Equalization

Many common tasks in the sound reinforcement industry are misunderstood. One of the most common misunderstandings is the reason for system equalization. The 1/3-octave equalizer has become ubiquitous and yet it often fails to provide the solution that was envisioned when it was first mounted in the equipment rack. Is this a failing of the technology or the application?

In many cases, system equalization is intended to solve problems that were designed into a sound system. Many issues, such as loudspeaker placement, interaction between drivers within the loudspeaker system, interaction between mics, or even the early reflections into the mics or out from the loudspeakers. Tonality and excessive feedback can be attributed to the way the mics and loudspeakers are oriented and not just the frequency response of the system. These features may already be fixed in place before the system equalization is attempted. The result is a compromise that tries to make the best of a bad situation.

Surprisingly, only a slight amount of equalization is required for most high quality loudspeakers. Three or four bands of a good parametric filter will take out the audible bumps in the frequency response. If you are making good use of a time domain analyzer (a Crown TEF20, in my case) to assist in the process, the phase response of the system will also be improved. I base all of the equalization on listening to the system and then measuring the direct sound from the device. The measurement will exclude all but the first few milliseconds of energy reflected back from the room. This allows me to see the driver response that I can change with equalization, while ignoring the room response, which I can't change with any electronic equalizer.

The initial stage in the equalization process is the examination of the way the loudspeakers interact with each other and the surrounding surfaces. Solving each of these problems as they present themselves, such as adding acoustical absorption to the surfaces nearest the loudspeaker or readjusting the loudspeaker focus, may yield much greater audible benefits than the equalization, itself. When I am called in to equalize a sound system, I usually find that very little time is required to do the equalization, but considerable time is invested in checking out the rest of the signal chain.

I find that some installers and end users expect to adjust equalization to compensate for changes in the response across the coverage pattern of a single loudspeaker system. Unfortunately, equalizers can't get there. You must select an appropriate loudspeaker and precisely align it with the others in an array, before the coverage uniformity will improve. The attempt to solve these types of problems with equalization simply swaps the bad sounding seats with the good sounding ones or, alternatively, makes all of the seats sound equally bad. There are situations where the best way to equalize a sound reinforcement system is to dismantle the loudspeaker clusters and start over.

We have seen some serious improvements in equalizers over the past few years. When undertaking the precise equalization of a well-designed and carefully installed system, the DSP-based units have made the task much faster. Not only do they allow you to copy equalization curves between like-systems, they are the first filters to provide precise control over the parameters. If you have ever tried to set a conventional analogue parametric to 6458 Hz and found it jumping between 6104 Hz and 6878 Hz, you will know what I mean.

Equalization technology has definitely been advancing. We just need to make sure that the sound systems we intend to equalize have been configured to make it worth the effort. Just remember, if the sound varies more than 6 dB (in level or frequency response) between any two seats, then it is too soon to touch the equalizer.

United Entertainment Media Inc.
460 Park Avenue South, 9th Floor
New York, NY, 10016
Ph. 212-378-0400
FAX 212-378-2160

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