Knowledge of how acoustic stimuli are processed by the auditory system is important in the development of new digital audio technologies. Audio codecs, which are essential components in new multimedia and broadcast services, depend on the characteristics of the auditory system to compress audio information for efficient transmission and storage at low bit rates. Also, objective quality measurement schemes, which also depend heavily on psychoacoustic knowledge, have been developed to simulate subjective ratings of audio quality. Therefore, our lab conducts research to refine and test psychoacoustic models.

The objective of research in psychoacoustics is to understand the relationship between the physical characteristics of a sound and the perception of the sound by a human listener. For example, the physical energy of a sound is perceived as loudness, or the physical frequency of a sound is perceived as pitch. The function that transforms the physical into the perceptual domain is typically a non-linear function of some kind. For example, perceived pitch was described as a function of frequency by Stevens and Volkmann in 1940 using a scaling method that required listeners to select frequencies that doubled the pitch of given sounds. The relationship they discovered is seen below.

Investigation of the relationships between physical and perceptual quantities in different contexts also helps us to understand the limitations and variability of human perceptual response. For example, listeners vary in their ability to hear sounds below a threshold energy that is frequency-dependent. Also, listeners vary in their ability to hear high frequency sounds. Young listeners are often able to hear frequencies as high as 17-18 kHz, while older listeners often are insensitive to frequencies beyond 8 kHz. Further, some elements of a sound that are physically present in a signal may not be perceived because of masking by other elements adjacent in time and frequency. Such masking also shows considerable variability across listeners.

Recent developments in audio coding and compression have made extensive use of psychoacoustic knowledge. Psychoacoustic models are used in audio coders to predict which elements of a sound are likely inaudible. By ignoring such irrelevant information, the bandwidth required to transmit the audio signal may be significantly reduced. The performance of coding algorithms typically varies with different types of audio content, and some implementations may be more successful than others in the use of psychoacoustic knowledge.

Objective measurement methods have also been developed with the help of psychoacoustic knowledge to evaluate and compare the performance of the new generation of audio codecs. One such method, named PEAQ (Perceptual Evaluation of Audio Quality), was standardized by the International Telecommunications Union (ITU). An example of the performance of this method may be seen in the accompanying figures where objective codec quality measurements are compared with corresponding subjective ratings

Some elements of the ITU quality measurement model came from a predecessor named Perceval developed by CRC as a tool for measuring the quality of audio codecs. Perceval's computational model of peripheral auditory processes was developed under a contractual arrangement with Sherbrooke University (Quebec, Canada). A task-specific cognitive model, which analyzes the representation obtained from the ear model, was added subsequently in our laboratory. The cognitive model outputs perceptual or cognitive variables appropriate for modeling audio quality assessments or auditory detection thresholds. Perceval reproduces basic psychoacoustic phenomena, and estimates perceptual degradations that correlate well with average listener quality ratings.

Psychoacoustic models aid in optimizing the performance of audio processing and measurement devices. This is possible because the basic function of the ear is understood well enough to be practically useful. However, there are obvious limitations that need to be addressed to achieve even better performance. Our laboratory has developed a facility and an on-going research program to fill apparent gaps in what is known about perception of audio.