Microphone acoustical polar pattern converter

A microphone acoustical polar pattern converter conveniently converts the acoustical polar pattern of a microphone from cardioid to supercardioid and back at will. The converter includes primarily a body of a tubular body portion fitted forward of the microphone head. Most preferably, the body is formed of open pore plastic, with a pore size of 70 microns.

BACKGROUND OF INVENTION 
This invention relates to directional acoustic microphones. 
As is well-known in electroacoustics, directional properties are usually 
imparted to microphones by subtracting the sound pressure at one point 
from the sound pressure at an adjacent point, ordinarily from 1/8" to 1" 
away. Consider a tube, 1" long, with a vibratile diaphragm in the middle. 
The motion of the 
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diaphragm will be proportional to the difference in sound pressure between 
entries A and B. There will be a time difference of 0.077 milliseconds 
from entries A and B for sounds originating at the left or right due to 
the velocity of sound. This is the maximum time difference for this 
construction and, hence, maximum output. Sounds originating in the plane 
of the diaphragm will arrive at exactly the same time and will produce no 
output. There is, of course, an additional delay of 0.0385 milliseconds 
inside the microphone (from entry A or B to the diaphragm), but this delay 
is added to both sounds and disappears in the subtraction. 
If we establish an axis through the center, call the right entry 
180.degree. and the left 0.degree., then the polar pattern from this 
microphone will have a zero at 90.degree. and 270.degree. and will have 
equal and maximum sensitivity at 0.degree. and 180.degree.. This pattern 
is termed bidirectional, and occurs whenever the internal time delay is 
equal on both sides of the diaphragm. It is used when sounds from the side 
are to be rejected. 
If a cloth screen is placed over entry B, the right half of the tube is 
converted to a low-pass acoustic filter. Below the cutoff frequency, this 
filter introduces an additional time delay. If the filter introduces a 
delay of 0.077 milliseconds, then sounds approaching from the right 
(180.degree. ) will experience the following delays in milliseconds as it 
proceeds to the diaphragm by two paths: 
______________________________________ 
Entry A Entry B 
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.077 (tube length) 
.077 (filter) 
.0385 (1/2 tube) 
.0385 (1/2 tube) 
Total .1155 .1155 
A-B (difference) = 0, and hence sounds from the 
right will produce no output. 
For sounds approaching from the left (.degree.): 
.0385 (1/2 tube) 
.077 (tube length) 
.077 (filter) 
.0385 (1/2 tube) 
Total .0385 .1925 
A -B (difference) = .154 msec. 
For sounds approaching from 90.degree. or 270.degree.: 
.0385 (1/2 tube) 
.077 (filter) 
.0385 (1/2 tube) 
Total .0385 .1155 
A-B difference = .077 msec. 
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This is 1/2 of the delay at 0.degree., and hence 1/2 the output. 
This microphone will have a maximum output at 0.degree., 1/2 maximum at 
90.degree. or 270.degree., and zero at 180.degree.. This polar pattern is 
called a cardioid and is produced when the external delay is equal to the 
difference in internal delay from each of the two entries to the 
diaphragm. This pattern is used when rejection of sounds from the back of 
the microphone is desired. 
When an interfering sound is present at an angle between 90.degree. and 
180.degree. (say 125.degree. ), a pattern called the supercardioid may be 
used, which has a null response at that angle. This pattern is produced 
when the internal difference of delay is equal to 0.577 times the external 
delay. 
The foregoing examples show how the polar pattern may be changed by 
changing the delay of the filter in the rear (B) entry. This is the 
variable normally used to design a microphone polar pattern for a specific 
application and is an element of construction; i.e., not adjustable from 
the outside. Also, this explanation has not dealt with features which make 
the response to various frequencies of sound have uniformity. 
SUMMMARY OF THE INVENTION 
In the foregoing context, the object of the inventor was to increase the 
usefulness of a cardioid polar pattern microphone by providing facile 
modification of the cardioid polar pattern of the microphone to a 
supercardioid polar pattern. 
Another object of the inventor was to accomplish the foregoing while 
maintaining the frequency response characteristic of the microphone along 
the principal axis. 
Another object of the inventor was to accomplish the foregoing in a 
non-electronic apparatus which is relatively inexpensive, reliable, long 
lasting and easily used without major, intricate, or internal modification 
of the microphone. 
Consistent with fulfilling these objects, the present invention includes, 
in a principal aspect, a microphone acoustical polar pattern converter. 
The converter functions by adding an acoustic filter and time delay to the 
front entry of the microphone, which decreases the overall internal time 
delay and generates the supercardioid pattern. Physically, this comprises 
an annular converter body alignable with the microphone principal axis in 
forward proximity of the front entry of the microphone element. 
Preferably, the body defines an empty central cavity forward of the 
microphone element, and has its annular wall formed of open-celled 
material such as porous plastic. The pores of the material function as 
multiple ports along the microphone axis, which transmit sound from 
exterior to the body through the body of the cavity, and then to the front 
entry of the microphone element. The converter, as described, 
preconditions sound entering the microphone, such that the acoustic polar 
pattern of the microphone and converter combination is supercardioid. 
This and other aspects of the invention are more fully explained in a 
detailed description of a preferred embodiment, which follows.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The following terms are used in describing the preferred embodiment of the 
invention: 
(1) Microphone Element--An acoustic transducer that converts acoustical 
energy to electrical energy. Unless stated, the polar pattern of this 
element is unrestricted. 
(2) Cardioid Polar Pattern--A polar pattern in which responsiveness drops 
approximately 6 dB (decibels) at 90.degree. from the principal axis and 
drops to null at 180.degree. from the principal axis. 
(3) Supercardioid Polar Pattern--A polar pattern in which responsiveness 
drops approximately 8.7 dB at 90.degree. from the principal axis and in 
which two nulls appear spaced from 180.degree. from the principal axis. 
(4) Nulls--Locations of substantially decreased responsiveness such that in 
comparison to principal axis responsiveness, the responsiveness is 
effectively null or zero. 
(5) Open Pore Material--Material formed as by sintering to have surface 
pores and internal pores which are open to each other to form labyrinth 
paths through the material. 
(6) Pore Size--Nominal diameter of pores which may vary in diameter above 
and below nominal. 
Referring to FIG. 1, the preferred embodiment of the invention is an 
acoustical polar pattern converter (hereafter converter) 10 within a foam 
cover 12. Referring to FIG. 2, the converter 10 and cover 12 are fitted to 
a microphone, with the cover over the microphone head 14. 
The microphone head has a principal axis 15, along which the head 14 points 
(to the right in FIG. 2). The head 14 includes internally an axially 
directed microphone element. 
The front entry of the microphone is shown by 49 of FIG. 3, and the rear 
entry is shown as circumferential openings 50. 
In FIG. 4, the converter 10 includes a body 16, a front support 17 and a 
rear support 18. The front and rear supports 17, 18 hold the body 16 to 
the microphone head 14. 
The body 16 comprises three components: an annular or tubular body portion 
20, an end cap 22, and a fitting portion 24. All three components are 
integrally formed of an open pore material, and preferably porous plastic. 
Most preferably, the body 16 is formed of high density polyethylene with a 
pore size of 70 microns. This most preferred material is available from 
Porex Technologies Corporation of Fairburn, Ga. 
The annular body portion 20 has an annular wall with a uniform, radially 
measured, annular wall thickness. The annular wall encircles a central 
cavity 25 defined in the body portion 20 and closed remote from the 
microphone by the end cap 22. 
The central cavity 25 is concentric with the principal axis of the 
microphone, as is the body portion 20, circular end cap 22, and fitting 
portion 24. The fitting portion 24 encircles and defines a fitting recess 
26 which is open to the cavity 25. The fitting recess 26 has a diameter 
greater than cavity 25. The fitting portion 24 thus has an internal 
diameter such as to be press fittable on the tip of the microphone head 
14. 
FIG. 7 reveals the supercardioid pattern of the microphone with converter. 
Frequencies of 4,000 Hz and 8,000 Hz chart the most predominant 
supercardioid patterns. As shown in FIGS. 7 and 8, the microphone with 
converter experiences two nulls at approximately 125.degree. from the 
principal axis. The nulls are most noticeable at higher frequencies. 
Referring again to FIGS. 1 through 6, the body 16 of the converter 10 is a 
"gist" of the converter 10. The body 16 is the element of the converter 10 
which provides the acoustic polar pattern conversion just described. 
The body 16 is, in part, press fittable on the microphone head 14 as 
stated. The body 16 is also held to the microphone head 14 by the supports 
17, and 18. The body 16 slides within a cage of the front support 17. 
Referring to FIGS. 5 and 6, the cage is formed of a plurality of axially 
elongated, cross-sectionally square, circumferentially spaced ribs 30, 32, 
34, 36, 38, and 40. The ribs are joined to a forward ring 42 and a 
rearward tube 44 in a single structure preferably of Lexan (TM). 
The tube 44 of the front support 17 includes a lip 46, shown in FIG. 4. The 
lip 36 enters a groove 48 on the rear support 18, to attach to the rear 
support 18. The rear support 18 slips and clips over the microphone head 
14. The foam cover 12 slips over the supports 17 and 18 for aesthetics, 
and to cushion inadvertent impacts against the converter 10. 
The preferred embodiment and the invention are now described in such full, 
clear, concise, and exact terms as to enable a person of skill in the art 
to make and use the same. To particularly point out and distinctly claim 
the subject matter regarded as invention, the following claims conclude 
this specifications.