Adventures in Room Correction

A most interesting thread, and a subject that rarely get's much airtime on hifi forums. So, whilst it is here, and as it pertains to a subject in which I have some interest, the I may as well stick my oar in.

It is universally true that it is difficult to treat spurious mode-related resonance in a small listening-room environment. The vast wavelegths limit the use of simple absorption for reasons already covered - although a workable bass trap need be nothing more than a roll of loft-insulating fibreglass. Stack one or two in the corners and you will have wideband (relatively speaking) low bass absorption. That said, the builder-merchant style of decor is not to everyone's taste, leaving effective, and modestly discrete low-bass absorption to more refined methods.

Panel traps are one very valid means, as too are designed absorbent bass traps. Yet, whilst undoubtedly effective at broadband (again, relatively speaking) low bass absorption, they can; if over-done, kill a room. Now, this may be the brief in a recording venue, but rarely that of a listening one - there is a distincition in venue requirements, and one all too often overlooked. Now, don't get me wrong, there is nothing wrong with the use of broadband low bass treatment as a base layer of room treatment. Indeed, there are rooms that may need (as a function of benign room size and furnishing, etc) nothing more.

But, in other cases it is the broadband nature of such products that means that it is ineffective at; specifically, single problematical resonance frequencies. More so, when you beging to consider such anomalies, you realise that the problem may not be soley one of amplitude, but of the ability of the mode to sustain, or to ring. This is where 2-dimensional frequency sweeps fail us. We need to introduce time into the equation, and more specifically in the guise of spectral decay.

So, how do we achieve narrowband absorption and control resonance at the same time?. Well the oft mentioned Helmholtz resonator is one means, and one i've done quite a bit of work on. Once i'd established my target frequency (as much by listening as by testing), then I assumed my brief to be to design a frequency-specific device to reduce the modal amplitude by as much as possible. This was a mistake that I realised only with the benefit of hindsight

Anyway, and to cut a long story short, resonator size has been mentioned a few times, to which all I will say is: tuned to 61Hz (app.) my resonators (two of them) are (in tube form) about 18" tall, by 9" in diameter (working diameter). Given that in the process of tuning them I managed (to the best of my knowledge) a maximum amplitude reduction in the order of 3-4dB (and operhaps, higher, but i'd have to confirm that). Albeit of a fantastically narrow bandwidth of operation - such that system Q may well have been in the thousands Anyway, they are not large devices by any means, yet demonstrably effective. More so when I realised that they were better served controlling resonance (as in ringing) rather than out and out amplitude. After final tuning, through the design addition of wadding to the resonant cavity (essentially to lessen the resonant effect of the cavity, and reduce amplitude effect in favour of reducing modal resonance), I settled for a measured maximum of 1.25dB.

This may not seem like a lot, but as I said, this is relative to the 2-dimensional amplitude function. When considered with time, in terms of spectral decay, then I assume the effect to be wholly positive. In audible terms the reduction is marked, and the effect on sonic replay likewise. You can find more about the project in my article, which is available on the Audioholics website, here:

http://www.audioholics.com/techtips/roomacoustics/Helmholtzresonatorabsorber.php

 

Excellent post, IMHO. It had struck me that the analyses I have seen are concerned only with the frequency domain. For instance, is there such a thing as a subwoofer that has built-in digital correction is the time domain as well as the frequency domain?

 

Yes I too have found a much greater improvement in the RT (reverb time) of the bass in my room from having the bass traps than I have noticed in the actual frequency amplitude responce. Though they did help both. I have not measured the RT but it is quite clear when listening. Thats why I have both the DEQ and room treatment. The bass traps fix any 'ringing' or 'boom' and the DEQ fixes any remaining amplitude problems.

I think if you used EQ on its own, you would find it a bit strange because although the amplitude would be correct you would still hear the 'ringing' caused by bad reverb times - which is just as much a trigger to you brain that there is something wrong with the sound as the frequency amplitude is. This is IMO the reason why so many people dislike room corrective EQ. One really needs to sort out the rooms RT60 before you start EQing or your mind will still detect the sound as being different at certain frequencies. Once it is low enough you more or less have a clean slate to work with the corrective EQ.

Killahertz, nice post. Have you tried any panel traps in the same way? My experience with both resonators similar too yours (although I admit I didn?t spend long tuning them) and the panel traps I use, is that the panel traps had a much larger effect on the RT60 of the bass as it was over a larger range. Once you had tuned the resonators to have the 1.25db effect in the frequency amplitude, what sort of Q did you have on them?

 

Thanks.

With regard to panel traps my experience extends only to their theoretical application and design, not practical. My room, whilst relatively small, has dimensions that are modally benign, which is confirmed in theory (i'm happy that said theory holds, as a result of the space being of basic rectangular shape, and solid-wall construction), and also in practical listening assessment. Modally benign, that it, with the exception of the frequency anomalies I have targetted with my resonant absorbers.

I'm satisfied, then, that I don't need specific broadband absorption, which limits the practical need for panel traps. But, whilst I would consider them for practical experience alone, room size means that I have limited wall space - especially when you consider I have multiple absorbent panels already in place (as part of the room's specular HF treatment, along with ceiling-mounted diffusion). Leaving floor space (and the corrners) as the only viable locations for such treatment, and hence the main reason for considering the free-standing resonators.

As for the Q of the resonators - well I can realistically (I believe )tell you their theoretical maximum Q, which is a considerable 131. I'd estimate thier current Q to be somewhere about 1/5th of that. These are truly frequency-specific devices, and hence the need for careful design and tuning.

With regard to your panel traps what material are you using for the membrane?. And the cavity behind the membrane, I see you vary it with different panels - which is a sound idea, and would add to their overall broadband functionality - but do you use any form of absorbent in the cavity to widen operational bandwidth at all?.

 

Good discussion. A quick couple of points:

1. Panel absorbers can be built by anyone at quite low cost, and can have a noticable effect (improvement). You can find several suitable designs online, whether you are after absorptive traps (broad-band control) or Helmholtz (resonant, more focused on rather specific problems.).

2. Don't expect such absorbers - or even the commercial panels - to have any impact on a measured 'sweep' of the room. Benefits appear in the time domain (reduced ringing) rather than frequency domain*. By this I mean that straight FR sweeps will still show a peak; yet, a ssuitable bunch of 'bass traps' - while it won't reduce the level of bass in-room -should improve the ability to discern double-taps on the bass drum, bass timbre and so on.

3. Appropriate damping in the room has no bearing at all on how much your music of choice annoys the neighbours. You wouldn't believe the number of people that confuse absorption with isolation[/].

 

felix,

Good points all of them, and especially No.2. I can understand and equate to the point you make with regard to bass drum clarity. My own reference throughout testing was the double bass of John Clayton (from Diana Krall - A Night In Paris). Real depth and clarity, with pace and timing - and not easy to get right in any system, especially one where the listening room has been negated.