Decibels - Phonometers

Leggi questa pagina in Italiano Lire cette page en Franšais
CDROM Multimedia Audio Course Enjoying this Course?
Download the full version!

3.9.1. dBspl meters

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 (fallen into disuse): 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).

The Z-type circuit, when available, is intended to have a flat response.



3.9.2. Vu meters

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 average value of a signal 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.



3.9.3. PPM meters

PPM stands for Peak Program Meter. This kind of meter measures the signal level with a faster response to transients, due to a much faster raise time (10ms compared to 300ms of Vu meters). This meter is often used for measuring digital signals and its measuring unit may vary referring to dBfs [dBfs (Full scale) ] values.

Most meters used in digital hardware and and software are not PPMs, they just indicate digital peak level, which is essential to avoid clipping, but completely useless as an indication of subjective loudness.

An interesting one is the K-meter designed by Bob Katz, it displays both digital peak and RMS on the same scale (see picture). The white dots are not as usual the maximum of the coloured bars: they are independent and indicate digital peak level, while the bars are RMS with a reaction time similar to a Vu.

Decibels and dynamics - K-Meter

K-Meter