page_title: Hearing aid application
section_title: HAA basic principles of work
hierarchical_section_title: Hearing aid application / HAA basic principles of work
caption_attribution_description: English: General signal processing scheme of hearing aid applications
context_page_description: Hearing aid application is software which, when installed on a mobile computational platform, helps with hearing. Mobile devices may include smartphones, tablets or smart watch.
context_section_description: The principles of working HAA are similar to those of traditional hearing aids.
  Most HAA provides two modes: settings mode (passing through the in situ-audiometry procedure) and the hearing correction mode.
  Setup mode is an in situ-audiometry procedure that allows the user to independently measure hearing thresholds using tone audio signals that are generated with a gradually increasing amplitude over a period of some seconds. Audio signals are generated according to the following sequence of frequencies: 125, 250, 500, 1000, 2000, 4000, 6000 and 8000 Hz.
  Some HAA do not have an integrated audiometry system. In this case, the adjustment of the sound amplification level (similar to the equalizer) is made by the user based on their own subjective feelings.
  Hearing correction mode represents an audio signal processing system. The audio signal processing system implements a broadband method for changing the correction of the spectral envelope of a signal using a filter with a finite impulse response whose frequency response is generated based on user-defined hearing thresholds during the in situ-audiometry procedure.
  Figure 2 shows a general signal processing scheme in a hearing aid application.
  Audio input signal, acoustic feedback (echo)  and background noise are captured by the microphone, summed and then divided into sub-band components with a lower sampling rate using an analysis filter bank (AFB).
  Estimation of noise is based on the minima controlled recursive averaging of past values of noise.
  Cleared of noise subband signal is cleared of acoustic echo by adaptive filtering.
  The subband audio components that are cleared of echo and noise are amplified by multiplying by the corresponding coefficients, which are calculated based on the level of input and output audio signals, ambient noise energy, as well as hearing thresholds (hearing loss frequency characteristics) of the user.
  The gain unit also performs the function of dynamic range compression, thereby compensating for the function of non-linear amplification of the human cochlea.
  The processed broadband signal is synthesized using a synthesis filter bank (SFB).
  AFB and SFB are can be realised as DFT-modulated filter banks, which is one of the most popular filter bank type used in modern hearing aids.
  The output signal can be multiplied by the total gain, which provides a comfortable sound level. This coefficient is adjusted by the user using an external controller directly in HAA interface.
  HAA, as a rule, provides high speed (i.e. minimum sound delay) and high sound quality due to a simple signal processing algorithm as well as due to the effective implementation of application functional blocks.
  Some HAA may not have an acoustic cancellation block.
caption: Figure 2 - HAA general signal processing scheme
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