The interaural timing and intensity differences are used by the auditory system to determine the location of the source relative to the head. The sound is, then, concentrated by the eardrum and middle ear bones. These latter structures affect the upper and lower limits of hearing and their effects are included in AIM, but they do not set up any new dimensions and they are not central to our research. LopezPoveda and Meddis have developed a new model of the transfer function of the concha including diffraction, reflection and interaction phenomena. The basilar membrane and outer haircells in the cochlea together perform a frequency analysis of the pressure wave.
Frequency is the second dimension of hearing.
The surface defined by the set of lines is the model's representation of basilar membrane motion in response to this vowel. The filter has an asymmetric amplitude response and the asymmetry can be varied with level to explain auditory masking over a wide range of frequencies and levels. The new version of the filterbank will appear as an update in AIM R8.2. The mechanical response of the basilar membrane to sound can also be simulated using a nonlinear filter system recently developed by O'Mard and Meddis. The system is characterised by a compressive response near CF and a linear response below CF, a shift of best frequency at high signal level, wider filters at high signal levels, combination tones, and two-tone suppression.