What is sharpness?

Definition

Sound perception is a subjective phenomenon depending on both physical and cognitive factors. The aim of psychoacoustics is to better understand the human perception of sound and improve Sound Quality.

Sharpness is a psychoacoustics metric that provides a numerical measure of sensation based on the amount of high-frequency components in a sound. Its unit is the “acum”, 1 acum corresponding to the sharpness of a broad-band noise centered on 1 kHz, with a width of 1 critical band and a level of 60 dB. It is a linear scale : double the frequency content, double the sharpness.

Sharpness calculation as described in DIN 45692 is based on a prior loudness calculation according to Zwicker method. It is computed as a weighted sum of the specific loudness levels reported to the global loudness. Several weighting functions exist (by Aures, Fastl, Von Bismarck) but they all increase toward the highest frequency bands to model the fact that the hearing system is more sensitive to high-frequency components level.

Illustration

Dehumidifier electric pump


Car electric motor


Although their overall dBA level is the same, these two sounds are not perceived in the same way. The car noise seems high-pitched, narrow and aggressive while the noise from the dehumidifier seems deeper, wider and more bearable. The difference between the two sounds is visible on the third-octave spectra in dBA below : the car motor spectrum shows louder high frequency components than the dehumidifier’s one.

Third-octave spectra comparison

This feeling is well described by the sharpness that takes into account the frequency-sensitivity of the human ear. The car motor sharpness value then reaches 2,04 acum whereas the dehumidifier motor only gets to 0,84 acum.

Consequently, sharpness is a metrics that represent well the human perception of sounds with high frequency components. Its calculation can be useful to understand some e-NVH aspects and optimize the sound produced by an electric motor.

See also

Read more