While sound pressure measurements should give a reading of the sound pressure in dB SPL, in the context of human hearing it is more practical to provide also a value which corresponds more closely to the hearing sensation or loudness in phones. Normal equal-loudness-level contours for pure tones For frequencies lower than 3-4 kHz and higher sound frequencies, the ear becomes less sensitive. As it could be seen in the figure below, the frequencies 3-4 kHz are the most sensitive within sound frequency range from 20 Hz to 20 kHz that can be heard by human ear. Isophonic curves relate the characteristic of a given tone expressed in dB SPL to its subjective loudness level expressed in phones (see figure 1 below). In order to assess loudness of a sound the isophonic curves are explored.

The loudness of a sound is not equal with its sound pressure level and differs for different frequencies. This subjective or perceived magnitude of a sound by an individual is called its loudness. The human ear is not equally sensitive to sounds (tones) of the same sound pressure levels but different frequencies.

Table 1: Typical sound pressure levels for daily life soundsģ.3.3.2. In the logarithmic scale the range of human ear’s audible sounds is from 0 dB SPL (hearing threshold) to 120-140 dB SPL (pain threshold) (see table 1 below). Since using such a large scale is not practical, a logarithmic scale in decibels ( dB) was introduced which is also in agreement with physiological and psychological hearing sensations.ĭB of sound pressure level ( dB SPL) is defined as: 20 log 10 p1/p0 where p1 is actually measured sound pressure level of a given sound, and p0 is a reference value of 20μPa, which corresponds to the lowest hearing threshold of the young, healthy ear. Human ear’ audible sound pressure levels range from 20 μPa ( hearing threshold) till 20 Pa (pain threshold), resulting in the scale 1:10,000,000. One parameter of the acoustic ( sound) wave which is generally used to assess sound exposure to humans is the sound pressure level expressed in μPa or Pa.

Decibel scale sound