Equalizing the room By Bob McCarthy

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“I am going to equalize the room.” We’ve all heard that statement so many times that we scarcely think about what it literally means. We know that in practical terms it means adjusting an equalizer to suit your taste. It may be done with the latest high-technology analysis equipment, voodoo magic or simply tweaking away “until it sounds right.”

In any case, are we really “equalizing the room”? What exactly are we doing? There are lots of disagreements on this topic but all agree on one thing: You can not change the architecture of the room with an equalizer.

You can, however, equalize the response of the speaker system. Where the room fits into all this is a matter of debate; It is much more than semantics and has very real practical consequences on our approach to sound system alignment.

What do equalizers “equalize” anyway?

Let’s assume that we have a speaker system with a flat (or otherwise desirable) free field frequency response. That is to say, it requires no further equalization. There are three categories of interaction that will cause the frequency response to change, to become, for lack of a better word, “unequalized.”

The first of these interactions are between speakers. When a second speaker is added the combination results in a modified frequency response at virtually all locations. This is true of all speaker models and all array configurations, regardless of any claims to the contrary.

The summation of the two responses varies the frequency response at each position, depending upon the relative time arrival and level between the two speakers. As additional speakers are added the variations in response increase proportionally.

The second category is the interaction of the speaker(s) with the room. These are generally termed coupling, reflections or echoes. The mechanism is similar to the speaker interaction above. The response varies from position to position, depending upon the relative time arrival and level between the direct and reflected sound.

Both of the above effects are the result of a summation in the acoustical space of multiple sources, either speaker and speaker, or speaker and reflection. Therefore the solutions for these interactions are very closely related.

The third interaction is caused by the effects of dynamic conditions of temperature, humidity and changing absorption coefficient. However, the effects of these interactions are small by comparison with the other two, so we will not touch on them further here.

Are any of these problems solvable with an equalizer? The answer is a qualified “Yes”. The magnitude of the above problems can be reduced by equalization, and substantial progress can be made toward restoring the original desirable frequency response.

If equalizers were totally ineffective, then why have we been loading these things into our racks for the last 35 years? However, in a practical sense the equalizer can only provide complete success in equalizing the response when applied in conjunction with other techniques such as architectural modification, precise speaker positioning, delay and level setting.

To what extent is the speaker/room interaction equalizable? This has been a matter of debate for more than 15 years. In particular the advocates of various acoustical measurement systems have come down hard on these issues. What we are doing is equalizing, among other things, the effects of the room on the speaker system. Why is this controversial? It stems from the historical relationship of equalizers and analyzers. Let’s turn on the Way-back machine and take a look.

Early analysis

In ancient times (the 1970s), the alignment of sound systems centered around a crude tool known as the Real-Time Analyzer (RTA) and a companion solution device, the graphic equalizer. The analyzer displayed the amplitude response over frequency in 1/3 octave resolution and the equalizer could be adjusted until an inverse response was created, yielding a flat combined response.

It takes a negligible skill level to learn to fiddle with the graphic EQ knobs until all the LEDs line up on the RTA. It is so simple that a monkey could do it, and the result often sounded like it. Although these tools were the standard of the day, they have severe limitations, and these very limitations can lead to gross misunderstanding of the interaction of the speakers to each other and the room, resulting in poor alignment choices.

One such limitation is the fact that the RTA lacks information regarding the temporal aspects of the system response. There is no phase information nor any indication as to the arrival order of energy at the mic.

The RTA cannot discern direct from reverberant sound, nor does it indicate whether the response variations are due to speaker interaction alone and speaker/room interaction. Therefore the RTA provides no help in terms of critical speaker positioning, delay setting or architectural acoustics.

Second, the RTA gives no indication as to whether the response at the mic is in any way related to the signal entering the loudspeakers. The RTA gives a status report of the acoustical energy at the microphone, with no frame of reference as to the probable causes of response peaks and dips.

These peaks and dips could be due to early room reflections or speaker interactions, which can respond favorably to equalization. However, the irregularities in response could be from late reflections, noise from a forklift engine or reflections from a steel beam in front of the speaker.

The equalizer will be ineffective as a forklift or steel beam remover, but the RTA will give you no reason to suspect these problems. A system that is completely unintelligible could look the same as one that is clear as a bell.

Third is the fact 1/3-octave frequency resolution is totally insufficient for critical alignment decisions. In addition, there is the misconception that a matched analyzer/filter set system is desired. It is not. The analyzer should be three times the resolution of the filter set in order to be able to provide the visible data needed to detect center frequency, bandwidth and magnitude of the response aberrations.

A 1/3 octave RTA is only able to reliably determine bandwidths of an octave or more. What appears as a 1/3 octave peak may be much narrower. What appears as a broad 2/3 octave peak, may actually be a high narrow peak placed between the 1/3 octave points. What will your graphic equalizer do with this?

Unfortunately the absence of this critical information lulled many users into a sense of complacency predicated on the belief that equalization was the only critical parameter for system alignment. In countless cases, equalizers were employed to correct problems they had no possibility of solving, and could only make worse.

Graphic equalizers have no possibility of creating the inverse of the interactive response of the speakers with the room. Simply put: “You can’t get there from here.” (For further discussions, see “Equalizer Inequality” in June ’99 issue of Mix.)

The audible results of all this tended to create a generally negative view of audio analyzers. Many engineers concluded that their ears, coupled with common sense could provide better results than the blindly followed analyzer. As a result, though RTAs were often required on riders, they only received cursory attention on show day.