than 0.5 dB between 25 Hz and 16 kHz. Note that the specification mentions only sinusoid waveforms. Given the electromechanical principle of the meter, the deviation of the needle is actually approximately proportional to the average of the part of the signal with more than approximately 0.4 V instantaneously because of the two copper-oxide rectifiers always in series, which transfer function curve has a knee around 0.2 V. Signals generally do not have a sinusoidal waveform by far, even if they all fall within the VU-meter bandpass. The reading is the average of the voltage, and is not an indication of the power of the signal, which is proportional to the average of the square of the voltage, or the root-mean-square (RMS) value. As a conventional VU reading, however, it served its purpose as an indication a) of the overall level and dynamics of the signal and b) of the proximity to the maximum admitted level, to the operators of recording and broadcasting equipment. Maintenance staff could also use it as a measurement apparatus, to check for losses in transmissions and level alignment, provided that they used exclusively sine waves as test signals. Impedance The VU meter and its attenuator should present a 7,500-ohm impedance to the circuit it is applied to, measured with a sinusoid signal that sets the indicator to 0 dB. History The VU-meter was originally developed in 1939 by the combined effort of Bell Labs and broadcasters CBS and NBC. In the 1970s–80s, neon-filled, planar dual displays with up to 201 segments per stereo channel were popular among broadcasters as fast bar graph VU meters. Other level meters The consumer audio industry often uses volume indicators that do not comply with any standard. BBC sound engineers would refer to the VU meter as the "Virtually Useless" meter, preferring the PPM. Academic research has shown that an SVI or VU meter behaves differently to the average value and RMS meters. The ballistics shown by this instrument in response to signals with large crest factor position its readings halfway between both. For example, an increment of 3 dB in the crest factor of an audio signal gives approximately a fall of −3 dB in a RMS meter, −6 dB in an average meter, and −4 dB in a VU meter. See also Audio equipment Crest factor Decibel Loudness monitoring Mixing console Noise pollution Nominal level Peak meter Peak