U.S. patent number 3,793,489 [Application Number 05/255,206] was granted by the patent office on 1974-02-19 for ultradirectional microphone.
This patent grant is currently assigned to RCA Corporation. Invention is credited to Jon Rinaldo Sank.
United States Patent |
3,793,489 |
Sank |
February 19, 1974 |
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.
Inventors: |
Sank; Jon Rinaldo (Haddonfield,
NJ) |
Assignee: |
RCA Corporation (New York,
NY)
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Family
ID: |
22967306 |
Appl.
No.: |
05/255,206 |
Filed: |
May 22, 1972 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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834347 |
Jun 18, 1969 |
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Current U.S.
Class: |
381/92; 381/357;
381/86 |
Current CPC
Class: |
H04R
1/342 (20130101) |
Current International
Class: |
H04R
1/34 (20060101); H04R 1/32 (20060101); H04r
001/40 () |
Field of
Search: |
;179/121D,1DM,138VL,138R |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Olson et al., Bigradient Uniaxial Microphone, RCA Review, Dec.,
1956, pp. 522-533..
|
Primary Examiner: Claffy; Kathleen H.
Assistant Examiner: Kundert; Thomas L.
Attorney, Agent or Firm: Norton; Edward J. Squire;
William
Parent Case Text
This is a continuation, of Application Ser. No. 834,347, filed June
18, 1969 and now abandoned.
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.
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.
* * * * *