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Hard proof that equalization kills room modes

In recent years there has been much debate about how effective equalization is in reducing the negative impact of room modes. Room modes are characterized by a peak in the frequency response and extended ringing in the time domain. They create low frequency coloration and lack of articulation and are perhaps one of the most prevalent acoustical problems, since they exist in all small rooms. This article provides measured evidence of the efficacy of equalization in reducing both the peak and ringing associated with room modes.

Equalization should be thought of as the last component of the optimization process. First an appropriate acoustic design should be completed taking into account the use of the space (two channel listening room or home theater; number of seats; family room vs dedicated, etc), the radiation characteristics of the speakers and the number of subwoofers permitted. Once the design is complete installation of the acoustic treatment and equipment needs to take place. Next is the process of calibration, where the acoustical targets for the level of reflections, modal decay, frequency response and RT60 are confirmed by measurements and speaker placement, levels, delays, phase and crossover slopes adjusted. Finally comes equalization, when all other adjustments have been made.

The correct application of equalization to address room modes is not a straightforward subject. Historically equalization filters were calculated based on a simple inversion of the frequency response, with no consideration as to the location / Q of modal resonances, the correctability of the issue or the location of speaker boundary interference related suckouts. It is probably these simple correction filters that have caused the debate around the efficacy of equalization.

The theory behind correction of room mode resonances is described clearly by Floyd Toole:

“Room resonances at low frequencies behave as “minimum phase” phenomena, and so, if the amplitude vs. frequency characteristic is corrected, so also will the phase vs. frequency characteristic. If both amplitude and phase responses are fixed, then it must be true that the transient response must be fixed – i.e. the ringing, or overhang, must be eliminated” (Toole, The Acoustical Design Of Home Theaters, 1999)

There has been debate about whether this theory applies in small rooms and also whether small rooms have any areas that are minimum phase. On the latter point, John Mulcahy, creator of the acoustic measurement package Room EQ Wizard has written a useful article that explains how a measurement called excess group delay can be used to understand which areas of a room’s response are minimum phase. On the former point we present the measurements in this article as proof that the ringing can indeed by reduced or eliminated by correctly deployed equalization.

The following measurements were taken in a dedicated 9 seat home theater designed by my friend and acoustics accomplice Jeff Hedback of HdAcoustics. On this project we split the workload – I did the calibration and Jeff did the acoustic design. Practical limits of space and cost limited the amount of low frequency absorption that could be deployed, so it was known from the outset that equalization would be appropriate in helping to improve performance. The design was completed in such a way that the seat to seat variation in frequency response was low, allowing equalization to be applied effectively. It was confirmed during the verification measurements that a couple of parametric EQ filters would be beneficial in improving the sound quality in the room. Two equalization filters were applied, at 25Hz and 50Hz. Since room modes have a very narrow bandwidth – typically around 5Hz – you need narrow bandwidth or high Q filters to properly address them. The filters used had a bandwidth of 0.35 octaves. The attenuation at 25Hz was 13dB and at 50Hz was 6dB.

Before the equalization was applied there was a slight subjective loss of articulation of piano, bass guitar and drums due to the resonance at 50Hz. Furthermore there was also a significant ‘energizing’ of the room at 25Hz such that low frequency effects from movies were slightly blown of out proportion and were even making the screen noticeably shake! After the equalization there was a very clear improvement in articulation and that energizing of the room at 25Hz was almost completely nullified, creating a very articulate and dynamic bass presentation. Certainly one of the better home theaters I have had the pleasure of calibrating, and a great testament to the combined skills of a home theater installer, a specialist calibrator and a specialist acoustic designer all playing their part in achieving the results.

Three different measurements are presented: the frequency response, the time/energy/frequency response and the impulse response. The frequency response measurement clearly shows how the equalization filters have flattened out the response. The time/energy/frequency measurement, presented both as a waterfall and a spectrogram, clearly shows the reduction in decay times. The impulse response measurement most clearly shows the reduction in ringing.

Frequency response

Frequency response measurements before and after EQ

Time / energy / frequency

Waterfall measurement before and after application of EQ

Spectogram measurement before application of EQ

Spectogram measurement showing reduction in ringing from EQ

Impulse Response

Impulse Response measurement showing reduction in ringing from EQ

 

Proof indeed that properly applied equalization can reduce both the frequency response peaks and time domain ringing associated with room modes.

10 thoughts on “Hard proof that equalization kills room modes”

  1. Interesting post Nyal. Did you happen to measure the decay at locations other than where the measuring microphone was placed when the EQ was calibrated? You can do a lot of neat stuff for one location, but usually it all disappears a foot or two away. Even a few inches away a peak can become a null, and this happens even at very low frequencies.

  2. Hi Ethan! Nice to have you on my blog and thanks for reading and leaving a comment!

    Indeed I did take decay measurements at other seats – in fact all of them. I will post some more measurements from another seat as a follow up.

  3. 3 years late, I know …

    Interesting stuff, but I don’t think that the waterfall graphs are really meaningful. I mean, the yellow graph (no EQ) it’s 90dB at 25Hz, while the blue one (EQ) it’s 72dB. Obviously the last one will fall below 50dB (the minimum level represented in the graph) before than the yellow graph. You should have level matched both resonances (25Hz and 50Hz) to see if there is a real improvement in the time domain.

    The same would apply to the spectrogram and impulse response, in my opinion.

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  6. Hi Nyal,

    Because this topic will never die and remains continually misunderstood, I too have done a treatment on the topic. I too used concepts presented by Toole, but of course, Earl Geddes also gets a lot of credit on this thinking as well. Basic properties of minimum phase systems requires that if things change in the frequency/amplitude domain, they will also change in the time domain.

    What I think has been missed in some past articles, and remains an issue of contention, is if the slope changes. People see that the sound decays into noise, but they make the astute point that if the overall energy level is lower, it has less distance to go until it decays into noise. That isn’t the same as saying the number of db’s per ms that is drops has actually increased. The answer there is in my article, and it is true that it does. The best way to show this is to show filtered impulse responses and Schroeder integral lines. Also available in REW.

    https://www.avnirvana.com/threads/eq-does-improves-bass-decay.2506/

    and for anyone attending AXPONA this year, I will be giving a Masterclass on REW, and it will cover a case study that looks at this an acoustic treatment, using filtered IR’s as a means to show the improvements each made.
    http://www.axpona.com/sessions_detail.asp?id=4679

    I agree with Ethan’s general statement that EQ in and of itself, while effective at eliminating ringing, is not a solution unto itself in many situations. In small office systems I see no reason not to use EQ. I have not found that a null turns into a peak over inches, usually feet (At least anything that is low enough Q to be a concern), and if you always sit in the same spot, who cares if it changes. However, for most people and most home theaters, the addition of multiple LF sources and some LF damping is certainly good, as it allows the EQ to do its job more effectively over a wide range of spaces. In fact with modern DSP you can now even cancel or weaken these room modes in a manner that mimics how bass traps work. I think EQ cannot be dismissed, there are no bass traps on the market and no multi-sub solutions that can be as effective as EQ can be, it must be part of the solution. All of these options work together to maximize the effect.

    1. Nice article! I like the use of filtered IR and the Schroeder integral. I kind of gave up trying to prove anything, as to me it was blindingly obvious and there in the data. So obvious it didn’t need any proof!

      The answer to low frequency issues is use as many tools as you can: multi-subs, EQ, acoustic treatment, room construction, speaker/listener position, etc. I don’t understand “acousticians” who stick to acoustic treatment to try and deal with sub-100Hz issues. The design of the sound system and room construction is more important to the end result in that region that any acoustic treatment.

      Did you read my bass optimization ebook yet?

      1. Hi Nyal,

        I have read much of your stuff. I agree with your approach completely. When I built my theater I designed the room first. The first step was optimizing dimensions as much as possible to minimize the negative effects of modes. The second stop was to use damped decoupled walls because they help dampen the low bass. The third was to optimize the number and placement of LF sources (Which in my case includes the mains, I overlap everything). Then once that was done, I treated the room and applied EQ. It was a no brainer. While the acoustic treatment was critical for the overall room sound and feel, it actually had relatively little effect below 100hz. It was EQ and the multiple subs that had the most pronounced effect. I’m sure the room dimensions and walls mattered too, but I have no way to measure what would have happened with different dimensions or different wall construction.

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