Type I meters measure the level of noise energy arriving at the measuring microphone over a specified time period, which usually varies between 5ms and 2 seconds. However, their accuracy diminishes rapidly at long distances or when the source is a continuous sound rather than a sound burst.
Type II meters measure the level of sound energy over a specified frequency range (wavenumber range), which takes into account both the amplitude and the phase of the noise arriving at the microphone. The frequency range is controlled by variable attenuators or phase shifters (in the case of a delay line) and may be selected for typical noise levels.
Although the standard eight-point measurement method offered by industry has proven satisfactory for many applications, free field measurements sometimes require a double or triple arrangement to ensure accurate measurement. In this case, the microphone signal is processed to obtain a complex spectrum with four or six components; the amplitude of the two or four strongest components is then used for the final measurement.
Anechoic chambers are used to measure sound-producing systems, such as loudspeakers (including earphones and studio monitors), PA systems, mixing consoles, etc. Two types of anechoic chambers exist: anechoic chambers with free field conditions and anechoic chambers with free field conditions with numerous absorbing boundaries. In the former, walls and floor should reflect sound fields (are absorbers) as close as possible to those outdoors (for a more complete discussion on absorbers, see below).
A wide variety of noise measurements exist. While many are based on the same basic measurement principle, they are usually performed for specialized purposes. In addition to the eight-point measurement technique, sometimes more complicated methods are used, such as 20-point or 45-point measurements.
For indoor environments where free field conditions are not available, anechoic chambers with absorbing walls and floors must be used, a standard measure placed on the interior walls of the chamber. d2c66b5586