Patent Publication Number: US-7590253-B2

Title: Hearing aid having switchable first and second order directional responses

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 09/999,133 filed Nov. 1, 2001, which is a continuation of U.S. application Ser. No. 09/624,805 filed Jul. 24, 2000, now U.S. Pat. No. 6,327,370 issued Dec. 4, 2001, which is a continuation of Ser. No. 08/955,271 filed Oct. 21, 1997, now U.S. Pat. No. 6,101,258 issued Aug. 8, 2000, which is a continuation of U.S. application Ser. No. 08/632,517 filed Apr. 12, 1996, now abandoned, which is a continuation of U.S. application Ser. No. 08/046,241 filed Apr. 13, 1993, now U.S. Pat. No. 5,524,056 issued Jun. 4, 1996, all of which are incorporated by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to improvements in the use of directional microphones for hearing aids that are used in circumstances where the background noise renders verbal communication difficult. More particularly, the present invention relates to a microphone system for such a hearing aid. 
     BACKGROUND OF THE INVENTION 
     Individuals with impaired hearing often experience difficulty understanding conversational speech in background noise. What has not heretofore been well understood is that the majority of daily conversations occur in background noise of one form or another. In some cases, the background noise may be more intense than the target speech, resulting in a severe signal-to-noise ratio problem. In a study of this signal-to-noise problem, Preasons et al, “Speech levels in various environments,” Bolt Beranek and Newman report No. 3281, Washington, D.C., October 1976, placed a head-worn microphone and tape recorder on several individuals and sent them about their daily lives, obtaining data in homes, automobiles, trains, hospitals, department stores, and airplanes. They found that nearly ¼ of the recorded conversations took place in background noise levels of 60 dB sound pressure level (SPL) or greater, and that nearly all of the latter took place with a signal-to-noise ratio between −5 dB and +5 dB. (A signal-to-noise ratio of −5 dB means the target speech is 5 dB less intense than the background noise.) As discussed in a review by Mead Killion, “The Noise Problem: There&#39;s hope,”  Hearing Instruments  Vol. 36, No. 11, 26-32 (1985), people with normal hearing can carry on a conversation with a −5 dB signal-to-noise ratio, but those with hearing impairment generally require something like +10 dB. Hearing impaired individuals are thus excluded from many everyday conversations unless the talker raises his or her voice to an unnatural level. Moreover, the evidence of Carhart and Tillman, “Interaction of competing speech signals with hearing losses,”  Archives of Otolaryngology , Vol. 91, 273-9 (1970), indicates that hearing aids made the problem even worse. More recent studies by Hawkins and Yacullo, “Signal-to-noise ratio advantage of binaural hearing aids and directional microphones under different levels of reverberation,”  J. Speech and Hearing Disorders , Vol. 49, 278-86 (1984), have shown that hearing aids can now help, but still leave the typical hearing aid wearer with a deficit of 10-15 dB relative to a normal-hearing person&#39;s ability to hear in noise. 
     One approach to the problem is the use of digital signal processors such as described in separate papers by Harry Levitt and Birger Kollmeier at the 15th Danavox Symposium “Recent development in hearing instrument technology,” Scanticon, Kolding, Denmark, Mar. 30 through Apr. 2, 1993 (to be published as the  Proceedings of the  15 th Danavox Symposium ). This approach, using multiple microphones and high-speed digital processors, provide a few dB improvement in signal-to-noise ratio. The approach, however, requires very large research expenditures, and, at present, large energy expenditures. It is estimated that the processor described by Levitt would require 40,000 hearing aid batteries per week to keep it powered up. One of the approaches described by Kollmeier operated at 400 times slower than real time, indicating 400 SPARC processors operating simultaneously would be required to obtain real-time operation, for an estimated expenditure of 60,000 hearing aid batteries per hour. Such digital signal processing schemes therefore hold little immediate hope for the hearing aid user. 
     First-order directional microphones have been used in behind-the-ear hearing aids to improve the signal-to-noise ratio by rejecting a portion of the noise coming from the sides and behind the listener. Carlson and Killion, “Subminiature directional microphones”,  J. Audio Engineering Society , Vol. 22, 92-6 (1974), describe the construction and application of such a subminiature microphone suitable for use in behind-the-ear hearing aids. Hawkins and Yacullo (see above) found that such a microphone could improve the effective signal-to-noise ratio by 3-4 dB. 
     First-order directional microphones, however, are not without their drawbacks when utilized in the in-the-ear hearing aids employed by some 75% of hearing aid wearers. The experimental sensitivity of a first-order directional microphone is typically 6-8 dB less when mounted in an in-the-ear hearing aid compared to its sensitivity in a behind-the-ear mounting. These results come about because of the shortened distance available inside the ear and the effect of sound diffraction about the head and ear. An additional problem with directional microphones in head-worn applications is that the improvement they provide over the normal omni-directional microphone is less than occurs in free-field applications because the head and pinna of the ear provide substantial directionality at high frequencies. Thus in both behind-the-ear and in-the-ear applications, the directivity index (ratio of sensitivity to sound from the front to the average sensitivity to sounds from all directions) might be 4.8 dB for a first-order directional microphone tested in isolation and 0 dB for an omnidirectional microphone tested in isolation. When mounted on the head, however, the omnidirectional microphone might have a directivity index of 3 dB at high frequencies and the directional microphone perhaps 5.5 dB. As a result, the improvement in the head-mounted case is 2.5 dB. 
     An approach exploiting microphone directional sensitivity was pursued by Wim Soede. That approach utilizes 5-microphone directional arrays suitable for head-worn applications. The array and its theoretical description are described in his Ph.D. dissertation “Development and evaluation of a new directional hearing instrument based on array technology,” Gebotekst Zoetermeet/1990, Delft University of Technology, Delft, The Netherlands. The array provided a directivity index of 10 dB or greater. The problem with this array approach is that the Soede array is 10 cm long, requiring eyeglass-size hearing aids. It is certainly not practical for the in-the-ear hearing aids most often used in the United States. While there may be many individuals whose loss is so severe that the improved signal-to-noise obtained with such a head-worn array would make it attractive, a majority of hearing aid wearers would find the size of the array unattractive. 
     Second-order directional microphones are more directionally sensitive than their first order counterparts. Second-order directional microphones, however, have always been considered impractical because their sensitivity is so low. The frequency response of a first-order directional microphone falls off at 6 dB/octave below about 2 kHz. The frequency response of a second-order directional microphone falls off at 12 dB/octave below about 2 kHz. At 200 Hz, therefore, the response of a second-order directional microphone is 40 dB below that of it&#39;s comparable omni-directional microphone. If electrical equalization is used to restore the low-frequency response, the amplified microphone noise will be 40 dB higher. The steady hiss of such amplified microphone noise is objectionable in a quiet room, and hearing aids with equivalent noise levels more than about 10-15 dB greater than that obtained with an omni-directional microphone have been found unacceptable in the marketplace. For similar reasons, first order microphones have likewise not gained wide acceptance for use in hearing aids. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved speech intelligibility in noise to the wearer of a small in-the-ear hearing aid. 
     It is a further object of the present invention to provide the necessary mechanical and electrical components to permit practical and economical second-order directional microphone constructions to be used in head-worn hearing aids. 
     It is a still further object of the present invention to provide a switchable noise-reduction feature for a hearing aid whereby the user may switch to an omni-directional microphone for listening in quiet or to music concerts, and then switch to a highly-directional microphone in noisy situations where understanding of conversational speech or other signals would otherwise be difficult or impossible. 
     It is a still further object of the present invention to provide an automatic switching function which, when activated, will automatically switch from the omni-directional microphone to a directional microphone whenever the ambient noise level rises above a certain predetermined value, such switching function taking the form of a “fader” which smoothly attenuates one microphone and brings up the sensitivity on the other over a range of overall sound levels so that no click or pop is heard. 
     These and other objects of the invention are obtained in a hearing aid apparatus that employs both an omnidirectional microphone and at least one directional microphone of at least the first order. The electrical signals output from the directional microphone are supplied to an equalization amplifier which at least partially equalizes the amplitude of the low frequency electrical signal components with the amplitude of the mid and high frequency electrical signal components of the directional microphone. A switching circuit accepts the signals output from both the omnidirectional microphone and the directional microphone. The switching circuit connects the signal from the omnidirectional microphone to an input of a hearing aid amplifier when the switching circuit is in a first switching state, and connects the output of the equalization circuit to the hearing aid amplifier input when the switching circuit is in a second switching state. 
     Several switching circuit embodiments are set forth. In one embodiment, the switching circuit is manually actuatable by a wearer of the hearing aid. In a further embodiment, the switching circuit is operated automatically in response to the level of sensed ambient noise to switch directly between the first and second switching states. In a still further embodiment, the switching circuit is operated automatically as a fader circuit in response to the level of sensed ambient noise to gradually switch between the first and second states thereby providing a gradual transition between the microphones. 
     In a further embodiment of the invention three different types of microphones are employed: an omnidirectional microphone, a first order microphone, and a second order microphone. The microphone outputs are gradually switched to the input of the hearing aid amplifier in response to the sensed level of ambient noise. 
     In one embodiment of the invention, the directional microphone is of the second order. The second order microphone is constructed from two first order gradient microphones that have their output signals subtracted in a subtracter circuit. The output of the subtracter circuit provides a second order directional response. Optionally, diffraction scoops may be disposed over the sound ports of the first order gradient microphones to increase their sensitivity. Hearing aid performance may be further increased by employing a windscreen in addition to the diffraction scoops. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and features of the present invention may be further understood by reference to the following detailed description of the preferred embodiment of the invention taken in conjunction with the accompanying drawings, on which: 
         FIG. 1  is a schematic block diagram of one embodiment of a hearing aid apparatus constructed in accordance with the teachings of the invention; 
         FIG. 2  is a polar chart showing the directional response of an omnidirectional microphone; 
         FIG. 3  is a graph of the frequency response of an omnidirectional microphone, a first order directional microphone, and a second order directional microphone; 
         FIG. 4  is a polar chart showing a directional response of one type of first order directional microphone having cardioid directivity; 
         FIG. 5  is a polar chart showing a directional response of one type of a second order directional microphone; 
         FIG. 6  is a schematic block diagram of a hearing aid apparatus of the invention that utilizes two first order directional microphones to produce a second order directional response; 
         FIG. 7  is a more detailed circuit diagram of the circuit of  FIG. 6 ; 
         FIG. 8  is a schematic diagram of a hearing aid apparatus having automatic ambient-noise-level dependent switching between microphones; 
         FIG. 9  is a schematic diagram of a hearing aid apparatus having automatic ambient-noise-level dependent switching between microphones wherein the switching is performed by a fader circuit; 
         FIGS. 10-12  are graphs showing various signals of the circuit of  FIG. 9  as a function of sound pressure level; 
         FIGS. 13-15  are schematic block diagrams of various constructions of a hearing aid apparatus and its associated components employing automatic switching between an omnidirectional microphone, a first order directional microphone, and a second order directional microphone; 
         FIGS. 16 and 17  are cross sectional views showing the mechanical construction of various microphones suitable for use in the various hearing aid embodiments set forth herein; 
         FIG. 18  is a perspective view of a hearing aid constructed in accordance with the invention as inserted into an ear; 
         FIG. 19  is a cross sectional view showing certain mechanical structures of one embodiment of a hearing aid in accordance with the invention; 
         FIG. 20  is a perspective view showing an alternate mechanical construction of the second order microphone shown in  FIG. 19 ; and 
         FIG. 21  is a front view of the diffraction scoop used in  FIG. 19 . 
     
    
    
     It will be understood that the drawings are not necessarily to scale. In certain instances, details which are not necessary for understanding various aspects of the present invention have been omitted for clarity. 
     DETAILED DESCRIPTION OF THE INVENTION 
     A hearing aid apparatus constructed in accordance with one embodiment of the invention is shown generally at  10  of  FIG. 1 . As illustrated, the hearing aid apparatus  10  utilizes both an omnidirectional microphone  15  and a directional microphone  20  of at least the first order. Each of the microphones  15 , 20  is used to convert sound waves into electrical output signals corresponding to the sound waves. 
     The free space directional response of a typical omnidirectional microphone is shown by line  21  in  FIG. 2  while the corresponding frequency response of such a microphone is shown by line  25  of  FIG. 3 . The directional and frequency response of a typical omnidirectional microphone make it quite suitable for use in low noise environments when it is desirable to hear sound from all directions. Such an omnidirectional microphone is particularly suited for listening to a music concert or the like. 
     The free space directional response of one type of a first order directional microphone is set forth by line  26  in  FIG. 4  and the corresponding frequency response is shown by line  30  of  FIG. 2 . As illustrated, the first order directional microphone tends to reject sound coming from the side and rear of the hearing aid wearer. As such, the directivity of a first-order directional microphone may be used to improve the signal-to-noise ratio of the hearing aid since it rejects a portion of the noise coming from the sides and behind the hearing aid wearer. The first order directional microphone, however, experiences decreased sensitivity to low frequency sound waves, sensitivity dropping off at a rate of 6 dB per octave below approximately 2 KHz. 
     The free space directional response of one type of a second order directional microphone is set forth by line  31  in  FIG. 5  and the corresponding frequency response is shown by line  35  of  FIG. 2 . As illustrated, the second order directional microphone is even more directional than the first order microphone and, as such, tends to improve the signal-to-noise ratio of the hearing aid to an even greater degree than the first order microphone. The second order directional microphone, however, is even less sensitive to low frequency sound waves than its first order counterpart, sensitivity dropping off at a rate of 12 dB per octave below approximately 2 KHz. 
     Referring again to  FIG. 1 , the output of the directional microphone  20  is AC coupled to the input of an equalizer circuit  40  through capacitor  45 . The equalizer circuit  40  at least partially equalizes the amplitude of the low frequency components of the electrical signal output from the directional microphone  20  with the amplitude of the mid and high frequency components of the electrical signal output. This equalization serves to compensate for the decreased sensitivity that the directional microphone provides at lower frequencies. The equalizer circuit  40  provides the equalized signal at output line  50 . 
     As explained above, the equalizer circuit  40  raises the noise level of the hearing aid system. The noise level is significantly raised when a second order microphone is equalized. This noise is quite noticeable to the hearing aid wearer when the hearing aid is used in low ambient noise situations, but tends to become masked in high ambient noise level situations. It is in high ambient noise level situations that the directionality of the directional microphone is most useful for increasing the signal to noise ratio of the hearing aid system. Accordingly, the equalized electrical signal output from the equalizer circuit  40  and the electrical signal output from the omnidirectional microphone  15  are supplied to opposite terminals of a SPDT switch  55  that has its pole terminal connected to the input of a hearing aid amplifier  60 . The electrical signal output from omnidirectional microphone  15  is AC coupled through capacitor  62 . The hearing aid amplifier  60  may be of the type shown and described in U.S. Pat. No. 5,131,046, to Killion et al, the teachings of which are hereby incorporated by reference. 
     The SPDT switch  55  has at least two switching states. In a first switching state, the electrical signal from the omnidirectional microphone  15  is connected to the input of the hearing aid amplifier  60  to the exclusion of the equalized signal from the equalizer circuit  40 . In a second switching state, the equalized electrical signal from the equalizer circuit  40  is connected to the input of the hearing aid amplifier  60  to the exclusion of the electrical signal from the omnidirectional microphone  15 . Microphone selection, such as is disclosed herein, allows optimization of the signal-to-noise ratio of the hearing aid system dependent on the ambient noise conditions. As will be set forth in more detail below, such selection can be done either manually or automatically. 
       FIG. 6  shows another embodiment of a hearing aid system  10 . The hearing aid system  10  employs two first-order directional microphones  65  and  70 . The electrical signal output of directional microphone  70  is AC coupled to the positive input of a summing circuit  75  while the electrical signal output of directional microphone  65  is AC coupled to the negative input of the summing circuit  75 . The directional microphones  65 , 70  have matched characteristics. The resultant electrical signal output on line  80  of the summing circuit  75  has second order directional and frequency response characteristics and is supplied to the input of the equalizer circuit  40 . 
     A more detailed schematic diagram of the system shown in  FIG. 6  is given in  FIG. 7 . As illustrated, the electrical signal output of first order directional microphone  65  is AC coupled through capacitor  85  to the input of an inverting circuit, shown generally at  90 . The inverting circuit  90  includes an inverting amplifier  95 , resistors  100  and  105 , and balance resistor  110 . The electrical signal output of first order microphone  70  is AC coupled through capacitor  115  to resistor  120  which, in turn, is connected to supply the electrical signal output to summing junction  80 . 
     The signal at summing junction  80  is supplied to the input of the equalizer circuit  40 . The equalizer circuit  40  includes inverting amplifier  125 , resistors  130  and  135 , and capacitor  140 . The equalized electrical signal output from the equalizer circuit  40  is supplied to switch  55  on line  145 . 
     The components of the embodiment shown in  FIG. 7  may have the following values and be of the following component types: 
     
       
         
           
               
               
             
               
                   
               
               
                 Component 
                 Description 
               
               
                   
               
             
            
               
                 100, 105  
                 27K 
               
               
                 85, 115 
                 .027 M F 
               
               
                 110 
                 25K variable   
               
               
                 120 
                 15K 
               
               
                 130 
                 100K 
               
               
                 135 
                 1M 
               
               
                 140 
                 560pf 
               
               
                 95, 125 
                 LX 509 
               
               
                   
                 of Gennunm Corp 
               
               
                   
               
            
           
         
       
     
     In an alternative embodiment of the switching system, the SPDT switch  55  can be replaced by an automatic switching system that switches between the directional microphone and the omnidirectional microphone dependent on sensed ambient noise levels. Such alternative embodiments are shown in  FIGS. 8 and 9 . 
     The embodiment of  FIG. 8  includes a directional microphone  20  of at least the first order and an omnidirectional microphone  15 . The output of directional microphone  20  is supplied to the input of equalizer circuit  40  through capacitor  45 . The equalized output signal from the equalizer is supplied on output line  50  to an FET switch  150 . The output signal from omnidirectional microphone  15  is supplied through capacitor  62  to a further FET switch  155 . 
     Each FET switch  150  and  155  includes two complementary FETs  160  and  165  arranged as series pass devices. Where the DC signal level at the input of hearing aid amplifier  60  is 0 V (such as with the hearing aid amplifier design set forth in the above-noted U.S. Pat. No. 5,131,046), only a single FET (i.e., an N-channel FET) need be employed. The FET switches  150  and  155  receive respective control signals from a noise comparison circuit, shown generally at  170 , to control their respective series pass resistances. 
     The noise comparison circuit  170  includes a noise sensing circuit portion and a control circuit portion. The noise sensing circuit portion includes an amplifier  175  that accepts the electrical output signal from omnidirectional microphone  15 . The amplified output signal is supplied to the input of a rectifier circuit  180  which rectifies the amplified signal to provide a DC signal output on line  185  that is indicative of the ambient noise level detected by omnidirectional microphone  15 . 
     The control circuit portion includes comparator  190  and logic inverter  195 . The DC signal output from the rectifier circuit is supplied to the positive input of comparator  190  for comparison to a reference signal V REF  that is supplied to the negative input of the comparator  190 . The output of comparator  190  is a binary signal and is supplied as a control signal to FET switch  150 . The output of the comparator is also supplied to the input of logic inverter  195 , the output of which is supplied as a control signal to FET switch  155 . 
     In operation, the signal V REF  is set to a magnitude representative of a reference ambient noise level at which the hearing aid apparatus is to switch between the directional and omnidirectional microphones  20  and  15 . For example, the signal V REF  can be set to a level representative of a 65 dB ambient noise level. When the sensed ambient noise level thus rises above 65 dB, FET switch  150  will have a low series pass resistance level and will connect the equalized output signal at line  50  to the input of the hearing aid amplifier  60  while FET switch  155  will have a high series pass resistance and will effectively disconnect the electrical signal output of omnidirectional microphone  15  from the input of the hearing aid amplifier  60 . When the ambient noise level drops below 65 dB, FET switch  155  will have a low series pass resistance level and will connect the electrical signal output of microphone  15  at line  200  to the input of the hearing aid amplifier  60  while FET switch  150  will have a high series pass resistance and will effectively disconnect the equalized signal output on line  50  from the input of the hearing aid amplifier  60 . To avoid excessive switching at ambient noise levels near 65 dB, the comparator  190  may be designed to have a certain degree of hysteresis. 
     The reference signal V REF  may be variable and may be set to a level that is optimized for the particular hearing aid wearer. To this end, reference signal V REF  may be supplied from a voltage divider having a trimmer pot as one of its resistive components (not shown). The trimmer pot may be adjusted to set the optimal V REF  value. 
     A further embodiment of a hearing aid apparatus that employs automatic switching is set forth in  FIG. 9 . The circuit of  FIG. 9  is the same as that shown in  FIG. 8  except that the noise comparison circuit  170  is replaced with a fader circuit, shown generally at  205 . 
     The fader circuit  205  includes an amplifier  210  connected to receive the electrical signal output of omnidirectional microphone  15  through capacitor  62 . The amplified signal is supplied to the input of a logarithmic rectifier  215  such as is shown and described in the aforementioned U.S. Pat. No. 5,131,046, but with reversed output polarity. The output of the logarithmic rectifier  215  is supplied as a control signal VC 1  to FET switch  155  and is also supplied to the input of an inverting amplifier circuit  220  having a gain of 1. Where the output range of the logarithmic rectifier is insufficient to drive FET switch  155 , an amplifier may be used the output of which would be supplied as the control signal VC 1  and to the input of inverting amplifier circuit  220 . The output of inverting amplifier  220  is supplied as a control signal VC 2  to FET switch  150 . 
       FIG. 10  is a graph of the control voltages VC 1  and VC 2  as a function of sound pressure level. As the ambient noise level increases there is an increase in the sound pressure level at omnidirectional microphone  15 . This causes an increase of the level of control voltage VC 1  while resulting in a corresponding decrease of the level of control voltage VC 2 . Similarly, as ambient noise level decreases there is a decrease in the sound pressure level at omnidirectional microphone  15 . This causes an increase of the level of control voltage VC 2  while resulting in a corresponding decrease of the level of control voltage VC 1 . 
       FIG. 11  is a graph of the resistances RS 1  and RS 2  respectively of FET switches  155  and  150  as a function of sound pressure level. As the ambient noise level and, thus, the sound pressure level, increases, there is a corresponding increase in the series resistance RS 1  of FET switch  155  and a decrease in the series resistance RS 2  of FET switch  150 . At the input to the hearing aid amplifier  60 , there is thus an increase in the relative level of the signal received from directional microphone  20  and a decrease in the relative level of the signal received from the omnidirectional microphone  15 . As the ambient noise level and, thus, the sound pressure level decreases, there is a corresponding increase in the series resistance RS 2  of FET switch  150  and a decrease in the series resistance RS 1  of FET switch  155 . At the input to the hearing aid amplifier  60 , there is thus a decrease in the relative level of the signal received from the directional microphone  20  and a increase in the relative level of the signal received from the omnidirectional microphone  15 . At some sound pressure level, here designated as SPL 1 , the omnidirectional microphone  15  is effectively completely connected to the input of the hearing aid amplifier  60  while the directional microphone  20  is effectively disconnected from the input of the hearing aid amplifier  60 . At a further sound pressure level, here designated as SPL 2 , the directional microphone  20  is effectively completely connected to the input of the hearing aid amplifier  60  while the omnidirectional microphone  15  is effectively disconnected from the input of the hearing aid amplifier  60 . In between these two sound pressure levels, there is a gradual transition between the two microphones. At sound pressure level SPL 3 , the contributions of both microphones are equal. 
     As is clear from the foregoing circuit description, the fader circuit gradually decreases the relative amplitude of the equalized signal supplied to the hearing aid amplifier while gradually increasing the relative amplitude of the electrical signal supplied to the hearing aid amplifier from the omnidirectional microphone as the level of ambient noise decreases. Likewise, the fader circuit gradually increases the relative amplitude of the equalized signal supplied to the hearing aid amplifier while gradually relative decreasing the amplitude of the electrical signal supplied to the hearing aid amplifier from the omnidirectional microphone as the level of the ambient noise increases. 
     The fader circuit  205  may be designed so that the voltage at the input to the hearing aid amplifier  60  is a monotonic function of sound pressure level. This characteristic is illustrated in  FIG. 12 . A hearing aid apparatus having such characteristic would not present any noticeable deviation in sound output to the user as the apparatus transitions through the various sound pressure level states with variations in ambient noise levels. 
     As will be recognized by those skilled in the art, an amplified telecoil may be substituted for omnidirectional microphone  15  in  FIG. 8 , with V ref  chosen to provide a switch in the output of comparator  190  when a sounding telephone is brought to the ear. Control of FET switch  155  is through the signal output of comparator  190  and control of FET switch  150  is through the output of inverter  195 . This functions to connect the output of the telecoil to the input of hearing aid amplifier  60  and disconnect microphone  20  (which may be either an omnidirectional or directional microphone) whenever sufficient magnetic signal is available at the telephone thus avoiding the necessity of activating a manual switch whenever the hearing aid wearer uses the telephone. In some telecoil applications, the fader circuit of  FIG. 9  may be used. 
       FIG. 13  shows an embodiment of a hearing aid employing an omnidirectional microphone  230 , a first order directional microphone  235 , and a second order directional microphone  240 . The directional microphones  235 ,  240  are AC coupled to respective equalizer circuits  245 ,  250 . The output of equalizer circuit  245  is supplied to FET switch  255  and the output of equalizer  250  is supplied to FET switch  260 . 
     Ambient noise is sensed at omnidirectional microphone  230 , the output of which is supplied to amplifier  265  and therefrom to logarithmic rectifier  270 . The output of microphone  230  is also AC coupled to FET switch  275 . The output of logarithmic rectifier  270  is supplied to a first inverting amplifier circuit  280 , a second inverting amplifier circuit  285 , and directly to control FET switch  275 . The gain of the inverting amplifiers  280  and  285  are chosen so that the omnidirectional microphone output signal dominates at the input of hearing aid amplifier  60  in low ambient noise conditions, the first order directional microphone output signal dominates at mid-level ambient noise conditions, and the second order microphone output dominates at high ambient noise conditions. 
       FIG. 14  shows an alternative design of the circuit of  FIG. 13 . In this arrangement, two first order microphones  290  and  295  are employed along with omnidirectional microphone  230 . First order microphone  295  functions both as a first order directional microphone and as a portion of a second order directional microphone when the output of microphone  290  is subtracted from the output of microphone  295  at junction  300 . Equalizer  245  is not utilized in this circuit for the sake of economy and will not drastically effect hearing aid performance since the lack of low frequency sensitivity of a first order microphone is within a tolerable range without equalization. 
       FIG. 15  shows an alternative circuit for driving the FET switch of the first order microphone  295  in  FIG. 14  or first order microphone  235  in  FIG. 13 . As illustrated, the output of logarithmic rectifier  270  is supplied to the input of an inverting amplifier circuit  305 . The output of inverting amplifier  305  is supplied to the input of a further inverting amplifier circuit  310 , to an FET switch  315 , and to the positive input of comparator  320  for comparison with a comparison voltage V COM . The output of inverting amplifier circuit  310  is biased by a voltage V BIAS  and supplied to FET switch  325 . 
     Comparator  320  compares the voltage at line  330  with the voltage V COM  and supplies a binary state signal output based on the comparison. The binary output is supplied as the control voltage to FET switch  345  and to the input of a logic inverter  335 . The output of logic inverter  335  is supplied as the control voltage to FET switch  315 . The outputs of the FET switches  315  and  325  are supplied as the control voltage for the FET switch associated with the first order microphone response. 
     In operation, V COM  represents the sound pressure level at which the first order microphone output to the hearing aid amplifier begins to be attenuated. The output of inverting amplifier  305  is supplied as the control voltage to the first order microphone FET switch through FET switch  315  for voltage levels below V COM  and gradually increases up to that point with increasing sound pressure level. For voltages above V COM , the output of inverting amplifier  305  is effectively disconnected from the first order FET switch and is replaced by the voltage output of inverting amplifier  310  which gradually decreases with increasing sound pressure level. The magnitude of V BIAS  is chosen so that there is a smooth transition of the control voltage output at line  340 . 
       FIG. 16  shows an omnidirectional pressure type microphone  15  commonly used in hearing aid applications. The omnidirectional microphone  15  includes a hollow body portion  345  having a diaphragm  350  disposed therein. An inlet tube  355  extends from the hollow body portion  345  and engages extension tubing  360  to form a sound port  365 . Sound received at effective sensing point  370  will be transmitted into the hollow body portion  345  to vibrate diaphragm  350  which transduces the sound wave into an electrical signal. 
       FIG. 17  illustrates a gradient first order directional microphone  20  that may be employed in the hearing aid apparatus set forth herein. The directional microphone  20  includes a hollow body portion  375  having a diaphragm  380  disposed therein that divides the interior of the hollow body portion  375  into two chambers  385  and  390 . A first inlet tube  395  extends from the hollow body portion  375  and is connected to extension tube  395  to define a first sound port shown generally at  400 . A second inlet tube  405  extends from the hollow body portion  375  and is connected to extension tube  410  to define a second sound port shown generally at  415 . A time delay acoustical network, defined generally at  420  may also be employed. As is understood by those of ordinary skill in the art, the effective port spacing D determines the sensitivity of the microphone as well as its high frequency response. Sound waves received at sound ports  400  and  415  will respectively travel to chambers  390  and  385  to cause a differential pressure force on diaphragm  380 . This differential pressure force is transduced by diaphragm  380  into an electrical output signal. 
       FIGS. 18-21  show various mechanical constructions that may be employed in the hearing aid embodiments described above. As illustrated, the hearing aid includes a housing  420  having an aperature over which a face plate  425  is disposed. The housing  420  is sized to fit within the ear  430  of a hearing aid user and contains the hearing aid amplifier and speaker (not shown) as well as an omnidirectional microphone and at least one directional microphone. A switch  435  may optionally be provided through the face plate  425  to allow a hearing aid user to manually switch between the omnidirectional microphone and the directional microphone. The sound port  440  of the omnidirectional microphone extends through face plate  425 . In the embodiment shown, the directional microphone is a second order directional microphone that is constructed from two first order gradient directional microphones  445  and  450  of the type described above. Each first order directional microphone includes a respective pair of spaced apart sound ports  400 ,  415 , and  400 ′,  415 ′. The sound ports  400 ,  415 ,  400 ′ and  415 ′ of the first order microphones may be arranged along line  455  as shown in  FIG. 18  so that they are generally collinear. The second order directional microphone formed from the two first order directional microphones will tend to be highly sensitive to frontal sound waves received in the direction shown by arrow  460  while being generally insensitive to rear sound waves received in the direction shown by arrow  465 . 
     An alternative construction of a second order microphone formed from two first order microphones is shown in  FIG. 20 . Rather than having all four sound ports connected through face plate  425 , this embodiment has three sound ports. The central sound port  470  is formed by interconnecting sound port  415 ′ of directional microphone  445  to sound port  400  of directional microphone  450 . The diameter of extension tube  475  is approximately 1.4 times the diameter of the extension tubes  395 ′ and  410  of sound ports  400 ′ and  415  to compensate for this interconnection. 
       FIG. 19  illustrates two additional mechanical structures that can be used to increase the signal-to-noise ratio of the hearing aid. First, a pair of diffraction scoops  480  may be disposed respectively above sound ports  400 ′ and  415 . The diffraction scoops  480  tend to increase the effective port spacing and thus increase the sensitivity of the directional microphone. A front view of a diffraction scoop  480  is shown in  FIG. 21 . Second, a wind screen  485  is disposed over the diffraction scoops  480  and at least a portion of face plate  425 . The wind screen  485  may be in the form of a porous screen or a multiply perforate molded housing. 
     The hearing aid apparatus disclosed herein results from a new understanding of the problems associated with the use of directional microphones in hearing aids. A first understanding is that directional microphones, particularly second-order directional microphones, offer the possibility of an expected directivity index of some 9.0 dB in head-worn applications. The improvement over an omni-directional head-worn microphone thus becomes an attractive 6 dB at high frequencies and nearly 9 dB at low frequencies. The improvement in effective signal-to-noise ratio for speech of 3-4 dB for a first-order directional microphone, might reasonably be extrapolated to an expected 6.5-7.5 dB improvement in single-to-noise ratio for a second-order directional microphone. 
     Although the equalization required for practical application of directional microphones in hearing aids itself results in increased noise, the applicants have realized a second understanding that in many, if not most, of those circumstances where the background noise level interferes with conversation speech, the background noise level itself will mask the added noise. Since an omnidirectional microphone may be switched to the input of the hearing aid amplifier under low ambient noise level conditions, the added noise does not present a problem for the hearing aid user. 
     While several embodiments of the invention have been described hereinabove, those of ordinary skill in the art will recognize that these embodiments may be modified and altered without departing from the central spirit and scope of the invention. Thus, the preferred embodiments described hereinabove are to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description. Therefore, it is the intention of the inventors to embrace herein all such changes, alterations and modifications which come within the meaning and range of equivalency of the claims.