via Shure Blog
Meters which monitor audio levels are typically one of two varieties: VU (Volume Unit) or PPM (Peak Program Meters). Though both perform the same function, they accomplish the function in very different manners. A VU meter displays the average volume level of an audio signal. A PPM displays the peak volume level of an audio signal. Analogy: The average height of the Himalayan Mountains is 18,000 feet (VU), but Mt. Everest’s peak is 29,000+ feet (PPM).
For a steady state sine wave tone, the difference between the average level (VU) and the peak level (PPM) is about 3 dB. But for a complex audio signal (speech or music), the difference between the average level (VU) and the peak level (PPM) can be 10 to 12 dB! This difference between the reading of a VU meter and a PPM is known as the crest factor.
A VU meter and PPM also have different ballistics (acceleration/deceleration rates). If a 1kHz steady state tone is fed into a VU meter, it takes 300 milliseconds (0.300 seconds) for the meter to stabilize. However, the PPM stabilizes within 10 milliseconds (0.010 seconds). As the VU meter displays an average volume of the audio signal, it must “sample” the audio signal over a longer time period than the PPM.
Because of the crest factor and the difference in ballistics, a VU meter and a PPM will display the same speech/music audio signal in very different ways. Therefore, using a steady state tone to line up a VU meter with a PPM is not effective unless these differences are taken into consideration. Analogy: A mini-van (VU) and a sport car (PPM) will cruise side by side at a constant 60 MPH (steady state tone). But they will not cruise side by side if each vehicle accelerates to 100 MPH and brakes to 20 MPH many times and as quickly as possible (speech/music signal). Though both vehicles are performing the same function, the location of each vehicle (position of each meter indicator) will be very different.
The VU meter closely corresponds to the level sensing mechanism of the human ear. It provides a useful indication of the subjective loudness of different programs and is very useful when matching levels between programs. But the VU meter does not give an accurate indication of peak signal levels because of its relatively slow ballistics. In practice, a VU meter will under-indicate the peak signal level by 8 to 20 dB.
Here is a rule of thumb when using a steady state tone to align a VU meter with a PPM. When the VU meter indicates “0” (typically a +4 dBm level), the PPM should be set to read 20 dB below its maximum full scale reading. For example, when the VU meter of a Shure FP mixer reads “0”, the PPM on a Sony Beta Cam with a “+12″ full scale reading should be set to read at “-8″. Like any rule of thumb, this one may vary depending on the actual specifications of the products in use.
Note: Meters marked with the symbols “VU” or “PPM” may not actually meet the international standards for such meters. The best advice is to listen critically while recording and not rely solely on meter readings.
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