Monitoring the loudness with an audio meter can’t prevent unnecessary noise and distortion. Pat addresses two types the Volume Indicator and Peak Program Meter.
Is your audio gear operating in the optimum part of its dynamic range? Does excessive noise, or occasional distortion plague your system? Does it become difficult during a long, loud event to discern the loudness of the system, forcing you to keep a sound level meter handy to avoid excess levels? All of these problems can be solved with an audio meter. I will describe some of the major meter types, and what they are for.
A number of Standards exist for the response characteristics of the meters used by audio products. The two major types are the Volume Indicator (VI) and the Peak Program Meter (PPM). The VI is often referred to as a VU meter, since the meter indication is the Volume Unit or VU.
So Much for Standards
In today’s audio world it is getting increasingly difficult to find a meter that adheres exactly to a Standard. This doesn’t mean that the meter isn’t useful, it just means that one can’t be completely certain of what they are monitoring when they observe it. I try to check the characteristics of meters every chance I get, just to get a feel for what is out there in the market place.
The Volume Indicator
The reading on a VI tracks the loudness component of the signal, which is approximately related to the Root- Mean-Square (RMS) voltage at the output of the device. This is a vital piece of information to the system operator, not only because it relates to loudness, but because it also relates to the applied power to the loudspeaker. The visual monitoring of a properly calibrated VI can prevent loudspeaker thermal damage and excessive sound level to the audience.
The zero reference on a VI is typically +4 dBu and the range is -20 dB to +3 dB. Since the meter is blind to peaks due to its slow response time (300 ms), a peak LED may be included. The VI is a live sound operators best friend, because it gives them visual feedback as to the loudness of the system that can be trusted when the ears become fatigued.
The Peak Program Meter
The reading on a PPM tracks the peaks of the audio waveform, but not quite. The true peaks can exceed the indicated peak by up to 8 dB. The meter’s ballistics were designed to ignore very short term peaks for which clipping would not be audible. PPMs are useful for getting the full, usable audio level out of a device. There are several flavors, each having a different scale, but on sound reinforcement mixers meter zero is typically around 1 VRMS and levels up to +16 dB are indicated. They are blind to loudness due to their short integration time.
The True Peak Program Meter
Peak monitoring is especially important when making a recording, where it is necessary to avoid clipping of the signal. True PPM’s usually have 0 dB the top of the scale, with meter zero indicating clipping of the waveform. The response time is instantaneous (practically) so clipping of even very short term peaks is indicated. Software WAV editors and digital audio workstations often include True PPMs. While quite useful in recording applications, where the objective is to completely avoid clipping, in the world of live sound some short term clipping is typically inaudible, and even unavoidable at high sound levels. The meter shown below is a Windows application that works with a hardware DSP.
In-the-Trenches Meter Test
The main thing that one needs to know when they encounter a meter of either type is “What is this thing telling me?” It is not always apparent from simply observing the meter, with or without program material. I have devised a number of test tracks over the years to determine meter characteristics, but I recently found a much simpler way to know what a meter is showing. It involves a piece of test gear that most audio people already have or should have.
The NTI Minirator™ is a battery-powered signal generator that produces a number of useful test stimuli for sound system work. These include pink noise, white noise, sine waves at 1/3-octaves and a number of others. One of the most useful features of this handy device is that the actual output level is indicated on the LCD screen. This voltage can be indicated in volts, dBV, or dBu. I keep mine set to dBV. Now, here’s the important part. When the meter is set to produce say -10dBV (0.316Vrms) at its output for a sine wave, the same level is produced if the user switches to other waveforms, i.e. pink noise. So, I have two signals that are the same RMS level with different (but known) crest factors, and they are just a menu selection away from each other. This is quite handy for quickly determining the properties of a meter. Feed a 400 Hz sine wave into the mixer, and bring the level up to 0 dB. Make sure your amplifiers are turned off or down, because if they are maxed out you could damage your loudspeakers by operating the mixer at these levels.
With 0 dB established, switch over to the pink noise stimulus.
The virtual Minirator (left) runs on your PC and uses the sound card output. It can be downloaded from http://www.nti-audio.com/Home/Products/DiscontinuedProducts/tabid/97/Default.aspx. The physical Minirator shown has be discontinued and replaced with this one. I suggest the MR-Pro version as it stores and plays WAV files.
Peak or RMS?
A VI will produce the same indication for a 0dBV sine wave as it does for 0dBV of pink noise, since it is tracking the approximate RMS level of the signal.
A PPM will read approximately 9 dB higher for 0dBV of pink noise than it does for a 0dBV sine wave, since it is tracking the signal peaks.
A True PPM will read 12-14 dB higher for pink noise than for the sine wave, since it is tracking the true peaks in the waveform.
So, with this simple test, you immediately have a pretty good idea of the type of meter that you have – VI, PPM or True PPM. The most useful meters are those that indicate both, or are selectable between several meter ballistics.
Audio meters aren’t just glorified “signal present” or “clipping” indicators. They allow you to see the level of the audio signal so that you can achieve the best performance from your system. Spend some time with your audio gear (especially the mixer) to understand what is indicated by the metering. This will allow you to get the best signal-to-noise ratio from the device, and provide some much needed assistance to fatigued ears when mixing live. pb