Spatial Audio Processing: MPEG Surround and Other Applications

3.5 Limitations of the human auditory system

3.5.1 Just-noticeable differences in interaural cues

Although the human auditory system is capable of estimating interaural cues with significant accuracy, it has several known limitations as well. For example, interaural cues have to change by a certain amount in order to be detectable. Such minimum required change is often referred to as just-noticeable difference (JND) or threshold.

For example, the just-noticeable change in ILD amounts to approximately 0.5–1 dB and is roughly constant over frequency and stimulus level [101, 123, 191, 282]. If the reference ILD increases, ILD thresholds increase also. For reference ILDs of 9 dB, the ILD threshold is about 1.2 dB, and for a reference ILD of 15 dB, the ILD threshold amounts between 1.5 and 2 dB [196, 225, 285].

The sensitivity to changes in ITDs strongly depends on frequency. For frequencies below 1000 Hz, this sensitivity can be described as a constant interaural phase difference (IPD) sensitivity of about 0.05 rad [153, 165, 191, 286]. The reference ITD has some effect on the ITD thresholds: large ITDs in the reference condition tend to decrease sensitivity to changes in the ITDs [123, 283]. There is almost no effect of stimulus level on ITD sensitivity [295]. At higher frequencies, the binaural auditory system is not able to detect time differences in the fine-structure waveforms. However, time differences in the envelopes can be detected quite accurately [22, 259]. Despite this ...