Understand Hearing Aid Specifications and Terminology


Audiogram is a graphic record of hearing ability for various sound frequencies that is used to measure hearing loss.

Automatic Gain Control (AGC) is a feedback system capable of adjusting the gain to an appropriate level for a wide range of input levels. For example, AGC can effectively reduces the output when the input signal is strong and raises the output signal when the input is weak.

Battery Current Drain is an indication on how long a battery would last in the device even if it is in storage.

Equivalent Input Noise Level is a rating of the overall noise performance of the hearing aid. It is a measure of how much noise a device will add to the input signal. For a hearing aid, it should be below 30dB.

Frequency Response shows the gain or amplification of the hearing aid at various frequencies. This should closely match your audiogram and is probably the single most important parameter.

Full On Gain (FOG) is the gain when the volume control is set to maximum. For example "HF Average Full-on Gain/Input 50dB SPL" refers to the average gain at high frequency for a 50dB input when the volume control is at its maximum. Normal conversation is at the 50dB level.

Harmonic Distortion occurs when new frequencies are generated that are harmonics of the original signal. Total harmonic distortion is a measure of the total harmonics generated for some standard reference input.

Hearing Level is a measurement of how well a person hears. An audiometer is used to obtain this measurement, which is always described in decibels (dB HL).

Induction Coil Sensitivity is how well the induction coil or telephone works. The induction coil prevents whistling (feedback) and stops outside noise from interfering with the sound from the telephone.

Output Sound Pressure Level 90 (OSPL90) is the output saturation sound level (SSPL) to a 90dB sound input and is measured over a frequency range. This is also called SSPL90. 90dB sound is very loud, equivalent to sound generated by a lawnmower or screaming child.

Reference Test Gain is the amount of gain with a 50dB input sound and the volume control at normal level.

Saturation Sound Pressure Level (SSPL) a measure of the maximum output of a hearing aid. For example, "SSPL90: <135dB" means that for a input of 90dB sound into the hearing aid, less than 135dB sound will come out. "HF Average SSPL 90" would refer to the average level of high frequencies (HFA) with a 90dB input. 90dB is a very loud sound and is frequently used as an input to test the limit of a hearing aid.

Sound Pressure Level (SPL) is a logarithmic measure of the effective sound pressure (loudness) of a sound relative to a reference value. It is measured in decibels (dB) above a standard reference level. The commonly used "zero" reference sound pressure in air is 20 ÁPa RMS, which is usually considered the threshold of human hearing at 1 kHz.

T-coil receiver within a hearing aid is a tiny piece of metal wrapped tightly with coils of copper. The T-coil detects and converts magnetic energy into electrical energy in much the same way that a microphone converts acoustic energy into electrical energy. T-coils can be thought of as mini-antennas, which receive magnetic information. Therefore, the T-coil receiver serves essentially the same purpose as the microphone.