This article appears here site with permission from the author, Bob McCarthy. Courtesy of Mix Magazine, and Curtis List who posted it on the LAUD users list.

From: "Curtis H. List" <clist@nospam.pop.voyager.net>

To The list members,

The article "Equalizing the room" by Bob McCarthy is a reprint from Feb.2000 "Mix" magazine. My sincere thanks goes out to Bob and the people at Mix for allowing me to share this with the list.

Equalizing the room.

"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 fifteen 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.

Secondly, 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.

Modern Analysis

Technological progress led to the development and acceptance of two

analysis techniques in the early 80s: Time Delay Spectrometry (TDS) and

dual-channel FFT analysis. Both of these systems brought to the table whole

new capabilities, such as phase response measurement, the ability to

identify echoes and high-resolution frequency response. No longer could an

unintelligible pile of junk look the same as the real McCoy on an analyzer.

The complexity of these analyzers required a well-trained, highly skilled

practitioner in order to realize the true benefits. Advocates of both

systems stressed the need for engineers to utilize all tools in their

system, not equalizers alone, to remedy the response anomalies. Delay

lines, speaker positioning, crossover optimization and architectural

solutions were to be employed whenever possible. And now we had tools

capable of identifying the different interactions.

But on the issue of "equalizing the room" a division arose. All parties

agreed that speaker/speaker interaction was somewhat equalizable. The

critical disagreement was over the extent the speaker/room interaction

could be compensated by equalization. The TDS camp advocated that

speaker/room interaction was not at all equalizable and therefore, the

measurement system should screen out the speaker/room interaction, leaving

only the equalizable portion of the speaker system on the analyzer screen.

Then the inverse of the response is applied via the equalizer and that was

as far as one should go. The TDS system was designed to screen out the

frequency response effects of reflections from its measurements via a sine

frequency sweep and delayed tracking filter mechanism, thereby displaying a

simulated anechoic response. The measurements are able to clearly show the

speaker/speaker interaction of a cluster and provide useful data for

optimization.

Such an approach can be effective in the mid and upper frequency ranges

where the frequency resolution can remain high even with fast sweeps but it

is less effective at low frequencies. Low frequencies have such long

periods that it is impossible to get high-resolution data without taking

long time records, thereby allowing the room into the measurement. For

example, to achieve 1/12th octave resolution, the equivalent to the Western

Tempered Scale, one must have a time record 12x longer than the period of

the frequency in question. For 30 Hz you will need a 360ms (12x30ms). If

fast sweeps are made to remove echoes from the measurement, the low

frequency data has insufficient resolution to be of practical use.

Dual-channel FFT analyzers utilize varying time record lengths. In the HF

range, where the period is short, the time record is short. As the

frequency decreases, the time record length increases, creating an

approximately constant frequency resolution. The measurements reveal a

constant proportion of direct sound and early reflections, the most

critical area in terms of perceived tonal quality of a speaker system. The

most popular FFT systems utilize 1/24th octave resolution, which means that

the measurements are confined to the direct sound and the reflections

inside a 24 wavelengths time period across the board. This is a good

practical level of resolution, allowing us to accurately equalize at around

the 1/8 octave level.

With the FFT approach, more and more of the room enters the response as

frequency decreases. This is appropriate because at low frequencies the

room/speaker interaction is still inside the practical equalizability

window. For example, the arena scoreboard reflection is 150 ms later than

the direct signal. At 10 kHz, the peaks and dips from this reflection are

spaced 1/1500 of an octave apart. At 30 Hz, they will be only 1/3 octave

apart. Thus the scoreboard is in the distant field relative to the

tweeters, and applying equalization to counter its effects will be totally

impractical. An architectural solution such as a curtain would be

effective. But for the subwoofers, the scoreboard is a near-field boundary

and will yield to filters much more practically than the 50 tons of

absorptive material required to suppress it acoustically.

Many years ago, the FFT camp boldly stated that the echoes in the room

could be suppressed through equalization. Unfortunately, these statements

were made in absolute terms without qualifying parameters, leaving the

impression that the FFT advocates thought it was desirable or practical to

remove all of the effects of reverberation in a space through equalization.

While it can be proven from a theoretical standpoint that the frequency

response effects of a single echo can be fully compensated for, that does

not mean it is practical or desirable. The suppression can only be

accomplished if the relative level of the echo does not equal or exceed

that of the direct and that no other special circumstances arise that cause

excess delay. (Excess delay causes a "non-minimum phase" aberration and is

outside the scope of this article.) If the direct level and echo level are

equal the cancellation dip becomes infinitely deep and the corresponding

filter required to equalize it is an infinite peak. As we know from sci-fi

movies, bad things happen when positive and negative infinity meet up.

Compensating for the response requires adjustable bandwidth filters capable

of creating an inverse to each comb filter peak and dip in the response. As

the echo increases, you will need increasing numbers of ever narrowing

filters. A 1ms echo corrected to 20 kHz will require some 40 filters

because there are 20 peaks and 20 dips varying in bandwidth from 1 to .025

octave. A 10 ms echo would need 400 with bandwidths down to an 1/400

octave. Obviously, it would be insane to attempt to remove all of the

interaction at even a single point in the hall. In the practical world, we

have no intention of attacking every minuscule peak and dip, but instead

will go after the biggest repeat offenders. The narrower the filters are,

the less practical value they have because the response changes over

position.

Practical Implications

It is indeed possible and practical to suppress some of the effects of

speaker/room interaction. If this was not possible, it would be standard

practice to equalize your rig in the shop, put a steel case around the EQ

rack and hit the road. The practical side of this is that we must be

realistic about what is attainable and what are the best means of getting

there.

The variations in frequency response due to both speaker/speaker

interaction and speaker/room interaction will always change with position.

Once you have seen high-resolution data at multiple positions, you can

never go back to thinking that your equalization will solve problems

globally. A system that has the minimal amount of the above interactions

will have the greatest uniformity throughout the listening environment and,

therefore, stand to gain the most practical benefit from equalization. If

it sounds totally different at every seat, let's just tweak the mix

position and head to catering. To minimize the speaker/speaker interactions

requires directional components, careful placement and precise arraying. In

areas where the speakers overlap, time delays and level controls will

minimize the damage in the shared area. To minimize speaker/room

interaction, the global solutions lie in architectural modification (it's

curtain time), the selection of directionally controlled elements and

precise placement.

Finally you are left with equalization. For each subsystem with an

equalizer, map out the response in the area by placing a mic in as many

spots as you can and see what the trends are. In particular, measure around

the central coverage area of the speaker. Stay away from areas of high

interaction, where the response will vary dramatically every inch. Examples

of this include the seam between two cabinets in an array or very close to

a wall. Each position will be unique, but if you place filters on the top

4-6 repeat offenders you will have effectively neutralized the response in

that area.

Conclusion

Modern analyzers are capable of displaying a dizzying array of spectral

data. But little practical benefit will come to us if we continue with the

antiquated approach of the RTA era. To fully take advantage of the benefits

of equalization, we must fully comprehend how to identify the mechanisms

that "unequalize" the system. With modern tools, it becomes possible to

analyze the response such that the interactive factors of speaker systems

can be distilled and viewed separately. This allows the alignment engineer

to prepare the way for successful equalization by using other techniques

that reduce interaction and maximize uniformity in the system. "Equalizing

the room" will remain in the domain of architectural acousticians, but with

advanced tools and techniques, we can proceed forward to better equalize

the speaker system in the room.

Bob McCarthy specializes in sound system design and alignment and can be

reached at bobmcc@nospam.primary.net

Copyright 2000 PRIMEDIA Intertec Publishing Corp.