It should be remembered that objective measurements of sound do not represent the subjective perception of sound by the human ear.
Noise is usually measured as sound pressure level (SPL). The unit of noise is the decibel (dB(A)), which is proportional to the logarithm of sound intensity (sound energy per square meter per second). A consequence of this logarithmic scale is that an increase in sound pressure level of 10 dB(A) corresponds to a doubling of subjective loudness perception:
The A-weighted sound pressure level measured in dB(A) allows for the fact that the human ear is most sensitive in the range 500 to 5000 Hz and is rather insensitive to very low and very high frequencies. Low-frequency noise is underestimated by the dB(A) scale, so that a 100-Hz tone raised by 10 dB(A) is experienced as four to five times louder (e.g. ventilation noise).
It is often believed that people become accustomed to noise, but in fact the annoyance increases with exposure time. The subjective response to noise depends not only on measurable facts, but also on many additional psychological factors ( Kjell,be,rg,,1990).
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