signals with a simple technology. Since a VU meter is a mechanical device, it can never reflect the instantaneous signal peaks of complex audio signals. The designers of the VU meter therefore took a different approach. They created a meter that did not measure peaks, but simply inferred them. A real VU meter has a very specific "ballistic characteristic". This means that it responds to changing audio signals at a very precise speed, rising from no signal to 99% of "0 VU" when a 1 kHz sine wave tone is applied for 300 milliseconds. When using a VU meter, the audio system is calibrated with a sine wave tone at a "reference level" for the system. At the reference level, the VU meter shows "0" for a sine-wave tone, but the engineer must know that, with music or speech, to always infer that peak levels are always between 6 dB and 10 dB higher than the reference level. The usefulness of the VU meter comes from the fact that for most types of audio sources the system engineer can count on these peaks being within this range and can design the audio system with confidence. Good engineering practice is to always build in a little extra "headroom", as it is called, to cover the strange conditions where an audio signal might exceed normal peak levels or the equipment operator fails to adjust the levels correctly. Typically the levels considered when designing systems using a VU meter are: Reference level (typically +4 dBu, valid with tones only); Standard output level (10 dB above reference, typical peak levels); Clip level (6 dB above standard output level, "headroom" to allow for unusual conditions) Standard characteristics The behaviour of VU meters is defined in ANSI C16.5-1942, British Standard BS 6840, and IEC 60268-17. Reference level Rise time The rise time, defined as the time it takes for the needle to reach 99% of the distance to 0 VU when the VU-meter is submitted to a signal that steps from 0 to a level that reads 0 VU, is 300 ms. The overshoot must be within 1 to 1.5%. The fall time is the same as the rise time, 300 ms. Frequency response The level specification is meant at 1000 Hz. The reading should not depart from the reading at 1000 Hz by more than 0.2 dB from 35 Hz to 10 kHz or more