the interdependence which exists between '.ho various branches of p'.iysical science, and suggests the prohability that the work of acoustical nsoarch in tlic future may be advanced liy experimental nielliods spn ially ucsigned for inves.tigation in other liehls. A revival will, of course, come in time for acoustics, as it has recently come for electricity, and it ought to come all the sooner because of the en-operation which jdiysieists may naturally look for from those who are cultivating the new fields of experimentiil psychology. In order to avoid the tedium of a bare enumeration of discoveries arranged chronologically. I propose to n fer, in tlie lirst instance, to the invention of the various experimental metnods which have !>een employed in acoustical research. A separate reference to these methods will enahle us to appreciate their potency in the idvancement of this science. The earliest of these methwls is duo to Chladni whose work "Die iV istik" appeared in the form of a French .translation in 18(i!t under the title "Traite d'Acousticpie do Ciiladni '". In this work were col- lected all the researches on the vibrations of bodies which ('iilndni had conducted with the aid of the new method (methode de sable.) This method consists in distingui>hin<., on the surfaces of vibrating bodies, 44 ROYAL SOCIETY OF CANAIU the parts which are vibrating from the parts which are in repose, by means of the sand which is driven from the former to collect on the latter. In th'^se experiments of Chladni on plates, etc., the violin bow was used for the first time to rcxluce the necessary vibrations. The bow had previously been used only for vibrating: cords, the " violon de fer", and other musical instruments. Chladni madu his discovery of sand figures in 1787, having been led thereto by Lichtenberg's dis- covery of electric figures. The transversal nodal lines given by Chladni's method in the case of rods vibrating longitudinally were readily explained. Not so, how- ever, the complicated nodal lines presented by vibrating plates, or the alternate lines which appear on the two sides of rods vibrating longi- tudinally, and which sometimes also appear on rods vibra*'ng trans- versally. It was not until 1833 that an explanation of the . .mtr of tliese phenomena was offered by Wheat.-tonfc's theory that the nodal lines were due to the superposition of transversal vibrations, corre- sponding to sounds of the same pitch coexisting with nspect to differ- ent directions in the plate. This theory was. confirmed experimentally in ]8(i4 Ijy Kudolph Ka-nig who constructed rectangular plates giving unison notes corresponding to different sets ,of nodal lines paraJlol to two aosai of the acoustician. A se<^ond optical metliod we o^e to Biot who. in 1S20. showed that the changes in density at the nodes of a transparent body vibrating longitudinally could be n'xhibitrd when the nooal line of the bmly is placed between the cross- ,:iirrors of i i)olarization apparatu=i. "During 46 liOYAL SOaETY OF CANADA the continuance of the vibrationB the image is highlj illuminated in the analyser and becomes darkened when the vibrations cease. This method was developed much further by Kundt in 1864 and by Mach in 1873. A third optical method was devised by Toepler and Boltzmann in 1870 for the purpose of exhibiting the clianges which take place at a nodal point of a vibrating column of air. This method consists in producing interference bands by means of two rays of intermittent light from the sam" source, one of which passes through the air in its normal state, and the other through a nodal point of the vibrating air column. A vibratory movement of the interference bands results, a movement which can be made as slow a.^ we p'ease, thus rendering it possible to deduce by stroboscop methods exact measurements as to the inovement of the air at the nodal point. Method of Manomelric Flames. — The object of the metliod of manometric flames, invented by Rudolph Kcenig in 1862, is to furnish an ocular proof of the variations in density at a point of the air tra- versed by waves originating in another body or in the air itself. A short description of the first apparatus based on this method appeared in PoggendorlFs "Annalen" in 1 864. Between that year and 1872 the method was appiief *o a series of instruments, the experiments being described ir the icv.ie Journal in a long memoir entitled "Les Flammes manometriqucs". Although this method is extrcnuly sensitive and capable of furnishing very accurate results, it has been prevented for a long time from rendering more efficient service on account of two causes: first, the want of snilicient brightness in the reflected images of the jumping flames, and second, the difficulty of observing the de- tails of these images owing to their niomentarj" appearance in the mirror. The former of these difficulties has now been overcome by the employment of acetylene and otlier gases, which at the same time allow admirable photographs of the flames to be taken, thu^^ obviating the second difficulty also. AYe owe an important paper on this subject to Professors E. L. Nichols and Ernest Merritt, published in 1898 in the "Physical Review". Kundt's Method. — In 1865 Kundt published his mcthori of using light powders for the purpose of exhibiting the vibratory char- acter of stationary air waves in columns and plates of air. During the existence of these vibrations the light powders arrange themselves in transversal stria which collect around the loops, and are wanting at th (ies. As in the case of the nodal linr-s on Chladni's plates, a 6h .o^actory explanation of these striae was for a long time wanting. In 1890 Professor Walter Koenig showed from hydrodynamical considera- [loudon] A CENTURY OF PROGRESS IN ACOUSTICS A7 tions, that the partic ^ of the powder necessarily arrange IhemselyeB in planes at i ht a jles to the direction of the vibratory movements, and that their oV i-ved distribution at the loops and nodes is in ac- cordance with the same laws. Method of Slow Movements. — Before th' invention of the \jt^ ceding methods the acoustician ocoasionally resort;ed to the device of deducing the vibrations of a sounding body from the behaviour of a similar body whose movements were of suiiicient amplitude to be seen by the eye, and so slow that they could be readily counted. In this way Mersenne counted the vibrations of a cord 15 feet long under a stretching force of 1 pounds, and foun'^ them to be 10 per second. In shortening the cord to 1-20 of its length, he obtained an audible sound whose pitch he concluded co^ "ponded to vl.)rations per second. In the same way Chladni employed a long . tliin metal .od, which gave in the first instance only 4 v lirations per second. He then shortened the rod until it ga\e t" auilibie sound, whose pitch he determined from the law expressii - the rolation between the lengtli and the number of vib. ^ us. Thit i lethod, however, which appears so 'mple in theory is sv^jjeot to larg? errors and gives in practice very poor results. The Stroioscopic Method. — Mersenne's and Chladni's method has accordingly given place to another — the stroboscopic — which allows the vibrations of the sounding body to be viewed dii-ectly, its move- ments relatively to a vibrating eye-piece being rendered as slow as we please. The first use of stroboscopic discs for the purpose of observing very rapid periodic movements was made by Plateau in 1836. His discovery, however, remained unnoticed, for Doppler in 1845 published a note on the same subject, without referring to Plateau's discovery. It was Tcepler who first made the m' hod generally known by employing it in a series of acoustical experiments, which he published in Poggen- dorff's "Annalen," volume 128. In the earlier applications of this method, the view of the vibrating body was rendered intermittent by looking through slits which were opened and closed in rapid succes- sion. This plan was modified by Macb who caused the vibrating body tc be illuminated by intermiaent light. If now we allow the stroboscopic images of a moving body t(i fall on a photographic plate, giving the plate a movement of translation which is arrested before eai.a appearance of the image, we thereby obtain a series of ])hotographs of t' "^ successive positions assumed by the body. If, further, matters are so arranged that the beginning and dur-'.ion of the phenomenon are traced on the images, we have a new metiiod, which is called Chronophotograr>hy. It was M. Janssen who first conceived the idea of taking autcmatically a series of photo- 48 ROYAL SOCIETY OF CANADA graphic images in order to determine the successive positions at dif- ferent times of the planet Venus in its passage across the sun. It was Janssen Iso who, in 1876, first suggested the idea of applying succes- sive photograms to the study of animal locomotion. The analysing of such movements was first accomplished by Muybridge of San Francisco. The method has been largely extended and perfected by M. Marey, who has employed it in studying the locomotion of all sorts of subjects, from men to insects. Electric Transmission. — The electric transmisirion of sound was first accomplishiHl by Philipp Reis in 1864. The discovery, however, not having been properly announced to the scientific world, did not receive the attention it deserved. Had it been published by Poggendorfl, to whom it is said a preliminary note on the subject was sent, Graham Bell's invention of the telephone (1876) woukl probably have been reached at an earlier date. The Wave-Siren Method. — The last of the methods to 1k> noticed is that employed by Rudolph Kcenig in his Wavc-siron. In this instrument a metal band or disc with curvilineal edges passes before a narrow slit from which issues a current of compressed air. By means of these discs we can produce either simple sounds, or sounds of various timbres, containing such harmonics as we please, the intensities and phases of the latter being varied at will. The first wave-siren was constructed in 1867, and the account of the first series of experiments was published in 1881. The mere enumeration of the methods of acoustical research which have been devised since the days of Chladni is an indication of the enormous advances which have been made in (hU branch of science. It remains now to state more particularly what these adtlitions to our knowledge of acoustical phenomena have been. This can be most conveniently done under the following heads, viz:— the velocity and diffraction of sound; its pitch, intensity and timbre; and tlie phenomena produced by the coexistence of two or more sounds. The Vehcity of Sound.— Ijong before the beginning of the last century it had been observed that the propagation of sound was not instantaneous. Mersenne in fact had tried to estimate the velocity by exiKTinients on ccliors. and by countint the time which elapses between the flash of a gun and the report. The latter exporinicnts were nlso repeated by Kircher a« well as by the Academy of Florence in 1660. Tlio same experiments were Mil)S(M|U<>ntly. in 1738, nndertaken by n.cmlierB of the Academy of Sciences at Paris; l»y r^avants, such as Kipstner. Benzenberg. (loldingham. and by otlmrs; but tlie n\<*ult.^ obtained did not gain the confidence of the scientific world. A new scries of experiments was accordingly undertaken in 1822, on tlie sug- [LOUDON] A CENTURY OF PROGRESS IN ACOUSTICS 49 gestion of Laplace, by members of the Bureau dos Longitudes, to determine the velocity in air and other media. These experiments, which were the beginning of truly scientific work in this subject, were performed by Prony, Arago, Mathieu, A. de Humboldt, Gay-Lussac and Bouvard, between Montlh6ry and Villejuif, cannon bemg fired at botli stations. The result obtained was 331 m. at zero temperature, with an increase of 0.6 m. for each degree above zero. In the course of these experiments it was observed that the cannon fired at ViUejuif were all distinctly heard at Montlhery, whilst the reciprocal reports wore BO faint that only a small number were hoard. Tyndall long after- wards, in 1875, explained this curious phenomenon, attributing it to the existence at Villejuif of a heterogeneous atmosphere, eaused by the heated air which came from Paris. Since the memorable experiments of the Bureau des Longitudes of Paris, varioufl individuals have from time to time undertaken to solve the same problem. Among these may be mentioned MoU and van Beck (at Utrecht), Gregory Woolwich, Stone and Captain Perry in voyages to the polar regions m 1822, 1824, and Kendall in the Franklin expedition in 1825. In some of these experiments the temperatures ranged from 2" to-40% the results obtained according with the theoretical values. In 1823 Stampfer and Myrback conducted experiments botwoon two stations in the Tyrol at a difference of level of 1364 m.; a similar experiment being undertaken in 1844 in Swit- zerland by Bravais and Martin with a difference of level of 2079 m. Both experiments confirmed the law that the velocity of sound in air ig independent of the pressure. In all these experimenta the exactness of the results was affected by the difficulty of estimating accurately the time between the percep- tion of the flash and that of the report. Differ, nt observers of cj.uree gave different estimates. This source of error was first eliminated by Victor Kegnault, who in his long series of researches between 1860 and 1870 made use of the graphical method, electric signals being employed to measure time intervals. Regnault's experiments were conducted in 7 tubes (part of the Paris sewers) varying in length from 70 ra to 4900 m., and of diameters from 0.11 m. to 1 .10 in. Experi- ments were also conducted in the open air by means of reciprocal .hots fired from two stations at a distance of 2445 metres. The number of the shots fired was 334. These researches of Regnault represent such an enormous amount of work that I shall attempt to give only the principal conclut'iona deducible from them : 1 In a cylindrical tube the intensity of the wave vanes, dimi- nishing with the distance. The narrower the tube, the more rapid is the diminution. S«T. 111.. 1(101. 4. BO ROYAL SOCIETY OF CANADA 2. The velocity of the sound decreases as the intensity diminishes. 3. The velocity approaches a limiting value, which is higher, the greater the diameter of the tube. The mean value in dry air at 0° in ;i tube of (liaiiictcr 1.00 ni. is 330.6 m. 4. The velocity is not affected by the mode of producing tlie .sound wave. 5. Tlie velocity in a gaa is independent of the pressure. 6. The ratio of the velocities in air and any other gas is -. / , where ^i is the density of the gas, supposed perfect. 7. The average of the results of all the experiments in the open aix is v=330.r m. at 0°. lii'frnault wns also the first to attempt direct experiments for deter- mining the velocity of musical sounds. In this case, however, the electric signals and the graphical recording apparatus were not &eiL<\- tive enough to respond to the front of the wave, and it became neces- sary to resort to the ear alone. In these experiments Regnault had the co-operation of Koenig as observer, with whose assistance it was shown that : 1. A note does not change sensibly when it traversee long distances in tubes of large diameter. 2. When the sounds are observed by the ear tl velocity of high notes appears to be less than that of low ones. Tlus may be due to the more ready response whicli the tympanum makes in the case of low notes. 3. In raversing tubes of great length, a note does not preserve its timbre, being resolved into its simple components. Regnault's experiments have recently been repeated by M. Violle in the large sewers near Grenoble and Argenteuil, some of Regnault's a)>paratus being employed for the purpose. The results of these experi- ments have not, however, Iwen yet published. Dijjradion. — The phenomenon of the diffraction of sound was first oxpc'rimentally shown by Ix)rd Raylcigh in 1880. I'Uch. — I5efitr<_' the last century, as already mentioned, Merscnno had attempte*! to delennine the vibrations of a cord by deducing lliom from very slow vibration > of the same cord when lengthened, riiliuliii's tonometer, which consisted of a vibrating metal rod of variable ' 'ngth, \\ii> baswl on the same principle. In 1819 dignianl do la Tour inventini the siren, a inucli ^upe^ioI• instrument, but incapable nf giving very exact results nut« ilh-tandin;,' the simplicity of il< mechanism. The same remark may be made of the toothed wheel invented by Savart in 1830. [loudunJ a century OF PUOGKESS IN ACOUSTICS 51 A most important step in advance was made in 1«:U by Henri Siheibler of Creield, who in that year invented his tonometer, con- sistiii}; of a series of oii forks goiiij;- from .1 (440) to its oetave (880), the vibrations increasing reguhirly by differences of eight, any two adjacent forks thus giving four beats per second. Curiously enougli, althongh Scheibler went to Paris and exhibited his touomet<.'r there, lie was unal)le to interest savants in his discovery; and il was not untd riie Lond.m Exhibition of IS&i that tiu' atteiUion of pliysieists and musicians was directed to the value of the instrument by Ko-nig. The appanitus in it> new form oiitained Ci.") forks going from C :>\•^ to ( ' ^ - lOU. Notwithstanding the great utility of tins tonometer to the acous- tician, it still left undertemiined the absolute pitch of the fundamental note, and lience of the whole series. This problem of realizing a sttiiidard of pitch remained jjractically unsolved even after the French (ion ctabli>luMl a standard lork. the error of which did not exceed 1-GOOO cd' a vibration. The chnk-fork melh(Hl enables us at the same time to iletennine readily the variations in the nundicr of vibrations due to a rise or fall of lemperature. Having established in this way an absolute standard of .".12 v.s. at 20° ('., Ku-nig commenced the consini.lion of a universal i.nioineter biiscd tlicrcon. a colossal niiderlaking wliidi he finish.Ml in 18!)r. after working on il for nearly a score of years. This tonometer consists of the following: 1. Four fork.- giving vibrations from 32 to i28. with diiTerences at first of \ v.s., and afterwards of 1 v.s. 2. Oni" humlr.d iind thirty-two large forks, tuned to give (without the sliders) the 127 harmonies of r-, -- (J4 v.s., <:., .•,„ <■,, r,,. <■«. being in duplicate. Kach fork can be lowered, by mean.s of slider*, to unison with the fork next Ih'Iow. The differences immediately obtainable by sliders are:-l v.d. between r, and r,; 2 v.d. l>etwcrn e„ and r,; 4 v.d. betwern r, and r,. 3. 40 resonators to reinforce forks of (2). 82 ROYAL SOCIETY OF CANADA 4. One large resonator of diameter 0.48 m and of length varying from 0.30 m to 2.30 m. 5. Eighteen forks for notes from c, to /^^ 43,390:6 v.s. 6. Fifteen forks for notes from g^ to 180,000 v.s. Under the head of pitch come two very difficult questions relating to the audibility of very low or very high sounds. With regard to the former, Helmholtz has shown that, if the vibrations are very slow, and do not follow the pendular law, (the fundamental being thus accom- panied by a series of harmonics) the fundamental may be quite in- audible, whilst the harmonic is heard distinctly. In such a case the harmonic is often mistaken for the fundamental. On the other hand, if we employ large tuning-forks, vibrating rods, or the wave-siren for the purpose of obtaining pendular vibrati .ns, we are still met with the difficulty of determining accurately the limit of audibility, owing to the fact that it not only depends on the intensity of the vibrations, but varies from one observer to another. In general it may be stated that it requires from 60 to 80 v.s. to produce a sound perfectly con- tinuous and possessing a musical character. In using very powerful high forks to produce beats, which were gradually diminished in number, Koenig found that the sensation of a continuous low sound ceased when tlieir number did not exceed 26. As to the high notes above (:\ = 8192, the amplitudes of the vihrations are generally so small, that the ordinary methods no longer seno to determine the pitch. For this reason it was at first the practice to tune forks alwve C\ by means of the ear. The high forks CdU- structed l.v JIarloye and presented in 1848 by Depretz to the Academy of .Sciences at Paris were constructed in this way. In 1858, however, Ko'iiig showed that even in the upper half of the octave c^-c,, the best musicians ceased to judge the intervals accurately, a fact which seemed to k'iow tliat it was extremely unlikely that forks giving notes two octaves higher could be tuned accurately liy the ear. For this reason Ko'nig effected the tuning of very high forks by means of the sounds resulting from tlieir beats. Tlic first series of forks tuned in this way were made by Ka-nig in ISTO. A set of similar forks constructed about tiie same time by Preyer, and going, ns he alleged, as high as c„„ were shown by .Melde in t8!)4 to Im? greatly out of tune, the intervals being wrong by as much ns a third, and even iin octave. In 189T Melde's results were eonfimied by Stumpf uid Meyer. In 1899 Ka'nig published .lis researches on very high note.^. In this memoir, after sbowiiig the exactness of the tuning attained by tlie sin.nds of l)eats in f; ;k-i lielween r. and f.,. he iimeeeds to i-tate *hat, by means of Kundt's method of using liglit powders, he had con- [LOUDON] A CENTURY OF PROGRESS IN ACOUSTICS B3 stnictcd a series ^1 high forks accurately tuned and proceeding accord- ing to th cerv'.ls of the perfc (major) scale, from c- to the enormous pitch of 180,000 v.^., aii that without reaching a limit to the number of such vibrations. As to the audibility of these high forks, it has been remarked by Koenig that those between c, and c„ are generally audible, whil -t Cjo and those above are entirely inaudible. He further remarks that the limit of audil)ility, which thus lii-s bitwwn r„ and c^o largely depends, as in the case of low sounds, on the intensity, and vaxies with the individual. . . Intemity.— With regard to the question of intensity of sound, it is only necessary to say that there exists here a great lacuna in our acous- tical knoAdedge, as we do not yet possess a means of measuring the physiological intensity of sound. Timbre— To Helmholtz belongs the credit of first elucidating the question of timbre by showing that the timbre of a sound depends upon the nimiber and intensity of the harmonics which sccorapany the fundamental. The question of timbre is thus intimately counecteil with the st'.dy of the phenomena produced by the coexistence of two or more sounds. With regard to such phenomena it was stated by Helmholtz that when two notes of differeut pitch are sounded together, they give rise to two other sounds, the pitch of which is measured lie one by the difference, and the other by the s\im of th(> vihratu- thi> two primary sniindn. Furtiier. that these re'-ultant >()uni1s uTr ..A due to beats. These propositions of Helmholtz are controverted by Koenig, wiio, on the contrary, bus pro\ed that the sovnds actually heard accom- panying two primary sounds are always due to beats. Koenig "saiTts, n^oreo-^r, that the sounds referred to by Helmholtz, evjn it we lould prove that they had a real existence, would always be inaudible, and therefore without oJIect on the acoustical phenomena. He further establi^ics the curious I'act that even inten 'lotions of a sound give rise to auuiher sound. As to timbre, Helmlioltz's theory was that it depended solely on the numl>er and relative intensities of the harmonics which accompany thefundaiiicntil.iuidthat itis not alTecti'd in niiy decree by ditr.renoes in tlie])]iat,esnr tiu'S(M>oini)om'iit>. Tlii- Inttuv iiropo of pha^e as regard- hiirmonics exercise a very iiiipurUiiil intluencp on tlie timbre of u sound, so that according to him timbre depends on ilie number, relative intensities and diiTerences of phase of the harmonics which accompany the funda- mental. Ka'nifr's cxiierinionls on this di-iuiel |Kiinl «err perlormed B4 ROYAL SOCIETY OK CANADA with Jiis large wave-«iren. Even this wider definition of timbre is, however, according to Koenig's most recent view and experiments, insufScientj as not being applicable to certain dadoes of timbrco — frr example, those produced by most musical instruments, especially stringed instruments. In these cases the fundamental is accompanied not only by harmonics, but also by other sounds which are not harmonic, the superposition of which produces series of waves which change their form suecessivdy. These wave formp have be(>n investigated by Koenig in a paper *'Sur les timbres a ondes de formes vari;ibius," in which he determines the conditions under which such timbres may be con- sidered musical, and concludes that in these cases the fundamental is accompanied by harmonics which continually change their relative intensities and their phnsc-diffcrcncis. In conclusion, 1 nuiy state tliat. according' to KoMiijr, the fact that (lilTerences of phase amongst harmonics produce diileren(?es of timbre is explained for the first time by his recent discovery that the intensity of a sound can be increased by the addition of another sound when the maxima of intensity in the \-ibrations iv the two cases correspond more or less exactly, an'' that several sounds produced together may reinforce a sound of lower pitch than any of them. For example, with the same six primary sounds, by changing their phases only, he produces not only timbres differing in intensity and in richness, but timbres in which, at onie time, the octave (2) and at another time the fifth above (3) is heard. The difference between these two timbres is, indeed, so great that when heard in succession, there appears to be an interval of a fifth between them, aJthough their fundamentals are pxiirtly the same. These experiments may be said to be the last on this difficult subject in the ytar> of tlie cinturv which hii-^ just closeJ.