Ultradirectional Microphone

Abstract

A directional microphone includes a microphone unit responsive to waves in the audio frequency range and means for causing the microphone unit to operate as a second order microphone in a lower portion of that frequency range. Further means for imparting highly directional characteristics to waves in the remaining upper portion of the audio frequency range are coupled to the microphone unit for transmitting these waves thereto, the microphone unit responding as a highly directional unit to waves in that upper portion.

Parent Case Text

This is a continuation, of Application Ser. No. 834,347, filed June 18, 1969 and now abandoned.

Description

This invention relates to microphones, and more particularly to a highly directional microphone responsive to a wide range of waves in the audio frequency range.

Directional microphones are generally used to discriminate against sound waves originating from undesirable sources. These microphones have found wide application in the entertainment industry, and especially in television and motion picture sound collecting systems. In these systems, there is a continuing need for improved directional microphones. This need is especially acute in the areas of mobility and directivity.

In the past, a variety of microphones have been developed which emphasize one or more of these areas. It has been found, however, that emphasis in one area, resulted in a compromise in another. Presently, no one microphone achieves the maximum desired efficiency in all areas. For example, in Olson U.S. Pat. No. 2,228,886, there is disclosed a highly directional microphone, which is of the wave or line type. Basically, this type of microphone is characterized by an elongated tubular structure having longitutionally spaced holes at progressively varying distances from an electro-acoustic transducer. The tubular structure, which may take a variety of forms, the most common being a plurality of open ended pipes, receives and transmits sound waves to the transducer, imparting ultradirectional characteristics to the waves. This structure has the particular disadvantage of being relatively long and cumbersome, imposing a limitation on its mobility; lengths of seven to 20 feet not being uncommon.

A second type of microphone is the pressure or gradient type, which has first order or cardioid directivity, which is not as directional as the line microphone. An example of a pressure microphone is shown in Olson U.S. Pat. Nos. 2,301,638 and 2,271,988. Olson U.S. Pat. No. 2,301,744 further revealed that these gradient microphones could be combined to yield second order or more highly directional characteristics than first order. Such a microphone is shown in Olson U.S. Pat. No. 2,640,110. It is explained in this latter patent that the upper frequency limit of the second order characteristics of the combined unit is given by

f.sub.c = c/d

Where

f.sub.c = frequency of the combined microphones

c = velocity of sound in air

d = distance between microphones

Above this upper limit, the combined units no longer cooperate as a pair, but rather, the characteristics are that of only one unit, the output of the ohter unit being blocked. Maximum effectiveness is achieved when each of the two microphones have similar frequency response characteristics. As a result, past efforts were directed toward making each unit in the combination as directional as possible, but, at best, could only be first order. Present second order microphones of this type utilize uniaxial microphones, such as Olson U.S. Pat. No. 2,680,787, which, when combined, result in a relatively small unit capable of mobile boom applications.

The present invention is directed toward improving the first order directional characteristics in the upper portion of the audio frequency range of the combined gradient type of microphone while retaining at least the second order characteristics in the lower portion, and yet, provide a microphone capable of mobile use.

Therefore, it is an object of the present invention to provide an improved microphone that is highly directional in the useful audio frequency range and is adaptable for mobile applications.

A microphone unit is provided which is responsive to waves in the audio frequency range. Means are provided to cause the microphone unit to be highly directionally responsive to waves in a predetermined lower portion of the audio frequency range. Additional means, capable of imparting ultradirectional characteristics to waves in a predetermined upper portion of the audio frequency range, are adapted to transmit these ultradirectional waves to the microphone unit, which unit cooperatively responds with both means mentioned in accordance with the frequency of the waves impinging thereupon.

A more detailed description will be provided in connection with the accompanying drawing in which;

FIG. 1a is a diagrammatical presentation of one embodiment of a microphone according to the invention.

FIG. 1b is a sketch of a further embodiment of a section of the microphone of FIG. 1a.

FIG. 1c is a sketch of another form which the section of FIG. 1b may take.

FIG. 2 is a schematic showing the circuit of the microphone unit in FIG. 1.

FIGS. 3a-3d are directional patterns at various frequencies.

Referring to FIG. 1, microphone 10, illustrating one embodiment of the present invention, is shown having a forward section 8 and a rearward section 6. Section 6, the microphone unit, includes two microphones 40 and 41. The microphones 40, 41 are coupled by leads represented by connection 46 and circuit 42 in a manner to be described. Power supply 44 powers the microphone section 6. Casing 22 secures microphones 40, 41 and circuit 42 to form the microphone section 6, suitable means, not shown, serving to join the section 8 to the casing 22 to form microphone 10.

In discussing the present invention, a brief discussion is first given as to the state of the art with which this invention pertains. Microphones 40 and 41 have substantially the same frequency response and sensitivity. Each of these units are first order directional microphones. An example of this type of microphone is described in Olson U.S. Pat. No. 3,007,012, which has a circuit described in Morgan U.S. Pat. No. 2,920,140. This is an electrostatic unit having high directional efficiency. The principles of a first order or cardioid microphone are exaplained in Olson U.S. Pat. No. 2,301,638 and in "Acoustical Engineering" by H. F. Olson Van Nostrand, Third Edition, 1957 pp. 275-315. A further refinement is a unidirectional microphone which has cardioid directivity, but is responsive to waves emanating from only one direction, which is also shown in U.S. Pat. No. 2,301,638. A directional microphone may also be uniaxial wherein the axis of maximum directivity coincides with the longitudinal axis of the microphone, an example of which is shown in Olson U.S. Pat. No. 2,680,787.

By combining two first order directional microphones, higher order of directivity may be obtained. The theory of this operation is explained in Olson U.S. Pat. No. 2,301,744, wherein two units, having first order characteristics when combined according to the patent, produce a second order microphone. That is, the pattern of directivity is better than cardioid. See FIG. 25 of the Olson patent. Referring to FIG. 3d of the present specification, an example of a first order cardioid pattern is shown, while FIG. 3a shows a second order pattern. The patterns in FIGS. 3a-3d are a plot of microphone output voltage versus the direction of the source of sound. This voltage is given by:

(1) e = e.sub.o (1 + Cos .theta.) Cos .theta.

where

e = output volts at an angle .theta.

e.sub.o = output volts at .theta. = 0.degree.

.theta. = direction of incident sound

(See FIG. 1, where .theta. is shown with respect to the direction of the axis 39 and to the direction of incident sound 49).

This is a low frequency pattern and is independent of frequency up to an upper frequency limit defined by

(2) f = c/d

In Olson's "Acoustical Engineering" pp. 315-318, there is revealed a second order unidirectional gradient microphone which includes two microphones, each being similar to that shown in Olson U.S. Pat. No. 2,680,787. Another example of this type is Olson et al, patent 2,640,110 in which the microphones have a common axis and are axially aligned in tandem. When two microphones are combined in accordance with U.S. Pat. No. 2,640,110, they are spaced from each other at a distance which controls the maximum frequency, which distance is determined in accordance with equation (2). This distance defines the amount of separation between the vibratory elements of the transducers in each respective microphone unit. With respect to higher frequencies, circuitry is introduced which cuts off the most rearward of the two microphones, such that the response of the pair is actually that of only the most forward of the two units. An example of this circuitry is shown in Olson U.S. Pat. No. 2,640,110. These microphones vary in size according to the application. In one case, such as Olson U.S. Pat. No. 2,640,110, the microphone is large and intended for stationary use. In another case, such as shown in RCA Review Dec. 1956 pp. 522-533 by Olson et al., the microphone is small and readily adaptable for mobile applications. But, in any case, the above noted limitation as to directivity in the upper audio frequency range is present.

In the invention disclosed herein, microphones 40 and 41 of FIG. 1 are combined in a like manner for illustration, as a uniaxial first order microphone having an upper frequency limit in accordance with the relationship f = c/d. Just below this limit means are provided which cancel the most rearward unit 41 while the most forward unit 40 is kept active, responding to waves in the upper portion of the useful audio frequency range, which in this case is from 1,500 hertz to approximately 10,000 hertz. The circuitry 42, which combines the outputs of units 40 and 41 in phase opposition to cause the units to have second order characteristics as well as process the output of unit 40 in the upper frequencies, is shown by FIG. 2 and will be described below. It is sufficient to note here, that the pressure responsive elements 37 located at end 38 of units 40 and 41, face the same direction and are axially aligned along axis 39 facing sound waves traveling primarily in direction 48. The two units together produce with circuit 42 a second order microphone in a lower portion of the useful frequency range which in this case is from 50 hertz to approximately 2,000 hertz. Casing 22 may be any conventional design which will secure units 40 and 41 and circuit 42 together in accordance with the required alignment. Casing 22 has ports 47 in peripheral spaced locations in approximate radial alignment with pressure responsive elements 37 at ends 38 of the two units 40 and 41. These ports 47 permit sound waves to pass through case 22 in the lateral direction and be received by units 40 and 41.

Section 8 of microphone 10 forms, in conjunction with microphone 40, a highly directional microphone similar to that known as a line or wave type. This type, in the most common configuration, includes an electro-acoustical transducer and a plurality of open ended pipes coupled thereto. An example of this type of microphone is shown in Olson U.S. Pat. No. 2,228,886. These pipes, or tubular elements, as explained therein, are of uniformly progressively varying lengths such that the free, or pickup ends, lie along a straight line. They are preferably parallel and arranged to direct acoustical waves onto the vibratile member of the transducer. The diameters of each of the pipes are preferably the same and small compared to the size of the vibratile member. the cross section area of the tubular structure, which is herein defined to include all of the pipes, is substantially equal to the lateral cross section area of the transducer. The theory of operation of this type of microphone is further explained in U.S. Pat. No. 2,228,886. The frequency pattern of the line microphone is governed by the relationship: ##SPC1##

where:

e = output voltage at angle .theta.

e.sub.o = output voltage at .theta. = 0.degree.

.theta. = direction of incident sound (See FIG. 1)

L = Length of the tubular element

.lambda. = wavelength

The progressively varying lengths of the openings with respect to the transducer result in acoustical cancellation of incident sound waves received in the lateral direction in accordance with known principles. The directional characteristics improve as the length of the line increases with respect to the wavelength. Where a line microphone has a length of 4.lambda., it is at least as directional as the second order microphone. These characteristics improve with an increase in frequency. However, since it is length which improves this type of microphone, it is this same length which limits its utility. For example, at a frequency of 100 hertz, the wavelength is approximately 331 cm. To achieve second order characteristics, the line microphone length would have to be approximately 1324 cm or 43 feet.

Referring now to FIG. 1, section 8, in one form includes a plurality of tubular elements 25-34 having progressively varying lengths. These elements may be a bundle of small diameter hollow pipes which terminate at end 49 secured in fixed relationship to microphone unit 40 by casing 22. Facing end 49 is the vibratile element 37 of unit 40 located at end 38 thereof. The other end 24 of section 8 terminates at various uniform distances from end 49, each tube 25-34 of section 8 being open to the ambient such as at end 35 of element 25. The elements 25-34 impart highly directional characteristics to waves of incident sound 49 received at the open ends 24, transmitting the waves to microphone unit 40. Section 8 imparts ultradirectional characteristics to waves in a predetermined upper portion of the audio frequency range, passing nondirectional waves in the lower portion to microphone unit section 6. In particular, section 8 imparts directivity to waves in that portion of the frequency range at which section 6 no longer responds as a second order microphone.

To provide continuity of directivity, section 6 and section 8 may be designed such that their effective frequency ranges overlap. Unit 40, being responsive to waves throughout the useful audio frequency range (e.g.: 100 to 10,000 hertz), responds to all waves which are transmitted thereto by section 8. The portion of those waves which are ultradirectional in the upper portion of the frequency range result in unit 40 responding as an ultradirectional microphone in this predetermined upper frequency range (e.g.: 1,500 hertz to 10,000 hertz), which heretofore was limited to first order characteristics. Section 8 can have a length which is substantially reduced (e.g.: from 7-20 feet to approximately 2.25 feet), permitting mobile applications. Thus, the limitation due to the great lengths required for the lower frequencies in the prior art is no longer a consideration. The single microphone 40 cooperates with section 8 as a line microphone, but also cooperates with microphone 41 and circuit 42 as a bigradient second order directional microphone.

Variations of the structure 8 in FIG. 1a are tubular structures 9 and 5 of FIGS. 1b and 1c, respectively. Structure 9 is an elongated tapered tubular element 3 having uniformly spaces ports 51-61 along its longitudinal axis. The ports serve the same function as the varying length tubular elements 25-34 in FIG. 1a. Ends 50 and 62 terminate similarly as ends 49 and 35 respectively. Another variation, is structure 5 of FIG. 1c which is shown having an elongated tapered tubular element 2 which includes a longitudinal slot 71 for coupling the inner tubular cavity to the ambient along the length of the structure. Ends 70 and 72 terminate similarly as ends 49 and 35 of the structure in FIG. 1a.

In the embodiment illustrated in FIG. 1, microphones 40 and 41 are aligned on a common axis 39 parallel to the axis 48 in which maximum directivity is desired. Axis 39 and 48 may be aligned by any means, mobile or fixed (not shown), as required. The vibratory element 37 in each unit is located approximately at end 38, and is the diaphram shown in Olson U.S. Pat. No. 3,007,012, or a similar electroacoustic element. The spacing between the two microphones 40, 41 is established in accordance with the maximum frequency of the desired effective second order directional characteristics in the lower frequency portion. In the example shown, this distance is 15 cm., which establishes a maximum effective second order frequency response of approximately 2,000 hertz. The wavelength at this frequency is approximately 15 cm. By making the structure 8 a minimum length of approximately four times .lambda., or, in this case about 70 cm., ultradirectivity characteristics are insured. Ultradirectional characteristics in a line microphone vary with frequency, but even at 1,500 hertz, the directional characteristics of the line segment of the microphone according to the invention are at least as directional as that of a second order microphone (see FIG. 3b). Thus, by having unit 40 responsive with unit 41 as a bigradient microphone (section 6) and responsive to the waves transmitted thereto by the tubular structure of section 8 at an appropriate crossover point, a continuous highly directional frequency response from 50 to 10,000 hertz is provided.

Turning now to FIG. 2, microphones 40 and 41 are each respectively connected to terminals 80 and 81 at which is applied power from a power supply (not shown) for operating the two microphones and circuit 42. Each of the microphones 41 and 40 include circuits similar to that shown in Morgan U.S. Pat. No. 2,920,140, for example. Conductors 82 and 83, and conductors 84 and 85 are connected to the respective outputs of microphones 40 and 41, respectively. Conductors 82 and 84 are respectively connected to output terminals 91 and 92 of FIG. 2. Conductors 83 and 85 are connected together via a path which includes a serially connected inductor 86. A capacitor 87 and resistor 88 are connected in parallel, each having one end connected to conductor 83 and the other end to conductor 84. Between conductors 82 and 84 are serially coupled resistor 99, inductor 90, and capacitor 89 in that order, respectively. Also serially coupled between conductors 82 and 84 are a switch 93, resistor 97, and inductor 98, in that order, respectively. Switch 93 has three switchable positions 94, 95, or 96. Terminal 94 is an open circuit connection. Terminal 95 is connected to one end of resistor 97 the other end of which is connected to inductor 98 and to terminal 96. Circuit 42 functions as a combining and equalizing circuit. Inductor 96, capacitor 87, and resistor 88 serve as a low pass filter, cutting off the rear microphone 41 above approximately 1,500 hertz. This prevents a phase cancellation hole in the frequency response at the cut off frequency at 2,000 hertz where the spacing between units is one wavelength. Resistor 99, inductor 90 and capacitor 89 are resonant at approximately 1,000 hertz where the spacing is one half wavelength to prevent a peak in the response. Resistor 97, inductor 98, and switch 93 form a variable cut off low frequency equalizer to attenuate undesired acoustic rumble. The resonant circuit including elements 89, 90 and 99 serves to reduce the response between 500 and 2,000 hertz resulting in a low frequency response that is flatter than 6db per octave characteristic which would otherwise be present. The high frequency response of the microphone according to the embodiment of the invention shown is uniform and equal to that of microphone unit 40 without the tubular structure. The high frequency cut off is determined by the increasing impedance of the pipes which are in series with microphone unit 40. This cut off point is given by the relationship

(3) f.sub.2 = 1/2 .pi.R.sub.T C.sub.e

where:

f.sub.2 = cut off frequency in hertz

R.sub.T = Acoustical resistance of pipes CGS acoustical ohms.

C.sub.e = Acoustical capacitance of an electrostatic microphone unit cm.sup.3 /sec.

FIG. 3a is the pattern of the bigradient microphone portion (units 40 and 41) at a frequency range of 50 to 1,000 hertz. FIG. 3b is the pattern at 2,000 hertz, and FIG. 3c is the pattern at 4,000 hertz. At the higher frequencies, the pattern is at least as directional as that in FIG. 3c or better according to the particular implementation of the tubular structure. FIG. 3d is a cardioid response of a single directional microphone. At 2,000 hertz, L/.lambda. is 4 and at 4,000 hertz L/.lambda. is 8 giving a 2 to 1 variation in L/.lambda.. Whereas, in a single line microphone responsive throughout the audio frequency range, L/.lambda. may vary 100 to 1 or more. Thus a considerable improvement over the bigradient directional microphone and the line type microphone is achieved. Distances, lengths, and proportional sizes given herein are illustrative only. The combined length of the pipe structure 8 and the microphone 6 shown herein may have an overall length of less than 3 feet, the tubular structure being approximately 2.25 feet long. This is more than 50 percent shorter than known line microphones, and yet, has an improved directivity pattern over a greater frequency range than that overwise possible for the size.

Claims

What is claimed is:

1. A unidirectional microphone comprising:

a second order bigradient uniaxial microphone including two first order uniaxial microphone units coupled in series opposition, said microphone units having substantially the same directivity and sensitivity along a first axis and being spaced apart facing substantially the same direction along said first axis, said second order microphone having a response symmetrical about said first axis, said second order microphone including means coupled to said units for causing said second order microphone to have second order directional characteristics in a lower portion of the audio frequency range which overlaps an upper portion of the audio frequency range,

an elongated acoustic pipe structure for imparting highly directional characteristics to waves in said upper portion of the audio frequency range along a second axis which substantially coincides with the longitudinal axis of said acoustic pipe structure and having a response symmetrical about said second axis, and

means for coaxially coupling said acoustic pipe structure to said bigradient microphone such that the directivity of said pipe structure faces substantially the same direction as the directivity of said bigradient microphone and said first and second axes coincide such that the responses of said acoustic pipe structure and said bigradient microphone are symmetrical about a single axis whereby said second order microphone and said acoustic pipe have substantially the same acoustic axis, said microphone units each including a microphone element which is spaced from the other element a distance that is less than the acoustic length of said pipe structure,

said acoustic pipe being coupled to the microphone unit which is forward the other unit in a direction towards said same direction and said second order means includes means for acoustically cancelling the rear microphone unit in said upper portion of said frequency range.