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Characteristics of Hearing Aids

ODHHS Information and Technical Assistance Series

Characteristics of Hearing Aids
(Source: Starkey Labs, Technical Services)
Although hearing aids come in all shapes and sizes, they all have the same basic components. Sound is picked up from the environment by a microphone. The function of the microphone is to change the sound from an acoustic signal to an electronic signal. The electronic signal is then processed and amplified by the hearing aid. The hearing aid volume is adjusted with a volume control.
All hearing aids require a battery to supply power to the system. Once the sound has been amplified, it goes to a receiver. The receiver works like a speaker changing electronic signals back into acoustic sound. The sound is then delivered to the ear canal through a tube in the earmold or hearing aid shell. Most hearing aids also include on-off switches. Some may also have one to four screw-set controls, telephone pick-up switch, and/or direct audio input.
Characteristics of Hearing Aids
Hearing aids are usually described in terms of their "electroacoustic characteristics." The procedures for testing hearing aids are standardized by the American National Standards Institute [ANSI S3.22-1989]. These standards require that hearing aids be tested in a sound treated chamber which meets its specifications.
The hearing aid must be attached to a microphone in the test box with a 2cc coupler. Although the 2cc coupler measurement does not represent the hearing aid performance in a real ear, it does provide a standardized system for comparing different hearing aids. The primary features of a hearing aid which are used to describe its function include:
Saturation Sound Pressure Level [SSPL]
The SSPL [also called SSPL 90] is a measure of the maximum output of a hearing aid, the level beyond which the hearing aid cannot amplify. It is important that the maximum output of the hearing aid does not exceed the Uncomfortable Loudness Level [UCL] of the hearing aid wearer. UCL is measured in Hearing Level [HL] and hearing aids are measured in Sound Pressure Level [SPL]. In order to convert from HL to SPL the conversion is +20 dB [for speech only]. For example, a patient with a UCL of 90 dB HL for speech would be uncomfortable with a hearing aid with an SSPL 90 greater than 110 dB SPL.
Full-on Gain
The term "gain" refers to the difference between the input signal and the output of the hearing aid. Gain is the amount of amplification provided by the hearing aid --- how much power it adds to the environmental signal. "Full-on" refers to the volume control setting. The full-on gain is the maximum amount of gain that the hearing aid can generate.
Frequency Response
It is assumed that the hearing aid wearer does not use the hearing aid with the volume set to full-on. The reference test gain [RTG] is a method of approximating the actual use gain of the hearing aid.
Hearing aids can vary in frequency from broad band, to high frequency emphasis, to low frequency emphasis. In order to give an idea of what the frequency range of a hearing aid is, the High Frequency Average HFA of the frequency response curve [obtained at RTG setting] is determined [the HFA is the average output of 1000 Hz, 1600 Hz, and 2500 Hz].
Total Harmonic Distortion
Equivalent Input Noise
The Equivalent Input Noise is the amount of noise which is present in the hearing aid without any input present. The electronic current passing through the amplification system of the hearing aid will cause a low level of noise. Higher gain instruments tend to have more input noise. This is not usually a problem because the wearer of such a high gain hearing instrument will not hear the noise. Ambient noise present in any room is usually enough to cover any noise generated by the hearing aid circuit. Typically the equivalent input noise is less than 3--35 dB SPL.
Battery Current
The battery current of different hearing aids can vary widely. It is important to have an estimate of battery current so that the hearing aid battery life can be predicted. Battery current is tested with the volume control in reference test gain position and with an input of 1000 Hz tone at 65 dB SPL. Often the hearing aid test equipment will have a "battery pill" which draws current from the hearing aid. Battery current values can range from 0.2 mA [milliAmps] to 5.0 mA and can be as high as 15-20 mA for body style hearing aids.
Other Hearing Aid Parameters
All hearing aids are tested with the above requirements. In addition to these, any hearing aid with a telephone telecoil can have telecoil frequency response measured. Also, automatic gain control [AGC] hearing aids require testing of the input-output characteristics and attack/release times.
All of these tests are required to be performed according to the ANSI standard for hearing aids. Some variation will occur from one hearing aid to another. ANSI also states tolerances for this variation. It is important that all equipment used to test hearing aids be calibrated on a regular basis. Equipment that is not in proper calibration will produce erroneous results.