VU Meter

VU meter

The volume unit (VU) meter measures the volume of audio levels in digital audio workstations (DAWs) like Cubase or other (hardware) devices. The VU meter is sometimes also denoted as ‘standard volume indicator’. The Acoustical Society of America standardized it in 1942 (ANSI C16.5-1942; Wikipedia).

One use of the VU meter is gain staging which I do not discuss here. For a nice video about gain staging using VU meters see [here].

VU meters display signal levels in volume units (VU), a measure of average volume level. By definition, a reading of 0 VU describes a 600-ohm resistance in which 1 milliwatt (mW) of sine-wave power (see my post about Sound Waves) flows at 1kHz. Most VU meters have a scale from -20 to +3, and rise and fall times of 300 milliseconds (ms). If a constant sine-wave of 0 VU is applied, the meter will achieve 0 VU in 300 ms. Readings on the VU scale are approximate to the base 10 of the power ratio referenced to that level. For a pure 100-Hz sine wave, 0 VU equals 0 dBm or 0.775V root means square (RMS; see my post about Decibels). In practice, however, VU meters contain an isolating resistor so that 0 VU equals +4 dBm (power of about 2.5mW applied across a 600-ohm load) for a pure sine wave (see Engineering 360).

Note: a power level (i.e., amount of energy transferred per unit of time; Unit is Watt) of 0dBm corresponds to a power of 1 mW

VU meters display the average volume level of an audio signal whereas PPMs display peak volume. The difference between the reading of a VU meter and a PPM is called the crest factor. VU meters and PPMs also differ in terms of acceleration and deceleration rates (ballistics). Whereas a VU meter takes 300 ms to stabilize when a 1 kHz steady-state tone is introduced, a PPM stabilizes in just 10 ms. Because of their ballistics, then, VU meters must sample the audio signal over a longer period of time than audio meters such as PPMs. VU meters are designed to provide a very slow response and are optimized for perceived loudness instead of peak performance.

Programme modulation

Although often overlooked, the VU meter has two scale. The primary calibration is marked in decibels relative to 0VU. But there’s a secondary scale beneath that shows the programme modulation level between 0 and 100%. ‘Modulation’ is a term rooted in AM radio broadcasting, and it refers to the strength of the audio signal being broadcast: 0% modulation means that the carrier is present but conveying no audio signal, while 100% modulation means it’s carrying as much audio amplitude as is possible
without overload. The 100% modulation level aligns with 0VU (this is coincidental; it relates to the meter’s slow ballistics) and 0% is a little below the -20VU mark. In use, a steady 1kHz or 440Hz tone at the desired Operating Level (whatever that may be) should read 0VU, while a varying audio programme should stay below the 100% mark most of the time

Testing in Cubase

I have setup a small project in Cubase to test the VU meter and compare it to peak meters. I used the VU meter from Waves  and the VU meter deluxe from Klanghelm. In addition, I used the standard peak meter (K20-scale; see here) in the Control Room from Steinberg, and the Dorrough meter from Waves,  which provides both the peak levels and the average level. The integration time of the Dourrough plugin is not specified by Waves but it is probably 600 ms following the hardware devices, and thus twice as long compared to the VU meters. All volume faders were set to 0 dbFS.

Sinus wave -20dB. Overhead 20dB.

First I tested with the 1kHz sine wave of -20dBFS. The RMS level of this wave is -3dbFS. Both VU meters were set with a 20dB overhead. This implies that if the VU meter shows 0, there is 20dB of overhead available for the audio peaks (which we don’t have in case of a pure sine wave). The image below shows the meter readings (click to enlarge). The K-20 meter correctly shows 0dB (note that the K-20 scale implies the 0dB is assigned to -20dbFS) and, in addition, shows the RMS value at -3dB. The Dourrough meter shows -20dBFS (in this case the average (RMS) and peak correctly coincide; see here for an explanation). Both VU meters show 0VU. The Klanghelm meter additionally shows the peak level (-19.5dB; slightly off). The Klanghelm meter can also show the RMS value (second image below) and correctly shows 0dB (actually RMS+3dB as indicated in the meter AES-17 standard; see here for an explanation).

The selection (-20,-18, or -14) on the VU meter also represents a reference level of +4dBu, which is usually represented by “0” on professional analog VU meters.

Sinus wave -20dB. Overhead 14dB.

I again used a 1kHz sine wave of -20dBFS but now decreased the amount of overhead to 14dB. Now both VU meters show -6 VU (see image below). In this case 0 VU corresponds to -14dBFS. Increasing the signal by 20dB will give 0dBFS on a peak meter and +14 VU on the VU meter (which cannot be displayed as the maximum is +3 VU). Thus imposing a smaller overhead (14 dB instead of 20 dB) implies that the VU meter has to show a lower level to accommodate to maximum possible increase in the signal (+20dB).

Sinus wave -20dB. Overhead 23dB.

I again used a 1kHz sine wave of -20dBFS but now increased the amount of overhead to 23dB. Now both VU meters show exactly +3 UV (see image below). In this case 0 VU corresponds to -23dBFS. Thus to reach the maximum VU level 23 VU the VU meter should show +3 VU since the maximum increase of our signal is 20dB (to reach 0dBFS). Thus imposing a larger overhead (23 dB instead of 20 dB) implies that the VU meter has to show a high level to accommodate to maximum possible increase in the signal (+20dB). We can even impose more overhead but the VU meter will not be able to show this since its maximum is 3 VU.

Scales

In the figure below we see a graphical representation of the tests I just showed in Cubase. Actually the amount of overhead chosen corresponds to different standards. Note that dBFS is not defined for analog levels and depends on the standard chosen. For example 0VU (+4dBu) depends on the standard referred to (Figure below):

• -20 dBFS is the Digital AES reference standard.
• -18 dBFS is the Digital EBU reference standard.
• -14 dBFS is commonly used in post-production and certain mastering situations

Figure. Analog and digital audio levels. Figure copied from ZedBrookes. The European Broadcasting Union (EBU) Technical Recommendation R68-2000 and the Society for Motion Picture & Television Engineer (SMPTE) RP 155 – 2004 operating practices are based on two different audio reference levels, namely –18 dBFS1 and –20 dBFS respectively”

Note: dBu (decibel unloaded) is a unit of measurement of the ratio of an amount of voltage to (voltage measured as RMS):

Thus, a recording level of +4 dBu corresponds to V=1.2283 volts.

Video

References

VU meters: “Virtually Useless” or very useful? (Sound on Sound)

Questions and Answers about the VU meter (pdf)

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EBU Technical Recommendation R68-2000 (pdf)

1 file(s) 16.22 KB

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