Electrodynamic microphone having an adjustable tube for changing the directional characteristic of the microphone

Sept. 8,

1970 A. H. VAN LIEMPD 3 527 902

ELECTRODYNAMIC MICROPHONE HAVING AN ADJUSTABLE TUBE F0R^>CHANGING THE DIRECTIONAL CHARACTERISTIC

OF THE MICROPHONE Filed March 21, 1967

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INVENTOR.

ADR I ANUS H. VAN LIEMPD

BY

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United States Patent Office _Patented

3,527,902

ELECTRODYNAMIC MICROPHONE HAVING AN ADJUSTABLE TUBE FOR CHANGING THE, DIRECTIONAL CHARACTERISTIC OF THE MICROPHONE 5

Adrianus Hendrijus vanLiempd, Emmasingel, Eindhoven, Netherlands, assignor, by mesne assignments, to U.S.

Philips Corporation, New York, N.Y., a corporation of Delaware

Filed Mar. 21,1967, Ser. No. 624,942 , _n

Claims priority, application Netherlands, Mar. 26, 1966, 6604014

Int. O. H04r 1/38

U.S. Cl. 179-121 11 Claims

ABSTRACT OF THE DISCLOSURE

An electrodynamic microphone in which the back side of the diaphragm communicates with a closed air chamber formed by an adjustable sleeve within the microphone 20 housing. The back side of the diaphragm communicates either with the open air or with the closed air chamber, depending upon the position of the displaceable sleeve. Adjustment of the sleeve provides a change in the directional characteristic of the microphone with little or no 25 change in frequency response.

The present invention relates to an electrodynamic microphone having a housing consisting of a cap perme- 30 able to sound and a shaft, while in the cap there is mounted a microphone capsule having a diaphragm which communicates on the front side with the ambience and on the rear side with a closed air chamber located at least in part in a sleeve provided in the shaft. _3g

Such a microphone is known from the British patent specification 755,143. Due to the closed air chamber in the shaft, this microphone has a comparatively flat frequency characteristic, while the directional characteristic is cardioidal. 40

The invention is characterized in that the sleeve is provided in the shaft so as to be displaceable and is operated by a mechanical switch so that the rear side of the diaphragm communicates with the ambience. Thus, the advantage is obtained that with a substantially unchanged 45 frequency characteristic, the directional characteristic can be considerably varied in a simple manner. When the rear side of diaphragm of the microphone is completely closed from the ambience, the directional characteristic of the microphone exhibits a substantially uniform course, that 50 is to say that this so-called omnidirectional characteristic in which the sensitivity of the microphone is substantially equal in all directions. When the rear side of the diaphragm is not closed from the ambience, however, the sensitivity of the microphone on one side is an optimum, 55 whereas it is 0 on the opposite side.

One embodiment according to the invention includes a sleeve enclosing the air chamber and is characterized in that the sleeve is closed at one end and is open at the other end and can be displaced inside the shaft in a longi- 60 tudinal direction, while in one position of the sleeve the open end engages the rear side of the microphone capsule.

By a simple manipulation, that is to say, by a displacement of the sleeve, any directional characteristic can be obtained between the so-called omnidirectional character- 65 istic and the cardioidal characteristic. A modification of this embodiment provides a microphone in which the sleeve tightly fits into the shaft and the mechanical switch consists of a cam provided on the outer periphery of the sleeve and located in a longitudinal slot in the wall of 70 the shaft. In this embodiment, a helical spring presses against the closed end of the sleeve.

In a particularly favourable embodiment of the invention, the mechanical switch further includes a ring which surrounds the shaft and is provided on its inner side with an inclined guide engaged by the cam.

When the ring is rotated, the cam slides along the inclined guide into the ring; a force is then applied to the cam which causes the sleeve to move inside the shaft.

A quite different embodiment of the invention is characterized in that the air chamber is limited on one side by the shaft and on the other side by a rotatable sleeve which is arranged in the shaft with tight fit. The end of the sleeve facing the capsule is provided with one or more eccentrically arranged apertures corresponding in one position with apertures of the rear side of the capsule which communicate with the rear side of the diaphragm, one or more apertures being provided on the outer periphery both in the sleeve and in the shaft which correspond with each other in the said position. Instead of being displaced inside the shaft, the sleeve is now rotated. The directional characteristic of the microphone can be improved by providing the apertures in the sleeve embodiment with an acoustic resistance.

In a further particularly advantageous embodiment the mechanical switch consists of a cam provided on the outer periphery of the sleeve and located in a transverse slot in the wall of the shaft.

The invention will now be described more fully with reference to the accompanying drawing.

In the drawing:

FIG. 1 is a longitudinal sectional view of a microphone in accordance with the invention,

FIG. 2 is a perspective view of a detail thereof; and

FIG. 3 is a partial longitudinal sectional view of another embodiment of a microphone in accordance with the invention.

FIG. 1 is a longitudinal sectional view of an electrodynamic microphone in which the housing 1 consists of a cap 2, permeable to and a closed metal shaft 3. Inside the cap 2 there is arranged a microphone capsule 4, the diaphragm 5 of which communicates with the ambience through a member 6 for protecting the diaphragm and with the rear side 7 through ducts (not shown). A sleeve 8 is located in the shaft 3 and is pushed at one end, by means of a helical spring 9, against a rubber ring 10 on the rear side 7 of the microphone capsule 4. The shaft 3 is surrounded by a rotatable ring 11 which is provided, on the inner side, with a plastic sleeve 12. This sleeve has a recess 13 into which fits a cam 14 on the inner side of the ring 11. The sleeve is further provided with an inclined guide 15« on which a pin 15b bears. This cam is disposed freely in a longitudinal slot 16 and is screwed to th? sleeve 8.

When the ring 11 is rotated, the pin 15b is forced to move forwards or backwards through the guide 15«. In the latter case, the sleeve 8 gets free from the capsule 4 and the sound can reach the rear side of the diaphragm 5 in the cap 2 through the ducts (not shown) in the capsule 4. The directional characteristic is then cardioidal. If the sleeve 8 fully engages the ring 10, the sound cannot reach the rear side of the diaphragm. The directional characteristic is then omnidirectional.

FIG. 3 shows another embodiment. In this figure, the shaft 3 is partly broken away. The shaft 3 encloses, with a tight fit, a short rotatable sleeve 8 on which the cam member 15 is provided. The cam member is adapted to move freely in a transverse slot in the shaft.

The end of shaft 3 is provided with apertures 17 which cooperate with apertures 18 in the sleeve 8. The apertures 18 are covered with gauze 19.

The sleeve is further provided with other apertures 20 which in turn cooperate with apertures 21 in the shaft 3.

The apertures 17 and 18 correspond with the apertures

3,527,902 and 21 in one position of the sleeve 8 and the shaft 3. The sound can then reach the rear side of the diaphragm. The directional characteristic of the microphone is cardioidal in this position.