US1911772A - Fornia - Google Patents

Description

y 39, 1933- F. RIEBER DIRECTIVE SOUND PICK-UP Filed July 14, 1931 INVENTOR, FRANK R/EBER.

ATTORNEY Patented May 30, 1933 UNITED STATES PATENT orrlcs FRANK BIEB EB, OF SAN FRANCISOO, CALIFORNIA, ASSIGNOR TO BOUND LABORATORY CORPORATION, LTD., 01' SAN FRANCISCO, CALIFORNIA, A CORPORATION 01 CALI- FOBNIA Application filed July 14,

My invention relates to sound pick-ups for the electrical recording or transmission of sound, and its broad purpose is to provide a pick-up wherein the ratios between direct and reflected sound of both high and low frequencies from a given source are adjusted to substantially the values of normal human perception.

Among the objects of my invention are:

fprovide a pick-up having substantiall uni orm response to direct sound of hot high and low frequencies, while discriminating a ainst reflected or secondary sound of high requency; to rovide asound directive system which wil intercept a portion of a received sound wave, concentrate said wave, and redirect it as a' wave-front of smaller diameter and higher concentration in its direction of original travel; to provide 20 a sound pick-rup system wherein the advantages of a baflie may be combined with the advantages of a directive reflector; to provide a directive sound receivin s stem wherein the'micro hone is complete y s ield- 5 ed from sound 0 both hi h and, low pitch proceeding from the rear 0 the system; and to provide a directive sound receiving system which is completely adjustable 1n direction, both as to azimuth and altitude, and

3 having means whereby it may be directed so that the sound source lies upon the axis of the pickup.

Other objects of my invention will be a parent or will be specifically pointed out in the description forming a part of this'specification, but I do not limit myself to the embodiment of my invention herein described, as various forms may be adopted within the scope of the claims.

Referring to the drawing:

Figure 1 is a side elevation of a directive pick-up embodying my invention.

Figure 2' is a vertical axial sectional view, the plane of section being parallel to the plane of ro ect1on of Figure 1.

radio broadcastmfix and particularly since the advent of ta 'ng moving pictures it has been recogo nized that sounds picked up by microphone and reproduced by a loud speaking mecha- Since t e early days 0 DIRECTIVE SOUND PilCK-UP 1031. Serial no. 550,045.

the process of sound pick-up that every effort is made to avoid them. Moreover, be cause the microphone has a peared to be an er-sensitive to the efi'ect o reverberation, it as been customary to osition the microphone as close as possib e to a speaker or other source of sound so that the directly received energy would so far over-balance the secondary sounds due to reflection from walls, furniture, etc, that the latter could be neglected.

In moving picture practice the micro phone must ordinarily be concealed from the camera, and this has led to the necessity of mounting the microphones on booms which could be swung to follow the actors about the stage while keeping the microphone out of the field-of view of the camera. The effect roduced by this practice has been that to t e ear, the speaker, remains stationary, while to the e e, heappears to move about, and in cases w one this was fatal to the desired illusion, changes in intensity of the sound have been accomplished by means of ,faders, which attenuated the s eech currents from the microphone to the esired level.

Attempts have been made to utilizaparabolic reflectors to concentrate the sound from a particular source u on-the microphone, I

thus partially exclu mg echoes and reverberations, and permitting freer motion of the speakers, but these attempts have only been artially successful owing to the that e frequency responseof such a pickup system has not been uniform, and that it has apparently discriminated greatly against sounds of ower pitch.

- A typical sound reproduction problem arises in the case of speech in a room having normal reverberation, i. e., reverberation su ficiently low so that conversation may be carried on without noticeable strain or dimculty in hearing. If a micro hone is positioned in the place of one of the listeners to such a conversation, the reproduction from the microphone circuit will be so clouded with interference, reverberation, and general noise, as to make the conversation practically unintelligible, or at least extremely diflicult to listen to. Again replacing the microphone by a listener, and this time covring or blocking one of his ears so that he is capable of monaural perception only, the same confusion and interference which were noted in the reproduction from the microphone pick-up obtain.

From this, and numerous other experiments, it becomes evident that the interferent sounds are not added or built up by the microphone, but that they actually are presout to the same extent as picked up by the microphone, and affect the listenin ear mechanically to the same degree. Tie binaural effect of the normal listener, however, enables that listener to disregard sounds roceeding from undesired directions, an to concentrate his attention upon the desired sound. Thus,although the interferent sounds afl'ect the ear of the listener, they do not affeet his perception. The mechanism involved in thus disregarding undesired sounds is obscure, but experiment shows that base differences in the received sound as tween the two ears of the listener play an important part in the operation. It follows that the directive effects relate almost wholly to sounds of sufficiently short wavelengths to produce a marked phase difference as between the two ears, and that low pitched sounds are not directionally discriminated against.

i If a directive pick-up. be used which is equally eflective directionally as regards sounds of both long and short wave-lengths, the-reproduced sounds have an unnatural quality and are apparently lacking in tones t of iower pitch, alt ough the reverberatory effects are very largely excluded.

Considered broadly, my invention comprises a system of sound reflectors or concentrators which is highly directional as re- 33; gards sounds of relatively high pitch or frequency, and decreasingly directional as reards sounds of lower pitch. In its preerred form the directive system comprises aparaboloidalprimary reflector, which redi cts the sound back in the general direction ofits source,toward the focus of the para-- bola. Adacent the focus is mounted a secondary re ector, preferably in the form of a convert paraboloid, whose focus is substan- W tially coincident with the focus of the primary reflector. Sounds received from the primary reflector b the secondary reflector are again redirecte in the direction of their original travel, but with a greatly concen- IN trated wave-front, to the microphone which is mounted centrally of the primary reflector, preferably in an aperture therein which is screened from direct sound by the secondary reflector. Sounds whose wavelengths are shorter than the diameter of the primary reflector are picked up and concentrated thereby as above mentioned. Itis well known, however, that a reflector must be large in comparison to the wave-length it is receiving in order that reflection phenomena may predominate over diffraction phenomena. With increasing wave-lengths the effect described becomes less and less noticeable, while the sounds become more and more capable of bending by diffraction, 80 around the secondary reflector and enterin the microphone without being redirecte For such sounds the primary reflector acts as a baffle, so that the pressure doubling effect which is always present when the wave-lengths shorter than the diameter of the microphone impinge thereon, also becomes effective for sounds of long wavelengths. Not only do long wave-lengths affect the microphone, but reflected sounds, curving by diffraction about the obstruction offered by the deflecting system, also react upon it and, to a certain extent, undergo the pressure doubling effect. The result is that thesounds of high frequency and short wave-lengths which reach the microphone are primary sounds only. As the wavelength increases there is an increasing ratio of secondary sound reaching the microphone. If the primary reflector be made with a diameter of, say, from to 40 inches. and the secondary reflector from one-fourth to onesixth the diameter, the sounds of all frequencies reaching the microphone bear approximately the same primary-secondary ratio as in ordinary binaural perception.

The usefulness of the apparatus described is not limited to'the one case thus discussed in detail. It has been found that the sounds which are ordinarily extremely diflicult to reproduce, such as strikin matches, tea-ring paper, the sound of sur and the many minor sounds which give life or back-ground to almost all activities, are produced at their natural value. The sound pick-up may be mounted at a considerable distance, in many cases 20 to feet, and in some cases as much as a hundred yards, from the sound source, and an entire play or act may be taken without changing the microphone set-up. Actors may approach or retreat from the microphone, and the sound level changes only as it normally should in res onse to their movements. There is thus 0 tained an effective depth in the sound action which adds greatly to the illusion of reality.

The maximum effect of the ick-up system is obtained when the soun source lies directly upon the axes of the parabolic re thus obtained a third dimensional or lator support 20, covered with sound-absorb eral depth, which is capable of being used to great advantage. A periscopic sight, whose line of collimation coincides with the axis of the reflector system is preferably provided. If it be desired to make a single voice dominate a situation, the reflecting system may be pointed by the aid of this sight so that the mouth of the speaker lies exactly upon the axis. In'this case, the voice of that particular speaker will rise somewhat above those even immediatel adja'cent him, while the suppression o the other voices is not sufficiently severe to be objectively noticeable.

A preferred embodiment of this inven" tion is shown in the drawing, and will next be described in detail. A suitable tripod or foot 1 having a vertical column 2 carries a yoke 3 which is journaled within the column for rotation in azimuth. In the design shown, the combined height of'the stand thus formed, extending from the foot of the tripod to the trunnion bearings 5 at the upper ends of the arms of the yoke, is

approximately nine feet, but other models have been built and operated satisfactorily where the over-all height of the apparatus was approximately five feet.

Trunnioned in the bearings 5 is a ring 6, carrying a lever 7 which is joined by a link 8 to a hand lever 10. The hand lever is pivoted in a collar 11 secured to the stem of the yoke 3, so that the yoke may be rotated in the stand by swinging the handle, or the ring tilted vertically by raising or lowering the handle.

Secured to the ring 6 by a series of clips or lacings 12, is a paraboloidal primary reflector indicated by the general reference character 13. The reflector comprises an inner reflecting surface or shell 15, referably formed with a central axial ori ce 16. Covering the outer surface 'of the shell 15 is a sound-absorbing coating 17 of felt or similar material, which in turn is covered with a protecting-covering 18, which may I also be of metal or which may beof fabric if desired.

Mounted within the reflector is a bracket ing padding 21, and arranged adjustably to support a stern or shaft 22 on the axis of the parabola. ,This. shaft carries upon ltSflHIlQI end a flange 23 to which is secured the convexly This secondary reflector may" made of wood, metal, oran suitably rigid material whose surface wil sound reflector.

paraboloidal secondary reflector 25.

act as a satisfactory Covering the back of the orifice 16 is a container 26, within which is mounted a microphone pick-up of conventional ty e, preferably a condenser microphone 27. 'I he sound-absorbent coating 17 is continued as a cover 28 over the container 26, and the container and coating are rotected by a ca 30,

provision bein made or the entrance 0 the microphone ca le 31.

Mounted on an extension 32 of the bracket 21, on the axis of the two paraboloids, is a mirror 33, mounted at 45 with the axis and the objective 35 of a 'periscope or periscopic sight, whose eye-piece 36 is carried by a'tube 37 projecting downward from a bracket 38 mounted ,on the lower rim of the par'aboloid. A second 45 reflector 40 completes the o tical system of the sim le'periscope thus ormed, whose line of col imation therefore lies upon the axis of the reflector system.

In operation, the secondary reflector 25 is usually adjusted so that its focus coincides with that of the primary reflector '15. ,The operator of the pick-up rotates the system in azimuth and tilts it in altitude until the periscope or sight indicates that the sound source lies upon the axis of the "system or closely adjacent thereto.

When the pick-up is positioned at suflicient distance from the sound source so that fronts. of arriving sound waves are substantially plane, the waves of relatively high frequency are redirected toward the focus of the primary reflector. Because of the mathematical properties of the parabola, the distance traveled by each portion the sound wave to the focus would be identical with the distance traveled to a plane the same distance behind the apex of the parabola that the focus is in front of it, and all portions of the wave-front therefore are in phase, or would he were they allowed actually to reach the focus. Before reaching the focus,

however, they are redirected by the second-' form of the reflector assures that all portions of the waves arrive in phase at the microphone diaphragm. Those waves 0 which 'would otherwise have. reached this diahragm without reflection are intercepted by the-secondary reflector.

In the case of wave-lengths which "are long in comparison with the diameter of the reflector, the phenomena of reflection is subordinated to diffraction phenomena. Such waves bend around the secondary reflector ahd act directly upon the microphone. Be-

cause the secondary reflector is preferably made of such diameter that its projection in the plane of the micro hone is a circle of one quarter wave-lengt diameter with respect to the longest wave which it is desiredto receive at full intensit the pressure-doubling eflect becomes eflective and the reflector acts as a baflle, givin the microphone substantially the same. e ciency for such wave-lengths as it has for the shorter ones. It is to be noted, however, that the effect upon the micro hone for these low frequencies is only don were the baflle not present, instead of being approximately 36 to 1 as it is for the high frequencies, the secondary reflector being assumed to be one-sixth of the diameter of the primary reflector.

In the actual response to sound, however, the difference in order of response is not nearly as great as these figures would indicate. Secondary low frequency sounds are received from the walls of the room wherein the device is located, or from other surrounding structures, and are received by diffraction from these sources. Only sounds coming from directly behind the reflector system are completely cut off by a combination of baflle action and the insulation covering the microphone and reflector system from the rear.

The relationships. described, corresponding to maximum sharpness of focus, are strictly accurate only for plane wave-fronts, i. e., for sounds originating far from the pick-up. it several actors,-an orchestra, or other relatively large sound source is to be used, it is desirable that the pick-up field be made divergent, while if a single actor close to the pick-up must dominate the received sounds, a convergent pick-up field is of advantage. This flexibility is ro vided by the adjustability of the secon ary reflector. Gonvergence of focus is secured by moving the latter awa from the apex of the primary reflector, w ile divergence is secured by moving the a ices of the reflectors together. Such ad ustments need be very slight, and are accomplished empirically, the Josition of optimum effect being immediate y perceptible to a monitor. The depth of focus is great, however, when the paraboloids are confocal, and it is mldom that the adjustment need be changed in recording a given situation.

I am aware that paraboloidal reflectors for sound are not, per se, new, having been described in my Patent No. 1,299,616, and by Aitken, Horn, and others, Horn in particular being concerned with the differentiation between primary or direct and secondary or reflected sounds. In so far as I have been able to ascertain, however, the necessity for Secondarysounds of low pitch to impart naturalness is my own discovery, and none Is what it would be 1 of the other inventors of whom I am aware has utilized a secondary reflector to redirect the reflected sounds in their original direc-.

tion, giving the advantages above set forth.

I claim:

1. A directive sound pick-u comprising a microphone, a baflle surroun ing said microphone and having a minimum extension therefrom equal to one-ei hth of the wave length of the lowest pitc ed sound to be received, and means for insulating the back of said micro hone from sound waves.

2. A directive sound pick-up comprising a paraboloidal reflector having a central openmg therein, a microphone arranged within said opening so that the reflector constitutes a baflle therefore, sound absorbent material dis sed to cover the back of said reflector an said microphone to shield said microphone from sound waves proceeding from behind said reflector.

In combination with a microphone and a reflecting system comprising a primary reflector for gathering and focusin the sound waves and a secondary reflector or redirecting said waves upon the microphone and mounted to intercept unreflected waves directed toward said microphone, means for directing the axis of said reflecting system toward a sound source, and sighting means including an optical reflector screened from said microphone by said secondary sound reflector.

4. In combination with a microphone and a directive sound reflective system therefor, a mounting for said reflective system adjustable in both azimuth and altitude, and sighting means for said system havin a line of collimation substantially coinci out with the axis of said reflective system.

5. In combination with a paraboloidal sound reflector and a microphone and a secondary reflector mounted coaxially therewith, a mounting for said paraboloidal reflector adjustable in azimuth and altitude, and sighting means having a line of collimation substantially coincident with the axis of said reflective system.

6. In combination with a paraboloidal sound reflector anda microphone and a secondary reflector mounted coaxially therewith, a mounting for said paraboloidal reflector adjustable in azimuth and altitude,

and sighting means including an optical reflector screened from said microphone by said secondary reflector and havinga line of coflimation substantially coincident with the axis of said reflective system.

In testimony whereof, I have hereunto set my hand. I

FRANK RIEBER.

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