US1606967A - Acoustic apparatus and method - Google Patents

Description

Nam 16 1926. i,606,5967

c. A. EQPRAGUE ACOUSTIC APPARATUS AND METHOD Filed April 1, 19 18 n all lhlosr. 3., 1926.

,i' n? E CLAREHQE A. SPRAGUE, OF EAST ORANGE, NEW JEBSEY.

ACOUSTIC APPARATUS AND METHOD.

Application filed April 1,

This inception relates to acoustical instruments and methods.

An object of the invention is to provide a substitute for the diaphragm ordinarily employed in many acoustical instruments.

A further object ofthe' invention is the provision oi": an improved form of diaphragm of general application.

A further object 18 the rovision of an acoustical instrument which is free from certain defects ordinarily introduced by the vibratory elements of such instruments.

Other objects will be apparent from a reading oi the following specification.

its is well known, every diaphragm has a natural period of its own by virtue of which its response to sounds of different frequenelse is most vigorous at those frequencies near its own. This resonance eilect produces distortion when a diaphragm is employed with voice. controlled or sound. reproducing instruments.

lit has been proposed to overcome this defeet by employing a diaphragm whose natural frequency is above the range of essential voice frequencies, or to provide dampmeans to keep the amplitude of vibration of the diaphragm small and uniform. The former expedient has the disadvantage that a diaphragm whose natural frequency is high enough to avoid resonance effects is very small and a small dis lacement of the central portion introduces igh tension and brings the diaphragm to rest so that it will not transform suiilciently large amounts'of energy. The second expedient mentioned has the disadvantage that it is impossible to provide damping means which will not dampen desired as well as undesired vibration, so that the device is inetficient.

A further disadvantage of a metallic diaphragm having a frequency within the audible range is that whenever it vibrates the character of the overtones is such as to introduce a metallic quality, which is often very noticeable in telephone reproduction. It is therefore desirable to have metallic diaphragms of such natural frequency as to render their vibration inaudible.

I propose to employ a small relatively freely moving-piston in place of the diaphragm. I prefer to have this piston very light and as rigid as possible, so that it will easily follow the movement of the air or other gas in contact with it. When a wave passes from air to a tightly stretched or 1918. Serial N0. 226,128.

heavv diaphragm much of the energy is refiected instead of being absorbed. This is obviated to a great extent by making the piston very light so'that with a relatively small restoring force considerable displacements may be produced.

The piston is preferably held in its initial or neutral position by a small, light spring. The natural frequency of the vibrating system formed by the piston and spring is preferably above the range of essential frequem cies impressed upon the piston. When speech is impressed, the natural frequency of the vibrating system may be i000 or 5000 per second, or higher, the essential voice frequencies lying below this range. The natural frequency of the system may also be within the range of essential impressed frequencies, but a disadvantage above mentioned is then present.

The important feature is that the piston is light enough to follow the movement of the air with greater fidelity than a stretched orheavy diaphragm, and that it is free to vibrate with relativelyigreat amplitude be fore being brought to rest by the suspending structure as in the case of a diaphragm.

ith this arrangement there arises the disadvantage that the piston when used in a telephone transmitter is easily displaced bodily by the air currents set up by the voice of a speaker. To eliminate these air currents I propose to place a diaphragm across the mouthpiece at or near the lar s end of the same and to so construct this iaphragin that it has a frequency above the audible limit. The metallic character given to themeproduced speech is thus eliminated.

My reason for placing the auxiliary diaphragm at the large end of the mouthpiece instead of near the small end is that I am thus able to compensate for the smallness of the amount of energy which can be transmitted by a single diaphragm of exceedingly high frequency; for the greater area of cross section at the larger end of the mouthpiece permits the useof a plurality of i such diaphragms inparallel, so to speak. That is, the total energy transmitted is the sum of that transmitted by the individual diaphragms, just as the amount or" energy.

transmitted by a plurality of electric condensers connected in'parallel in a line is the sum of that transmitted by the individual condensers. This energy is all concentrated upon the single small piston. which is, able not to absorb anti hi'aosmit mush oi it by virtue of its small mass and relatively 'gresh freedom of motion. i I reform use a light, ri 'd. piston mount. ed 11 onslidable means, soc as a pin having hearings as free from friction as fisible, as described more in detail below. e piston Y can he made to transmit'the energy of its motion to a microphone box or other movl0 able element through'thomeans upon. which it is mounted or otphsrwis'a,

The improved diaphragm which I prefer to employ inoonnsction with the piston is notv limited tosuch use, but will find a. wide variety of uses in the aconstig arts, and the is not limited to use therewith.

piston My invent-ion will he better understood by ref-Bronco to the accompanying drawings in which Fig. 1 is an elevationol view, partly in section of s telephone transmitter according to my, invention. y r Fig. 2 is as its emery plain view, partly in section, of t orrsnge monc shown in 2 Fig. 1. s

. Fig. 3 is so elevotioosl side view, partly in i. section, of a. form of say compound die.-

phrsgm.

.Fi is an elevstioniilfroni, view of the 39' dis agm of Fig. 3.

ig.-5 is a view in swarm of s modified form of piston.

Fig. 6 is a iragmentsry view, partly in section, of the mo ification of Fig. 5 on on enlarged male. i

\ Fi. Z is aview in elevation of s-(lifiereni 'nio'ostion of piston, also upon an enlarged seals. f H

In the drawings like reference characters wyrefer to like @rts thro hoot.

A conical A pod soun I deflecting element 1, which is hora shown gs smooth iece of a. telephone tritser "is rovirie with a. v ocono of @he'kind escs o above. This PlStOl'i' is hero shown as operating; a microphone 2. The microphone box is held to the mouthpiece b means'of strapsB. The pin 6 upon which e pisfion is'mounied is attached to the front of an ordinary microphone box 9 such as that"; now generally employeri. intolephono'transmitters. The front electrode 8 is suspended by the diaphragm 7 which may be of mica. ."l. isdiaphragm is small-and may v well be much smaller than that ordinarily 5 used. The front end of the in 6 slides in a, bearing in the support 5 w ose base 13 is riveted or screwed tothe mouthpiece 1. The granular carbon is shownut, 10, the rear electrode at9 and the lealtlwiros at 11 and 12.

The coinpounddia hragm shown in Figs. 3

- and 4 isdesignnte by numeral 14 in Fig. 1 and is held in place by a. ring 20, which ma be fastened to the mouthpiocs by so The, eompound iaphmgm of s. 1.5 of mortal or etizcr suit is maooms? terial placed or stretohezi over the face oi; a. circular metal plate 15 of relatively very reat thickness and mass. The membrane may be bent over the edge of the plate and held in place by the clamping ring 19. The

membrane is preferably put under very high tension by on known means so that Its noturol period 0 vibration is very small. This may be done, for example, by stretching a.

sheet of the diaphragm material, lacing thei.

face of plate 15 thereogainst, ilscmg ring 19 upon the opposite side ofthe iaphragm. row" terisl co-axially with plate 15 and pressing the ring in plaoeon the edge of the plate, all in o welhknown manner. The plate 15 has a" plnrolity of openings 16. A second'plsio 18 which is a, duplicate of plane- 15 cxcoptihot it. is preferably not so thick is clamped ovcr ills scratched membrane and held in. lace hy screws or bolts l? or by other c om mg means. The membrane is thus made to unotioo as a plurality of small diaphragms each of which hes a natural frequency high as com ered with that of the membrane as s whole. The natural frequency of each of the 1 small (liaphragms may es'sil be made so be:

by screw 36 which has its bearing in support 1 32. The forward spring maybe attached to an arm 35 similar to arm 5 of Fig. '1. Springs 33 and 3s are shown by way of exomglo.

"When 'avery high natural period is desired, a spring consisting of but s singleturn' or a portion of a turn of relatively very heavy wire is employed. The spring 2% may mks the form of a pin, such as that shown in i3,

Fig. 7. No novelty is claimecl for the'form of spring omploye, as it is well known how 50 design springs so "as to produce with a small mssso vibrating system having a. high "1 natural frequency of vibrahon.

In Fig. 7 is shown a form ofmounting of.

more general application. Qno' side of the piston 50 isshown as lmattaohed. The pin 41 base bearing 42. A spring pin 43 is emplayed; to position the plston. Adjustment. may be made by means of the auxiliary spring 'and adjusting screw 45. This arrangement may be utilized, for example, with a. microphopio box. such ,as 30. Figs. .5 and 6, bv placing the left-hand face of the piston 40 in Contact with the granular microphonic niaterial. An oil cup 46 may be provided.

The term acoustic as used in this spcci- .fication todescribc a type of wave is intended to be taken in its brnadestsense, i. e-., with respect not only t osound waves of audible.

frequency, but also similar materialwaves of frequency below or above the audible limit It will be understood that many changes maybemade'in the embodiments of my 111- vention shown and described withoutfd-eparting from the s irit of my invention as defined in the fol owing claims. It is obvious It will often be desirable to make use of the invention in one aspect only, many such aspects being defined in the following claims. What I claim is:- 1. The method of utilizing vibratable sheet material in acoustic instruments for the setting up of desired waves of speech frequencies so as to eliminate acoustic wave distortion due to resonance effects, when the material is of a character and in a form ,to produce such distortion, which comprises re straining the sheet along lines forming boundaries to many very small, less restrained areas of high natural fundamental frequency as compared with the range of fre- -quencies essential in speech reproduction,

applying periodic force directly, simultaneously and in the same amount to said small areas, said force being within the speech range of frequencies, and preventing the sheet material from vibrating along any of said lines of restraint in a manner to intro-' duce natural frequencies within the speech v range.

' '2. The method of reducing acoustic wave' distortion in' vibratable-sheet,material of acoustic instruments when such material is of a character and in a form to introduce distortion due to the fundamental and liarmonic-frequency resonance effects, which method comprises restraining the material along lines forming boundaries to many small,.; less restrained areas or segments so that the natural fundamental frequency of vibration of each of these segments is above the range of audibility and preventing the sheetmaterial from vibrating along any of the lines of restraint in a manner which would introduce natural frequencies within the range of essential speech frequencies.

3. An acoustic vibrating diaphragm system comprising sheet vibratable material, a

'rigid framework having many small openings therein, means for holdmg said sheet material over said openings under tension to form diaphragms in saidopenings, each diaphragm having a fundamental natural frequency above the range of frequencies essential in thereproduction ofspeech, and means for applying driving force directly, and

rigid framework having many small openings therein, a second framework similar in contour to said first and having corresponding openings therein, means for holding said frameworks together'with the openings in one registering with those in the other, with said sheet material therebetween and in firm contact with the peripheries of saidopenings to form diaphragms 111 the openings, the size of the openings, the character of the sheet material and the tension being chosen to give each diaphragm a fundamental natural frequency above the range of frequencies essential in the reproduc on of speech, and means for applying driving force directly and simultaneously to each of said diaphragms.

5. In an acoustical instrument, the combi-.

nation with the system of claim 30f a wave deflecting wall enclosing a conical shaped space filled with gas having said system at he large end thereof, si -vibratable piston of very. small size and mass, means for suspending said piston in said opening concentrically therewith, a-second vibretable element, and a rigid connecting means extending between said. vibratable element and the face of said fhce of said wall, the vibratable system comprising said vibratable element and said piston having a natural fundamental frequency of vibration above the range of essential voice frequencies.

6. The combination with an electric C11- piston most remote from the 1 cuit of means for acoustically ciontrolling the resistance of said circuit, comprising movable means of such. sensitiveness that slight air currents interfere with its proper functioning, and means for protecting such movable means from air currents set up by the breath of a speaker, comprising a vi bratable wall spaced from said [movable means and adapted to receive voice-sound waves from a speaker and to transmit them toward said movable means, said vibratabie wall having no natural period of vibration within the range of essential voice frequencies transmit-ted.

7. Means for producing waves within. the range of speech and music frequencies comprising sheet vibratable material, means for restraining said material along lilies forming boundaries to many small areas of high natural fundamental frequency as compared with the range of frequencies essential in speech reproduction so that the ear is substantially unresponsive to the resonant frequency of any of said areas, and means for applying periodic force directly, simultaneously and in the same amount to each ofsaid small areas so that each will vibrate similarly and so that a plurality of said izs ' and music frequencies.

- igaoasaa small areas 'n ot vibrate together. as'a unit having a "natural fundamental ire quency within therange of. essential speech -8. Means for prodncin Waves within the range of speech and music frequencies comprising sheet vibratable material, means forv restraining said material along lines forming boundaries to many small areas'of high natural fundamental frequency as compared with the range of frequencies essential in speech reproduction soithat the ear is substantially nnresnonsive to the resonant frequency of an of said areaa, means for applying ,perio ic iorce directly, simultaneousl and in the same amount to each of said sma areasso that each will vibrate similarly and so that a plurality of said-small areas will not vibrate together as a unit having a natural fundamental 'frequenc within the range of essential speech and music frequencies, and a chamber into, which said wave producing means works to set up sound waves therein.

. CLARENCE A. sPnAonn I

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