Thanks @Xanthe this gives me a useful guide
Hi Xanthe,
If I may, I’d say that acoustic foam, 3 inch thick, will do its job down to roughly 300Hz.
Rockwool or glass fiber (like OC-703) is perhaps a better option.
It may sound rhetorical, of course, but I do consider peaks as destructive as nulls. As I see it, adding or removing is degradation. And, in a way, peaks are worst then nulls. They are far more noticeable.
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Here is an online app that will help you calculate the “perfect” room size to start with if you plan to build a listening room from scratch.
The link includes some dimensions. The ones I intend to use if I have the opportunity to build a dedicated room :
→ amroc - THE Room Mode Calculator
Of course, there is no such thing as a perfect room, but with the right shape and dimensions, and the correct type and amount of treatment, one may reach a nearly flat frequency response 
Beyond agreeing with Thomas, my statement was purely to suggest talking room treatment in a language that most people understand. We are not all rocket scientists and getting a nice room response suiting your taste with acoustic treatment really does not have to be rocket-science from my experience. ATB Peter
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With reference to @Thomas ’s reply, have a look at this thread
in particular posts 6 & 23 where you will find info and links to spreadsheets perfect for informing room design with plain rectangular rooms.
Of course, rectangular is not ideal, and if building from scratch other designs may be much better. I always wondered about pentagonal with sloping ceiling, but more info is available on what is known as the golden trapagon (Google…).
There is a lot of documentation around parallelepipedic rooms. Their relative predictability makes them the easiest choice when building from scratch, but perhaps not the best choice.
I might have the opportunity to build my own listening room. It’ll be around 40 m2 with the ceiling at around 3,6 meters.
Ceiling and floor will be parallel, but non-parallel side and front-back walls is a possibility. But doing so might lead to unpredictable and potentially nasty room modes.
I read some papers related to room modes in non-parallelepipedic rooms, but I’m not sure I could do the math.
If you have some links to interesting information, I would be grateful.
Hi Thomas, I have posted links to anything specific of interest in the thread I referenced.
I’ve thought myself of building a room with in-wall (“soffit mounted”) speakers, with a thread asking others’ experiences (none). But whether the needed house move will happen I don’t know, COVID having resulted in shelved, possibly cancelled, plans.
In this context, where we are considering the effects of wave interference pattern, the words constructive and destructive have specific meanings.
Agreed, of course.
But, sometimes, being too specific or precise may lead to some confusion, especially for those lacking a certain understanding.
“Constructive” could well be interpreted as “more” and therefore “better”. Which obviously isn’t the case.
The problem there isn’t the material, it’s the lack of thickness, to act as a realistic bass trap (even for mid bass) a porous (i.e. flow mode) absorber needs to be at least 300mm thick (hence the very much approximated 120Hz lower cut-off).
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It strikes me that the best solution for this would be to use FEA, this would be capable of anaysing not only the air’s behaviour but also the interaction with any non rigid surface.
On the other hand using constructive and destructive in both the physical sense and the perceptive sense could lead to confusion.
“The constructive nature of the result of course makes it more destructive, and creating a partially destructive pattern at that point would be highly constructive.”
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You need to look for a very high but as yet un-renovated end 1930’s property - you may just be luck enough to find such a room already constructed!
Other than that, wall off part of a normal room with a cavity wall built from high density concrete blocks; the void created will be you ‘speaker cabinets’. (Selecting the drivers and designing the speaker’s structures and crossovers for such a system is a whole new can of worms.)
I have just moved my system from a small listening room (3.3m x 3.0m x 2.3m) to our larger living room (5.0m x 4.8m x 2.8m). Due to room layout constraints I have had to position the speakers firing across the slightly shorter length. The larger space has allowed me to find the theoretical optimum listener and speaker positions, but I now have to finalise the actual positions via laborious testing.
During my pre-moving ‘due diligence’ research into room acoustics, speaker positioning and room treatment, I calculated the theoretical ‘optimum’ position for listener and speakers and rack (Fraim). I have had a listen with unspiked speakers at this theoretical optimum positioning and immediately noticed deeper and more controlled bass and a more airy soundstage with greater definition. As the speakers now have room to breathe, I have removed the foam bungs from the rear speaker ports.
I also produced a graph of the theoretical standing waves (constructive interference, nodes or whatever you want to call them) based on the empty room dimensions. It looked reasonably sparse but, as the room is almost square, I was expecting one or two low frequency standing waves, and indeed the graph showed one in particular around 31 Hz. As this is such a low frequency, on the edges of an old man’s hearing, I thought it wouldn’t be a problem. I can however hear it when the music goes deep. Not overpowering by any means, but I am going to try and improve things by optimising listener/speaker positioning, re-installing speaker spikes, furniture repositioning/replacement and with/without speaker bungs.
As I say the low frequency standing wave(s) is not too bothersome and is significantly outweighed by improvements in SQ elsewhere, and I know there is more to come.
I am aware that it is difficult to treat low frequency humps with velocity traps as they are less effective at low frequencies and need to be very bulky to have any effect. Pressure traps are even more tricky in that you must correctly identify the frequency and position of the standing wave(s) to be treated. Get the frequency and position slightly wrong and pressure traps are ineffective, an expensive mistake.
I know I can never remove standing waves, particularly low frequency ones, but I will try to improve things as much as possible without bespoke room treatment e.g. (bass traps) as I don’t want to go down that path in this very old property.
Does anyone have any ideas how best to treat low frequency standing waves (in the 30 Hz region) without resorting to bass trapping? The last thing I want to do is destroy the lovely enhancement in the mids and highs.
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I would be tempted to try with the bungs loosely fitted, partially blocking the port etc to see if there is some middle ground?
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The problem is the property has to meet other needs as well… The house I had in mind (70s bungalow) has a room large enough to build a good size listening room within, apart from limited height unfortunately. COVID has resulted in changed plans, with house move postponed and uncertain, so whether that ever comes about I don’t know. And yes, solid walls needed - which makes going ahead a substantial decision (pun not intended) so much planning first…
In an ideal world I’d have a house built to my design - but there is simply no land of suitable size in the locations to which we would want to live out our years after retirement (the rare plots that do come up invariably have planning permission for multiple houses, pushing the land price up ridiculously).
Absorbing at 30 Hz, isn’t an easy task. Just forget the usual porous absorbers, they have literally no effect.
The only way to go is to precisely identify the frequency and build something targeting that specific frequency (Membrane based absorber, Helmholtz Resonator or diaphragmatic absorber).
There is another, but costly, option : active absorption, like the PSI AVAA C20.
Moving the listening position, if possible, is something to try. In fact, the listening position has massive impact on the perceived low end.
It is easy to try : just walk around the room while listening to low frequencies (under 250 Hz).
I uploaded for you some pink noise here (low frequencies only) :
→ http://www.serra.se/LowF_Pink_Noise.zip
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Hi Nigel,
Just forget my previous answer. I fear my “techy answer” won’t be of any help.
My advice would be : just forget your 30 Hz issues. It’s no big deal really, unless you’re an organ or electronic music amateur. Even so 30Hz doesn’t happen so often.
I would worry a bit more about the upper part of the low end spectrum.
If you have a room mode at around 30Hz, it means you have it too at around 60Hz, 120Hz, 240Hz and so on.
You can deal with 60Hz, and up, with 6 inch (or more) porous absorbers. If you make them yourself it’ll cost you the price of two bottles of Islay single malt 
(and you can make a lot of them).
The link below may be useful :
→ Porous Absorber Calculator