In high-fidelity equipment, especially the more sophisticated amplifiers (“amps”), loudness meters are sometimes used. These are calibrated in decibels, a unit that you will often have to use, and interpret, in reference to electronic signal levels. A decibel is an increase or decrease in sound or signal level that you can just barely detect, if you are expecting the change.
Audio loudness is given in volume units (VU), and the meter that indicates it is called a VU meter. The typical VU meter has a zero marker with a red line to the right and a black line to the left, and is calibrated in decibels (dB) below the zero marker and volume units above it (following figure). The meter might also be calibrated in watts rms, an expression for audio power. As music is played through the system, or as a voice comes over it, the VU meter needle kicks up. The amplifier volume should be kept down so that the meter doesn’t go past the zero mark and into the red range. If the meter does kick up into the red scale, it means that distortion is taking place within the amplifier circuit.
A VU (volume-unit) meter. The heavy portion of the scale (to the right of 0) is usually red, indicating the risk of audio distortion.
Sound level in general can be measured by means of a sound-level meter, calibrated in decibels (dB) and connected to the output of a precision amplifier with a microphone of known sensitivity (following figure). Have you read that a vacuum cleaner will produce “80 dB” of sound, and a large truck going by will subject your ears to “90 dB”? These figures are determined by a sound-level meter, and are defined with respect to the threshold of hearing, which is the faintest sound that a person with good ears can hear.
A meter for measuring sound levels. The output of the audio amplifier is rectified to produce dc that the meter can detect.