Hearing threshold

The hearing threshold is the minimum intensity of sound capable of impressing the human ear. Although this threshold is not always the same for all 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 at 70 dB, up to almost 400 Hz at 10 dB, this threshold tends to rise because these frequencies have a much lower sound. At frequencies above 10,000 Hz it rises to 20 dB, since 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 that are at higher or lower frequencies, greater pressure is required to excite the ear. This means that the ear's response to different frequencies is unequal.

The upper threshold of frequencies is dependent on age. With the passage of time, the hair cells of the organ of Corti deteriorate, which has the consequence that we perceive the high frequencies less and less. 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, maintaining the pressure at 15 dB, the sound could be heard perfectly.

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

The 0 dB are expressed in intensity as 10^–12 W/m² and in pressure variation as 2·10^–5 N/m².

Pressure for different frequencies

Below 2000 Hz and as the frequency decreases, the ear becomes less sensitive. Hearing thresholds for frequencies less than 2 kHz are:

Originally (curves calculated by Fletcher and Munson), the audibility threshold had been defined as the minimum pressure necessary to perceive a 1 kHz sinusoidal sound. The pressure necessary for this is 20 μPa (or an intensity of 0.98 pW/m2 at 1 atm and 25 °C), a value also taken as a reference for determining absolute values. That is, the audibility threshold is 0 dB for 1 kHz. However, more recent calculations of the curves (Robinson and Dadson) showed that, if the value of is kept as a reference value, the audibility threshold is +3 dB for 1 kHz.

The ear becomes less sensitive for frequencies above 4 kHz but not as much as at low frequencies. Fluctuations towards other frequencies are perceived due to placement in the sound field. The hearing threshold is the minimum sound pressure required for the ear to be excited. The limit is 130 dB, this being the pain threshold where it begins to cause damage to the hearing. The main one affected is the middle ear. Permanent hearing loss occurs when you spend a long time exposed to levels above 130 dB.

Most sounds commonly used in music have only components that appear in the form of partials above 5 kHz, and not as fundamental frequencies. Let's think that the highest C on the piano has a fundamental frequency of 4224 Hz. Determination of pitch there becomes difficult.

In acoustics, logarithms predominate when dealing with frequencies: representations, graphs and so on. The main reason is that the human ear interprets frequencies in an almost logarithmic way. On the frequency axis of any graph seen so far, the marks go from one frequency (for example 1000 Hz) to double (2000 Hz). The subjective appreciation of a listener will be that there is the same distance between a tone of 200 Hz and another of 400, as between one of 1000 Hz and another of 2000 Hz. However, the "distance" In frequency in the first case it is 200 Hz and in the second it is 1000 Hz.

The curves show two different ways of measuring the audibility threshold, the minimum audible pressure (MAP) and the minimum audible field (MAF). The fundamental differences between one curve and another (the area between 1.5 and 6 kHz) are mainly due to the resonances produced in the pavilion and the external auditory canal. The outer ear increases the sound pressure in the eardrum by about 15 dB for frequencies between 1.5-6 kHz. Middle ear transmission is most efficient for mid frequencies.