Psychoacoustics

Psychoacoustics is the study of subjective human perception of sounds. Effectively, it is the study of psychology of acoustical perception.

Limits of perception

The human ear can usually hear sounds in the range 20 Hz to 22 kHz. With age, the range decreases, especially at the upper limit. Lower frequencies cannot be heard but loud sounds can be felt on the skin.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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'.

Related Topics:
Phase - 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.

Related Topics:
DB - Hearing disability

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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.

Related Topics:
Equal-loudness contour - Bell Labs - 1933

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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.

Related Topics:
1956 - Anechoic chamber - ISO - 1986 - 2003

~ ~ ~ ~ ~ ~ ~ ~ ~ ~