US2640110A

US2640110A - Second order gradient directional microphone

Info

Publication number: US2640110A
Application number: US124461A
Authority: US
Inventors: Harry F Olson; Preston John
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1949-10-29
Filing date: 1949-10-29
Publication date: 1953-05-26
Anticipated expiration: 1970-05-26

Description

May 26, 1953 H. F. OLSON EIAL SECOND ORDER GRADIENT DIRECTIONAL MICROPHONE Filed Oct. 29A, 1949 3nnentor$ ahw Pegsron (Ittomeg Patented May 26, 1953 SECOND ORDER GRADIENT DIRECTIONAL MICROPHONE Harry F. Olson, Princeton, and John Preston,

Metedeconk, N. J assignors to Radio Corporation of America, a corporation of Delaware Application October 29, 1949, Serial No. 124,461

8 Claims.

The present invention relates to sound translating apparatus, and more particularly to a second order, pressure gradient responsive microphone with a high order of directivity over a wide range of useful sound frequencies.

Directional microphones are universally employed to discriminate against undesirable sounds normally picked up by the acoustical system of sound translating apparatus. It has been established that a directional sound collecting system with a directiviy pattern which does not vary materially with the frequency is the most desirable, being particularly useful in recording sound motion pictures, television, orchestras, radio, stage productions and in many sound reenforcing applications.

One of the most important applications is that of sound pickup in television. The action in television covers a long period of time and a relatively large range, with rapid changes in pickup points. In addition, it is becoming increasingly desirable that the microphone be kept out of the picture at all times. In the sound pickup systems in use today, the microphone is usually mounted at the end of a boom which can be moved around by the operator to cover the action and at the same time keep the microphone out of the picture. This system is quite cumbersome, and difficulty is experienced at times in covering the action. If a microphone with a high order of directivity is employed, it is possible to use a number of these microphones arranged in fixed, spaced positions to cover the entire area of action. As the action changes from one area to another, the appropriate microphone may be brought into action, as by fading between microphones through a control device or monitoring console. It will be seen readily that, under this improved sound collecting system, the action can be covered in a more satisfactory manner than by means of a boom, particularly when the action is complex and shifts rapidly from one part of the stage to another. In addition thereto, there is another outstanding advantage for this method of sound pickup, namely, that the sound is picked up from the front of the stage rather than overhead as in the case of boom microphone pickup. The net result is that better illusion is obtained because the sound corresponds to the picture.

It is, of course, well known that directional sound collecting systems may be divided into two classes, namely, wave and gradient types. In the case of gradient systems, which depend upon differences in pressure, two major problems are encountered, namely, (1) similarity of frequency response of the units to obtain proper balance, and (2) adequate sensitivity. We have been successful in overcoming these two problems by our present invention and have developed a second order pressure gradient microphone with a high order of directivity and sensitivity, and which is particularly adapted for use in a sound collecting system such as that outlined above.

A second order pressure gradient responsive microphone with a unidirectional directivity pattern may be obtained from a combination of two first order pressure gradient responsive microphones and a suitable delay network, or from the combination of two unidirectional microphones each consisting of a first order, pressure gradient responsive microphone and a 'zero order, pressure radient responsive microphone.

The theory of operation of both of these systems is explained in Olson Patent No. 2,301,744. Of the two systems, the latter system has been found more suitable, particularly when a broad frequency range is desired.

It is Well known that the upper limit of the useful frequency range of a second order, pressure gradient responsive microphone of the type employing two unidirectional microphones, such, for example, as the microphone shown and described in Olson Patent No. 2,301,638, is determined by the distance between the units. This upper frequency limit is given by c f v 3 1 where fc=upper frequency limit in cycles per second, c=velocity of sound, in centimeters per second,.

and d=distance between the units in centimeters.

The voltage output of a gradient microphone of the type mentioned above, in the low frequency range, that is, in the range for which d is given by d=distance between the units, in centimeters, and

i=wavelength, in centimeters.

If the upper limit is 10,000 cycles per second,

the distance between the units will be relatively small, as given by Equation 1. Under these conditions, Equation 2 shows that the voltage output in the low frequency range will be low. There are two alternatives, namely, (1) to use two systems of the type employing two unidirectional microphones, as mentioned above, one covering the low frequency range, with a relatively large distance between the units, and one covering the high frequency range with a relatively small distance between the units, or (2) to use a pressure gradient system of the type mentioned above for the low frequency range and a wave type system for directivity in the high frequency range. We have found the latter type of microphone to be the preferred type from the standpoint of overall size and weight, and since it requires fewer elements, it is easier and less costly to produce.

The primary object of our present invention, therefore, is to provide a second order pressure gradient responsive microphone with a high order of directivity over a wide range of useful sound frequencies.

Another object of our present invention is to extend the useful range of sound frequencies of a second order, pressure gradient responsive microphone while at the same time preserving a high order of directivity and sensitivity.

It is also an object of our present invention to provide an improved second order pressure gradient responsive microphone which has linear dimensions which are relatively small, and one which is simple in construction, yet highly efficient in use.

In accordance with our present invention, our improved second order, pressure gradient responsive microphone comprises a pair of unidirectional microphone units disposed in tandem along a common axis and with the vibratile elements thereof mounted in predetermined, spaced-apart relation and facing in the same direction so that their directional axes of maximum response are substantially in common. An acoustic resistance in the form of a labyrinth pipe structure is mounted behind each vibratile element with an opening in each pipe behind the vibratile element of a sizeto impart a unidirectional characteristic to each microphone unit. The labyrinth pipe structurealso serves as a framework for supporting the respective magnetic structures with their associated vibratile elements, as well as the output transformer and electrical crossover network, therebetween. The two microphone units are effectively connected so that their signal output voltages are in opposite phase relation and function as a second order, pressure gradient responsive microphone below substantially a predetermined sound frequency. A capacitor is connected in circuit with one of the microphone units for cancelling the signal output voltage thereof above this, predetermined sound frequency so that the microphone assembly functions as a first order pressure gradient microphone. Thus, the microphone assembly operates as a first order, pressure gradient responsive microphone in'the higher range of sound frequencies, and as a second order, pressure gradient responsive microphone in the lower range of sound frequencies.

The novel features characteristic of our invention, both as to organization and method of operation, as well as additional objects and advantages thereof, will be understood better from the following detailed description, when read in connection with the accompanying drawing in I which Figure 1 is a perspective view, as seen from one side, of a second order, pressure gradient responsive microphone in accordance with one form of our present invention, the protective screen enclosure having been removed,

Figure 2 is a perspective view of the microphone shown in Fig. 1, as seen from the front,

Figure 3 is an enlarged side view of the labyrinth connector mounted behind the ribbon element of one of the microphone translating units and employed to couple the vibratory element of this unit to the damped labyrinth pipe,

Figure 4 is an end view of the labyrinth connector shown in Fig. 3,

Figure 5 is a curve showing the directional characteristic of either of the unidirectional microphone units shown in Fig. 1,

Figure 6 is a curve showing the combined directional characteristics of the two unidirectional microphone units shown in Fig. 1, and

Figure '7 is a diagrammatic view showing the microphone output circuit arrangement.

Referring more particularly to the drawing, wherein similar reference characters designate corresponding parts throughout, there is shown a second order, pressure gradient responsive microphone I comprising two unidirectional microphone units 3, 5 of the dynamic type each of which is similar to a unidirectional microphone of the kind shown and described in the above mentioned Olson Patent No. 2,301,638.

Each of the unidirectional microphone units 3, 5 comprises a magnetic structure 1 having an air gap 8 Within which a conductive ribbon or other vibratile element 9 is disposed for vibratory movement in response to sound wave energy impinging on opposite sides thereof. The ribbon element 9 is exposed on one side to incident sound waves, and the other side of the ribbon S is enclosed and terminates in an accoustical resistance in the form of a labyrinth structure ,or closed pipe i I which is filled with suitable damping material. The labyrinth structure II, for convenience of manufacture and assembly, comprises two parts, namely, an elongated, folded pipe 13, and a labyrinth connector 14 which is disposed directly behind and encloses the ribbon 9. An opening [5 is provided in the labyrinth connector 14 directly behind the ribbon 9 to provide acoustic inertance to sound waves approaching the ribbon 9 through the opening, and a screen ll of fine mesh is mounted over the opening to provide an acoustic resistance for the opening I5, in a manner more fully disclosed, for example, in our copending application Serial No. 687,419, filed July 31, 1946. By suitable choice of constants, each microphone unit is constructed to produce a directional response characteristic having substantially the same cardioid pattern, such as that illustrated by the curve 18 in Fig. 5 of the drawing.

The two, folded pipes l'3 are spaced from each other laterally, as clearly shown in Figs. 1 and v2, the microphone units 3 and 5 being mounted therebetween in spaced-apart relation and in proximity to the ends of the two labyrinth structures.

The effective length of each of-theclosed pipes 13, whichare required to maintain proper phase relations in the low frequency range, is about inches. Therefore, in order to provide a compact unit or assembly, the closed pipes l3 are folded in, the manner shown in Figs. 1 and '2.

While the respective folds IQ of the pipes l3 can be disposed in any suitable manner, they should be spaced apart a sufiicient distance to provide a completely open framework to provide accessibility for sound waves to opposite sides of the ribbon elements 9 and to reduce objectionable resonances which may occur between the labyrinth structures. The labyrinth structures or frameworks Ii are appropriately spaced apart so that the magnetic structures 1 of the units 3, as well as the transformers 2|, the condenser 25 and other elements of the electrical crossover network can be mounted or supported therebetween.

The two microphone units 3, 5 should be very smooth and free of irregularities in response, and the sensitivity of the two units must be the same within a fraction of a decibel over the entire operating frequency range in order to function efiiciently. If these conditions are not maintained in the gradient frequency range a high order of cancellation will not be obtained. In order to provide these conditions, special techniques should be followed. For example, the distance between the ribbon elements 9 of the two microphone units 3, 5 should be approximately one-half the wave length of the highest frequency to which that one of the microphone units 3, 5 which is cut off at substantially a suitable mild-frequency, as hereinafter set forth, is responsive and small compared to the wave length at the lowest frequency to which the system is responsive. The outputs of the two microphone units 3, 5 should be connected in series and in phase opposition so that, together, the response corresponds to the pressure gradient of the pressure gradients in the range below said mid-frequency. As shown in the electrical circuit of the microphone in Fig. 7 of the drawing, a capacitor 25, which has a value suflicient to cut off the signal output of one of the microphone units 3, 5 at approximately the upper end of the operating range of the system as a second order pressure gradient responsive microphone (that is, at substantially the aforementioned mid-frequency), is connected in circuit with the output of the microphone unit to be cut off so that the remaining microphone unit is alone responsive to sound waves above substantially that mid-frequency cut-off frequency. Thus, according to one second order, pressure gradient microphone, which was designed and constructed similar to that illustrated in Figs. 1 and 2, the two microphone units 3, 5 were disposed 12 inches apart with the exposed sides of the ribbon elements 9 facing in the same frontal direction. The configuration of the respective parts of the magnetic structures i and the elements of the compound acoustical networks which terminate the ribbons were chosen so that diffraction and the acoustical network cooperate to form a directional system. The upper end of the operating range of this system, as a second order, pressure gradient responsive microphone is approximately 1000 cycles. In the overlap frequency band from 1000 cycles to 2000 cycles, the system operates as a combination gradient and diffraction system, and in the frequency range above 2000 cycles, the one unit, preferably the front unit, operates alone.

From the foregoing description, it will be apparent that we have provided an improved, compact, highly directional second order gradient microphone employing two unidirectional microphone units so related as to provide a directional pattern given by the expression (1+cos 0) cos 0, where 0 is the angle between the direction of the incident sound and the major axis of the microphone, as represented by Figure 6. The pickup distance of the microphone described herein is considerably greater than that of the unidirectional microphone with a cardioid characteristic for the same reproduced reverberation and ambient noise.

While we have described and illustrated but a single modification of our second order pressure gradient responsive microphone, it will, of

course, be obvious to those persons skilled in the art that various changes and modifications are possible within the spirit of our invention. Therefore, we desire that the particular form of our invention described herein shall be considered as illustrative and not as limiting.

What is claimed is:

l. A sound translating device comprising a pair of microphone units each of which comprises (1) a magnetic structure having an air gap with a magnetic field therein, (2) a conductive element mounted in said field for vibration in response to acoustical waves, and (3) means closing one side of said conductor providing acoustic resistance and inertance to sound waves approaching said conductor from said one side, and (4) a pair of labyrinth pipe structures, each of said pipe structures being associated with a separate one of said microphone units, said means for closing one side of each of said conductors constituting coupling means between said conductors and their respectively associated labyrinth pipe structures, said conductors being disposed in spaced apart relation, and means for eifectively connecting the signal output voltages of said microphone units in opposite phase relation, said last mentioned means including means connected in circuit with one of said microphone units for cancelling the signal output voltage thereof above a predetermined frequency.

2. A sound translating device according to claim 1, wherein said conductors are mounted substantially parallel and in tandem along a common axis with the side opposite said one side facing in the same frontal direction.

3. A sound translating device according to claim 2, wherein said microphone units have substantially the same sensitivity, and wherein said conductors are spaced apart a distance of the order of one-half wave length of the highest frequency to which said microphone unit is responsive and small compared to the wave length at the lowest frequency to which said one microphone unit is responsive.

4. A sound translating device according to claim 3, wherein said labyrinth pipe structures comprise separate, folded frameworks, wherein said magnetic structures of each of said units are mounted between said frameworks, and wherein the respective folds of each of said frameworks are spaced apart, thereby to provide an open framework.

5. A sound translating device comprising a pair of relatively elongated, reversely folded labyrinth structures, said labyrinth structures being spaced from each other laterally, and a pair of microphone units mounted between said labyrinth structures in spaced relation to each other, each of said microphone units including means for providing said units with a unidirectional response characteristic and comprising a magnetic structure having an air gap with amagnetic field therein, .a conductive element mounted in said field responsive to sound waves impinging thereon, and coupling means mounted behind each of said conductive elements for closing one side thereof, said coupling means being connected respectively to separate ones of said labyrinth structures.

6. A sound translating device comprising a pair of relatively elongated, reversely folded labyrinth structures, said labyrinth structures being spaced from each other laterally, and a pair of microphone units mounted between said labyrinth structures in proximity to the ends thereof and in spaced relation to each other, each of said microphone units including means for providing said units with a unidirectional response characteristic and comprising a magnetic structure having an air gap with a magnetic field therein, a conductive element mounted in said field responsive to sound waves impinging thereon, and coupling means mounted behind each of said conductive elements for closing one side thereof, said couplingmeans being connected respectively to separate ones of said labyrinth structures.

7. A sound translating device comprisinga pair of relatively elongated, reversely folded labyrinth structures, said labyrinth structuresbeing spaced from each other laterally,and'a pair of microphone units mounted between said labyrinth structures in spaced relation to each other, each of said microphone units including means for providing said units with a unidirectional response characteristic and comprising a magnetic structure having an ,airgap witha magnetic 'iield therein, a conductive element mounted in said field responsive to sound wavesimpinging thereon, and coupling means mounted behind each of said conductive elements for closing one side thereof, said coupling means being connected respectively to separate ones of said labyrinth structures, each of said coupling means having an opening therein disposed directly behind the conductive element associated therewith.

8. A sound translating device comprising a pair of relatively elongated, reversely folded labyrinth structures, said labyrinth structures being spaced from each other transversely, and a pair of microphone units mounted between said labyrinth structures in proximity to the ends thereof and in spaced relation to each other, each of said microphone units including means for providing said units with a unidirectional response characteristic and comprising a magnetic structure having an air gap with a magnetic field therein, a conductive element mounted in said field responsive to sound waves impinging thereon, coupling means mounted behind each of said conductive elements for closing one side thereof, said coupling means being connected respectively to separate ones of said labyrinth structures, each of said coupling means having an opening therein disposed directly behind the conductive element associated therewith, and acoustic resistance means disposed over said openings.

HARRY F. OLSON. JOHN PRESTON.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,237,298 Baumzweizer Apr. 8, 1941 2,301,638 Olson Nov. 10, 1942 2,301,744 Olson Nov. 10, 1942 2,305,599 Bauer Dec. 22, 1942