Enjoying this Course?Generally they contain a very sensitive microphone and they are calibrated so as to pick up sound pressure levels with a response that simulates the human ear.
Usually a switch is present on the device that changes its response to the sound source, thus activating different circuits:
Circuit A: the response curve of the circuit corresponds to the human ear's isophonic curve of 40 phons [Isophonic curves - Equal loudness contours ] and allows modest sound pressure levels such as those present in a normal conversation to be measured.
Measurements carried out with this circuit are expressed in dB(A).
Circuit B: the response curve of this circuit corresponds to the human ear's curve at 70 phons. It is ideal for measuring pressure-levels between 55 and 85 dBspl.
Measurements carried out with this circuit are expressed in dB(B).
Circuit C: the response curve of this circuit corresponds to the human ear's curve at 100 phons. It is ideal for measuring values beyond 85 dBspl.
Measurements carried out with this circuit are expressed in dB(C).
Sometimes a D-type circuit is present for very high pressure-level measurements.
The Z-type circuit, when available, is intended to have a flat response.
Zero, in Vu meters always indicates the SOL, therefore in professional equipment it indicates +4dBu (1.2 V) whilst in Home Hi-Fis indicates -10 dBu (0.25V). Vu Meters measure the RMS (Root Mean Square) of a signal, in other words, its effective value, and are used for analogue equipment, especially for recorders. They are not built for visualizing all the transients of the signal, due to the inertial mass of their indicators.
PPM stands for Peak Program Meter, and as its name implies, it gives us the peak values of signals and not its effective value. Moreover, the meter follows all the signal transients and is especially used for measuring digital signals. Its measuring unit is dBfs [dBfs (Full scale) ] .
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