US1616639A - High-frequency sound-transmission system - Google Patents

Description

` c. A. SPRAGUE HIGH FREQUENCY SOUND TRANSMISSION' SYSTEM Filed June s'. 192x Tr-4f( 1414141414/ other purpose.

Patented Feb, 8, i927.

Application le Jue 2l,

This invention relates to acoustic signaling' systems and more particularly to a system of this kind in which acoustic waves of freque cies abovethe limits of audibility 5 are ployed to reinforce, in the rear of an ditorium, the ordinaryacoustic waves' set up by the voice of a speaker or other sounds produced at the'front of the auditorium for instruction, entertainment or The invention, so vfar as known, is a marked departure r from' acoustic systems which Ahave been previouslyl devised, and several features employed are capable of broad application in acoustics and signaling.` In order to' clearly describe the invention, however, its application to auditoriums will first be described. This is the field in which the invention in its entirety will probably indits most useful application.

` The need of' a system for enabling auditors in the rear portions oftheatres and churches and other assembly places to hear distinctly the sounds emanating from the stage orfrostrum has become urgent.- Recently so-called loud speaking telephone systems haved been developed which ina measure lill this need, but all such'systems have one or more of the defects V'mentioned below, all of which are substantially overcome in the system according to the present invention. l

If the reinforcing horn or instrument'is near the speaker, the reinforcement isgreatest in the front of the room, where it should be least. To those near the speaker the sounds appear unnaturally-'loud and this ef- 'fect .is sometimes so great vas to be very annoying.

- lf the reinforcing horn or instrument is in the rear of the room, or if many such instruments are distributed throughout the room -those near these instruments may receive the impression that the speaker is at their rear. The range ,of frequencies employed (the audible range) is a `diiiicult one in which to .operate because in transforming sound en-. ergy to electrical energy and vice versa, distortion may be introduced, for example, by the generation of audible harmonic waves,

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By employing carrier frequencies above the audible range during these transformations, as in the system of this invention, the harmonies produced are inaudible and havelno effect upon the ear of the auditor.

When a single reproducing horn is einployed, the distortion due to resonance within the horn is marked.` This effect is overcome by using inaudible frequencies.

Refering now to the accompanying draw- 00 lings, Fig. 1 is a diagrammatic view of an auditorium vequipped with this invention; Fig. y2 isa diagram of the high frequency electrical -circuit' arrangement disposed near the speaker; Aand Fig. 3 is a diagram ofthe 05 high frequency electrical circuit arrangement located in the rear of the auditorium. The auditorium 1, provide-d with seats 2 and platfrm 3 sequi ped with apparatus 4 placed near the spea er for transforming 70 a portion of the energy of `the speakers voice into high fre uency sound waves for transmission towar the rearof the room.

-These waves are modulated in accordance Mechanical 15' with the speech frequencies. means can be used for producing these high frequency soundwaves, but apparatus well adapted for this purpose has not-yet beenF developed' and it is accordingly preferred atthe resent time to use electrical appaso ratus o wellv'known design.

' Referring to Fig. 2, an oscillation generating system O of the audion type employing atube 6 is adjusted to generatefalter- Dating currents of a frequency above the audible range in the oscillation circuit 7, 8,

9, in well known manner, the frequency bev ing determined b the amount of inductance and capacity in t is circuit. rlhe instrument '9 is a device for setting up sound waves of 90 enerated current and vmay consist of what is commonly called a condenser receiver,- many forms of which have previously been devised. A parabolicsound 'reflector 10, preferably thick-walled 95 rojects the sound waves ina beam bof su stantiallyparallel rays to the concave sound reflector 11 (Fig. l). The latter reects and focuses the sound beam upon the diaphragm of the sound receiving apparatus1 20 to be later described.'

A modulating. system 12 of the audion type receives hlgh frequency energy from generator 0 ad is simultaneously supplied with voice frequency energy from a telephone transmitting circuit 13. The telephone transmitter is located where ,a por.- tion of the speakers voice willreach it.

The audion' 14 ampliiie's the high frequency electrical waves impresse-d upon, it

and the amplif ing power of the tube 4is controlled by tie voltage impressed from the transmitting circuit 13. 'The result is to Aproduce high frequency variations in the output current the amplitudes of which accord with the speech waves impressed on the input circuit. This is a well known form of modulating system and is shown merely by way of example. It was invented by E. H. Colpitts and J. H. Van der Bijl and for a more detailed description the reader is referred to U. S. Patent (Reissue) No. 14,380 to Colpitts and U. S. Patent No. 1,350,752 to Van der,Bijl.

The high frequency modulated waves thus produced are impressed throughr trans' former 15 upon an electric Wave filter 16 of the type disclosed in U. S. patent to Campbell, No. 1,227,113, granted May 22, 1917, the filter being designed, in 'the manner described in the patent, to pass all frequencies above certain value and to suppress all frequencies below this value. It is 'now well known that a high frequency modulated wave is in effect a plurality of wavesv of different frequencies which may be separated from each other by electric filters. If C represents the frequency of wavesv generated by the generator 0, and S represents the instantaneous frequency of the composite speech wave, the modulated high frequency wave may be considered as made up of three waves having the frequencies C-I-S, C-S and C. The component of C frequency may be looked upon as the portion of the original wave which has not been modulated, there always being such a component unless special means are used to obtain complete modulation. Since S isa constantly changing frequency, G+S and C-S are likewise continually varying. For example, if C is 30,000 and S varies from 100 to 5000, G+S will vary from 30,100 to .-35,000 and C-S will vary from 29,900Y to,

25,000; .For a detailed disclosure of a mathematical analysis of a modulated electric wave see British patent 102,503 for an invention of John R. Carson relating to .modulation The filter 16 is designedl to transmit only the component C-l-S4 or the component C-S and tosuppress the others.

Therefore, 'although the transmitted fre quency is constantly changing, it never falls 'to awalue which is audible.

This band of changing frequencies is imvice 18 similar in function to 9 pressed through transformer 17 u on a defor transforming the electricI waves into sound Waves of like frequencies. A meaphone 19 transmits these sound waves the form of a beam a toward the rear ofthe auditorium.

A cross-section of this bea-m, due to the shape of the megaphone, is greater in its horizontal than in its vertical dimension.

If the beam a were to be employed alone, no effect would be produced upon the auditors; but by simultaneously producing a sound of frequency .Cp in the region infwhich an amplifying effect is desired, the detecting property of the ear is brought into play an the voice of the speakerbecomes audiblel even though his voice cannot directly be heard. This is due to the phenomenon of acoustic beats. Whenever two sound waves of different frequencies are simul-l taneously impressed upon a vibratable body which distorts the waves, their energy is absorbed and reappears in the form of new waves, not only ofthe frequencies of the impressed waves, but a third wave which has a frequency equal trthe sum ofthe im pressed frequency waves, a fourth having a frequency equal to the difference between those of the impressed Waves and others of frequencies harmonic to those of the im- 95 pressed waves, that is, integral multiples of these frequencies. The amplitudes ofV the sum and difference frequencies will in general be much greater than those of the higher harmonics and 'will be roportionate'100 to the product of the amplitu es of the impressed waves. Therefore this amplitude can be made great by increasing the am litude of one of the impressed waves. he vibratable elements of the ear have the property of distorting high frequency waves impressed upon them to produce such a series of waves as that just described. Thus, when thefrequencies G and C-l-S, for example, are impressed upon the ear there is produced a wave having a frequency equal to the difference lof these Waves, i. e.

In other words speech is reproduced. -At the same time other waves having frequencies of 2C-l-2S, 2C, 2C{ S etc. are-produced, 115

' but each of these is above the limit of audi- Ibility and produces no effect.

'If a wave of frequency C-S rather vthan C-i-S is impressed upon the ear simultaneously with a wave of frequency C, the result, so .farasvlao the production of audible frequencies 1s concerned, is the same, i. e., the difference frequency is S, and the sum frequency and harmonic frequencies are inaudible.

The purpose of the apparatusshown in 125' Fig. 3,is to set up in the rear of -the auditorium a wave of the frequency C andof constant amplitude, which reaches the auditors at the same time as the sound of fre'- quency C-l-S (or C -S) from the front of Lacasse the room. A sound generator, either mechanical or electrical, capable of producing frequency C, could be operated at `the, rear of the auditorium independently of the high frequency sound produced near the'speaker, if it were possible to maintainl the frequency C constant, or exactly the same as the carrier frequency pro ced at the front-of the room. It is so ditiicult to do this, however, that it is lpreferable to provide synchronizing means between the generators at the' front and the rear of the room. Any known means for producin the desired synchronism may be employe The system shown in Fig. 1 is being here described by Way of example. Strictly speaking, this is an amplifying rather` than a synchronous generating system in that the beam b of sound Waves is transformed into electrical waves of corresponding frequency, which are then amplified and reconverted into sound waves. -The lbeam b is reflected and focused by the concave relector 11 upon the diaphragm of telephone transmitter 20, which Ipreferably has a natural frequency equal to or greater than that of the impressed waves but which'may be provided with damping means. Any other form of device for transforming sound waves into electrical Wavesmay be substituted for the transmitter 20. The .Waves thus set up are amplified by the thermionic vacuum tube amplifier A and are reconverted into sound Waves by means of the converter 22 in circuit 21, 22, 23. The converter 22 may be of any known type but is shown as a so-called condenser-receiver. As the potential upon the condenser elements is varied in accordance with the Waves flowing in the circuit 21, 22, 23, they are vibrated and thus set up. sound Waves in the air. The circuit 21,' 22, 23 may be tuned to the frequency transmitted as may also the circuits of jsound transmitter 18 and sound receiverI 20. 'A damping resistance 23 may be inserted in circuit 21, 22, 23, as shown, and may be of.

such valve as to give -the circuit any damping characteristic desired. `A similar resistance may be used in ,the other circuits vjust mentioned. Thesource 21 of -directpoten# tial provides means for polarizing the condenser-receiver 22 and may be omitted if desired. It is to be understood that Whenever.A

a condenser-transforming device is employed it may if desired be lpolarized in well-known manner. `The sound waves thus set up are transmitted as a. beam c toward the front of the room, a crossfsection of this being, as in the case of beam a, greater in its horizontal .than in its vertical dimension. By means of the amplifier A any desired degree ofamplif ication may be obtained, a plurality of am'- plifiers being employed if necessary. The

wave amplitude of the beam c is preferably muchigreater than that offbeam a; yThe operation of thersystem should be apparent from the above description-` The sound beams a and o are simultaneously impressed upon the ears of the auditors throughout the room excepJ in a zone near the speaker, in which his oice can'readily be heard. `Assuming, for example, that the beam a consists of -a band' of frequencies C-i-S (the effect being the same, however, whether the band is one of G+S or C-S frequencies)l there is produced in the ears of each auditor who receives these .waves a series of waves of frequency or frequencies S in the manner described above, i. e., the ear functions as a detector to render perceptible the low frequency modulating com- "1 ponentof the impressed waves in a manner analogous to the operation of a so-called detector of electrical rwaves at a receiving radio telephone station.. y

The auditor therefore, if he is Within the range of the speakers voice, hears the speaker both directly and indirectl effects are additive. The e ect which is pro'- duced indirectly-is, however,` greatest in magnitude in the rear of the rgom and is progressively less from there toward the' front and may be made of such value as to render the sum of the indirect and'direct effects approximately Vuniform throughout the room." .The reason why the eect 1s greatest at the rear of the room and decreases progressively from there toward the front of the room is that, as explained above, the amplitude of the difference frequency'produced when two waves simultaneously impinge upon a.

element is proportional to the detecting product of the amplitudes of the impressed waves. Therefore by making the initial a'mplitude of the beam c very much g greater than that of the beam a the product of these amplitudes will be greatest where the amplitude of the beam o is greatest, that is, in the rear of the room.

Since the high frequency sound wave which is proceeding from the vicinity of the speaker is varying 1n accordance with speech, while the wave from the rear of the room is of constant amplitude and freqtlency, the effect upon the ear is the same as though the entire energy received pioceeded from-the vicinity of the speaker. This is considered' an important feature of this invention, since, by reason of it, the source or sources of plified waves ma be placed in the rear of the room or distributed throughout the room Without producing the incongruous effect of seeing the speaker in one direction and hea ing him in, another.

` As just suggested, as many sound sources of frequency C may be used as necessary to give approximately uniform sound intensityA throughout the room. It lwould, o'f course,-

and these two f lof beam a may be widened by a properly shaped horn or megaphone, or several sound transmitters such as 18A may be employed simultaneously and energized from the sameV '2 thel reflector 11 and the telephone transmitter 20. #In this way the two high frequency waves may be made to reach the auditors in any desired phase relation and since the pheno enon is one of beats the phasel of thegegt Wave with. respect to that of a wave e same frequency from the speaker d epends upon the phase relation between the sound beams a and c. The wave length of the wave having the frequency C isa few inchesor at most a few feet so that the phase relation between the two high fre uency waves of beams a and c may be varie from zero to 180 degrees by a change of a few inches'or at most a few) feet in the position of the producing or reflecting apparatus.

In case the installation is to be permanent and indoors, it is ordinarily preferred io omit the condenser 7 'the `sound producer 9, the reflectors 10 and 11 and the sound receiver 20 with its. battery, and to connect the amplifier A with the generator 0 by means of an electrically conductive circuit extending along the Hoor lor wall of the room. Such a circuit may, of coursefbe employed also when the installation is outof-doors if desired.

While the invention has been described.

as of particular utility in a sound amplifying system, it is also applicable to relatively long distance aerial or .submarine secret signaling.' 'I he transmitting operator thenv speaks directly intothe transmitter 13 and l the ear of theY receiving operator is placed .of the sound from small.

near the sound producer 22. The amplitude 22 may then be relatively An acoustic detecting instrument may be employed in the place of the ear, or as an auxiliary thereto.

The terms sound and acoustic are herein employed in their broader sense to denote material waves of frequencies lying outside the audible range as well y'as those within that range.

What is claimed is:4 1. An acoustic wave system comprising means for producing-,low frequency sound waves, a. source of high frequency Waves,

means for modifying said high4 frequency waves in accordance with` said low frequency waves, means for suppressing all of the comonents of the modied wave except one side band of frequencies, means for directively transmitting to a receiving point an acoustic wave comprising said side band frequencies, a source of acoustic waves of the'same fre- 'quency as the said first-mentioned high frequency source, a Wave distorting device for producing waves of sum and difference frequenciesgfrom impressed waves, and means for impressing said acoustic wave having side band`\frequencies and a wave from said last-mentioned-high frequency source simultaneously upon said Wave distorting device.

2. An adoustic s stem comprising means for producing audi le sound. waves, a source of high fre uency waves of frequency above the audible limit, means for modifying said high frequency Waves in accordance with said audible waves, means for suppressing all. of the co l ponents of the modulated wave except one side band of frequencies, means acoustic Wave comprising said side band frequencies, a source of acoustic waves of the same frequency as the said first-mentioned high frequency source, and-means for im-V pressing said acustic wave having side band frequencies `and a wave from said last-mentioned high frequency source simultaneously upon the ear of an auditor.

3. A- sound wave reinforcing s stem comprising means for producing au ible sound waves to be reinforced, a source of high frequency waves of frequency above the audible limit, means for modifying said inaudible wave in accordance with said audible waves, means for suppressing all the com onents of the modulated wave except one si e band of frequencies, means for transmitting to a receiving point an acoustic wave com rising said side band frequencies, a source o acous-f tic waves of the same frequency as said firstmentioned high frequency source, and means for impressing said'acoustic lwave having 'side band frequencies and a wave from said last-mentioned high frequency source simultaneously upon the ear..of an auditor. p

In a Wave transmissionv system, the method of reinforcing audible frequency waves at a desired point in space which comprises combining two inaudible wave's with each other and with said 'audible frequency t latediin accordance with signals, transmitting said modulated vwaves through space, and directively transmittino' and reecting said unmodulated waves whereby they are caused to combine at a` desired point with said modulated waves.

7. A signalx transmitting system which comprises means forproducingun'modulated l inaudible waves', means for producing inauwhich comprises generating and transmitting an inaudible acoustic wave having its frequency modied -in accordance with a signal separately generating an inaudible wave, and making the signal'audible by impressing upon the ear simultaneously 'therewith said second inaudible wave.

10. The method of acoustic wave signal-` ing which comprises impressing simultaneously upon`tlie ear of an auditor an acoustic wave of constant frequency and -a separately generated and transmitted wave of varying frequency. l

11. The method of acoustic wave signaling which comprises generating and trans-'I mittinga high frequency acoustlc Wave modulated in accordance with the signal, separately generating an unmodulated high frequency wave and simultaneously impressing said Waves upon theear of an auditor.

12. The 'method of acoustic signal transmission from substantially the position of an audible frequency source to an audience,

Vwhich comprises transmitting from substantiallythe position ofl said source, audible fre uency waves as modulations of an inaudible frequency acoustic Wave, and directively transmitting acoustic waves of said inaudible frequency from a pointother than that of said source.

13. The method of ,acoustic signal transmission fromv substantially the position of an audible frequency source to an audience,

' which compris transmitting ,audible frequency acoustic waves from said source,

'transmitting lfrom substantially the position of said source an inaudible frequency acoustic wavemodnlated by waves of said audible frequency and directively transmitting acoustic waves of said inaudible` frequency from a point other than that of said source.

14. The method of acoustic signal transmission from a desired point to an audience which comprises transmitting audible 'frequency acoustic Waves from said point to the audience, transmitting from said point waves of said audiblefrequency as modulations of an inaudible vfrequency acoustic wave, and transmitting acoustic waves of said inaudible frequency and of greater amplitude than that of said modulated waves from a point at the op osite side of the audience from said desire point.

15. The method of acoustic signaling froma desired point to an audience which comprises transmitting an' audible frequency acoustic Wave from said point to the audience, modulating an inaudible frequency acoustic wave in accordance with said audible .frequency Wave, transmitting the modulated components of said wave, to the audience, se arately directively transmitting the unmodu ated componentto a point on the opposite sider of-said audience from said point, generating by means of said unmod` ulated component at said second point a Wave of the same frequency but of substantially greater amplitude, and transmitting said wave-to the audience.

16. The method of 0acoustic wave transmission which comprises modulating an acoustic inaudible frequency carrier wave in accordance with a low frequency wave in such manner that the resultant modulated wave has a component equal` in amplitude to the product of the carrier and `low frequency' waves, transmitting said modulated wave to the ear of an auditor and impressin upon the ear simultaneously with the mo ulated wave a. second inaudible wave of the-same fr uency as said acoustic inaudible Wave.

el? A sound operated amplifying appa-l ratus for an auditorium comprising sound receiving means located ata point Where sound from a source has become attenuated, and electrical means for reinforcing sound waves controlled bysaid sound responsive means. J l

18.` The method of producing the sensation of audible speech upon the ear of an observer Which comprises impressing upon the natural physical medium in operative proxl'mity to the ear a compressional wave of frequency above theusual audible range 1nher'ently containing the elements of speech and another Wave of frequency! above the usual audible rangev y 19. A system comprising separate elements for producingcompressional waves-of frequency above the usual audible range, means for causing one of said waves to vary 1n accordance with speech, and means for im-V pressing said Waves upon the ear of an observer withpdesired relative intensities.

20. A system for producing in one and the same physical medium at least a part of the .frequency components of a compressional wave varyingn accordance with speech and an unmodulaed compressional component of said wave, and means for independently regulating -the intensity'of. said unmodulated 5 component. Y

21. A system comprising separate wave producers and associated elements for pro- 'ducing in th'e same body of meium a compressional wave speech side band', and a, eompressional wave corresponding to the 10 unmodulated component of said side band. AIn witness whereof, I hereunto subscribe my name this 2nd ay of June, A. D. 1921.

CLARENCE A. SPRAGrUE.v

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