Recognizing features important to perception enables scientists and engineers to concentrate on audible features and ignore less important features of the involved system. It is important to note that the question of what humans hear is not only a physiological question of features of the Ear but very much also a psychological issue.
Frequency resolution of the ear is, in the middle range, about 2 Hz. That is, changes in pitch larger than 2 Hz can be perceived. However, even smaller pitch differences can be perceived through other means. For example, the interference of two pitches can often be heard as a (low-)frequency difference pitch. This effect of phase variance upon the resultant sound is known as 'beating'.
However, the effect of frequency on the human ear has a logarithmic basis. In other words, the perceived pitch of a sound is related to the frequency as an exponential function. The 12-tone musical scale is an example of this; it evolved due to the way tones are perceived. When the fundamental frequency of a note (or tone) is multiplied by 2^(1/12), the result is the frequency of the next higher semitone. Going 12 notes higher (an octave) is the same as multiplying the frequency by 2^(12/12), which is the same as doubling the frequency.
The impact of this is that the raw frequency resolution of the ear is best judged in terms of semitones, or in 'cents' which is 1/100 of a semitone.
The "intensity" range of audible sounds is enormous. Our ear drums are sensitive only to the sound pressure variation. The lower limit of audibility is defined to 0 dB, but the upper limit is not as clearly defined. The upper limit is more a question of the limit where the ear will be physically harmed (see also hearing disability). This limit depends also on the time exposed to the sound. The ear can be exposed to short periods in excess of 120 dB without permanent harm, but long term exposure to sound levels over 80 dB can cause permanent hearing loss.
A more rigorous exploration of the lower limits of audibility determines that the minimum threshold at which a sound can be heard is frequency dependent. By measuring this minimum intensity for testing tones of various frequencies, a frequency dependent Absolute threshold of hearing (ATH) curve may be derived. Typically, the ear shows a peak of sensitivity (i.e., its lowest ATH) between 1kHz and 5kHz, though the threshold changes with age, with older ears showing decreased sensitivity above 2kHz.
The ATH is the lowest of the equal-loudness contours. Equal-loudness contours indicate the sound pressure level (dB), over the range of audible frequencies, which are perceived as being of equal loudness. Equal-loudness contours were first measured by Fletcher and Munson at Bell Labs in 1933 using pure tones reproduced via headphones, and the data they collected are called Fletcher-Munson curves. Because subjective loudness was difficult to measure, the Fletcher-Munson curves were averaged over many subjects.
Robinson and Dadson refined the process in 1956 to obtain a new set of equal-loudness curves for a frontal sound source measured in an Anechoic chamber. The Robinson-Dadson curves were standardized as ISO 226 in 1986. In 2003, ISO 226 was revised using data collected from 12 international studies.
If two sounds occur simultaneously and one is masked by the other, this is referred to as Simultaneous masking. A sound close in frequency to the louder sound is more easily masked than if it is far apart in frequency. For this reason, simultaneous masking is also sometimes called frequency masking. The tonality of a sound partially determines its ability to mask other sounds. A sinusoidal masker, for example, requires a higher intensity to mask a noise-like maskee than a loud noise-like masker does to mask a sinusoid. Computer models which calculate the masking caused by sounds must therefore classify their individual spectral peaks according to their tonality.
Similarly, a weak sound emitted soon after the end of a louder sound is masked by the louder sound. In fact, even a weak sound just before a louder sound can be masked by the louder sound. These two effects are called forward and backward Temporal masking, respectively.
Psychoacoustics is based heavily on Human anatomy, especially the ear's limitations in perceiving sound as outlined previously. To summarize, these limitations are:
Given that the ear will not be at peak perceptive capacity when dealing with these limitations, a compression algorithm can assign a lower priority to sounds outside the range of human hearing. By carefully shifting bits away from the unimportant components and toward the important ones, the algorithm ensures that the sounds a listener can hear most clearly are of the highest quality.
Yet to be done: