Audio Level Metering Primer
by Buzz Turner
This article was written for an audience of people who use sound with video, but it contains much useful information for audio and radio producers.
A friend of mine who is a gaffer has a great story. When he was just getting started in the business a cameraman he knew offered him several days of work as a sound person. He knew absolutely nothing about running sound but was desperate for the work. On the morning of the job my friend showed up and was handed a basic sound package consisting of a mixer, recorder, headphones and a mic. Once he figured out how to thread the tape he thought it might be time for an " advance" course in audio. Who should a gaffer turn to for advice on how to record sound? Why a cameraman of course!
Out of earshot of the producer my friend casually asked the Director of Photography (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
My trusty American Heritage dictionary defines a meter as; "meter (me tar) n. - 1. Any of various devices designed to measure or to indicate the amount or volume of something. 2. To measure with a metering device." The meters we will concern ourselves with are the ones commonly employed on audio equipment designed to measure audio levels.
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 VU meter began life in 1939. It was designed by a committee made up of American broadcasters and the good folks from Bell Labs. They both agreed that a system was needed that would allow them to exchange program audio over telephone lines without compromising dynamic range, headroom or adding distortion.
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 PPM
When I first went to work for the BBC I discovered that the British and most Europeans referred to the VU as "virtually useless". Throughout much of the world Peak Program Meters (also known as PRM for Peak Reading Meters) are the standard. Unlike the VU with a typical scale indication of -20 to +3 there are many different PPM scales. Although the scale gradients and ballistic standards may differ slightly from country to country the total meter range is typically 24 to 26 dB.
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 doesn't 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
One of the biggest problems when working with both VU's and PPM's in the same system is calibrating the different scales so that both meters correspond to one another. Let's say you have a mixer with a "true" VU meter. By true VU I mean a meter with a 300 millisecond rise and fall time. So far, so good. Send tone through the mixer and line up the output to zero VU. If you are feeding a Nagra it is common practice to then adjust the Nagra's modulometer to -8 dB.
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
Another problem when working with different types of meters in the same system is meter integrity. Just because a manufacturer has affixed a "VU" , "PPM" or "Peak" label to a meter scale doesn't necessarily mean it's so.
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.
* On a Nagra recorder there are two "crosses" that go from all black to black and white as indicators. One cross tells you if your pilot tone is operating and the other cross stays black until you engage the transport. When the transport reaches the correct IPS ("Inches Per Second," or "speed," as measured by an internal tach) the cross goes to black and white. So on a film shoot, the behind the scenes goes like this:
Director: Camera Ready?
Director: Sound Ready?
Director: Roll Camera.
Director: Roll Sound
At this point the AC (assistant camera person) leans in with the slate and slates the take.
Buzz Turner spent 10 years working in network radio. He now runs Turner Audio Sales in Newtown, CT.