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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
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
)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. 
