Filters

In signal processing it is often necessary to change the frequency response of a signal to emphasise or dampen certain frequencies, or to restrict a signal’s frequency content to some specific frequency region. This is what filters are for.
 

Band-split filters

One commonly used type of filters in hearing aids is band-split filters, which split the sound signal into more signals of limited frequency ranges.

This subdivision of the signal makes it possible to process for example the region around 500Hz separately from the region around 1kHz.
 

Important filter characteristics

A description of a filter typically includes the following important factors: 

• The cut-off frequency: the frequency where the filter starts to affect the signal significantly 
• The steepness: how much the filter will affect the signal once the cut-off frequency is crossed
• The delay: how much the signal will be delayed at different frequencies by passing through the filter

The cut-off frequency depends on how the filter will be applied. If it is a filter for dampening low frequencies, it will have a low cut-off frequency of, for instance, 70 Hz.

The steepness and the delay are connected. Generally, the steeper the filter, the greater the delay introduced in the frequency response. Therefore, filter design will often involve a tradeoff between obtaining the required dampening or amplification and achieving the lowest possible delay.