Note: Descriptions are shown in the official language in which they were submitted.
21 74798
DIRECTIONAL MICROPHONE ASSl~MBLY
Te~hnicalField
This invention relates to directional microphones and, more particularly, to a
structure for holding one or more microphone elements.
5 B~ kground of the Invention
Microphones having a directional char~cterictic are useful in many
applications. One known technique for achieving directionality is through the use
of a first-order-gradient (FOG) microphone element which comprices a movable
diaphragm with front and back sl~rf~es enclosed within a capsule. The capsule
lo includes openings on each side thereof for admitting sound p.~s~u~e to interact with
the front and back surfaces of the diaphragm. In r~ln~nse to this interaction, an
electrical signal is generated that is pio~ollional to the differential sound pres~ on
the opposite ~u~Ç~ces of the diaphragm. Sounds are ignored that come from
directions in which the wave front arrives at the front and back surfaces of the15 diaphragm at the same time. In this situabon the in~ rleous sound ~les~ule oneach surface of the diaphragm is identical, so the diLr~.e.llial sound pressure is zero.
Sounds em~n~ting from other directions reach one surface of the diaphragrn before
the other according to the delay encountered in traveling an effective path length "d"
between the opposite surfaces. This delay creates directionalit,v, but it also affects
20 the frequency response chata.;le.istic because path length "d" corresponds to a
different fraction of a wavelength at each dirr~ l frequency.
One known microphone device, ~e~iFn~tec~ WM 46AAD201, is available
from National/Panasonic and provides a cardioid polar response characteristic. AFOG microphone element is enclosed within a rigid, 2-piece plastic housing which25 includes openings that permit sound waves to enter the housing on each side of the
FOG microphone. The housing and the FOG microphone are mutually held
together by glue or other bonding material so that each side of the FOG microphone
is exclusively influenced by sound waves entering the app~o~liate opening in thehousing. Unfortunately, the construction of such a device is labor intensive due to
30 the use of bonding materials - an extra step that requires curing time. Moreover, if
applied hllplop~lly, leaks may result, thus ch~ngin~ the acoustic directional
characteristic .
2 21 7479~3
One directional microphone assembly of merit which uses tubes to couple a
microphone element to the desired sound pickup points is shown at FIG. 2 of the
Knowles Electronics, Inc. Technical Bulletin TB-2 1, "EB Directional Hearing AidMicrophone Application Notes." Ul~ol lwl~L~ly, no structural means is provided for
~pollillg such an assembly within sound-input equipment, and the tubes do not
appear easily sealed against the e~luipmel" surface.
More recelllly, a directional microphone assembly has been proposed that
employs a two (2) housing arrangement, which does not require the use of a
bonding material to hold the structure together (see U.S. patent 5,226,076 issued
0 July 6, 1993). Although this prior microphone assembly ope.dtes satisfactorily in
certain applications it does have some limitations. For example, as the distance "d"
becomes larger than the width of the microphone clelllent both the dil~ ~,livilypattern and frequency response of this prior assembly changes, which is undesirable
in some applications.
I s Therefore, it is desirable to provide a housing for a microphone element
which is of relatively simple construction so that m~nllf~-~tnre and installation are
facilitated.
Summary of the Inv~nl~;^n
A unitary housing made from an acoustically-opaque, resilient material is
employed with a microphone clelllellt to form a directional microphone assembly.The microphone element includes a diaphragm which moves under the influence of
sound p.~s~u.e applied to its opposile surfaces to gen~ lale an electrical signal which
is proportional to the di~l~.llial sound ples~ule. The unitary housing includes a
first acoustically-transparent channel for communicating sound ples~lre from a first
port in the unitary housing to one surface of the diaphragm, and a second
acoustically-transparent channel for coll~ icating sound p~ e from a second
port in the unitary housing to the other surface of the diaphragm.
In an illustrative embodiment, the unitary housing comprises a small unitary
"boot " having a s~face including the ports for coupling acoustic energy to the
acoustic channels, an inner chamber for housing the microphone element and a
predet~nninecl opening in a surface of the boot for ~cces~ing the inner chamber.The microphone element is inserted into the inner chamber via the predetP~nined
3 21 74798
opening, which is ~limen~ioned to seal itself around the inserted microphone
element so that acoustic energy cannot leak out of or into the inner charnber through
the opening. The distance between the ports is relatively short so as not to change
the directivity pattern or frequency response of the resulting microphone assembly.
In an illustrative embodiment of the invention, the unitary housing is molded
from Ethylene-Propylene-Diene-Monomer with Polyl,r~,~ylene, which is a rubber-
like m~t~ri~l that is resilient. It forms an excellent seal around the perimeter of the
microphone element so that sound pressure in one channel does not leak into the
other. Moreover, the rubber-like m~teri?~l forms a seal with the surface of sound-
o input equipment where it is housed.
It is a feature of the present invention that the directional microphone
assembly may be conveniently embedded within or mounted behind an exterior
surface of sound-input e~luipllle.lt with the channel openings deployed on one or
more of its surfaces.
Brief Descripti.~ of the l-rawillg
FIG. 1 is a front perspective view of a unitary boot arrangement illustrating
the invention;
FIG. 2 is a rear pe~ecli~e view of the unitary boot arrangement employed
in the present invention;
FIG. 3 is a top cross-sectional view of the unitary boot arrangement
employed in the invention;
FIG. 4 is a side cross-sectional view of the unitary boot arrangement
employed in the invention;
FIG. S is a front cross-sectional view of the unitarv boot arrangement
employed in the invention;
FIG. 6 shows a telephone hand set employing an embodiment of the
invention;
and
FIG. 7 shows a personal conl~ulel employing an embodiment of the
invention.
net~iled Descru)tiQn
GENERAL
4 21 74793
rLs~r~ Microphones
Single port microphones are capable of sensing in~t~nt~neous sound plcs~we
at their input sound port and producing an electrical output voltage signal
corresponding to the magnitude of the sound plcS~wc. Such microphones are
5 known in the art as "~les~we microphones". A sound port admits sound, i.e.,
acoustic energy, into microphone assembly which interacts with one side of a
diaphragm to produce an electrical voltage. The other side of the diaphragm resides
in a closed region whose volume affects the compliance of the diaphragm. Pressure
microphones are equally responsive to sounds coming from any direction and,
lo therefore, their response patterns are ornnidirectional.
F-irst-Order-Gradient Microphones
Gradient microphones are those which achieve a directional polar lejpollse
ch~le~istic by me~slmng the differential ples~ on opposite sides of one or
more diaphragms of a microphone element. A first-order-gradient (FOG)
microphone typically includes two input sound ports positioned on opposite sides of
microphone element diaphragm. The sound ports are sel,al~lc;d by an effective
distance "d" which ~cl~lese~ the di~t~nce that a sound wave must travel around the
FOG in going from one sound port to the other. Mo~ emc ,lt~ of the diaphragm areconverted into voltages at the output of the rnicrophone element. The magnitude of
20 the voltage output of the FOG microphone is a function of the in~ eous
difference in sound ples;,wc on the opposite sides of microphone element
diaphragm. Recall that the velocity of sound in air at 70 degrees Fahrenheit is 1128
feet per second, so that a f=2250 Hz audible signal has a wavelength of about six
inches. Thus, even small separation ~ t~ncçs provide sufficient phase difference25 between the sound ports so that the FOG microphone has a bidirectional polar
response pattern. in fact, the polar response pattern is largely independent of
frequency as will be seen below in equation (2). Note, that the polarity of the output
voltage is (1et~rmined ~y the particular side of the diaphragm that is first impinged
upon by the moving acoustic wave front. Note also that the FOG rnicrophone is
30 u~ e~ollsive to sounds coming from certain directions that are known as nulls.
This property is of use in the present invention.
2~ 74798
The spatial separation "d" between the sound ports leading to opposite sides
of the diaphragm of the FOG microphone element may be varied. The pl~,s~ule
gradient ~p, in the far-field, has the following relationship to "d":
~p oc sin(2 kd cos~) (1)
where: k = f
~ = polar orientation of the impinging wave front with
respect to the major axis of the miclophone; and
o c = wave velocitv.
Equation (1) may be simplified for small values of kd to become:
~p oc 2 cos ~ (2)
The sensitivity or frequency response of a FOG microphone is defined by equation( 1 ) for (2) the direction ~ = 0 . It is known that the frequency response and the
directivity pattern may be changed by altering the gradient microphone itself. For
example, acoustic recict~nce 1~ may be introduced into one of the sound ports of the
20 FOG microphone. Such resistance alters both the directivity pattern and the
frequency response of the microphone assembly. More generally, the directivity
pattern D(~) associated with FOG microphones ope.d~ g in the far field, and where
kd < 1 is given by the following relationship:
( ) l+B ( )
d/
where: B= R c ; and
6 21 747~8
Ca= 2
pc
In equation (3), p is the density of air, V is the volume of the acoustic regionbehind the diaphragm, and Ca is the acoustic compliance (similar to capacitance)5 between the diaphragm and Ra From equation (3), a cardioid directivity pattern is
achieved when B is set equal to 1, which is to say that the time constant RaCa is set
equal to the time it takes for a sound wave to travel dict~nce "d". A FOG
microphone element, suitable for use in connection with the present invention, is the
EM1 18 m~mlf~r,tured by the Primo Microphone, Inc. Another popular shape of the
10 microphone assembly directivity pattern is known as a super cardioid. It is obtained
when d, Ra~ and V are adjusted such that B is set equal to the square root of 3.Further, by increasing the value of B to 3, a hy~elcaldioid directivity pattern is
created. Each of the selected microphone configurations has its own set of
characteristics such as: (i) the location (in degrees) of its null; (ii) distance factor - a
15 multiplier indicating how many times more than the distance from a pres~u.e
microphone to the sound source that a directional microphone can be and have thesame signal-to-random incident noise ratio; (iii) front-to-back response ratio etc.
P~2EFF~RRF.n EMBODIMENT
FIG. 1 is a front pcl~ec~ive view disclosing a low profile unitary housing
20 100 for a FOG microphone element that effectively limits the tli~t~nce "d" between
sound ports 101 and 102 of the microphone assembly and the FOG microphone
element contained therein to a relatively short distance. Indeed, it is desirable to
have distance "d" be the width of the microphone element which will be inserted
into the inner chamber of unitary housing 100, as shown and described below. This
25 rectangular block boot structure is molded from vulcanized (cured) rubber or other
suitable resilient m~teri~l and replaces the baffle of the prior microphone assembly
described above. The unitary housing is made from an acoustically opaque material
which does not transmit sound pressure as efficiently as air. As indicated above,
one suitable m~t~?ri~l that is commercially available is Ethylene-Propylene-Diene-
30 Monomer with Polypropylene.
7 21 74798
However, housing 100 includes ports 101 and 102 which admit sound
pressure, via acoustically transparent channels 103 and 104, respectively, into the
inner chamber where the microphone element 201 resides (see FIGs. 2-5).
Microphone element 201 includes a pair of wires (not shown) that exit the unitary
housing 100 through a self-sealing hole 105. Unitary housing 100 is resilient and is
sized to form a seal with the microphone element 201 so that the sound ~les~ in
one of the channels is not leaked to the other channel around the microphone
element. Advantageously, by using a resilient material for the housing, the need for
adhesives to achieve sealing is elimin~tç-l
FIG. 3 is a top cross-sectional view of the resilient unitary housing assembly
l00, illustrating the its interrelationship with FOG rnicrophol1e element 201 (FIG. 2),
ports 101 and 102, ch~nn~l~ 103 and 104, hole 105, opening 200 (FIG. 2) and inner
charnber 300. Note that inner chamber 300 is ~im~n~ioned to house and support
microphone element 201 such that a seal is formed around microphone element 201
so that acoustic energy does not leak from one ofthe ~ nn~lc 103, 104 to the other.
Additionally, note that opening 200 is ~lim~n.cjoned to be smaller in size than the
cross-section of microphone element 201 such that when microphone element 201is
inserted into inner chamber 300 a seal will be formed around it and opening 200so
that acoustic energy cannot leak out of or into inner chamber 300 via opening 200.
FrG. 4 is a side cross-sectional view of resilient unitary housing 100
including microphone element 201 inserted into inner chamber 300. Note the
dimensional relationship of opening 200 to the dimensions of microphone element
201 thereby forming the desired seal for the acoustic energy. The sealing effected
by resilient housing 100 is also shown around microphone cl~ ent 201 and
ch~nn~ 103 and 104.
FIG. S is a front cross-sectional view of the unitary boot arrangement
employed in the embodiment of the invention. Shown are the rlimPn~ional
relationships of microphone element 201 to channels 103 and 104 and to opening
200 in the resilient unitary housing 100.
Note that in FIGs. 3-5 the dimensions shown are approximately five (5)
times those of one embodiment of the inventive microphone assembly.
APPL~CA TIONS
21 74798
`~ The present invention may be installed in any sound-input equipment that
uses a directional microphone. Sound-input equipment such as a telephone hand
set, speaker phone, personal co~ utel or the like are ~, ~resell~li~e examples.
FIG. 6 discloses one application of the present invention within a telephone
hand set 600. Telephone hand set 600 includes a speaker 601, and the unitary
housing assembly 100 including a microphone element to form a directional
microphone arrangement is positioned along one surface of telephone hand set 600,
as shown. Hand set 600 can be a standard hand set, a cordless phone, a cellular
phone or the like.
FIG. 7 shows a personal colllput-,l arrangement including a speaker phone
employing an embodiment of the invention. Specifically, shown is personal
computer 700 including loudspeaker 701, and the unitary housing assembly 100
including a microphone element to form a directional microphone arrangement is
positioned on one surface of personal colll~u~l 700, as shown. This arrangement
allows for hands free telephony operation. Although a desk top type personal
colllpul~ l is shown for illustrative purposes, it will be appalenl that an embodiment
of the invention may be included in speakerphone arrangements in other similar or
imil~r eq~lirm~nt, one example being a lap-top personal computer.
Additionally, it should be noted that since the lengths of acoustic channels
103 and 104 are relatively short, there will be little effect in the directivity patterns
or frequency response of, for example, cardioid or dipole directional microphoneassemblies.
Although a particular embodiment of the present invention has been shown,
it is clear that modifications are possible within the scope of the invention. Such
modifications include, but are not limited to, the use of other resilient m~t.-ri~l~ for
fabricating the housing, the use of housings that are not molded, and openings in the
housing that are non-rectangular or that do not reside in the same plane. Further,
rather than using a single FOG microphone element, the use of two electrically-
interconnected, ~les~u.e microphone elements is contemplated within the spirit of
the invention.
