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A VU meter is the familiar name for a volume-indicator (VI) instrument: a specially calibrated meter designed to show the level of speech and music. Its reading is not interchangeable with dBm, and it is accurate only when interpreted with the meter’s reference impedance, attenuator setting, and response characteristics in mind.
What are VU and VI?
Broadcasting and telephone engineers designed the instrument in 1939 to measure speech and music in communication circuits. The unit on its scale is the VU, while the instrument itself is properly called a volume indicator, or VI, instrument. “VU meter” is widely used, but is technically imprecise: VU names the calibrated scale, not the instrument.
VU and dBm are related level measurements made with specially calibrated voltmeters, but they describe different kinds of signal. dBm labels a steady-state sinewave level; VU is used for program material such as speech and music. A meter’s indication for a changing program signal therefore should not be treated as though it were a direct dBm measurement of a steady tone.
Why impedance changes the reading
A VU or dBm scale is correct only at the impedance for which the meter is calibrated. Common reference impedances are 150 ohms and 600 ohms. If the measured circuit has a different impedance, the scale gives an apparent level, not the true level for the 600-ohm reference.
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For a 600-ohm reference, correct an apparent reading using:
True VU = Apparent VU + 10 log10(600 / Z measured)
Here, Z measured is the impedance in ohms at which the level is measured. For example, the correction is zero at 600 ohms; at a lower impedance the correction is positive, and at a higher impedance it is negative. Confirm the meter’s calibration reference before applying this 600-ohm formula; a meter calibrated for 150 ohms requires the appropriate reference instead.
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How the VI instrument is built and why its scale reads low
The VI instrument described by Don Davis and Eugene Patronis uses a 50-microampere D’Arsonval movement and a copper-oxide bridge rectifier. The instrument and rectifier together are 3,900 ohms. A 3,600-ohm build-out resistor raises the total to 7,500 ohms. In the stated 600-ohm-line arrangement, this produces a 4 dB loss.
Consequently, a properly installed instrument fed 0 dBm across a 600-ohm line indicates −4 on the meter scale before the attenuator setting is included. The low indication is not evidence by itself of a fault; it reflects the instrument’s loss and how the reading is interpreted.
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How to read and calibrate a VU instrument
For the 600-ohm reference, the true level combines the instrument indication, the attenuator or sensitivity setting, and the impedance correction:
True VU level = Instrument indication + attenuator or sensitivity setting + 10 log10(600 / Z act)
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Z act is the actual measured impedance in ohms. The equivalent form sometimes shown without the setting term assumes that the instrument indication already includes the attenuator or sensitivity setting. Do not omit that setting when it is separate from the meter indication.
- Set up the meter for its specified reference impedance, such as 600 ohms, and connect it correctly to the circuit being measured.
- Apply a steady 1,000 Hz sinewave at 0 dBm (0.001 W) for calibration.
- Adjust the meter or calibration arrangement so the corrected, true level is 0 VU. In the typical 600-ohm example, the meter indicates −4 and the attenuator is set to +4.
- When measuring program material, combine the indicated value with the attenuator or sensitivity setting and any required impedance correction to report the true level.
VI instruments may use different scale types: a VU scale intended for recording-studio use or a modulation scale intended for broadcast use. Check which one is fitted before interpreting a reading.
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Why VU readings do not show short peaks accurately
The VI instrument responds too slowly to track short-duration peaks in complex waveforms. Davis and Patronis describe observed VI readings and peak levels as about 10 dB apart; this is an approximate relationship, not a universal correction for every signal or meter. A true +8 VU speech reading can therefore correspond to a required steady-state sinewave capability of +18 dBm to avoid overload. The meter’s program-level indication alone does not guarantee adequate peak headroom.
What to check when comparing VU meters
Two meters can show different values without measuring the same reference or presenting the same kind of scale. Compare the following before relying on their readings:
- Reference impedance: 150 ohms or 600 ohms, and whether the actual circuit impedance matches it.
- Setting convention: whether the attenuator or sensitivity setting is already included in the displayed indication.
- Scale type: recording VU or broadcast modulation.
- Calibration: whether a 1,000 Hz, 0 dBm steady tone produces a corrected 0 VU result.
- Movement, rectifier, and ballistics: their design affects the response and how much short peaks are understated.
The instrument construction, calibration example, and approximate peak lag here are described in the 2008 article by Don Davis and Eugene Patronis, printed with permission from Focal Press, a division of Elsevier, and identified as drawn from Sound System Engineering. They describe that account and should not be taken as confirmation of current standards compliance for any particular meter.
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