Generally they are built with a very sensitive microphone inside and they are calibrated so as to capt sound pressure with a response that mimes that of a human ear.
Usually an equipment-calibrating switch is present to measure the sound source. This makes sure that one measuring circuit is activated rather than another:
Circuit A: The response-curve of the circuit corresponds to the isophonic curve of 40 phons for a human ear [Isophonic Curves (Equal Loudness Contours) ] and allows measurements of modest sound pressure-levels such as those present in a normal conversation.
Measures done with this circuit are expressed in dB-A.
Circuit B: The response-curve of the circuit corresponds to the curve at 70 phons for the human ear. It is ideal for measuring pressure-levels between 55 and 85 dBspl.
Measures done with this circuit are expressed in dB-B.
Circuit C: Activates a circuit with a flat reponse-curve. Ideal for reading values beyond 85 dBspl. Sometimes it is present within a D-type circuit used for very high pressure-level measurings.
Measures done with this circuit are expressed in dB-C.
Zero, in Vu Meters, always indicates the SOL, thus in professional equipment it indicates +4dBu (1.2 V) whilst in Home Hi-Fi's indicates -10 dBu (0.25V). Vu Meters measure the RMS of the 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 inert mass of the indicators.
PPM stands for Peak Program Meter, and as its name informs us, it gives us the peak-values of signals and not its effective value. Moreover, the measurer 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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