by Ron Meyer
President, Professional Sound Corp.
Thirty years have gone by since Bell Telephone Laboratories, NBC, and CBS joined together to develop and standardize the Volume-Unit meter. The VU meter as we commonly know it, has remained unchanged since its introduction and is widely used in the audio field, even today. Though the VU meter has been in use for a long time, many people still have misconceptions about its properties and use.
The VU meter was designed to have ballistics (dynamic characteristics) that roughly approximates the human ear. In this manner the meter proved useful in determining signal levels for audio broadcast and recording. The meter itself has a response that is RMS or "average" reading. This stipulates that it will respond with a reading that averages out the complex wave forms that typically make up an audio signal, and this can lead to problems when recording audio that is of a transient nature. The transients may have a peak level that is 10 to 15 dB higher than average and are virtually ignored by the VU meter, thus causing overload during recording. To compensate for this, many users and manufacturers incorporate a six to eight dB lead into the calibration of the meter. This in effect helps to overcome the transient response to the problems.
The VU meter is based on the telephone company's 1939 reference level of zero dBm being equal to 1 mW into 600 ohms, or as the better known 0.775 volts into 600 W. This reference level was chosen by the telephone company as the best compromise between crosstalk and signal to noise ratio in their system. It should be noted that the VU meter is a relative reading meter in that its zero is always relative to some other standard such as +4 dBm outputs on today's audio gear.
Even though the quality of electronic equipment has improved dramatically over the past 50 years, these early standards developed by the telephone companies are the basis for most electronic measurements used today.
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