Microphones for Hearing Aids
In a typical audio application, a
microphone
is the device which captures sound waves as they are emitted from their source and passes them on to be
converted into electrical signals for a variety of uses. In a
noise cancellation application
, microphones are used to register noise waves which need to be silenced. The
microphones
pass these waveforms on into the system to generate the anti-noise waveforms which are then emitted to
cancel out the noise.
Microphone Preamplifiers for Hearing Aids
A
microphone pre-amplifier
boosts signals, typically to line-level (the level needed to transmit analog signals point-to-point, between
devices.) Further amplification may be necessary. The noise performance of a pre-amp is critical since the
final signal-to-noise ratio of downstream amplifiers can be magnified. For an audio input application, such
as microphones, the pre-amp is physically mounted near the sensor that feeds it, to reduce noise and
interference.
Audio Codecs for Hearing Aids
A
codec
is a device that can encode and/or decode a digital data signal. A codec encodes data for transmission
and/or decodes it for playback. Essentially, a codec has one or more
Digital-to-Analog data converters
(DAC) and
Analog-to-Digital converters
(ADC) in a single package. A codec is essential for the use of
Digital Signal Processors
(DSPs) in audio applications, since it can convert real-world analog signals (like sound) to digital signals
for the DSP, and back again from digital to analog for the human ear.
Audio Amplifiers for Hearing Aids
A
power
or
audio amplifier
is typically used to greatly increase the signal strength, or amplitude, of a current or voltage signal. In
audio applications, late stage "power" amplifiers in a signal chain can be used to increase the power output
of a signal such that the signal can physically move, or drive, the diaphragms in a loud speaker.
DSPs for Hearing Aids
A
digital signal processor
(DSP) is a specialized microprocessor usually with a modified Harvard architecture and a single cycle
multiply-and-accumulate required for the fast operational needs of processing and for mathematically
manipulating real-world signals like voice, audio, and video. In a
noise cancellation
application, the DSP examines the characteristics of the input noise waveform. The DSP then generates the
anti-noise waveform which effectively negates the input noise waveform. The human ear then hears less
"white" noise as the cancellation occurs in real, or near-real time.
RF Interfaces for Hearing Aids
An
RF
, or
Radio Frequency interface
, is a wireless method to transmit information via high frequency radio waves, through the air.
Wireless
technology enables the transfer of information over short or very long distances without cables. Wireless
communication is possible using a wireless transmitter and corresponding receiver. If both devices are in
the same package it is called a transceiver. A
wireless receiver
refers to the receiving end of the information transfer and requires less energy to operate than the active
transmitting portion where the transfer originates.
Power Management for Hearing Aids
Power management
for a hearing aid is a crucial technology element. Size must be as small as possible and nothing can produce
heat that will injure the wearer. Power management objectives include: small size, low heat,
ultra low power
so the device is long lasting on a single coin-sized battery, and overall reliability and low noise
introduction from collective systems.
AC/DC Adapters for Hearing Aids
An
AC/DC adapter
is a type of external power supply, often enclosed in what looks like an over-sized AC wall-plug. Other
names include power adapter, power converter, plug-in adapter, adapter block, domestic mains adapter, line
power adapter, or simply AC adapter. AC adapters are used with electrical devices that require DC power but
do not have the internal circuitry to accomplish the conversion of AC to DC.
Medical-grade power supplies
are designed to meet the
IEC60601-1
medical equipment safety standard and may include typical output voltages of 3.3 to 48 Vdc, a variety of
package style (open-frame, enclosed, encapsulated, etc.), mounting options, thermal management, and
environmental features, among others.
Battery Chargers for Hearing Aids
Battery charging
circuits are used to recharge batteries and are available in linear or switching topologies. They can be
completely autonomous in operation or used with a microcontroller. Generally speaking, integrated chip
charging technology can produce charge currents in a range from around 625 nA, up to around 4.5A. Battery
charger ICs often do more than just charge; they can protect from overcharging, regulate voltage, and manage
charging from irregular sources such as energy harvesters or very low voltage sources. In portable
technology, battery chargers are indispensable.
Fuel Gauges for Hearing Aids
A
battery fuel gauge
, or state-of-charge (SOC) indication, has evolved from a simple warning to a more complex system level use
of the information, such as soft shutdown to prevent data loss. Considerations that affect accurate
accounting for remaining battery charge include the effects of aging, self discharge and temperature
variations. The value for full capacity is obtained and algorithms are used to calculate the remaining
capacity. The drawback of this approach is that self-discharge is difficult to model since it is a function
of aging and temperature.
Switches for Hearing Aids
Modern hearing aids include In the Ear (ITE) and Behind the Ear (BTE) models. Various switches are used on
these products to adjust volume, change memory programs, or modify settings. Important design factors to
consider include haptics, small form factors, cycle life, and resistance to ear wax, sweat, and other body
chemicals.
