Hearing threshold

The threshold of hearing is the minimum intensity of sound capable of impressing the human ear. Although this threshold is not always the same for all the frequencies that the human ear is capable of perceiving, it is the minimum level of a sound for it to be perceived.

The normal value is between 0 audiometric dB (equivalent to 20 micropascals) and 25 audiometric dB, however, at very low frequencies, such as approximately 40 Hz to 70 dB, up to almost 400 Hz at 10 dB, this threshold tends to to go up because these frequencies have a much lower sound. At frequencies above 10,000 Hz, it rises to 20 dB, because due to the sharpness of these waves, the hearing threshold requires greater pressure. The hearing threshold, for the average human, is set at 20 µPa (20 micropascals = 0.000 02 pascals), for frequencies between 1 kHz and 4 kHz, with small variations in intensity between the two. For sounds at higher or lower frequencies, more pressure is required to excite the ear. This means that the response of the ear to different frequencies is uneven.

The upper threshold of frequencies is dependent on age. Over time, the capillary cells of the organ of Corti deteriorate, which has the consequence that we perceive less and less high frequencies. A frequency of 125 Hz at a level of 15 dB (pure tone) would be almost inaudible to the human ear. By varying the frequency around 500 Hz, keeping the pressure at 15 dB, the sound could be perfectly heard.

Each frequency has a pressure level necessary for the ear to detect the same loudness in all of them. At 2 kHz the threshold of hearing is fixed at 0 dB and at 4 kHz it is even less than 0 dB, since 3600 Hz is the resonant frequency of the human ear.

0 dB is expressed in intensity as 10–12 W/m² and in pressure variation as 2 10–5 N/m².