This is the hearing threshold in silence (for a typical human at age 20) on logarithmic, linear and Bark (=perceptual) frequency scales. Below these levels nothing is audible.
This is the hearing threshold in the presence of two different tones. Part of the threshold moves upwards, possibly hiding signals that would otherwise be audible. This is called masking. Note however that even in the presence of a loud (90dBA) sound, a considerable part of the threshold is still at the silent level. This explains why very small distortion levels can still be audible.
(model: Psychoachoustics, Zwicker 1990)
(model: Psychoachoustics, Zwicker 1990)
On stereo signals, the ear-brain system can achieve a significant processing gain. In stereo hearing, just by using two ears, directional information translates into amplitude and phase differences between the two ears. If there are phase differences, then a processing gain of up to 15 dB has been observed. That means you can suddenly hear a tone that would be 15 dB below the noise in mono. A simple experiment I did with a headphone (above) already showed 11 dB processing gain at low frequencies.
This explains why small distortion levels are more audible on live stereo recordings that contain directional acoustic information.
This explains why small distortion levels are more audible on live stereo recordings that contain directional acoustic information.
This figure is speculative. I'm not aware of a study that showed if stereo hearing processing gain can be applied to hearing threshold masking. Its an intriguing thought anyway.