# Inner hair-cell (ihcProc.m)¶

The IHC functionality is simulated by extracting the envelope of the output of individual gammatone filters. The corresponding IHC function is adopted from the Auditory Modeling Toolbox [Soendergaard2013]. Typically, the envelope is extracted by combining half-wave rectification and low-pass filtering. The low-pass filter is motivated by the loss of phase-locking in the auditory nerve at higher frequencies [Bernstein1996], [Bernstein1999]. Depending on the cut-off frequency of the IHC models, it is possible to control the amount of fine-structure information that is present in higher frequency channels. The cut-off frequency and the order of the corresponding low-pass filter vary across methods and a complete overview of supported IHC models is given in Table 20. A particular model can be selected by using the parameter ihc_method.

Table 20 List of supported IHC models
ihc_method Description
'hilbert' Hilbert transform
'halfwave' Half-wave rectification
'fullwave' Full-wave rectification
'square' Squared
'dau' Half-wave rectification and low-pass filtering at 1000 Hz [Dau1996]
'joergensen' Hilbert transform and low-pass filtering at 150 Hz [Joergensen2011]
'breebart' Half-wave rectification and low-pass filtering at 770 Hz [Breebart2001]
'bernstein' Half-wave rectification, compression and low-pass filtering at 425 Hz [Bernstein1999]

The effect of the IHC processor is demonstrated in Fig. 24, where the output of the gammatone filter bank is compared with the output of an IHC model by running the script DEMO_IHC.m. Whereas individual peaks are resolved in the lowest channel of the IHC output, only the envelope is retained at higher frequencies.

 [Bernstein1996] Bernstein, L. R. and Trahiotis, C. (1996), “The normalized correlation: Accounting for binaural detection across center frequency,” Journal of the Acoustical Society of America 100(6), pp. 3774–3784.
 [Bernstein1999] (1, 2) Bernstein, L. R., van de Par, S., and Trahiotis, C. (1999), “The normalized interaural correlation: Accounting for NoS thresholds obtained with Gaussian and “low-noise” masking noise,” Journal of the Acoustical Society of America 106(2), pp. 870–876.
 [Breebart2001] Breebaart, J., van de Par, S., and Kohlrausch, A. (2001), “Binaural processing model based on contralateral inhibition. I. Model structure,” Journal of the Acoustical Society of America 110(2), pp. 1074–1088.
 [Dau1996] Dau, T., Püschel, D., and Kohlrausch, A. (1996), “A quantitative model of the “effective” signal processing in the auditory system. I. Model structure,” Journal of the Acoustical Society of America 99(6), pp. 3615–3622.
 [Joergensen2011] Jørgensen, S. and Dau, T. (2011), “Predicting speech intelligibility based on the signal-to-noise envelope power ratio after modulation-frequency selective processing,” Journal of the Acoustical Society of America 130(3), pp. 1475–1487.