US 2346394 A
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Description (OCR text may contain errors)
Patented Apr. 11, 1944 stretti fiom soUNn PICKUP APPARATUS Michael Rettinger, Encino, Calif., assigner to Radio Corporation of America, a corporation of Delaware Application June 21, 1941, Serial No. 399,148
This invention relates to sound pickup apparatus such as high quality microphones used in making sound recordings and radio broadcasts, and particularly to windscreens or shields for such microphones.
In the art of translating sound waves into electrical currents for either simultaneous reproduction or for the recording thereof, the translators or microphones are frequently used in locations where there is considerable air movement or strong winds. On broadcast or motion picture locations the wind may be nautral, although, in motion picture work, strong winds are created on the sound stages by artificial means. In either case, however, the moving element, such as a ribbon or diaphragm, is subject to air currents and the noise produced thereby, both of which introduce distortion into the recorded or reproduced sound.
The principal object of the present invention, therefore, is to increase the efficacy of a soundpressure-to-voltage transducer.
Another object of the invention is to maintain the sensitivity, frequency characteristic and directional response of a microphone substantially constant when shielded from air currents.
Another object of the invention is to provide a light, easily removable and eicient protective shield for a microphone when used in windy locations.
A further object of the invention is to provide a shielding system for a microphone in which the efficiency may be varied in accordance with the velocity of the air currents encountered in use.
The use of perforated metallic shields covered by a thin silk fabric is well known in the art, as illustrated by H. F. Olson, Patent 1,987,413, of January 8, 1935, this patent also disclosing the use of a plurality of screens. However, it has been found that the particular type of shields disclosed by this patent, although providing better protection for the microphone than previous arrangements, are, nevertheless, not sufficient to maintain the sensitivity, frequency characteristic and directional response required in high quality sound recording or broadcasting. The present invention, therefore, has been found to be a distinct improvement over the particular construction shown in this patent.
Although the novel features which are believed to be characteristic of this invention will be pointed out with particularity in the appended claims, the manner of its organization and the mode of its operation will be better understood by referring to the following description read iny conjunction with the accompanying drawing forming a part thereof in which Figure l is an elevational view of one type of microphone embodying the invention;
Figure 2 is a cross-sectional view of the microphone of Figure 1 taken along the lines 2--2 thereof;
Figure 3 is a detailed view taken along the line 3-3 of Figure 2 showing the mounting of the screens;
Figure 4 is an elevational view of another type of microphone embodying the invention; and
Figure 5 is a graph showing the frequency characteristic of a microphone embodying the invention.
Referring now to the drawing in which like elements have been given identical numbers, a housing 5, which may or may not enclose a preliminary stage of amplification, supports, on a bracket 6 having a circular extension l, a crystal microphone unit 9 of the disc type. Conductors 8 connect the crystal or amplifier to the other equipment not shown. Surrounding the crystal 9 is a spherical screen or casing Il, the upper opening of which is welded or otherwise attached to a ring I2 which is mounted by screws I3 to the housing 5, as shown in detail in Figure 3. This screen is preferably perforated similar to the disclosure of the above-identified patent and is covered with a silk or similar fabric i4, this fabric being either internally or externally attached to the screen in any suitable manner, such as by gluing, so as to prevent movement between the screen and fabric.
Such a spherical windscreen has been found to be very efficient from the standpoints of reduction of wind noise and of maintaining constant the normal frequency characteristic and directional response of the microphone. However, such a screen is not sufficient protection for the microphone in high wind velocities but permits the air currents to enter and affectthe microphone. The former practice was to simply add additional layers of silk or similar fabric to the shield Il. This not only reduced the effect of wind currents, but also had a material effect on the frequency response of the microphone by reducing the high frequency response a very appreciable amount. To illustrate this reduction in high frequency response, reference is made to Figure 5 wherein the dotted curve shows a 7.5 db. reduction at 10,000 cycles by the use of four layers of -80 count silk.
To prevent this loss in high frequency response, applicant has used a second spherical ment in the form of air waves.
screen I6 constructed similarly to the screen ll in that it has an outer metallic portion and an inner fabric layer Il. This screen is mounted on an annular ring bracket formed of two semiannular sections 20 and 2l (see Fig. 2) having diametrically positioned ears, one of which is threaded to facilitate the fastening together of the sections 20 and 2l by threaded bolts 24 and 25.
The shield is divided into halves and hinged at 26 so that the shield may be easily attached to and detached from the housing 5. When the shield I6 is in position, it will be noted in'Fig. 3 that the lower portion of the ring 20 abuts a ring 28 welded to ring l2 which positions the screen I6 the proper distance from the screen ll. The screen I6 is attached to an outwardly extending fiange I8 of the ring 20 by means of rivets 29. A metal band 30 may be used to cover the joint between the two sections of the screen, this band being painted different colors over portions thereof' to indicate the front and back of the microphone in case this is required.
It will be noted that the screen IB is also spherical and that all points thereon are equidistant from the screen H. This is a particularly important feature of the invention inasmuch as it was found that the shortestl distance between a double shield controls the efciency of the shields from the noise reduction standpoint. That is, should the second shield be separated from the inner shield by a half inch at one point and three inches at another point, the shield would be no more effective than if the separation at all points were one-half inch. It has also been found that, by the use of a second shield with a single fabric thereon instead of additional layers of silk to a single shield, not only is there obtained the same amount of noise reduction, but there is substantially no reduction in the high frequency response as obtained with the use of multiple fabrics.
This is illustrated in Figure by the solid curve between one thousand and ten thousand cycles, this curve substantially following the normal characteristic of the microphone. Thus, by comparing the two curves which represent the same amount of Wind noise reduction (i. e., wind acting in the nature of extremely low frequency sound having random incidence), the effect on the frequency characteristic of using a double shield rather than four layers of fabric on a single shield is readily observable. Although the above-mentioned patent describes the use of a double screen, it will be noted that the outer screen is shown as being of a rectangular or square nature. This construction has been found to nullify to a large extent the benefits of the double screen because sharp corners tend to create air turbulence in contact with the screen itself which is transmitted to the moving ele- Applicants screen, therefore, is of a spherical nature, thus reducing to a minimum any whistling and turbulence of air regardless of direction of movement.
In Figures l, 2 and 3, the spherical type of screens are illustrated to conform to the type of microphone shown at 9. However, various types of microphones are in use, one standard type being illustrated in Figure 4. This microphone is of the magnetic type having an elongated ribbon 33 positioned in a magnetic field provided by a magnet 32. These units are shown mounted upon a bracket 6 similar to the crystal unit 9 in Figures l and 2. To produce the highest efciency of shielding for such a microphone, it will be noted that the shields are of an elliptical nature to conform in a general manner to the shape of the moving element. Thus, there is provided in the modification of Figure 4 an elliptical shield 35 and a second elliptical shield 36, these shields being of the same perforated nature as those of Figures l and 2 covered with a fabric of similar nature.
The inner shield 35 may be mounted to the housing 5, as shown in Figure 3, while the shield 36 may be mounted similarly to the arrangement shown in the spherical embodiment. That is, the shield 36 is divided into half sections, being hinged at 33 and having upper ring sections 39 and lill held together by bolts, one of which is shown at di. It has been found, however, that, in cases where the wind has an extremely high velocity, two shields are insufficient, and a third shield, shown by the dotted lines 43, is added. That is, this shield is hinged at ifi and has upper semi-ring portions and ril and fastening bolts, one of which is shown at M3. The shields 36 and 43 are readily attachable to and detachable from the housing 5, and the soundman, after a test, may add shields according to the wind conditions at the point of sound pickup. It will also be noted that the shield 133 is arranged equidistant from the shield 35, the outer shield rings abutting the flange 2B of the second shield and providing a positioning point.
Thus, the above-described wind shields or screens are designed to conform without sharp corners to the particular conguration of the actuating element of the microphones, to be arranged equidistant from each other and to have curvilinear surfaces to reduce air turbulence to a minimum. This construction has been found highly efficient for reducing noise Without interfering with the sensitivity, frequency characteristic and directional response of the microphone. In some instances, one screen is permanently maintained around the microphone for not only the purpose of reducing the noise, but also to provide a physical protection for the microphone. The additional screens are added as needed, these screens being of a spherical, elliptical or otherwise curvilinear nature to confo-rm with the one permanently attached.
In the above description, the shape of the screens not only conforms to the general configuration of the moving element, but substantially all points in one screen are equidistant from similar points in its associated screen or screens. A1- though this feature is applicable to most microphones, there are certain types which exhibit a greater sensitivity to wind noise in a direction normal to the diaphragm. The windshields for such types will be arranged eccentrically with respect to their moving elements, the shields not necessarily being spherical or elliptical but having completely curvilinear surfaces and arranged parallel with one another.
What I claim as my invention is:
l. In combination with a microphone having a nlipllills and a movable element theren, a shield for said microphone comprising a casing hav-ing apertures therein, said casing and housing completely enclosing said movable element, a closely woven fabric contacting the surface of said casing, and a second casing surrounding said first casing and attached to said housing, said housing and said second casing completely enclosing said first casing, said second casing being also provided with a plurality of apertures, and having the same shape and being spaced from said first casing an equal distance at substantially all points including the points of attachment of saidcasings to paid housing.
2. A combination in accordance with claim l in which the surfaces of said casings are curvilinear over their entire areas.
3. A combination in accordance with claim 1 in which said casings are substantially spherical.
4. A combination in accordance with claim l in which said casings are in the form of substantially concentric spheres.
5. A combination in accordance 'with claim 1 in which the movable element of the microphone is an elongated ribbon and'said'casings are elliptically shaped to conform to the general conguration of said ribbon.
6. In combination with a microphone having a movable element and a housing, a shield for said movable element comprising a plurality of perforated casings, said casings in combination with said housing completely enclosing' said movable element centrally located with respect to said casings, said casings having curvilinear sui'- faces conforming to the general conguration of said movable element and being equally spaced from each other.
7. A combination in accordance with claim 6 in which at least one of said casings is divided into half sections having a hinge at one point and iianged collars at opposite points to facilitate attachment and detachment from said microphone.
8. A microphone Wind screen assembly for attachment to said microphone housing comprising a plurality of concentrically arranged perforated casings, a closely woven fabric on each of said casings, means for mounting each of said casings to said microphone housing, said casings being spaced equidistantly from each other including the points of attachment to said housing and in combination with said housing completely enclosing the movable element of said microphone, and means on each of said mountings for predetermining the spatial disposition of one of said casings with respect to its adjacent casing.
9. A wind screen assembly in accordance with claim 8 in which said mountings for the external casings of said assembly are adapted to be attached to and detached from said housing.