Tutorial on microphone technologies
`for directional hearing aids
`
`By Stephen C. Thompson
`
`Directional microphones have been available for use in hear-
`ing aids since the early 1970s. The clinical benefit of improved
`hearing in noise with a directional microphone has been under-
`stood since at least the 1980s. Nonetheless, hearing aids with
`directional microphone responses did not gain significant mar-
`ket acceptance until the mid-1990s. Why is that?
`Part of the reason, certainly, is improvement in the design
`and directional performance in the more recent systems. How-
`ever, directionality also has disadvantages in some listening
`situations. Modern directional systems all provide a method
`of switching between a directional mode and a non-direc-
`tional mode so that the wearer can easily put the hearing aid
`in the appropriate mode for each listening situation. Many
`researchers believe that the primary reason for the wide accep-
`tance of modern directional systems is this flexibility.
`This article explains the different technologies that can be
`used to create directional microphone patterns. Regardless of
`the exact technology, all directional microphone patterns have
`the same major benefits and the same limitations.
`
`Directional hearing aids,
`the subject of this special
`issue of The Hearing Jour-
`nal, all include two or more
`microphones. All have
`both a non-directional
`mode of operation and one
`or more directional modes
`of operation. They all con-
`tain at least one omnidi-
`rectional microphone for
`use in those environments where it is needed. The second
`microphone may be either a two-port directional micro-
`phone or another omnidirectional microphone. When two
`omnidirectional microphones are used, their electrical out-
`puts are combined to provide the directional pattern. In
`either case, sound from two acoustic ports is used to pro-
`vide a directional response for the hearing aid.
`Of course, it is not strictly correct to call any mode of
`hearing aid operation a non-directional mode, because, in
`operation, a head-worn hearing aid always has some direc-
`tionality from the user’s head in addition to whatever addi-
`tional directionality is provided by the microphone(s). To
`be strictly correct and speak only of the directionality that
`originates from the microphone, we should speak of the
`free field directional behavior of the system. The free field
`behavior of a hearing aid is the behavior of the device when
`it is measured by itself, not on a head, and not placed near
`
`“...regardless of the exact techno-
`logy, all directional microphone
`patterns have the same major
`benefits and the same limitations...”
`
`any other large object that could affect the sound field.
`Figure 1 shows several of the free field directional patterns
`that are possible.
`The directional behavior of the hearing aid can result
`from two different designs of the microphones. In the first
`design, a two-port directional microphone, the direction-
`ality comes entirely from the microphone. This type of
`microphone has been available for use in hearing aids since
`at least 1971. The design of the directional microphone
`can be adjusted by the microphone manufacturer to pro-
`vide a range of directional patterns, examples of which are
`shown in Figure 1.
`In a hearing aid, the microphone then provides a sin-
`gle fixed directional pattern. In order to provide the capa-
`bility to switch to a non-directional mode of operation, a
`second microphone, one with an omnidirectional free
`field pattern, must also be included in the aid. This method
`of using both a directional and an omnidirectional micro-
`phone can be called a “directional with omni” system.
`The second way to provide a directional response in a
`hearing aid is to use two
`omnidirectional micro-
`phones, and to combine
`their electrical output sig-
`nals to provide a direc-
`tional pattern. Methods
`to do this are discussed
`later. Clearly, such a sys-
`tem can provide an
`omnidirectional free field
`pattern by simply using
`either one of the two microphones alone. This type of sys-
`tem can also provide multiple, selectable directional pat-
`terns by changing the way the signals are combined to form
`the pattern.
`To better understand the operation of directional micro-
`phones, it is necessary first to understand the operation of
`the standard, non-directional microphone.
`
`THE OMNIDIRECTIONAL MICROPHONE
`A diagram of an omnidirectional hearing aid microphone
`of a type manufactured by Knowles Electronics is shown
`in Figure 2. The microphone is basically a closed box that
`is divided into two small volumes by a thin polymer
`diaphragm.
`Sound pressure enters the microphone through a small
`tube shown at the left, and then travels to the region called
`the “front volume” of the microphone. In the front vol-
`
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`ume, the sound pressure creates a small
`motion of the diaphragm. On the other
`side of the diaphragm, the “back volume”
`contains a metal plate that is coated with
`an electret material that holds a perma-
`nent electrical charge. The motion of the
`diaphragm near the charged back plate
`creates a small electrical signal that is
`amplified to become the electrical output
`signal of the microphone.
`
`THE DIRECTIONAL
`MICROPHONE
`The directional microphone is very sim-
`ilar to the omnidirectional microphone
`described above except that it has a sec-
`ond sound entry port. In use, the two
`ports are normally aligned horizontally,
`along a line that points in the direction
`the user is facing. Thus, one port is called
`the “front port” and the other the “rear
`port” or “back port.”
`As Figure 3 shows, the front port brings
`sound into the front volume of the micro-
`phone and the back port brings sound
`into the back volume. This rear port often
`contains a screen resistance, whose pur-
`pose will be discussed below. Since both
`sides of the diaphragm now have an
`acoustic pressure, the diaphragm motion
`
`is driven by the difference in acoustic pres-
`sure between the front and back volumes.
`Consequently, a directional microphone
`of this type is often called a pressure-dif-
`ference microphone or a pressure-gradi-
`ent microphone.
`If the acoustic pressure in the front and
`back volumes were the same, then the
`pressure difference would be zero and the
`microphone would have no output. Luck-
`ily, this does not occur because of the sep-
`aration between the two sound entry
`ports. This means that for sounds that
`originate in front of the user the sound
`arrives at the front port a little sooner than
`it arrives at the back port. This time delay
`causes a small phase shift between the pres-
`sure signals in the front and back volumes.
`As Figure 3 illustrates, the time delay
`and phase shift vary for different arrival
`angles of sound. For each different arrival
`angle, there is a different time delay and
`a different output for the microphone.
`The small pattern at the center of Figure
`3 shows the case where no damping screen
`is used in the microphone. Sounds com-
`ing directly from the side have no time
`delay, and therefore the microphone has
`no output in this direction. When a
`damping screen is used, its resistance value
`can be selected to give any of the free
`field patterns shown in Figure 1.
`The subtraction process that takes
`place in the difference microphone
`generally causes the microphone sen-
`sitivity to be significantly lower than
`that of an equivalent omnidirectional
`microphone. Figure 4 shows the fre-
`quency response of the sensitivity of
`a directional microphone and of a
`similar omnidirectional microphone.
`The decrease in sensitivity is signif-
`icant, especially at low frequencies,
`and is greater when the separation
`of the two ports is smaller.
`
`Figure 1. Free field directional responses
`that are possible with directional micro-
`phones or dual-microphone processing.
`
`Figure 2. In a simple microphone, sound pressure
`enters the front volume and causes a small vibration
`of the diaphragm. The directional microphone has
`two ports. The diaphragm is driven by the pressure
`difference in the front and back volumes.
`
`DUAL-MICROPHONE
`PROCESSING
`The other approach to achieving a
`directional pattern in a hearing aid
`is to use two omnidirectional micro-
`phones and combine their outputs
`to create the directional signal. Fig-
`ure 5 shows the general form of the
`processing that is needed to produce
`a directional pattern. The output sig-
`nal of one microphone is subtracted
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`from that of the other. The specific direc-
`tional pattern may be changed among the
`possibilities shown in Figure 1 by adding
`a simple delay circuit, as shown in Figure
`5. The electrical processing can be done
`in either analog or digital circuitry within
`the hearing aid.
`The two methods of obtaining direc-
`tionality have much in common. Figure 6
`
`shows functional block diagrams of the two
`methods. In the two-port directional micro-
`phone, the acoustic signals from the two
`ports subtract by driving the diaphragm
`on its opposite sides. The difference signal
`is then converted to an electrical signal by
`the microphone. In the dual-microphone
`directional system, the acoustic signals at
`the two microphone ports are each con-
`
`Figure 3. Sound from different directions arrives at the two ports with different time delays
`and generates different microphone responses.
`
`Figure 4. The frequency response for a directional microphone, or for two microphones
`processed to give directivity, falls at low frequency.
`
`verted to electrical signals and then sub-
`tracted in the electrical circuits.
`In each system, the specific pattern
`shape (one of the options shown in Fig-
`ure 1) is controlled by a a component in
`the design of the system. For the two-port
`directional microphone, it is the damp-
`ing screen in one of the microphone ports
`that forms part of an acoustic filter to set
`the pattern. In the dual-microphone sys-
`tem, the pattern is set by the characteris-
`tics of an electrical filter.
`The functional similarity of the two
`methods means they are also alike in
`many aspects of their performance. For
`example, the frequency response curves
`of Figure 4 apply to both designs. The
`comments on internal noise and on wind
`noise later in this article also apply equally
`to both designs.
`One way in which the systems differ
`is that the pattern of the two-port direc-
`tional microphone is set by the micro-
`phone manufacturer through the selection
`of the damping resistor at the time of
`manufacture. In the dual-microphone
`system, it is possible to change the char-
`acteristics of the electrical filter under
`control of the DSP to obtain different
`directional patterns in different situations.
`Thus, a hearing aid can provide a num-
`ber of patterns that can be selected under
`manual or program control, or it can pro-
`vide an adaptive directional system that
`continually modifies the pattern with the
`objective of maximizing the SNR in a
`changing environment.
`
`MICROPHONE MATCHING
`One important consideration in the
`design of a hearing aid with two omni-
`directional microphones is unique to that
`design. When there are two microphones,
`their inherent sensitivities must be well
`matched for the patterns to have their
`intended shapes. Significant microphone
`mismatch can degrade the patterns in
`unintended ways.
`Hearing aid manufacturers can do sev-
`eral things to ensure that the microphones
`are well matched. First, they can purchase
`microphones in matched pairs from the
`microphone component supplier. To do
`this, the microphone manufacturer must
`measure the sensitivity of all microphones
`and select the units of a matched pair to
`have sensitivities that match within a spec-
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`ified tolerance.
`A second approach to sensitivity
`matching is to compensate for the differ-
`ences that remain by adjusting the gain
`of the amplifier for one microphone rel-
`ative to the other. Hearing aid manufac-
`turers commonly do this as a final step in
`the assembly of a hearing aid.
`A final possibility, called “dynamic
`matching,” is performed in some DSP
`aids. Using dynamic matching, the proces-
`sor constantly examines and compares the
`relative sensitivity of the two microphones.
`If they are perfectly matched, the signals
`from the two microphones should have
`the same overall level and frequency
`response. Any differences in level or
`response can, in principle, be corrected
`by changing the gain and frequency
`response of the processing for one of the
`microphones. While no such process can
`function perfectly, the various forms of
`dynamic matching may provide an impor-
`tant improvement in matching over the
`life of the aid.
`Of course, it is essential that the micro-
`phones maintain their match throughout
`the life of the hearing aid. Since the first
`dual-microphone hearing aids were intro-
`duced, there has been an underlying con-
`cern that a relative “drift” in the sensitivity
`of the microphones could degrade the
`instrument’s directional performance.
`Perhaps the most significant source of
`performance drift in the field is partial or
`complete clogging of the microphone ports
`with debris. It seems inevitable that sig-
`nificant port clogging will affect the pat-
`terns. Anecdotal evidence from hearing
`aid manufacturers does not indicate a sig-
`nificant problem with directional perfor-
`mance drift in microphones that are not
`clogged or otherwise obviously defective.
`However, further studies are warranted.
`
`DISADVANTAGES OF
`DIRECTIONALITY
`Internal noise
`To this point, we have not mentioned any
`disadvantages of directional patterns in
`hearing aids. However, there are two unde-
`sirable features of directional patterns,
`which in certain circumstances make it
`best to switch the hearing aid to an omni-
`directional mode.
`One disadvantage is internal noise
`
`from the hearing aid. In a quiet environ-
`ment, internal noise may be noticeable.
`Several studies have shown that the low
`level of internal noise that is inevitably
`generated in the microphone and ampli-
`
`fier circuits is not normally audible to the
`hearing-impaired user.1,2 However, these
`studies have assumed that an omnidirec-
`tional microphone is used in the aid.
`When a directional microphone or a
`
`Figure 5. A directional pattern is generated by two omnidirectional microphones whose
`outputs are subtracted.
`
`Figure 6. The two ways of obtaining directionality do the same things in a slightly different
`order. The two-port directional microphone does it all in the acoustics of the microphone.
`The dual-microphone system does the same things electrically.
`
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`phone is located very close to the source
`of the noise, while for normal hearers the
`eardrum is farther from the source and
`shielded somewhat from it at the end of
`the ear canal. This affects the signal level
`of all microphones, whether or not they
`are directional.
`Secondly, directional patterns are rel-
`atively more sensitive to sounds from the
`near field than to sounds from farther
`away. This is why wind noise is received
`at a much higher level in directional
`microphone patterns than in omnidirec-
`tional patterns. Also, note that the wind
`
`noise effect is the same in a dual-micro-
`phone directional implementation as in
`a two-port directional microphone.
`For the directional hearing aid user,
`this means that the aids should be
`switched to the non-directional mode in
`wind noise.
`
`SECOND-ORDER DIRECTIVITY
`The directional patterns described so far
`are all in a category called “first-order dif-
`ference patterns.” One method of further
`improving the SNR in a noisy environ-
`ment beyond the level that can be achieved
`
`pair of matched microphones is used to
`form a directional pattern, the sensitivity
`falls (see Figure 4). The sensitivity affects
`the level of the sensed signal, but the inter-
`nal noise level is approximately the same
`as that of an omnidirectional microphone.
`Consequently, the signal-to-noise ratio
`(SNR) at low frequencies is significantly
`lower in a directional pattern.
`A hearing aid that uses low-frequency
`amplification to enhance the audibility of
`low-frequency signals will naturally
`amplify the low-frequency internal noise
`as well. Figure 4 indicates that perhaps 20
`dB of additional amplification is needed
`in a directional pattern to achieve the same
`level of audibility as in an omnidirectional
`microphone. This amount of amplifica-
`tion is quite likely to raise the level of inter-
`nal noise above the environmental
`ambient noise in a quiet environment.
`Thus, the omnidirectional pattern
`is more appropriate in quiet.
`In a noisy environment where
`the benefits of the directionality
`are needed, the internal noise of
`even the directional pattern will be
`well below the ambient level. In
`hearing aids that allow the user to
`control the directional mode man-
`ually, the user must be counseled
`that the directional mode should
`generally not be used in quiet sit-
`uations. Some hearing aids provide
`for automatic switching from the
`omnidirectional to the directional-
`mode in backgrounds where the
`noise level exceeds a manufacturer-
`determined threshold.
`Wind noise
`The presence of wind noise can create
`another situation in which directional
`microphone patterns seem noisier than
`omnidirectional patterns. In this case, the
`noise is not generated by the microphone,
`but originates in the turbulent air flow as
`wind moves past the head and shoulders.
`We all hear wind noise in even a moder-
`ate breeze, and it can be quite loud in
`many situations.
`An explanation of wind noise is pre-
`sented in the box on this page. The impor-
`tant feature is that the noise is mostly
`generated within 10 cm of the ear.3
`This wind noise presents two difficul-
`ties for hearing aid users. First, the micro-
`
`WIND NOISE: WHAT IT IS AND WHY IT’S A PROBLEM
`The term “wind noise” is used to describe several different ways that wind can gen-
`erate sound. For example, wind can cause a loose shutter to bang against a house
`or it can cause a flag to rustle and snap. In these cases, the wind has caused an
`object to move, and the motion makes a sound.
`In other cases, wind moving past an
`object can create a howling sound, even
`though the object does not vibrate. Here,
`the sound is caused by turbulence that
`is created in the moving air as it passes
`by the object. This turbulence, which
`cannot be seen, is very similar to the tur-
`bulence in a fast-moving stream as the
`water flows around and over large rocks.
`We have all experienced this kind of
`wind noise while inside a house during
`a windstorm. The sound of the howling
`wind originates in the turbulence of air
`motion past the walls and roof.
`The form of wind noise that most inter-
`feres with our ability to hear and commu-
`nicate is the noise generated by air flow
`around our own head. Here the sound
`is generated within centimeters of our
`ears, and may be heard at quite a high
`level because of this close proximity.
`Studies of this phenomenon have been performed by researchers at the National
`Acoustics Laboratories (NAL) near Sydney, Australia.3 Figure 1A shows a photo of
`KEMAR in the NAL wind tunnel. A thin layer of smoke in the wind shows the char-
`acter of the air flow as the wind moves from left to right in the figure.
`Before it encounters the head, the wind is moving smoothly, and the smoke streams
`in a straight line. After passing KEMAR, the smoke spreads out considerably following
`the swirls and eddies in the turbulent air flow. For different angles of wind incidence,
`turbulence may be generated by the pinna and be even closer to the ear.
`Turbulent generation of noise very close to the ears can have a major impact on
`the ability to understand sounds from more distant sources. This is true for people
`with normal hearing as well as for hearing aid wearers. However, wind noise may
`be more problematic for people wearing hearing aids, because the microphone is
`closer to the wind noise source and is not as effectively shielded as the eardrum at
`the end of the ear canal. The noise source close to the head is especially problem-
`atic for directional hearing aids because the directional patterns are relatively more
`sensitive to sounds from the near field than from farther away. An omnidirectional
`microphone pattern has much better SNR in wind noise situations.
`
`Figure 1A. KEMAR head in a wind tunnel, with wind
`from the right. Wind is initially very smooth and quiet.
`After passing the head there is considerable turbulence
`and noise that originates close to the head.
`(Photo courtesy National Acoustic Laboratories.)
`
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`the higher frequencies that are more
`important for speech intelligibility with
`a first-order pattern at lower frequencies.
`The first commercial example of a sec-
`ond-order system with three microphones
`uses this type of hybrid processing.4
`
`Stephen C. Thompson, PhD, is Senior Member of Technical Staff
`in the R&D Department at Knowles Electronics LLC. Correspondence
`to Dr. Thompson at Knowles Electronics LLC, 1151 Maplewood Drive,
`Itasca, IL 60143; steve.thompson@knowles.com.
`
`REFERENCES
`1. Agnew J: Audible circuit noise in hearing aid amplifiers.
`J Acoust Soc Am 1997;102(5, part 1):2793-2799.
`2. Lee LW, Geddes ER: Perception of microphone noise in
`hearing instruments. J Acoust Soc Am 1998;104(6):
`3364-3367.
`3. Dillon H, Roe I, Katch R: Wind Noise in Hearing Aids.
`Presentation at the Australian Hearing Symposium,
`Hearing Aid Amplification for the New Millennium,
`November 1999, Sydney, Australia.
`4. Powers TA, Hamacher V: Three-microphone instrument
`is designed to extend benefits of directionality. Hear
`J 2002;55(10):38-45.
`
`with the first-order patterns of Figure 1
`is to combine the signals from either two
`directional microphones or three omni-
`directional microphones.
`The outputs from these microphones
`are combined in a way called second-order
`difference processing. In an ideal system,
`this could provide a little more than 3 dB
`improvement on the directivity index com-
`pared to the theoretically ideal first-order
`directional system. Figure 7 shows exam-
`ples of the free-field patterns that can be
`obtained from a second-order system. The
`patterns are noticeably narrower than the
`first-order patterns of Figure 1 and would
`provide improved reduction of noise.
`The challenge with a second-order sys-
`tem is that microphone matching
`becomes significantly more important.
`Small errors that can easily be tolerated
`in a first-order system are unacceptable in
`a second-order system. To overcome this
`challenge, a hybrid system can be used
`that combines a second-order pattern at
`
`Figure 7. Examples of free field patterns
`from a second-order directional system.
`
`Circle 102 on Reader Service Card
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`November 2003 • Vol. 56 • No. 11
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`Directional microphone hearing aids
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`The Hearing Journal
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