Patent Grant
8107652
The present invention relates generally to microphones and, more particularly, to an omnidirectional electret condenser microphone element designed for use in an acoustically noisy environment.
Electret condenser microphone elements are well known in the art and used in a variety of applications, for example landline and cellular telephones, broadcast and recording systems, communication headsets, and computer microphones. Such microphone elements can be designed to be either directional or omnidirectional, depending upon the desired application and performance requirements.
The designs for directional and omnidirectional microphone elements can differ in a variety of ways. One of the principal distinguishing design features between these two microphone designs is in the placement of the sound port(s), also referred to herein as an acoustic aperture(s). In a directional microphone, there are at least two spatially separated sound ports, this feature leading to a decreased pick-up of low frequency background ambient acoustical noise. In contrast, in an omnidirectional microphone element the sound port is located in a single spatial position, even if the microphone includes multiple ports. As a result of this design, omnidirectional microphone elements are less susceptible to the pick-up of wind noise than directional microphone elements. Wind or turbulent-type noise is present any time air is flowing past the microphone aperture(s), such as in automotive environments or from a fan.
Although in general omnidirectional microphone elements are less susceptible to wind noise than directional microphone elements, their lack of spatial discrimination can allow decreased signal quality when used in environments in which the primary audio signal source, e.g., the intended speaker, is surrounded by a high level of background ambient noise (e.g., traffic noise, machinery including engine and HVAC noise, and background vocal noise). Additionally, as omnidirectional microphone elements have a generally flat frequency response from about 50 or 100 Hz to about 10 kHz, they are further prone to picking up background ambient noise since the sound pressure levels associated with typical background ambient noise increase at lower frequencies. In contrast, background ambient noise is less problematic for directional microphone elements which exhibit a natural response roll-off at lower frequencies, and importantly because they spatially discriminate against acoustical noise from selected directions.
A circuit board 117 fits within, and covers, the casing opening located at the distal end opposite front face 103. One or more signal processing elements 119 (e.g., a field effect transistor or FET) are attached to circuit board 117 and contained within casing 101 as shown. Electrode patterns on circuit board 117, represented by raised contact regions 118, are used in conjunction with electrically conductive casing 101 to couple signal processing element 119 to electrode plate 107. Metallized diaphragm 113 is coupled to signal processing element 119 via an electrically conductive spacer 121 and a raised contact region 123 located on the bottom surface of circuit board 117. Spacer 121 is typically ring-shaped. A second electrically insulating spacer 125, typically ring-shaped, is used to prevent shorting of spacer 121 to casing 101 as well as insuring that spacer 121 is properly positioned relative to contact region 123. End edge portion 127 of casing 101 is folded over and crimped, thereby compressing circuit board 117, spacer 121, and metallized diaphragm 113 against each other and holding the individual components in place. Solder bumps 129 are used to electrically couple the microphone element to the intended device (i.e., cell phone, camcorder, etc.).
As known by those of skill in the art, there are numerous possible configurations for a conventional omnidirectional electret condenser microphone element. The microphone element described above relative to
In at least one conventional omnidirectional electret condenser microphone element known to the inventors, means are provided to achieve quasi-static pressure equalization between internal microphone volume 131 and the ambient environment. It will be appreciated that pressure equalization means can be specifically designed into the element, for example utilizing a leakage passageway as described more fully below, or by taking advantage of the normal mismatch between components within the microphone assembly. Quasi-static pressure equalization is often desired to avoid potentially damaging the diaphragm when the microphone is subjected to sudden and major pressure changes, for example those commonly encountered during air shipment. At the same time, and as known by those of skill in the art, the means used to provide pressure equalization must allow only minor air leakage between the ambient environment and volume 131, otherwise the microphone element will fail to operate properly and to provide the desired electro-acoustic response. In a typical omnidirectional electret condenser microphone element utilizing pressure equalization means, the leakage passageway is small enough that only frequencies below the audio band, for example near 5-10 Hz, are affected.
As shown in
What is needed in the art is an omnidirectional electret condenser microphone element, such as the conventional unit described above, but in which the design has been modified to reduce the pick-up of background noise, thereby providing an enhanced signal-to-acoustic background ambient noise ratio. The present invention provides a means for achieving such a microphone.
The present invention provides an omnidirectional electret condenser microphone element with improved low frequency background ambient acoustical noise rejection. The omnidirectional electret condenser microphone element of the invention includes a plurality of passageways in acoustic series that couple at least one acoustic aperture of the microphone element to an acoustic cavity formed within the microphone element. At least one of said plurality of passageways is of a predefined size that is determined to provide the desired frequency response roll-off within a predefined frequency range. In at least one embodiment, the roll-off resulting from the plurality of passageways is greater than 2.0 dB between 300 and 100 Hz. In at least one other embodiment, the roll-off resulting from the plurality of passageways is greater than 3.0 dB between 300 and 100 Hz.
In one aspect of the invention, an omnidirectional electret condenser microphone element is provided with an acoustic roll-off of at least 2.0 dB between 300 Hz and 100 Hz, the microphone element comprised of an electrically conductive casing with a first end portion that includes at least one acoustic aperture, a circuit board disposed within the casing and closing an opening at a second end portion of the casing, a diaphragm disposed within the casing, and a plurality of passageways that produce the microphone's acoustic roll-off and that couple the at least one acoustic aperture to an acoustic cavity formed within the casing and interposed between the diaphragm and the circuit board.
In another aspect of the invention, an omnidirectional electret condenser microphone element is provided that is comprised of an electrically conductive casing with a first end portion that includes at least one acoustic aperture, a circuit board disposed within the conductive casing and closing an opening at a second end portion of the electrically conductive casing, an electrode plate disposed within the electrically conductive casing, an electret material applied to a surface of the electrode plate, a metallized diaphragm disposed within the electrically conductive casing, an electrically insulating spacer interposed between the electret material and the metallized diaphragm, an electrically conductive spacer interposed between the metallized diaphragm and the circuit board, a signal processing unit disposed on the circuit board and electrically connected to the electret material and the metallized diaphragm, an acoustic cavity formed within the electrically conductive casing and defined by the metallized diaphragm and the circuit board, a first passageway coupling the at least one acoustic aperture to an air volume defined by an inner surface of the electrically conductive casing and an outer surface of the electrically conductive spacer, and a second passageway in acoustic series with the first passageway and coupling the air volume to the acoustic cavity. Preferably the first and second passageways produce an acoustic roll-off of at least 2.0 dB between 300 Hz and 100 Hz; alternately the first and second passageways produce an acoustic roll-off of at least 3.0 dB between 300 Hz and 100 Hz.
In another aspect of the invention, a method of providing acoustically-driven roll-off between a first frequency and a second frequency within an omnidirectional electret condenser microphone element is provided, the method comprising the steps of (i) providing a plurality of passageways in acoustic series within the microphone element, the plurality of passageways coupling at least one acoustic aperture of the microphone element to an acoustic cavity formed within the microphone element, and (ii) sizing the plurality of passageways according to the formula RA=20 log 10 [rFirst Frequency/rSecond Frequency], where RA is equal to the acoustically-driven frequency response roll-off, where rFrequency is equal to [C2 (a2+b2)0.5]/[{(ad)2+(1−bd)2}0.5], where d is equal to 1+C2/C1, where a is equal to ω C1R, where b is equal to ω2 C1 L, where ω is equal to 2πf, where f is the frequency, where C1 is the effective acoustical compliance of a diaphragm mounted within the omnidirectional electret condenser microphone element and is equal to A2/(8πS), where C2 is the acoustical compliance of the acoustic cavity and is equal to V/(ρc2), where A is equal to the area of the diaphragm, where S is equal to the radial tension of the diaphragm, where V is equal to the volume of the acoustic cavity, where ρ is equal to the density of air, where c is equal to the sound wave velocity in air, where R is the real part of the acoustic impedance of the plurality of passageways and is equal to 12 ρμD/(N W H3), where L is the imaginary part of the acoustic impedance of the plurality of passageways and is equal to 6 ρD/(5N W H), where μ is equal to the effective kinematic coefficient of the viscosity of air, where N is equal to the number of parallel legs forming a given leakage passageway, where D is equal to the passageway length, where W is equal to the passageway width, and where H is equal to the passageway height.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.
The inventors have found that it is possible to achieve many of the benefits associated with a conventional omnidirectional electret condenser microphone element while improving upon its background ambient noise rejection, more specifically improving the signal-to-noise ratio at low frequencies. The inventors have found it advantageous to roll-off the low frequency response, preferably by at least 2.0 dB, and more preferably by at least 3.0 dB, between an upper frequency of 100 Hz and a lower frequency of 300 Hz. This audio band was selected since the low frequency portion of the voice signal, while easily corrupted by acoustical noise, carries very little of the audio intelligibility of speech. For example, high-pass filtering a typical speech signal above 300 Hz will reduce the intelligibility by only about 3%. It should be appreciated, however, that the method described in detail below can be used to adjust the low frequency roll-off within other audio bands.
In order to accomplish the benefits of the invention, the microphone element is configured to include two or more leakage passageways of a predetermined size that couple the input acoustic aperture to the acoustic cavity located within the microphone and behind the diaphragm. By properly sizing these passageways, or at least properly sizing the controlling passageway, low frequency sounds in which acoustic background noise is concentrated are presented to both sides of the diaphragm, thereby partially canceling these sounds and thus decreasing their influence upon the diaphragm and hence microphone output. As described more fully below, the degree of acoustic cancellation is controlled by the frequency, the design of the leakage passageways, the tensioned diaphragm's effective acoustical compliance, and the acoustic cavity air volume.
A variety of techniques can be used to couple metallized diaphragm 709 to circuit board 711 while still providing one or more airways 713 that allow the air passing through passageways 701 to couple to acoustic volume 715. For example, the electrically conductive spacer 717 that couples diaphragm 709 to circuit board 711 can include one or more airways (not shown). In the preferred embodiment, however, the air passageways around spacer 717 are fabricated on the circuit board 711, specifically by including interruptions within the raised contact region on the bottom surface of circuit board 711. As shown in the bottom view of circuit board 711 provided by
With respect to electrically insulating spacer 721, as previously described this spacer is typically used to insure spacer 717 does not accidentally short out against casing 705. Spacer 721 also helps to insure the proper placement of spacer 717 relative to contact region 903. In one embodiment, spacer 721 is simply smaller than required, thus insuring that there are multiple air leakage pathways around the spacer. Alternately and as illustrated in the top view of
Although not required by the preferred embodiment described relative to
The roll-off, RO, in an omnidirectional electret condenser microphone element is equivalent to the sum of the roll-off, RE, that is due to the electrical signal processing of the element and the roll-off, RA, that is due to the inclusion of a controlled leakage pathway. As RE is negligible in a conventional omnidirectional microphone, in this case RO is equivalent to RA. RA, and thus RO, is given by the following equation:
RA=20 log 10[r300 Hz/r100 Hz](dB), where
r=[C2(a2+b2)0.5]/[{(ad)2+(1−bd)2}0.5], and
d=1+C2/C1, a=ωC1R, b=ω2C1L, ω=2πf.
In these equations, f is the frequency in Hz, C1 is the effective acoustical compliance of the diaphragm (e.g., diaphragm 709 in
C1=A2/(8πS)(m5/N), where
A=diaphragm area (m2), and S=radial tension (N/m).
and
C2=V/(ρC2)(m5/N), where
V=rear acoustic cavity volume (m3), ρ=density of air=1.21 kgm/m3, and
c=sound wave velocity in air=343 m/s.
R and L are the real and imaginary parts, respectively, of the acoustic impedance of the leakage passageway, which are given by:
R=12ρμ/(NWH3)(Nsm−5) and
L=6ρD/(5NWH)(kgms m−4), where
μ=effective kinematic coefficient of the viscosity of air=2.2×10−5 m2/s,
N=number of slotted regions constituting the acoustic passageway
D=passageway length in m, W=passageway width in m, and
H=passageway height in m and is the smallest dimension.
It will be noted that r in the above equations is the magnitude of the complex volume-displacement per unit pressure in units of m6/N impressed upon the acoustic aperture. Additionally, it should also be noted that the leakage system acoustical impedance shunts, i.e., is in parallel with, the diaphragm impedance.
In the preferred embodiment of the invention, illustrated in
In an exemplary embodiment, H is equal to 0.023×10−3 m, W is equal to 0.30×10−3 m, D is equal to 0.82×10−3 m, N is equal to 2, V is equal to 1.9×10−9 m3, S is equal to 30 N m−1, and A is equal to 5.0×10−6 m2. For this configuration, a roll-off of 2.5 dB is calculated between 300 and 100 Hz.
In a preferred embodiment, H is equal to 0.027×m, W is equal to 0.30×10−3 m, D is equal to 0.82×10−3 m, N is equal to 2, V is equal to 1.9×10−9 m3, S is equal to 30 N m−1, and A is equal to 5.0×10−6 m2. For this configuration, a roll-off of 4.0 dB is calculated between 300 and 100 Hz.
It will be appreciated that if the acoustical impedance of the other passageway (e.g., passageways 713) is significant, the two R and two L for each passageway can be summed.
As will be understood by those familiar with the art, the present invention may be embodied in any of a variety of microphone configurations without departing from the spirit or essential characteristics thereof. Accordingly, the disclosures and descriptions herein are intended to be illustrative, but not limiting, of the scope of the invention which is set forth in the following claims.
| Number | Name | Date | Kind |
|---|---|---|---|
| 20090257613 | Khamashta et al. | Oct 2009 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20100027817 A1 | Feb 2010 | US |
