Almost every sound system has an equalizer. It can be as simple as a channel strip tone control or as sophisticated as a multi-band parametric.

Some view the use of equalization as an art form, while others see it as distinctly technical in application. A prevailing myth is that an equalizer can correct all of the shortcomings of a sound system.

Of course, this is nonsense but used properly, equalizers can improve the sound quality and general performance of most sound systems.

Rather than covering every aspect of equalizers and their use, let’s have a look at ways of achieving the best overall results in the shortest period of time. The approach is technical, not artistic. Let’s work under the assumption that we have a means of measuring the magnitude of the frequency response of the direct field of the loudspeaker using one of the mainstream tools.

The procedure often followed for equalizing a loudspeaker is to place the measurement microphone on-axis and adjust for the flattest frequency response. This involves cutting and boosting some filters on the equalizer.

Those that are opposed to the use of boost filters may choose to arrive at the same resultant response by reducing (cutting) parts of the response to the lowest common denominator. This results in the same electrical curve, but without compromising headroom in the signal chain.

None of this is rocket science, and the process could be relegated to a computer algorithm.

As useful as this exercise is in producing a flat on-axis response curve, it ignores what is going on at other vantage points around the loudspeaker and may actually reduce the overall sound quality from the system/room for many of the listeners.

A modification to this approach: considering the off-axis as well as the on-axis response (not a new idea but this may speed the process). The goal of the equalization process is to produce a better sounding system for all of the audience. Yet a relatively small percentage of the audience sits in the on-axis position. It would, therefore, seem ill-advised to consider only the axial position when equalizing a system.

A possible solution is to average the response of a number of seating positions to arrive at the best “common denominator” curve for the equalizer. This is called a spatial average, and while useful, there are some major drawbacks, including:

—The measurement microphone is in a different acoustic environment each time you move it. This makes it difficult to isolate the direct field without a lot of setup work.

—Loudspeaker interactions can produce huge swings in the frequency response from seat-to-seat. A spatial average can’t correct this. This means that equalization should initially be conducted on one loudspeaker at a time.

—While the symptoms of acoustic problems can be observed with this method (i.e., uneven coverage) the cause cannot.

—It’s not practical or possible to measure at all positions around the loudspeaker, so the equalization curve is influenced by relatively few measurements.

Because any adjustments with an equalizer affect the total radiated energy, it is wise to give consideration to all of the radiated energy. The spatial average is not a bad idea, it’s just hard to implement.

Different Way To Same

Another way to consider off-axis listener positions is to determine the base equalization curve for the loudspeaker by observing its three-dimensional radiation balloon.

A properly gathered balloon will reveal the anechoic response of the loudspeaker at all listener angles.

Because the direct field of a loudspeaker is considered to be largely independent of the acoustic environment (at least at short wavelengths), direct field equalization based on balloon data has a strong theoretical basis.

Figure 1 shows the axial frequency response of a multi-way loudspeaker. The dip in response at 2 kHz is due to phase cancellation between multiple drivers in the box.

A boost filter at this frequency center will restore the axial response to flat. Observation of the polars and the entire radiation balloon at 2 kHz (Figure 2 and Figure 3 ) shows that even though they cancel on-axis, the devices come into phase at two off axis positions. At these angles, there is most likely a significant peak in the response for many of the audience members.

The “correction” made to the on-axis response will likely worsen the off-axis response where a greater number of listeners are located. Worse case is that one of the off-axis energy lobes covers a microphone position, so a boost filter will likely worsen the gain-before-feedback.

Devices that have a destructive phase offset at one listener position are likely to have an in-phase relationship at another. The on-axis notch might better be addressed by the use of precision signal delay between the elements to bring them into phase rather than feed them more energy.