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, ANSI S3.22-1987, specifies the electroacoustic tests that a manufacturer must perform and publish for each hearing aid before the instrument is shipped. 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 to 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 can improve 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.

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