Analyze This

Dec 1, 2007 12:00 PM, By Larry the O

How Loud Is It?

Measuring level is the most familiar metering function. While people frequently say that a level meter shows how loud a sound is, loudness is actually a perceptual attribute that is almost never directly measured. Level meters show signal amplitude or power in decibels and vary in their response characteristics and demarcation.

The ear responds primarily to the average level of sound. Therefore, averaged metering techniques such as RMS metering are useful when you are concerned with how level is being perceived. Having peak-level metering is more critical for equipment that can overload.

With music and most other signals (assuming that they have not already been subject to dynamic compression), a substantial difference typically exists between the peak and average levels, often as much as 20 dB. The ratio between the peak and average (RMS) levels of a signal is called the crest factor. Crest factor is important because it determines the amount of headroom that is required. In audio, crest factor is usually expressed in decibels, making crest factor calculations as easy as subtracting a signal's RMS value from its peak value.

Mastering engineer Bob Katz's K-System for level metering has been gaining acceptance among engineers and metering-software manufacturers. The K-System takes the idea of crest factor to the next level, defining three meter calibrations with different zero levels that indicate varying amounts of headroom (see Fig. 2). The three calibrations are used because recordings in different genres vary in crest factor. Pop-music recordings, for example, are typically highly compressed, producing a lower crest factor and higher average level, so that less headroom is needed on the meter; classical or audiophile recordings typically have a higher crest factor. Therefore, commercial pop recordings might use the K-12 scale, which defines only 12 dB of headroom, while classical recordings might use K-20, which sets zero at -20 dBfs. Whichever scale is used, according to the K-System, your monitors should be calibrated so that zero produces 83 dB SPL in the room. For a more complete explanation of the K-System, see part 2 of the paper “Level Practices” at www.digido.com/bob-katz/level-practices-part2-includes-the-k-system.html, on Bob Katz's Digital Domain Web site.

Viewing with VUs

Volume unit (VU) meters and RMS-based meters are intended to indicate average levels, but they use different methods to do so. Mechanical VU meters have long integration (rise and fall) times of 300 ms and use mechanical smoothing to achieve an approximation of averaging. RMS meters, in contrast, perform a root-mean-square calculation to derive an average (mean) power level over a period of time that is called an RMS window. Smaller window sizes make the measurement more responsive to short-duration events and lowlevel peaks, while larger sizes apply more smoothing but add latency.

In the VU's heyday, mechanical VU meters were useful because of how difficult it was to create a more accurate averaging meter. Modern software meters are rarely true VU meters, even when they are marked as such. RMS meters provide a more meaningful indication of average level.

Peeking at Peaks

Signal peaks can be much higher than average levels, and they are faster and shorter in duration. The peak program meter (PPM) was created to show signal peaks. The PPM-meter standard (European Broadcast Union technical document 3205-E) mandates an integration time of about 10 ms, which is fast enough to catch most peaks, but just slow enough to ignore spurious artifacts. Digital peak meters, which are the most common in DAWs, have no integration time — that is, they show the instantaneous peak value of the signal. While that method is truthful and can help to avoid clipping problems, it can lead you to focus disproportionately on peak values rather than average values, which give a better indication of what you are hearing. It is unfortunate that RMS meters aren't as prevalent as peak meters in DAWs and audio editors.

Hold functions highlight the hottest points in a signal by retaining the last high point that the level meter hit for a configurable amount of time (the hold time). Assuming that a higher peak has not already come along and reset the indicator, it will then fall to the current peak and remain there. Many engineers rely on the peak-hold indicator even more than they rely on the peak meter to monitor the maximum levels in the signal.

The Historical Record

Level history can be shown in several ways. A peak and/or RMS power-level history gives a continuous record of how the level evolves. That information is useful for, among other things, checking level in different parts of a song to make sure that the level of one verse or chorus is not too much higher or lower than that of other verses and choruses.

Level histograms give a different perspective on history. Histograms show frequency distribution, and so a level histogram indicates how often a given level occurs over time (see Fig. 3). When setting dynamics processors such as compressors and gain maximizers, a level histogram helps by letting you see where most of a song's energy falls.

The time-domain waveform displays found in DAWs and audio editors are level histories, too. Oscilloscopes, on the other hand, provide real-time waveform monitoring (see Fig. 4).

Implementing real-time analysis tools for multichannel surround can be a daunting task in several respects. As a result, it's rare to find surround monitoring other than within a DAW host. Programmable Analysis Software's (PAS) shareware Surround Meter is one of the few such tools that shows up in a Google search.