US1902442A - Sound reproducer - Google Patents

Description

March 21, 1933. A. N. GOLDSMITH ET AL 13023 SOUND. REPRODUCER Filed Nov. 5, 1928 SOURCE OF MODULATING CURRENT sourzcE 0F mew FREQUENGYE ,1 Remus-HQ, 500ml) WAVES mun MODULATOR DETEOTCR 7 /-/20 20 19 a 1/ j /M '16 Patented Mar. 21, 1933 i UNITED STATES PATENT OFFICE ALFRED 1v. aonnsm'rn Ann mvme' wour, or miw YoRx, n. Y., ASSIGNbRS 'ro RADIO CORPORATION or AMERICA, A CORPORATION 01* D LAWARE SOUND REPRODUGER Application fl led November 5, 1828. enial No. 317,149.

This invention relates to sound reproducers such as loudspeakers, which are adapted to reproduce sound waves without employing an electrically or mechanically vibrated diaphragm, or which, if desired, will reproduce sound waves by pressure changes without the use of a diaphragm. More par? ticularly this invention relates to sound reproducers in which means are provided for obtaining a modulated high frequency sound wave and in which additional means are provided for detecting the modulated high frequency sound wave to obtain audible frequency pulsations corresponding to its modulations.

V I It has been suggested that sound could be reproduced by modulating a source of high.

or inaudible frequency pulsations to obtain a modulated high frequency sound wave and by radiating this wave into the open air. Such an arrangement, however, will not operate due to the fact that the human ear is not sensitive to high or inaudible frequency sound waves even though the sound waves are modulated at an audible frequency. We

propose to overcome this inoperativeness by employing what may be termed an acoustic detector. The function of such a device is to detect the modulated high frequency sound waves and to radiate intothe open air an audible frequency sound wave corresponding to the modulation or envelope of the modulated wave. The modulated high frequency sound wave can be likened to the modulated high frequency carrier Wave of radio telephony and it can be detected in thesame manner as a modulated high frequency carrier wave to provide anaudible frequency wave.

It is an object of this invention to provide a sound reproducer consisting of means-for producing a modulated high frequency sound wave and means for detecting the sound wave to obtain an audible frequency wave.

A further object of this invention is to Figure 3is an end view of the acoustic detector illustrated in Figure 2;

Figure 4 is a section on line 44 of Figure 2;

of the acoustic detector employed in Figure Figure 5 is a plan view of a modified form of the acoustic detector;

Figure 6 is an end view of the modification illustrated in Figure 5;

Figure 7 is a section on line 7-7 of Figure 5; and

vFigure 8 is a, diagrammatic representation of a Venturi tube on which the theory of the operation of our acoustic detector is based.

Referring more particularly to the drawing 11 represents a radio receiver or a microphone or any similar source of energy for modulating the cdnstant amplitude high frequency sound Waves set up by the apparatus diagrammatically illustrated at 12. The source of high frequency sound waves illushated by reference character 12 may be a mechanically vibrated member, an electrical-1y. vibrated crystal, a Corona discharge meinber, or any other means for creating high frequency pulsations consisting of condensations and rarefactions. The modulating current from the member 11 may be at a radio frequency such as the modulated carrier wave used in radio telephony, or it may be atan audio frequency such as a detected carrier wave. The modulating current may be applied directly to the source at 12 or it may mechanically or electrically modu-.

modulated radio frequency currents thereto.

The modulated high frequency energy may also be obtained from a Corona discharge which is energized with modulated radio frequency energy or it may be obtained from a talking arc energized in the same manner I as shown by the patent to Bothe No. 1,567,230.

The beam of modulated high frequency pulsations is indicated at 13 and the acoustic detector for rendering the high frequency pulsations audible, is indicated at 14. The energy from the source 12 due to its hi h frequency and short wave length, can e radiated in the form of avbeam that is'adapted to be directed in-any desired direction. This beam of energy is directed toward the detector 14 which may be located adjacent the source 12 or which may be positioned some distance from the source. In the latter'case the decrease in the'energy received by the detector is hardly noticeable due to the fact that the energy can be transmitted in a beam without diffusing or greatly decreasing in strength.

Various types of acoustic detectors can be employed for obtaining an audible fre-.

quency wave from the modulated inaudible frequency wave just as various circuits can be used for detecting modulated carrier waves in radio telephony. An approved form of an acoustic detector is illustrated in Figures 2, 3 and 4:- and a-modified form is illustrated in Figures 5, 6 and 7. Referr ihg to the device illustrated by Figures 2, 3 and 4, the detector consists of a member 15 forming a plurality of sound wave openings or passage-ways 16 which are providedwith constrictions 17, in a manner similar to a Venturi tube. The constricted portions 17 of the sound wave openings or passage-ways 16 are connected by vents 18 with the atmosphere. The pulsations from which the audible frequency sound wave is obtained are set up in these vents 18 by the pressure variations taking place in these vents as th e result of the velocity variations of the a1r particles which are given motion by the sound waves passing through the passage-ways 16.

In order for the modulated high frequency sound wave to retain its identity and not be destroyed by interference it is advisable that the radial dimensions of the passage-ways be limited to a value not much larger than the wave length being transmitted. As the high frequency energy has a relatively short wave length it is usually necessary to employ a plurality of openings or passage-ways 16 arranged alongside of each. other in order to obtain suflicient energy for the reproduction of the sound.

The modification illustrated by Figures 5, 6 and 7, operates in the same manner as the detector illustrated by Figures 2, 3 and 4. In this modification, however, a diaphragm member 19 is provided and the vent 18 is modified to'form an opening 20 which will function to collect and amplify the sound waves from the diaphragm 19 in the same manner as the horn of an ordinary loudspeaker.

The diaphragm is made of any suitable material either metallic or non-metallic in nature and it may be clamped at its edges or suspended by suitable flexible suspending means. A preferable method is to suspend the diaphragm by means of a strip of velvet fastened between the edge of, the diaphragm and the supporting member. Bymaking the restoringforce of the suspending means sufficiently low, the naturalperiod of the vibrating system is obtained at a frequency belowed by the pressure changes resulting from the .Venturi effect.

It is usually preferable to construct the acoustic detector 14 from a material whose natural period of vibration does not lie within the audible frequency range.

In explaining the theory of operation of our acoustic detector reference is had to the Venturi tube diagrammatically represented by Figure 8. In a Venturi tube the pressure at the constricted portion, indicated at B in Figure 8, is not a linear function of the velocity or displacement of air particles at that point, but contains a term which is a function of the square of the velocity or displacement of these particles. The pressure is rep resented by the equation p=m'v'*l;nv (see Raleighs'Theory on Sound, volume 2, page 13, 1926 Edition) where m and nare constants and 'v is the velocity of t airiparticles. Strictly speaking, this equation is true for any sound wave but ordinarily the constant 92 is so small that its term is negligible compared with the m term. However, when the sound wave passage-way is constricted as in term W0 produces a pressure whichwill affect the human ear. As the pressure is sufficiently great to affect the human ear and as the pressure depends upon the square of the velocity, the ear will be able to respond to the envelope of the pressure impulses. This theory may be compared with the theory of detection by means of vacuum tubes.

In other Words as a sound wave, or a wave corresponding to a sound wave but having a frequency above audibility, moves through the air it sets the air particles in motion. The air particles move forward and backward through a path forming a closed cycle. The forward motion takes place, say, with the condensation impulse and the rearward motion with the following rarefaction impulse. The velocity of motion and the length of the path or the duration of the motion, depend upon the frequency and amplitude of the sound wave. Thus, at A and B the air particles will be moving forward and backward at velocities proportional to the pressure and rarefa ction impulses of the sound wave and having a definite relation to each other.' The ratio between the velocity at A and the velocity at B being approximately inversely proportional to the respective areas.

Referring again to Figure 8, as the pres sure at D varies proportionally to mv+nv the direction of motion of the air particles can be disregarded. That is, the pressure variation at D has a part which is the same, provided the velocity is the same, regardless of whether the direction of motion is from A to C or from C to This, in effect, is the same as if the impulses at B were all in the same direction, that is, as if the condensation impulses and rarefaction impulses were rectified so as to consist Wholly of con--- densation impulses or rarefaction impulses. As these impulses contain the same modulation or envelope as the original beam of high frequency energy, the ear drum can respond to the envelope of the impulses just as the telephone diaphragm can respond to the envelope of the detected carrier wave pulsations. Thus, an audible frequency sound wave is obtained at D.

The sound waves from the vents 18 or ffom the diaphragm 19 may be collected in amplifying horns or directed to some particular point as desired. This is not necessary, however,- as the sound waves emanating from these vents usually have sufficient amplitude so that amplifying horns are not required.

In the appended claims the term constricted passageway is intended to describe a passageway having a constricted portion.

While we have illustrated only two types of acoustic detectors it is to be understood that various other devices may be emploved for the detection of modulating high frequency sound waves and we do not desire to be limited by the arrangement shown but only by the scope of the appended claims.

We claim: 1. An acoustic system comprising a source of modulated super-audible frequency sound waves and an acoustic detector for obtaining an audible frequency sound wave therefrom.

2. An acoustic system comprising a source of modulated super-audible frequency sound I .waves, and means for obtaining audible sound waves from said modulated super-audible frequency sound waves.

sound wave, means for modulating said su-.

per-audible frequency wave, and means for obtaining an audible frequency wave from the modulated super-audible frequency wave.

6. An acoustic system comprising means for setting air particles into motion at a modulated super-audible frequency, and

means for obtaining an audible sound wave' from the motion of said air particles.

7. An acoustic system comprising means for providing a beam of modulated high frequency sound waves, and means independ-. ent of said first mentioned means and positioned at a distance from said first mentioned means for detecting said beam of high fre' quency sound waves to obtaln an audlble frequency sound wave therefrom.

8. An acoustic system comprising means for radiating a modulated super-audible frequency sound wave, and a member provided witha constricted passage-way for detecting said sound wave to obtain an audible sound wave therefrom. I

9. An acoustic device of the type adapted to respond to the envelope of 'a modulated sound wave comprising a member provided with a constricted passage-way so small that non-linear effects take place in the sound wave. I v

10. An acoustic device of the type adapted to respond to the envelope of a modulated sound wave comprising a member provided with a constricted passage-way sosmgll that non-linear effects take place .in the sound wave, and means cooperating with the constricted portion of said passage-way for radiating audible frequency sound pulsations.

11. An acoustic device of the type adapted to respond to the envelope of a modulated sound Wave comprising a sound wave passage-way having a constricted portion so small that non-linear effects take place in the sound-wave and a vent leading from said constricted portion for obtaining audible he I quency sound waves therefrom. s

' I 12. An acoustic device of the type adapted to respond'to the envelopeof a modulated sound wave comprising a member provided with a'plurality of sound wave passage-ways,

each of said passage-ways havin Y a constrict ed portion so small that non-linear efiects take place in the sound fwave,and avent leading from the constricted portion of eachof said passage-ways.

'13. An acoustic detector ,comprising :a member contalnmg a sound wave-passage- I way so smallthat non-linear effects take place in the sound wave, a constricted portionin said passage-way, and a vent leading from said constricted portion, the diameter of said soundwave passage-way being'not much larger than the wave length of the sound wave being detected.

14. An acoustic detector comprising" a member provided with intersecting passageways one of which is constricted to such an .extent that velocity variations in opposite directions are converted into pressure variations 1n a singledirection. I

15. An acoustic device of the type adapted torespond to the envelope of a modulated sound wave comprising 'amem'ber provided" V ble frequency sound wave.

' one of a plurality of intersecting passage- 1 17. An acoustic device of the type adapted to respond to the envelo of a modulated sound wave comprising awenturi tube, havin a portion so constricted that nob-linear e ects take place in the sound wave.

18. The method of detecting modulated high frequency sound waves which consists in passing the sound waves through a passageway having the internal form of a Venturi tube.

p 19. The method of detectin modulated high frequency sound waves w ich consists in passing said sound waves through a constricted passageway.

20. The method of detectin modulated high frequenc sound waves w ich consists in passing sai sound waves through at least ways at least one of which passageways has a constricted portion.

21. The'method of detectin modulated high frequency sound waves w ich consists in passin said sound waves through a pas .sageway aving a constricted portion, which 22. The method of detectin modulated high frequenc sound waves w ch consists in passin sai :sound: waves through a passageway tially at right angles to the direction of the passageway 'havingthe constricted portion.

modulated 23. The method of detectin high frequenc sound waves w ich consists in passing sai sound waves through at least one of twoeintersecting passageways which are, arranged so as to convertvelocity variations in opposite directions into pressure variations in a single. direction.

24. The'method of detecting modulated high frequency sound waves which consists in passing said waves through a passageway having a constricted portion and having a avlnga constricted portion, the constricted portion being in communication with a second passageway extendin substanventin communication .with said restricted portion.

L25. Themethod of detectin .modulated high frequency sound wavesw ich consists,

indistortingthe sound wave so as to obtain a pressure :d1

constricte passageway.

26. An acoustic detector comprising a member provided with a constricted passagewa and. a movable body adjacent the constricted portion of said passageway and op eratively associatedtherewith whereby preswhereby pressure variations in one of said assageways will cause movement of said ody and thereby produce pressure variations'in the other passageway.

28. An acoustic detector comprising a *member provided with a constricted passageway, a secondpassageway in said member terminating adjacent the constricted portion 'of the first passageway, said second passageway fiaring in cross section from its junction with the first passageway.

. 29. An acoustic detector comprising a member provided with a constricted passageway, a second passageway in said member terminating adjacent the constricted portion of the first passageway, and a movable body arranged at the termination of said second passageway to substantially shut off communication between said second passageway and the constricted passageway.

erential varying in accordance with the modulation of said modulated wave, I by passin said modulated wave through a 30. A system forcommunicating intelligence comprising means providing a source of lntelhgence 1n the form of vibrations, a

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