Interacoustics affinity 2.0 - Harmonic Distortion

Affinity
2.0
Additional Information Page 278
3.7.3 Harmonic Distortion
The pure tone components are called harmonics and occur at integer multiples of the fundamental
frequency. The output of a hearing aid, however, is rarely a pure tone and the signal will inevitably contain
elements at different frequencies not included in the input. These components are defined as distortion
products. As a result measuring these is called Harmonic Distortion.
Harmonic distortion can be measured for both microphone and telecoil. It is done by presenting a pure tone
to the hearing aid and analyzing the waveform of the output to measure the distortion components relative to
the total power of the input signal.
The Total Harmonic Distortion (THD) equals the total sum of all harmonics or in the Affinity
2.0
the 2
nd
and 3
rd
harmonic. The result is expressed relative to the wanted output signal component in percent.
It is important to know the harmonic distortion to be assured that the output of the hearing aid is not affected
by unwanted distortion. Distortion can be compared to the cocktail party effect as harmonics (noise) may
mask the wanted speech signal (fundamental frequency). A hearing aid with high distortion will therefore not
provide the client with the ideal speech understanding.
Below is an example:
The three bars represent
the total distortion, 2
nd
distortion, and 3
rd
distortion
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for marked frequency