The basic components of a contemporary hearing aid include a microphone, an amplifier, a receiver, and a power supply.A microphone is a transducer that converts the sound signal into electrical energy. The amplifier is a transformer that increases the amplitude of the electrical signal that is sent to the receiver. The receiver then changes the modified electrical signal back into sound energy that is directed into the ear.
A variety of microphones, amplifiers, and receivers are used, depending on the type and degree of hearing loss. The American National Standard Specification of Hearing Aid Characteristics specifies the electroacoustic tests that a manufacturer must perform and publish for each hearing aid before the instrument is shipped.1 The standard states the tolerance allowed so that the audiologist can perform the same tests to verify the performance of an instrument against specifications.
Currently used hearing aid microphones are primarily electrical devices that have good linear behavior over a frequency range of 50-6000 Hz. This range can be modified to be more appropriate for specific hearing losses.
Directional microphones have been developed that can vary with both the amplitude and the direction of the sound source relative to the microphone. They can reduce the sounds coming from the back of a hearing aid wearer compared with the sounds coming from the front by as much as 15 dB. This change can greatly improve the signal-to-noise ratio of the listener and thus the understanding of speech in the presence of noise.
Hearing aid amplifiers are transformers primarily composed of transistors that are built into an integrated circuit. These transistors provide a current source and serve a variety of functions. In these transistors, the primary function of the amplifier is to increase the power of the electrical signal received from the microphone.
Typically, hearing aids have 2 or more stages of amplification. The first stage is the preamplifier, which is at the level of the microphone. The preamplifier helps to amplify the initial input signal. At this level, the gain is relatively low.
Most amplification is supplied by the power amplifier. These amplifiers are typed in a particular class. The most common are referred to as class A, class B, and class D. They are distinguished by their power consumption, gain, and output abilities.
Each amplifier can be modified to limit the maximum output of the hearing aid. For linear amplification, the amplifier may be limited by peak clipping. This occurs when the electrical signal exceeds the maximum output of some component of the hearing aid circuit. This type of limiting causes various forms of distortion that have been found to reduce the intelligibility and the subjective quality of speech.
A hearing aid that has some type of level-dependent signal processing is termed a nonlinear hearing aid. Most nonlinear hearing aids reduce gain as input or output levels increase.
Nonlinear hearing aids are designed to amplify a wide range of sounds so that they are audible to the hearing-impaired listener without becoming uncomfortably loud. These aids usually use some form of compression circuit that reduces the gain of the instrument when either the input to the device or the output of the device exceeds a predetermined level. This process results in a comfortable amplification for the wearer and prevents the hearing aid from saturating.
Compression hearing aids can provide amplification of the speech components that are essential for intelligibility and can reduce impulsive or high-level sounds that normally cause discomfort.
The hearing aid receiver is an output transducer and handles more power than a microphone. Receivers in hearing aids are very small because of cosmetic considerations.
In general, larger receivers can supply larger output signals. Therefore, the small
receivers on hearing aids may be taxed to their output capabilities.The receiver must also be chosen to match its amplifier. A mismatch in design produces limited output and increases distortion.
Because of the receiver's open position in the external ear canal, it is vulnerable to damage from debris in the ear canal and from the aid being dropped. Manufacturers state that approximately 40% of hearing aids returned for service have damage or blockage to the receiver.
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