Filter definition

In acoustics, filters are often used to separate sounds with different pitch. The frequency range is divided into octaves. The frequency range where the highest frequency is twice the lowest frequency is called an octave.
Noise problems are often related to some frequencies and when the frequency is known (and thereby the wavelength) it may be easier to handle the noise problem. Different filter types are available.

A filter covers a number of frequencies from a lower edge frequency to an upper edge frequency.
Each filter has a center frequency that is in the middle of the filter band.
In Nor150 many filter levels are measured in parallell to cover a large frequency range. In the third octave filter mode there are 31 filters that covers the audible frequency range from 20 Hz to 20 kHz. Filters with lower frequencies, down to 0.1 Hz, are also available.
The frequency range from the lower to the upper edge frequency is also called the filter Passband.
Below the lower edge frequency, and above the upper edge, the frequencies are attenuated. This Attenuation shall preferably be as good as possible.
The filters and their behaviour are standardized. The current filter standard is IEC 61260 and the filters in the Nor150 is of class 1 according to this standard.