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The audible hearing frequency range over which a normal human ear can detect
is approximately 20Hz to 20kHz.
The range usually decays with age and generally this is of the higher frequencies.
If the level of a signal is sufficient and is of a frequency below 20Hz then it is
generally felt and not heard.
Frequency Weighted Filter
This refers to a special filter that weights or adjusts the spectral component of a
signal according to a specified function of frequency.
The specified functions of frequency are: -
A-Weighting
This is the most commonly used frequency weighted filter and was originally
developed to adjust the signal frequency spectrum to match the sensitivity of an
average human ear for sound pressure levels below approximately 50dB.
Now, this frequency weighting is used for all levels of sound pressure levels
which progressively attenuate the signal at the upper and lower ends of the
audible frequency range. It is widely used to measure environmental or industrial
noise.
B-Weighting
This frequency weighting was originally intended to adjust the signal frequency
spectrum to match the sensitivity of an average human ear for sound pressure
levels between approximately 50 and 90dB.
It attenuates progressively at the upper and lower ends of the audible frequency
range however the attenuation of the signal is less than with A-weighting.
This frequency weighting is not commonly used. As this weighting filter is not
required for noise assessments it is not available on Castle instrumentation.
C-Weighting
Another commonly used frequency weighted filter and was originally intended to
adjust the signal frequency spectrum to match the sensitivity of an average
human ear for sound pressure levels above approximately 90dB.
It attenuates progressively at the upper and lower ends of the audible frequency
range however the attenuation of the signal at low frequencies is much greater
than with A-weighting.
