The effect of loudness or level on perceived width
The research of Morimoto and Iida indicated that the relationship between the perceived source width of a sound and the interaural cross-correlation coefficient is affected by the reproduction level of the signal [Morimoto and Iida 1995
]. However, their experiment used wide-band musical stimuli which meant that the perceived increase in width may have been caused by the increasing audibility of the low frequency components of the signals as the level was increased.
We conducted an experiment that investigated whether reproduction level affected the perceived source width using simple narrow-band stimuli in order to control any confounding variables as much as possible. We found that loudness does have an effect on the perceived source width, with quieter sounds generally perceived to be narrower.
In view of this, the level of the signal in each of the audio windows for each of the frequency bands is measured. The binaural signals that are input to the model should be calibrated to a reference signal level, as mentioned previously
. This level measurement is a simple measurement of the average sound pressure level (SPL) within each audio window in each frequency band. The result is then converted into an estimation of loudness in phons based on the pure-tone equal loudness curves contained in [ISO 226 1987
], and is passed onto the level and frequency compensation component which equalises the results to a reference perceived scale, as discussed later