An Audio Level Metering Primer by Buzz Turner
Psst! Hey mister wanna
Out of earshot of the producer my friend casually asked the DP for a quick lesson in the finer points of location recording. "Sure, no problem. Wait till the director asks you to roll sound then yell speed. Turn it up so you can see the meter bouncing and stay out of the red. Nothing to it, have fun!".
While my friend's audio education lacked finesse it gave him the information to do a crude but acceptable job. For many working professionals the old adage of "keep it out of the red" is the sum total of their understanding of audio metering. The scope of this article is to explain the history of common metering practices, define and differentiate different types of meters and offer guidelines for coping with these technologies in the field.
Welcome To Meter School
Whether a meter is an analog design with a calibrated mechanical meter movement or a LED ladder type array all meters share two basic principles. A scale to define the unit of measure and an indicator. Meters are designed to have standardized response characteristics usually referred to as meter ballistics. The rise time of the indicator (how fast the meter responds to sound) and the fall time of the indicator (the time it takes the indicator to return to its position of rest) define the meter type. Audio level meters are available in two basic flavors; a Standard Volume Indicator better know as a VU and a Peak Program Meter or PPM.
Meet Mr. Average, The VU
The response of a VU meter is that of an averaging type meter. If you apply a 1 kHz tone at "0" VU a true VU meter will take 300 milliseconds for the indicator to reach zero on the scale. The rise time of a VU meter is therefore 300 milliseconds. The fall time is also 300 milliseconds.
The VU meter was designed to respond to sounds in the same way our brains perceive "loudness". The duration of a sound will affect our perception of how loud the sound is. A drum roll will sound louder than a single hit on a drum. Guess what? The VU will register higher on the drum roll. The bottom line is that a VU will work well on sounds of continuous duration but will not show rapid transient peaks.
The European Standard,
The significant difference between the VU and PPM are the ballistics of the meter. The PPM has a rise time of 10 milliseconds, thirty times faster than a VU meter! To allow an operator to read transient peaks the PPM has an extremely slow fall time. The peak indication provided by the PPM is valuable when we are recording or transmitting digitally as any signal that exceeds digital 0 FS (zero full scale) is lost. While the PPM doesnŐt offer a correlation with how loud something sounds, operators familiar with their use tend to peak music and dialogue differently thereby preserving the same perceived loudness levels regardless of program material.
Working With Both
The two meters will now read approximately the same. If the situation is reversed with PPM's on your mixer and VU's on your recorder use the same methodology and 8 dB differential. Adjust the mixer's line up tone to show -8 dB on the PPM then adjust the recorders input to read zero VU. Again, the different meters will read approximately the same.
To Tell The Truth
True VU and PPM meters are expensive to build. Analog meters that meet the ballistic standards can only be shrunk so small before there isn't enough real estate to meet the rise and fall times of a mechanical meter movement. Many manufacturers have switched to LED ladder arrays or LCD type displays. Those types of displays are fine except when the standard ballistics are ignored. This is unfortunately more often the case. This is especially true on "semi-pro" equipment, portable equipment with small mechanical meters or abbreviated LED scales and on video cameras.
When faced with unfamiliar meters, the best defense is a good offense. Check the unknown meter against a meter you are familiar with. If at all possible run a test on the device in question and establish the headroom available. With this information you can now calibrate the differential between the two devices and arrive at a happy compromise.
Remember that meters are not some kind of empirical standard to be blindly adhered to but rather a reference to use as a guide the same way you use a speedometer or ruler. If you understand how different types of meters work you can take advantage of the best that each system has to offer.
Buzz Turner is the owner of Turner Audio Sales in Newtown, CT and a sound recordist / mixer.