Technical Features Of Digital Hearing Aids
- A microphone picks up sound from the environment and converts it into an
electrical signal, which it sends to the amplifier.
- An Pre-amplifier
increases the volume of the sound and sends it to the
Digital Signal
Processing chip (DSP).
- The
DSP chip processes the sound according to its
internal program and hardware features and then the digitized result is sent to the receiver (or
speaker).
- A receiver/speaker
changes the electrical signal back into sound and sends it into the ear
where the eardrum then sends the impulses to the middle ear and the inner
ear.
- A battery
provides power to the hearing aid.
The Hearing Aid Microphone
Microphone Technology
Using special microphones or phase cancellation
signal processing, it is possible to configure a hearing aid
so that sounds from the side, and especially the
back of the hearing aid user are amplified less than sounds
originating from the front. Such technology can serve as a type
of "spatial" noise reduction if the user is correctly positioned.
Directional technology can be implemented on all hearing
aid styles except CICs (because of size constraints). However, if
implemented on a hearing aid, directional technology does not improve
localization of sounds.
Research has shown that many hearing aid users prefer directional
technology for listening in noise, usually when:
- the noise originates from behind the
listener
- the talker is in front of the listener
- the listener is close to the talker
- the room has low reverberation.
Some hearing aids automatically switch to such a
directional mode when the signal type and/or input intensity
are matched to the characteristics of the signal processing algorithm in the
hearing aid's DSP (Digital Signal Processing ) chip.
Such adaptive directional hearing aids can automatically track a
dominant single noise source (e.g., someone speaking to a hearing aid wearer
in a crowded room), by keeping the signal amplification from such a source
high while minimizing other sound sources.
Digital Signal Processing
Until recently, the majority of hearing aids
utilized analog signal processing. This has changed rapidly the past
few years, and today, nearly all hearing aids sold
in the U.S. utilize digital signal processing. The advantage of
digital processing is that less space is required,
allowing manufacturers to include many more "programmable
features" in a small package. Through the use of
digital signal processing, the hearing aid can conduct an
analysis of an incoming signal, and make a
reasonable classification of the content--speech versus broad-band
noise versus acoustic feedback (whistling) versus
music, for example. This classification can then be used to
trigger automatic activation of other special
features.
Digital Noise Reduction
One of the major capabilities of digital hearing aids, it
that it is possible for them to analyze an incoming signal and differentiate
speech from any other noises out their. This powerful capability is a
feature of what are called DSP's or "Digital Signal Processor " chips. These
powerful processors can sample incoming sounds at the unbelievable rate of
2.5 million samplings per second. This can be
accomplished simultaneously in several channels. If the
dominant signal is believed to be noise in a given
channel, there is a reduction in gain (amplification).
Recent technology has allowed hearing aids to filter out noises at a
party or restaurant so that dialog can be heard.
Adaptive Feedback Cancellation
Acoustic feedback (whistling from the hearing aid)
can be annoying, embarrassing, and in some cases, prevent the hearing aid wearer from using the correct
amount of gain. Many of today's hearing aids have an automatic
feature that quickly detects acoustic feedback and
cancels it. This feature is designed to manage transitory
feedback (e.g., caused by placing one's hand or a
telephone next to the ear), and is not a solution to a poorly
fitted ear mold or hearing aid.
Automatic Gain Control--Output (AGCo)
AGCo or output compression is used to put a
"ceiling" on loud sounds. It handles the output after the
amplifier, and can be adjusted to correspond to
the patient's threshold of discomfort (maintaining sounds
below this level).
Automatic Gain Control--Input (AGCi)
AGCi, or input compression, often referred to wide
dynamic range compression (WDRC) is used to "repackage"
the speech signal (and other incoming sounds) to
correspond to the reduced dynamic range of the hearing
aid user. That is, if the incoming sounds have a
60 dB range, and the patient only has a 30 dB range of useful
hearing, the sounds might be "compressed" by 2:1
to fit into the useful auditory region. The notion is that most
people with a hearing loss need more gain for soft
sounds than for average, and more gain for average sounds
than for loud. WDRC accomplishes this
automatically--in fact, if the WDRC is programmed correctly across
frequencies, many hearing aid users have little
need for a volume control.
Automatic Switching
Some hearing aids automatically switch to a
directional mode when the signal type and/or input intensity are matched to the characteristics of the algorithm. dominant single noise source (e.g., a car passing by
someone on a sidewalk), attempting to provide maximum reduction in gain toward the location of the source.
Multiple
Equalizer Bands
The majority of today's hearing aids have multiple
channels. Each channel represents a portion of the frequency
range important for understanding speech. One
advantage of multiple channels is that features such as gain
and compression can be programmed differently to
reflect changes in the patient's hearing across frequencies.
Multiple channels also are useful for implementing
other features such as digital noise reduction and feedback
cancellation (which will be discussed later). There
is no consensus regarding how many channels are enough
(or how many are too many)--to some extent, this
depends on the feature utilized within the channels.
Multiple Memories (Channels)
A memory is a location to store hearing aid settings
that are designed for a particular listening situation. It is
common for hearing aids to have two or three
memories. For example, in a hearing aid with three memories,
it is common that memory one will be for listening
in quiet, memory two will be for listening in noise, and
memory three will be for telephone. On the other
hand, many hearing aid users find that a single memory
works in a variety of listening situations, and may
only use one memory. Changing memories is accomplished
>by using a button (or toggle switch) on the hearing
aid, with a remote control device. In some digital hearing
aids, it happens automatically.
Telecoils
With this special circuit, electromagnetic signals
can be picked up from the handset of the telephone and
amplified in a manner similar to the amplifying
function of the hearing aid. Although many hearing aid wearers
report benefit with this circuit, there is
substantial variability across hearing aids. Telecoils are not available
in some smaller custom-made models due to space
limitations. Often, hearing aids with multiple memories
will devote one memory to the telecoil. In these
aids, the telecoil can be accessed through a push button on the
hearing aid or by the use of a remote control device.
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