Scott R. Foster wrote:Bob!
Neat site, great animations, clever fellows... but as I read to the conclusion of their story, I find the results are supportive of the objections myself and others here have long pointed to with regard to the narrow band approach to resonance control [Helmholtz bottles, tube traps, membrane absorbers, etcetera]. That being, that in a narrow band approach you have are tuning and tweaking a set of absorption wells in an attempt to achieve balance against an extraordinarily complicated vibration field... the sensitivity to initial conditions, and iterative nature of that field is a ripe field for results that are kinda sorta almost right... better over here, but worse over here... we've got the 73 Hz hum tamed, but now the 111 Hz drone seems worse.
To be fair, the author does explain some of the complexities... but he then goes on to ignore em because physics simply does not allow a mere mortal with a stack of cardboard tubes and a couple of "rule of thumb" acoustic formulae to address phyical realities like oblique modes and non perpendicular angles of incidence.
Also, there are what the site's author calls "artifacts". The left overs from the treatment process that re-emit into the room cause groan and rumble.
I know some the smartest folks in this forum [and that is amrt indeed] have used narrow band devices effectively - I aint saying they can't be effective - I restrain myself to comment for the benefit of new comers to our field of discussion who are looking for the "magic bullet"... and say to such folks, there aint no magic bullets but there are many paths to Buddha, and some are low cost and uncomplicated.
For a quick down and dirty fix to an elusive problem, I guess I can see resorting to the sonotube approach, and indeed this idea [cardboard concrete column form] is a clever / cheap way to go... especially when one is able to focus on results for a single listening position. For example this sort of thing might be a great quick, cheap portable solution for a guy trying to tame the worst of the resonances in the spare bedroom of his apartment where he wants to mix up some demo tapes for his garage band. I reckon noticeable improvement would be achieved - quick and cheap - and that's a very good thing.
BUT, for a similar cost and effort, and a lot less calculation [where a noobie could easily get lost - I have been fiddling around with this stuff for years now and I always get my first set of calcs wrong somehow], one could use a broadband approach such as SCA's and get something equally portable and there would be a lot less risk of endlessly tuning the thing to try and get results... in fact I'd reckon SCA would work better first try - I am even more willing to predict superior results if our hero will add a couple of polys to back walls to provide some diffusion to work in conjunction with the absorption of SCA's.
For a home theater guy I'd guess that either a SCA or SSC and a few polys would be a much better way to go as the listening positions are spread around the room and small imbalances in the tuned absorber array and their artifacts are likely to cause noticeable problems to some users / positions no matter how you arrange the pieces.
OTOH if $ are exchanged for time, effort, calculation, one could simply purchase some ML's and some form of back wall diffusion and probably solve the problem as good or better than either of the above approaches by any measure.
He did find that the three tubes that end up at the side of the room tend to want to interact with each other and that a bit of fidgeting with position has been needed to reduce acoustic artifact from these tubes.
Our experience in the HeadRoom listening room is that as you carefully move the tubes around, you can hear a faint growly sound when the tubes interact....
Interesting site and discussion. What I would like to know is how to quantify any of these things? For toilet tubes and helmholtzs and membranes how many Sabins do they equate to? Does anyone know the relationship?
Scott R. Foster wrote:Bob!
I find the results are supportive of the objections myself and others here have long pointed to with regard to the narrow band approach to resonance control [Helmholtz bottles, tube traps, membrane absorbers, etcetera].
Eric.Desart wrote:Hello Zaphod,
Your's are broadband absorption devices, working on a different principle.
So this is not related.
1) the size of the wavelength is much smaller than the diffuser width it acts as ping pong balls reflecting.
2) the size of the wavelength equals the diffuser width the wave gets scattered (reflected in random direction). This even happens with flat slats. That's why a John sayers slat type Helmholtz absorption approach will also act as diffuser in those higher frequencies.
3) the size of the wavelength is much larger than the width of the object, the wave will not notice the object.
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