US2608265A - Ribbon-type microphone diaphragm - Google Patents

Description

Aug. 26, 1952 RESPONSE /N 08 H. ECKARDT 2,608,265

RIBBON-TYPE MICROPHONE DIAPHRAGM Filed May 28, 1949 FIG] FIG. 4

200 400 500 M00 3200 was 12800 FREQUENCY /N K/LOCYCLES PER SECOND IN VE/v TOR H. E CKA R07 ATTORA/EV Patented Aug. 26, 1952 UNITED STATES PATENT OFFICE RIBBON-TYPE MICROPHONE DIAPHRAGM Helmuth Eckardt, Towaco, N. J assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application May 28, 1949, Serial No. 96,012

6 Claims. (01. 181-32) presently known constructions, the diaphragm may be fiat or transversely corrugated substantially throughout it length. In response to sound waves effective upon one or both faces thereof, the diaphragm vibrates in the magnetic field whereby potential variations respresentative of the sound waves are developed between spaced points on the diaphragm, specifically between the ends thereof.

In devices of presently known construction, the relationship between diaphragm vibration and developed potential, or the frequency response characteristic, is irregular. Specifically, in devices including diaphragms of the forms above mentioned, the response is quite non-uniform over a range of frequencies at the lower end of the operating range so that distortion or unfaithful translation of the suond waves results.

One general object of this invention is to improve the frequency response characteristic of acoustic devices, such as microphones, including ribbon-type diaphragms, whereby high fidelity translation of sound waves into electrical signals may be realized.

It has been determined that distortion such as mentioned heretofore is definitely associated with the manner in which the elongated or ribbon diaphragm vibrates in response to sound waves effective thereon. Specifically, such a diaphragm flat or transversely corrugated substantially throughout its length, vibrates in several different modes in addition to its fundamental mode. For the latter, the diaphragm vibrates generally as one section with maximum displacement midway between its ends. In the third mode, which occurs at a frequency about triple that of the fundamental, the intermediate portion of the diaphragm vibrates out of phase, substantially 180 degrees, with the two end sections. The combination of resonance and cancellation eifects results in marked irregularities in the response particularly in the range of several hundred cycles above the fundamental frequency. In general, because of the low mass of the diaphragm, the resonance frequency is low, for example of the order of 50 cycles. The third mode vibration, to-

2 gether with the fundamental resonance, results in substantial distortion in the range between about and 500 cycles in typical devices.

In accordance with one feature of this invention, the diaphragm is constructed to substantially suppress spurious vibration and especially vibration in the third mode.

In one specific construction illustrative of this invention, a diaphragm comprise a thin metallic ribbon supported at its ends and having one end portion corrugated transversely to increase its. flexibility, the opposite end portion formed to in. crease its mass and stiffness and an intermediate; or central planar portion.

The invention and the above-noted and otherfeatures thereof will be understood more clearly and fully from the following detailed descriptionwith reference to the accompanying drawing in;

which:

Fig. 1 is a face view of the magnet and dia--v phragm assembly in a ribbon-type microphone illustrative of one embodiment of this invention;

Fig. 2 is a side view, partly in section, of a diaphragm constructed in accordance with this invention, and illustrating the manner of support-- ing the diaphragm;

Figs. 3, 4 and 5 illustrate several steps involved in the formation of a diaphragm such as illus-.

trated in Fig. 2; and

Fig. 6 is a graph illustrating the improvementin response effected by diaphragms constructed in accordance with this invention.

Referring now to the drawing, the microphone unit assembly illustrated in Figs. 1 and 2 com-.

The diaphragm is in the form of a metallic ribbon and, as shown clearly in Fig. 2, comprises a planar intermediate portion IS, a transversely corrugated portion l6 at one end and a double thickness portion I! at the other end. One end of the ribbon diaphragm is clamped between a pair of metallic bars or blocks l8 secured together by screws l9 and afflxed to the pole-pieces by screws 20. The other end of the diaphragm is clamped between a metallic block 2| and a terminal 22, the terminal and diaphragm being insulated from the block 2| by an insulating strip 23. The block 2! is affixed to the pole-pieces by screws 24 and the terminal 22 is secured to this a region near one end thereof; Then, as villus= trated in Fig. 5, the other end portion of the strip is folded upon itself. and thei'end of the folded section is secured to the' body of the strip, as by a suitable cement indicated at 26 in Fig.

5. The ribbon is mounted between-the supports 4 thereby to obtain a uniform response over this range and consequently over the entire operating range of the device.

Although a specific embodiment of this invention has been shown and described, it will be understood that it is but illustrative and that various modifications may be made therein without departing from the scope 'and'spirit of this invention.

What is claimed is:

1. An acoustic diaphragm comprising a thin metal striphaving a. transversely corrugated flexible portion adjacent one end thereof, means on 'saidlstri for increasing the mass and stiffl8 and 2|, 22, tensioned to produce a prescribed resonance frequency therefor, for example of 50 cycles per second, and theexcess portions beyond the supports-cut away.-

i a' specific construction, the interm'diate' portion l5 may be A; ineli in length, the cor-'- riigatedp'or'tiorr l6 maybe 5/,- inclilong-and-"the unclamped part of the double thicknessportion' l1 "inay' be inchlong.

It will be appreciated that the completed diaphragm com-prises three distinct se'ctionsof different massand sti-ifness per unit'- length. Specifically, the section I 1 has the greatest-mass and stiffness, the sections- I 5 and" it have about the samemass; and the" section: I iihasthe'low'e'st stiffness; Thus,- considered as a vibratory sys term, the composite strip tends to liave' th r'ee'; (lif fefe'nt and relatively minor r'esonarices- Whereas a-st ri por uniform mass and stifins's throughout ha a single pronounced resonance and. as has beenpointed out heretofore for vibration-inthe' cycles are clearly evident." curve :B portrays the.

response ofthesame microphone but including a. diaphragm of the construction illustrated in Fig.2 and described hereinabove. Particularly to" be: noted are the relative. smoothnessof the response: over-the frequency range wherein curve A isdecidedljzirregular". The falling off ofthe response indicatedin curve Bin this range can be'compensat'ed foreasily by design of the 'electrical circuiti-ntowhich the microphone-operates,

I file of patent:

'"fiessfof abortion" adjacent the other end thereof, and means mounting said diaphragm at its ends;- I

2. A -ribbon diaphragm having a transversely corrugated flexible portion adjacent one end thereof, astifiened portion adjacent the other encl thereof; and an intermediate p ariar portion, and means-mounting said diaphragmat ends-.

3. A ribbon diaphragm 'lfavin'g a fiexi-b'lef tion-at one end-thereof,- an int rmediate section and ace-anon at the other endcfgrea'ter and stiffness per unit length-than said flexible and intermediate sections; and sup ort meansengaging said diaphragm at its ends.

4. A ribbon diaphragmcomprising a metallic strip Having portion adjacent one end thereof transversely corrugated 5 a second I strip over?" lying am secured to ix-portion or s'aid' fi 'rst 'strip adjacent the other end thereof, and means mounting saiddiaphragmat--its eiidsa 5". A diaphra m coin-prising inetallie'ribboiihavingin te mate-portion or single th ness," 9. transversely-ccrrugated flexible portionat one end and a double thickness portion at the other end, and nieans mounting said dianragm e'n'dsi- V I g i 61A diaphragm: mi accordance with: claim '5" of substantianys isizcrespectiveiy.

- "REFERENCES orrED Thefollowiffg references are or recorder the Number Name bate 1,689,513 Zimmerman Oct:- 30,-.1928 1,947,749 Vermeul'en etal. a": Feb-s 20,--1934 2,14154-20 Swickard- Dace-27,1938;

FOREIGNPATENTS Q 5 cease-y g fba'tejfj V I H Great Britain 1 Sept. 18; 1919 Germany Oct 22 1921 298,719 Gifatifirfitain- ;;OctL

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