US2113219A - Microphone - Google Patents

Description

April 5, 1938.

Fig. I

Filed May s1. 1934 H. F. OLSON ET AL lllllll MICROPHONE 2 Sheets-Sheer, 1

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2 Sheets-Sheet 2 mfg. 3

Oasi/H450 M S Y RWM M im will Hf Vl B Patented Apr. 5, 1938 PATENT OFFICE MICROPHONE Harry F. Olson, Collingswood, and Frank Massa, `Oaklyn, N. J., assignors to Radio Corporation of America, a corporation of Delaware Application May 31, 1934, Serial No. 728,202

3 Claims.

This invention relates to velocity microphones, and concerns particularly a velocity microphone having a cosine directional characteristc within a predetermined frequency range.

One object of the invention is to provide such a microphone which will avoid the pick-up of reverberated sounds.

Another object of the invention is to provide a microphone of the type referred to which is capable of being manufactured to any desired frequency-directional characteristics.

Another object of the invention is to provide an improved type of velocity or ribbon microphone.

1n the drawings: y

Figure 1 shows our improved microphone with the cover removed therefrom.

Figure 2 is an elevation of the pole-pieces and ribbons thereof.

20 Figure 3 is a top` view of the pole-pieces and ribbons with the ribbon clamps and conductors removed.

Figure 4 is a plot of the response of our microphone.

The directional characteristics of the velocity ribbon microphone are independent of the frequency. In certain types of sound motion picture sets the reverberation time is greater at the higher frequencies. Furthermore, it is also desirable in some recordings to increase the discrimination against reflected sound to obtain a more intimate effect. To accomplish these objectives requires a sharpening of the directional characteristics of the velocity microphone at the higher frequencies. It is the purpose of this invention to provide a velocity microphone employing two ribbons separated in space and which eX- hibits a cosine directional characteristic in the range below 1000 cycles and a progressively sharper characteristic above 1000 cycles.

The directional characteristic of the conventional velocity ribbon microphone is given by the line joining the sound source and the ribbon.

Let us consider a velocity microphone equipped with two parallel ribbons separated by a distance 55 D. The voltage output for a source of sound in the plane passing through the ribbons and vnormal to .axis of the ribbons is given by (WD sin e=e0 cos 0 cos where 0=angle between the normal to the ribbons and the line joining the sound source and the center line of the two ribbons,

D=distance between the ribbons,

}\= wavelength =v c'=velocity of sound, and f=frequency The theoretical directional characteristics are shown in Fig. 4. The observed directional characteris'tics are also shown in Fig. 4.

The essential details of construction are shown in Figs. 1, 2 and 3, Two corrugated aluminum ribbons .0001 thick (similar to the ribbons used in the usual velocity microphone) arerseparated by a distance of 2 inches between centers as shown in these figures. The magnetic structure is designed to replace the pole assembly in the usual velocity microphone.

The two ribbons are electrically connected, as shown in Fig. 2, in order that the eective inductance of the ribbon circuit Will be reduced to a minimum and electrical pick-up thereby pre- I vented. v

The general construction and operation of the usual velocity microphone referred to above is y shown, described and claimed in the patent to Olson No. 1,885,001 issued October 25, 1932 and, except for the arrangement of pole-pieces and ribbons, the present device is constructed in the same manner.

To describe in more .detail herein the construction of our microphone:

A permanent magnet I0 is mounted upon an appropriate casing II which houses an appropriate transformer, as described in the said prior patent. Upon the poles of the magnet I0 are mounted pole-pieces I2 and I3 of appropriate high-permeability iron which are so shaped as to provide a uniform and high magnetic iiux across the ribbons I4 and I5 which are constructed in accordance with the said prior patent.

Between the said ribbons is interposed a polepiece yoke -generally indicated at I6 and comprising the secondary pole-pieces Il and I8 and the magnetic connecting members I9 and 2U, distributing and concentrating the magnetic flux equally between the two ribbons.

The ribbons are supported by, and connected to, the clamps 2|, 22, 23 and 24, which are respectively insulated from the rest of the structure by the insulating members, preferably of Bakelite, hard rubber, or l the like, 25, 2B, 21 and 28.

'Ihe several leads 3D, 3l and 32 are connected as illustrated in Fig. 2, so that the ribbons are connected in series but not in opposition.

'This double ribbon microphone as shown in Fig. l has shown a measured ux density of 1450 eausses in each air gap. This is somewhat higher than half the flux density in a corresponding velocity microphone with a single air gap. The increased magnetic efficiency results from the reduction in pole face leakage.

Frequency response measurements made along the axis passing between the ribbons and normal to the plane of the ribbons in this microphone show the same characteristic as obtained with a standard Velocity microphone. The observed directional characteristics of the microphone at various frequencies are shown on the right hand portion of Fig. 4. On the left hand portion of the same figure are shown the theoretical directional characteristics which check fairly Well with the observed curves.

As described in the said prior patent, the dimensions of the several pole-pieces determine the individual frequency characteristics of the individual ribbons. However, in the present device, the cosine frequency directional characteristic referred to is determined by the lateral spacing between the ribbons.

We do not mean to be limited in the present invention to the specific construction, characteristics, and dimensions described above as the present preferred and commercial form of our invention, as it will be apparent that Within the scope of our invention we may provide any number of ribbons in a corresponding arrangement; or We may render the individual frequency characteristics of the ribbons asymmetrical by altering the size of the bales around them so as to make the curves of Fig. 4 lop-sided; or we may place the ribbons at a different angle to each other to modify the frequency-directional characteristics.

Having now described our invention, we claim:

1. A unitary device of the class described comprising a plurality of physically independent, series connected acoustically velocity-controlled elements located at a predetermined distance from each other Within the unit, and magnetic means common to both of said elements for supplying a magnetic field for each of them.

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