Abstract:
A microphone module and method for suppressing feedback in a microphone. The module has a casing with a hollow bore therethrough and a microphone mounted in one end of the bore. The other end of the bore is completely covered by a film mounted onto the top of the casing. The film has at least one slit therethrough in the film portion that covers the other end of the bore. The method includes introducing a sound wave to a film having at least one slit therethrough that separates the film into at least two parts; generating a sound wave from each film part; and conveying the generated sound waves in a sound tube to a microphone as a rejoined sound wave.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to microphones in general, and in specific, relates to microphones having feedback suppression. 
       BACKGROUND OF THE INVENTION 
       [0002]    The audio feedback effect, also called microphone feedback, occurs when a sound wave enters a microphone having a frequency that is the same as the frequency of a sound wave at an output of the microphone. 
         [0003]    Feedbacks could happen on the electronic equipment which receives and broadcasts sounds. When the External Feedback Path is formed, where sound waves generated by the broadcast point are received by the collecting point, sound waves are thus constantly repeatingly amplified.
   There are 2 major impacts of feedbacks.   
 
         [0005]    1. When feedback sounds are mixed with the original sounds, it would cause acoustic distortion. 
         [0006]    2. When feedbacks of the same frequency repeatingly accumulate, and volume gain is too large, piercing whistles occur. 
         [0007]    Cancellations in High Fidelity Acoustics: 
         [0008]    (1) A microphone cannot determine whether the incoming sounds or signals are from an objective sound source or from noises, such as background noises or internal microphone generated noises. When objective sounds are interfered with by noises, their sound waves are changed, and thus the acoustic quality is affected. 
         [0009]    (2) Traditional noise filters can solve this issue by treating the frequency of the incoming signals. If the noise and the sound source&#39;s frequencies are different, a high-pass filter (which allows only sounds below certain frequency to pass), a low-pass filter (which allows only sounds above certain frequency to pass), or a range-pass filter (which allows only sounds within certain frequency range to pass) can be used to filter out the noise. 
         [0010]    (3) However if the noise and the objective sound&#39;s frequencies are the same, or are close (such as multiple reflections of the objective sound), the objective sounds and noises are similar, and the filter cannot delete the noise. 
         [0011]    (4) In addition, irrespective of whether digital or analogue filters are used, or if frequency or time-domain filters are used, all are more-or-less subjected to mathematical transformations. The transformations result from distortion and time delay issues. Thus the better a filter is, the more complex design and mathematical conversions are required. For example the latest Wavelet filter could be used, but it is very expensive. 
       SUMMARY OF THE INVENTION 
       [0012]    A major difference between an objective, desirable sound signals and noise signals are in their incoming direction and energy. Objective sounds have a fixed direction and a stronger energy. The noises that originate from other sources and their various directions usually have a weak energy. A purpose of the present invention is to cause the objective sound signals to predominate over the noise signals. 
         [0013]    The present invention provides a mechanical solution to the feedback problem by shifting the phase of the input sound wave to the microphone. The phase shifting is done physically by separating the sound wave into at least two secondary waves and then re-combining them before they are impact on the microphone. 
         [0014]    A microphone module according to the present invention includes a body, an opening or area to receive sound waves, and a transducer diaphragm. The module also includes a film or diaphragm that extends over and is spaced from the sound wave receiving area of the microphone body. The film has at least one slit or cut through it which in one embodiment is located in a central portion of the film. The slit allows the sound wave to pass through it and results in the formation of at least two distinct acoustic waves, one generated by a film portion on each side of the slit. 
         [0015]    The structure of the film slit of the present invention allows sound waves from the directly ahead with a stronger energy to pass, but adds a filter effect to cancel out or reduce the effect of sound waves from other directions or with lower energy. In this way, there is no or only a little variance for the objective/target sound source&#39;s wave, and accordingly the acoustic quality is increased. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is an exploded, perspective, diagrammatic view of a microphone according to a presently preferred embodiment having a casing with a top that has a slit therein. 
           [0017]      FIG. 2  is a perspective, diagrammatic view of the microphone casing showing the slit location. 
           [0018]      FIG. 3  is a cross sectional diagrammatic view taken along lines A-A of  FIG. 21 , of a microphone surrounded by the microphone casing and showing a top portion with a slit and the internal chamber. 
           [0019]      FIG. 4  is a top plan view of the microphone casing. 
           [0020]      FIG. 5  is a diagrammatic cross sectional view showing schematically the division of an incident sound wave by the split in the film cover. 
           [0021]      FIG. 6  is a plan view of a film showing a presently preferred split or cross cut pattern. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    With reference now to  FIGS. 1-5 , the present invention will be described with respect to a presently preferred embodiment in which like numerals designate like elements throughout the several views. 
         [0023]    In describing an embodiment of the present invention, only diagrammatic representations will be used, at least because the present invention is subject to a large number of particular implementations, which those skilled in the art would recognize. 
         [0024]    Now, with a particular reference to  FIGS. 1 ,  2 ,  3  and  4 , there is depicted a microphone module  100  which comprises a diagrammatically depicted microphone  110  and a housing, guide tube or casing  120 . Microphone  110  can be, for example, a conventional condenser microphone. 
         [0025]    Guide tube  120  has an exterior surface  121  and an interior bore or chamber  122  extending completely there through. Chamber  122 , as depicted in  FIG. 1 , has a longer, upper section  124  (sometimes called the first section so that the orientation of the chamber is not at issue) and a contiguous lower, wider section  126  (sometimes called the second section). Lower chamber section  126  has a diameter and bore configuration so as to be able to receive the top or sound receiving part of microphone  110 , and to snuggly encompass microphone  110 , as depicted in  FIG. 3 . The area where upper chamber section  124  and lower chamber section  126  meet, bottom  129  of upper chamber section  124 , marks the end of the sound collecting space and thus its length. As discussed below, the length of upper chamber section  124  has an effect on the filtering characteristics and quality of microphone module  100 . 
         [0026]    Casing  120  as shown in  FIG. 1  has a top audio receiving end  128  and a bottom end  130 . The bottom audio transmitting end is depicted at  129 , as mentioned above. 
         [0027]    The interior shape of upper chamber  124  is depicted as being cylindrical, but it could be ovular or even rectangular. Although chamber  122  is depicted as having only one bore, casing  120  can be in more than one part and upper chamber  124  can be mounted directly to the end of microphone  110 . Also, an outer elastic housing (not shown) can surround casing  120  so as to better isolate casing  120  from external sounds and vibrations. 
         [0028]    Exemplary dimensions of casing  120 , for two different embodiments are:
       Microphone diameters (lower section  126 ): 9 mm and 6 mm;   Sound hole diameter of microphone: 4 mm and 2 mm;   Upper section  128  internal diameter: 4 mm and 2 mm; and   Upper section  128  length: 4 mm and 2 mm.       
 
         [0033]    Securely mounted on top end  128  of casing  120 , such as by an adhesive or some mechanical connection such as a screw or nail, is a disk-shaped thin film  140 . Film  140  has a minimum diameter so that it can completely close the upper end of chamber upper section  124  and is stretched tight across chamber  120 . In  FIGS. 1 and 2 , film  140  has the same diameter as does the upper end of casing  120 . In the present embodiment, film  140  is depicted and described as having only one sheet, but in other embodiments, film  140  could be comprised of a plurality of sheets or of a laminate having a plurality of layers. 
         [0034]    Located in the central portion of film  140  is a single thin slit  142 , which when film  140  is mounted on casing  120  fully extends across top end  128  of casing  120 . Slit  142  divides film  140  into a first section  144  and a second section  146 . 
         [0035]    Film  140  can be made of any flexible, but unbreakable or untearable material, such as a plastic film (e.g. PET, PEEN and OPP). Also, film  140  can be comprised of a flexible and thin metallic film. Further, although film  140  is depicted as being comprised of a single material sheet, film  140  could also be comprised a multipart, multi material sheet in which the parts could be concentric, or could be coplanar with slit  142  dividing the different materials. Obviously, this later design provides different sound reproduction effects as the produced waves will have different qualities (e.g. phase, amplitude, vibration) 
         [0036]    Film  140  has a thickness dimension in the range of about 0.01 mm to about 0.1 mm. The length of slit  142  can be as long as, or slightly longer than the diameter of the top of chamber  122  or it could be a length as short as one-half to nine-tenth the diameter of the top of chamber  122 . Slit  142  is preferable a simple, thin cut. 
         [0037]    The length of slit  142  that is equal to or larger than the diameter of end of upper chamber section  124  is preferred. Preferably, slit  142  is straight or linear, but it could have an arcuate shape that if extended would have a radius of  100   s  of millimeters to a few centimeters, somewhat depending upon the length of slit  142 . Also, as discussed above, slit  142  can actually be multiple slits that preferably intersect, such as depicted in  FIG. 6 . Obviously, a more complex plurality of signals would be generated. Also, slit  142  can be comprised of a plurality of cuts that do not intersect, such as parallel cuts that result in a plurality of vibrating separate film sections. Further, in the embodiment in which there are plural films, such as two or more axially spaced apart films, each film can have a slit that is aligned and located above the other, or they can be in different parts of the film body so as not to be vertically aligned.
   A slit  142  in a harder film  140 , is presently preferred to comprise or have a cross shape, and a slit  142  in a softer film  140  is presently preferred to comprise a straight line slit or parallel slits.   
 
         [0039]    Different locations of slit  142  with respect to the center of chamber upper section  124  has different results for piercing feedback suppression. If slit  142  is not in the center, there is a different size in first and second film sections  144  and  146  and a resultant different time shift of the sound wave. A slit  142  located in the center over chamber  122  is better than if it is not in the center of film  140 . Thus for either a single slit  142 , or for multiple slits, whether cross slits or parallel slits, the slits should be arranged symmetric to the center. 
         [0040]    The diameter of film  140  is related to the size of the microphone, and should be slightly wider than the size range of the sound receiving hole or holes in the microphone body (on the top and sound collecting end). The thickness of film  140  will affect the result of sounds passing through film  140 . When sounds are generated, high pitch sounds and low pitch sounds have the same level of energy. But as sounds spread away from the sound origin, high pitch sounds have more decay than the low pitch sounds. Thus when reaching a film  140  that is spaced from the sound origin, the low pitch sounds have more energy than the high pitch sounds. Thus, low pitch sounds are better able to pass (vibrate) a thicker film than high pitch sounds. Therefore, for the same film material, the thicker the film, the worse mid- and high-pitch sounds that would reach the microphone and that microphone design has a poorer performance at the mid- and high-pitch fields will not be good. For the same thickness of film, the softer the film material is, the better is the performance and results from mid- and high-pitch sounds. Films have a preferable thickness varying from 0.01 mm to 0.1 mm with material such as PET, PEEN and OPP. Various hardness of the film material is used to tune the microphone&#39;s performance for the desired result. 
         [0041]    Casing  120  is preferably only a few centimeters long and a few centimeters in width. Although casing  120  is shown as a cylinder, any exterior shape can be utilized. Casing is preferably made of an elastic or soft material that is slightly compressible, but could also be made of a solid hard material, such as a plastic or metal. Casing  120  can also be comprised of a ceramic material that is resistant to cracking or breaking. Casing  120  can also be comprised of two or more materials, but it is preferably that the interior walls forming upper chamber  24  be non-resilient and be reflective so as not to introduce any interferences into the passing sound waves. 
         [0042]    Similar as the ranges in the diameter of film  140  diameter, the length of chamber  122  affects the performance of microphone module  100  with various frequencies. If the length of chamber  122  is equal to or close to the inner diameter of chamber  122 , there will be a good result for high, mid and low pitch sounds, and good piercing feedback suppression from the sound source and microphone. When the length of chamber  122  is smaller than the inner diameter thereof, there will be a better result for mid- and high-pitch sounds, but the feedback suppression of piercing sounds is worse (i.e. at a closer distance from the sound source to the microphone). When the length of chamber  122  is longer than the inner diameter thereof, there will be a worse result for mid- and high-pitch sounds, but the feedback suppression of piercing sounds is better (i.e. at a closer distance from sound source to the microphone). 
         [0043]    Casing  120  can be made of a plastic, metal, ceramic material. The harder the material, the better are the isolation of possible vibrations from the casing material. 
         [0044]    In the operation of microphone module  100 , as depicted in  FIG. 5 , a sound wave  150  reaches the surface of film  140  and film sections  144  and  146  independently vibrate resulting in the generation of two sound waves,  152  and  154 . Sound waves  152  and  154  have the same frequency and if film sections  144  and  146  have substantially the same surface area, will have the same phase, but the amplitude will be reduced to half. There can also a phase difference (i.e. a time difference) between original sound wave  150  and sound waves  152  and  154 . Sound waves  152  and  154  pass through chamber  122  and are united and regenerated as a new sound wave at the bottom thereof. Due to the time difference between original sound wave  150  and generated sound waves  152  and  154 , there are small differences between the new and the original sound waves, which is sufficient to suppress any feedback. Obviously, the greater the number of generated sound waves, such as by the slits in  FIG. 6 , the greater the cumulative differences will be between the original sound wave and the reconstituted sound wave, and the created the feedback suppression. 
         [0045]    The present invention operates in theory as follows. 
         [0046]    A. Noise Cancellation 
         [0047]    Film  140  cancels feedback noises based on the following principles and reasons. 
         [0048]    (1) Noises come from the reflections of the objective sound source, from non-objective sound sources and reflection from non-objective&#39;s sound source, and white noises (which in general refers to all multiple reflections, refractions, and dispersions at a sound source&#39;s surrounding). 
         [0049]    (2) Orientation/Directional: Film  140  generates a large uni-directional effect, which filters out non-objective sound sources and white noises. Reflections of objective sound sources, non-objective sound sources, and white noises incident onto film  140  perpendicularly (i.e. in a normal direction) are not filtered. 
         [0050]    (3) The critical energy which drives the film and the energy transformation of the above processes are not linearly transformed. The film vibrates only when the incident sound wave has minimum amount strength. For example, those noises which come from an objective sound source&#39;s reflection, non-objective sound source&#39;s reflection, and white noises which are reflected or multiply reflected have energy decay after transfers and spherical spreading. Thus these low energy noises are thus filtered by film  140 . 
         [0051]    (4) By using the structure of guide tube  120 , a wind must pass through film  140  before reaching the microphone diaphragm. Thus wind pressure will not cause the microphone diaphragm to vibrate back and forth, but only to shift or move. Film  140  transfers sound energy by vibration. The shifting and movement of the film does not generate sound energy and thus noises because the energy is attenuated, absorbed, or reflected by the film. 
         [0052]    (5) There are 2 conditions which could still result in the generation of sound from a wind striking film  140 : the strength of the wind or the direction changes of the wind. When the wind&#39;s strength or direction changes, it changes the tightness of film  140 , which could cause an effect that is similar to vibration. This is especially true when there are more severe changes in the wind&#39;s strength or directions, which is a situation more like vibrations. This type of noise is more serious. 
         [0053]    When the wind blows toward the film  140  at a direction nearly parallel to the surface of film  140 , the slight angle variation causes a large sound pressure variation, and generates noises. The power of the wind pressures is much larger than sound waves. Thus, a wind component with film  140  resulting in less than 5% energy can make film  140  vibrate, and generate noises. Thus, when a wind blows nearly parallel to film  140 , there would be noises. (This phenomenon is similar to when wind flow a flag, the flag waves within small angles, and makes sounds.) 
         [0054]    A physical method of lowering feedbacks for microphone by using films has been described for various types of sound waves impacting on microphone module  100 . There is an elastic film at the input end of the microphone, and there is at least one cut in the film, as shown in  FIGS. 1 and 2 . Sound waves are energy that is transmitted by directional vibrations. A perpendicular component to film  140  makes film  140  vibrate and a parallel component does not. When film  140  is not cut film  140  is sealed tight and it is hard to make a contribution to the vibrations. Only small portion of can pass through film  140  and forms a penetrating wave while the rest is reflected and forms a perpendicular reflex wave. 
         [0055]    When film  140  is cut, the opening edges are free ends and the resulting film portions can easily vibrate, and form penetrating waves. When the generated sound waves reach microphone  110 , and are collected by microphone  110 , there is a time difference, but the time difference is small, and the distortion is usually acceptable. When there is no film  140 , as in traditional microphone, at the opening of the sound collecting end, though the incident wave comes parallel to the opening, some sound waves will enter the sound collecting end due to the diffraction effect. Thus certain sounds are still collected, and it is possible to totally block out the sounds. 
         [0056]    When there is no film, as in a the traditional microphone, at the opening of the sound collecting end, sound waves enter the sound collect opening in the transmission path which is not parallel with the sound collecting tube. There would be multiple reflections and other disturbances occur on the tube&#39;s wall. Various frequencies of reflections will cause various disturbances, and cause sound distortions. 
         [0057]    The invention&#39;s structure employs one or more films, but for the purpose of the following explanation, only a single film will be discussed. With respect to a film and its vibrations, sound waves enter the tube in the transmitting path which is nearly parallel to the tube&#39;s wall, produces less multiple reflections, thus there are no sound distortions. 
         [0058]    When sound waves from a sound source comes at an incident angle “theta” to the surface of film  140 , its sound wave arrives film A and B at difference time, and the 2 films vibrate independently. They could be seen as 2 new sound waves (see  FIG. 5 ), which have the same wave form with but half amplitude of the sound source, and there is the time difference and phase difference between the two new sound waves. The 2 new waves combine as one sound wave in inner chamber  122 . Because of the phase difference between the 2 sound waves, there is a slight difference between the new formed sound wave and the source&#39;s sound wave. The new formed sound wave is collected by the microphone, and outputted from the speaker. When the outputted sound wave returns to film  140 , the new wave arrives with a time difference from the original wave, and again new sound wave is formed in the tube with phase. And the accumulated phase difference increases, 
         [0059]    With the present invention, each time the wave feedbacks, it accumulates phase differences, and decreases the accumulation results, thus suppressing the feedback noises or whistles. For microphone feedback from microphones not employing the present invention, theoretically, the more times sound waves with same frequencies at zero phase difference feedback, the stronger will be the piercing whistles. However, with the present invention, the more times sound waves feedback, the phase difference increases, the accumulated difference of the wave form increases, thereby increasingly suppressing the piercing whistles. 
         [0060]    Other embodiments, alternatives, modifications, variations to the presently disclosed embodiments, as well as other dimensions, are obvious to those skilled in the art, and the scope of the present invention is determined by the attached claims.