Parabolic sound reflecting microphone holder
United States Patent Gibson et a1.  
 PARABOLIC SOUND REFLEC&#39;IING MICROPHONE HOLDER Inventors: Daniel Armstrong Gibson. 6 Dunloe Rd.. Toronto, Ontario; Robert Justin Ryan. 1 14 Newport Ave.. Scarborough. Ontario. both of Canada Filed: Apr. 2, 1973 Appl. No.: 347,056  
 11.5. CI 179/1 MF Int. Cl H04! 1/34 Field of Search 179/1 MF. 1 MG: 181/26.  
 181/31 B. 27 R: 340/8 FT References Cited UNITED STATES PATENTS 1.899.966 3/1933 Jensen .[179/1 MF 1 51 Apr. 29, 1975 Spotts 179/1 MF Richer 181/26 Hanson 179/1 MF Pfund 181/26 Hotchner 181/26 FOREIGN PATENTS OR APPLICATIONS United Kingdom 179/1 MF Primur E.\&#39;aminerWi11iam C. Cooper ABSTRACT A parabolic sound reflecting microphone holder is disclosed made of a transparent plastics material about 18 inches in diameter and from 0.080 to 0.125 inches thick having good acoustic properties in the low frequency range of from about 150 to 500 cycles per sec- 1 Claim, 3 Drawing Figures 1 PARABOLIC SOUND REFLECTING MICROPHONE .1 HOLDER FIELD OF THE INVENTION This invention relates to parabolic, sound reflecting, microphone holders and in particular to such a device that is small enough to be hand held, that is transparent for easy microphone aiming and that has good acoustic properties in the low frequency range of between I50 and 500 cycles per second.  
 BACKGROUND OF THE INVENTION Parabolic sound reflectors for use with microphones and other acoustic equipment are by no means new. World War II saw extensive development in this field, particularly by the Japanese. Subsequently parabolic sound reflectors havebe&#39;en used extensively for such things as recording bird sounds at a distance and for other applications which require a directional and long range microphone.  
  Generally speaking such parabolic sound reflectors of the prior art were made of metal spinnings or of glass fibre reinforced plastics. While such parabolic sound reflectors were useful in conjunction with a microphone and a tape recorder for recording relatively high frequency sounds, they could not give good fidelity when used to record relatively low frequency sounds such as the human voice unless they were made in a size such that they required the support of a tripod or similar device;  
  In other words, the prior art failed to provide a hand held and aimed sound reflector for use with, for example, the microphone of a portable tape recorder.  
  A second principle defect of the prior art sound reflectors is their opacity. Since to operate efficiently, the microphone must be at the focus of the parabolic surface and the axis of the parabolic surface must point directly at the sound source, aiming is an important factor in making use of a parabolic sound reflector.  
  Parabolic sound reflectors of the prior art are aimed in one of two ways. First, the sound reflector may be provided with a small aperture through which the operator may sight along a line parallel to the axis of the parabolic surface. This aperture shouldbe kept as small as possible so as to minimize the effect upon the sound reflecting properties of the device. Thus, aiming is quite difficult to achieve quickly and accurately since the field of vision is restricted and the movement of the sound reflector, if mounted upon a tripod or other support may be cumbersome. Second, the sound reflector may be aimed electronically. That is, by observing an instrument such as a recording level meter to determine in this way the position which gives maximum signal level and hence the positionof optimum aiming. This method is difficult to use in association with a moving target and even under ideal conditions is relatively slow and time consuming. Further, both methods of aiming are made more difficult by an increase in the diameter of the sound reflector, an increase which is necessary however as the need for fidelity in the lower frequency range increases.  
 THE INVENTION The present invention, therefore, is the result of a discovery which makes it possible to construct a sound reflector meeting three criteria, namely:  
 I. it must be small enough in diameter and light enough in weight to be easily hand held and hand aimed;  
  2. it must have a good acoustic response in the relatively low frequency range of from about ISO to 500 cycles per second, and  
  3. it must be fully transparent so that it can be aimed as easily and quickly and accurately as pointing a finger.  
  As will be seen from the description which follows, the PARABOLIC SOUND REFLECTING MICRO- PHONE HOLDER of the present invention fulfills these three criteria.  
 BRIEF DESCRIPTION OF THE DRAWINGS DETAILED DESCRIPTION OF THE INVENTION Before describing in detail the subject matter of the drawings, it is necessary to consider again, the three criteria which the sound reflector must meet, namely, lightness and size, good low frequency properties and transparency. The requirement that the parabolic sound reflector be transparent indicated that the plastics might be useful and tests were conducted on a number of plastics including acrylics, polycarbonates and butyrates all of which were transparent. The particular butyrate tested was a transparent sheet cellulose acetate butyrate available from Canadian Kodak Sales Limited in Toronto, Ontario. Against all other materials tested it showed a marked superiority, particularly in the low frequency range.  
 In order to enable the device to be hand held and hand aimed, a diameter of approximately 18 inches was taken to be the optimum size. The curvature of the parabolic surface is such that the focus lies in the plane containing the rim when the rim diameter is 18 inches.  
  Using cellulose acetate butyrate to form a parabolic surface the focus of which lies in the plane of the rim when the rim diameter is 18 inches, it has been found that the thickness of the material should be between 0.080 inches and 0.125 inches in order to present the best compromise between lightness and adequate rigidity to retain a reasonably accurate shape under most conditions.  
  As mentioned above, tests conducted with a number of parabolic sound reflectors made of various materials showed a parabolic sound reflector of cellulose acetate butyrate to be substantially superior, particularly in the low frequency range of about 150 to 500 cycles per second.  
  For example, at I50 cycles per second the signal level using a microphone sold under the trade mark SI-IURE 425 COMANDO and a cellulose acetate butyrate sound reflector having the size, curvature and thickness described above was about higher than that when using a similar sound reflector made from an acrylic material and about 125% higher than that when using a sound reflector made from a polycarbonate material.  
  At 175 cycles per second the superiority of cellulose acetate butyrate over acrylic and polycarbonate materials was shown to be approximtately 600 and 300% respectively.  
  At 250 cycles per second the superiority of cellulose acetate butyrate over acrylic and polycarbonate materials was shown to be approximately 1,400 and 900% respectively.  
  At 500 cycles per second, all three materials showed quite similar results.  
  Although the superiority of the cellulose acetate butyrate material is most marked in the low frequency ranges. its superior properties are noted throughout the range of frequencies up to the high audible ranges except for various spaced points at which its performance may be equalled or approximated by the other materials investigated. Overall, however, the cellulose acetate butyrate material demonstrates a consistent and marked acoustic superiority as explained above.  
  Metal and glass fibre reinforced materials do not give an adequate response in the l50 to 500 cycles per second range in sizes less than about 36 to 48 inches in diameter, a size which is too heavy and cumbersome for hand held use.  
  It is not possible to offer a substantiated explanation or theory for the superiority of cellulose acetate butyrate to other materials. No doubt the physical properties of the material enable it to resonate and respond in the low frequency ranges in a superior manner. However, these precise properties are not fully understood. The surprising fact is, nevertheless, that cellulose acetate butyrate provides surprisingly and unexpectedly superior results as has been explained.  
  Turning now to the drawings, a typical construction of a parabolic sound reflecting microphone holder embodying the present invention is illustrated.  
  In FIG. 1, a parabolic sound reflector is illustrated at made from cellulose acetate butyrate and having a wall thickness of between 0.080 and 0.125 inches and a rim diameter of approximately 18 inches. The rim may conveniently be reinforced by flanges such as 11 and 12 extending about the perimeter.  
  A shaft 13 passes through the sound reflector 10 and, on the side 10a of the parabolic shell adjacent the sound reflecting surface is provided a bracket 14 adapted to support a microphone 15 which will lie upon the axis 16 oflthe parabolic surface with the microphone head or diaphragm lying at the focus 17 of the parabolic surface.  
  On the opposite side of the parabolic shell the shaft supports a handle 18 by means of which the assembly may be hand held and aimed.  
  Conveniently, electrical connections to the microphone pass internally of the handle, the shaft 13, and the bracket 14 and may be connected to a cord exiting from the handle as indicated at 19.  
  Optionally, such features as an aiming guide 20 may be provided, and a microphone switch may be located at 21. Further, in order to enable the microphone to be removed and serviced or replaced, the bracket 14 is removably mounted upon shaft 13 by any convenient socket located at, for example, 22.  
  The method of securing the shaft 13 to the parabolic sound reflector 10 is illustrated in FIG. 2. At a point spaced from the axis 16, the reflector 10 is provided with a circular recess 23 having a central aperture 24 which is provided with four radially extending, mutually perpendicular slots 25. A circular flange 26 is provided having lugs 27 and a central cylindrical collet chuck 28 provided with external threads 29. The flange 26 is adapted to seat within the circular recess 23 with the lugs 27 entering the slots 25 to present rotation. The shaft 13 may then be passed through the collet 28 and the collar 30 passed over the shaft 13. The collar 30 is provided with four recesses 31 adapted to receive the lugs 27 where they extend through the thickness of the wall of the recess 23. A resilient O-ring 32 is then placed about the shaft 13 and finally a nut 33 is applied to engage threads 29 and firmly clamp the collet 28 onto the shaft 13 to hold it rigidly in place.  
  It will be noted that by loosening the nut 33, adjustments may be made of the shaft 13 relative to the reflector 10 so as to enable the microphone to be precisely located both on the axis 16 and at the focus 17 of the parabolic surface.  
  Although a particular form of the invention has been illustrated in the accompanying drawings and described herein, it is emphasized that the invention contemplates the use of other forms of handle, microphone and related hardware. The factors of the invention which are considered to be of paramount importance is the selection of cellulose acetate butyrate as the material from which the reflector is made. This material enables the reflector to achieve the necessary transparency as a result of which it is as easy to aim as pointing a finger. The curvature and diameter chosen provide for a light easily hand held and hand aimed assembly and the cellulose acetate butyrate material itself appears to provide for surprisingly and startlingly superior acoustic properties in the low frequency range of from about to about 500 cycles per second.  
 What I claim is:  
 1. A light weight, hand held sound reflecting parabola and microphone holder assembly comprising:  
 a. a parabolic shell having a sound reflecting surface,  
 b. a shaft passing through the parabolic shell parallel to but displaced from the parabolic axis, said shaft being adjustable in both senses of a direction parallel to the parabolic axis;  
 c. means carried by the shaft on the side of the parabolic shell adjacent the sound reflecting surface to support a microphone, said means extending from the shaft towards the parabolic axis so that the microphone may be supported on the said axis;  
 d. handle means carried by the shaft on the side of the parabolic shell remote from the sound reflecting surface to enable the human hand to grasp and support the sound reflecting parabola and microphone holder assembly;  
 e. the parabolic shell being:  
 1. circular in a plane normal to the parabolic axis;  
 2. made of transparent cellulose acetate butyrate plastics material;  
 3. of a thickness of between about 0.080 and 0.125  
 inches;  
 4. of a diameter of the order of about 18 inches;  
 and  
 5. of a curvature such that its focus lies in a plane&#39; containing the rim of the parabolic shell.