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Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ.

Re: Puh! Thanx, you saved me a bunch...

203.37.199.17

G'Day,

Funny you should mention 40Hz. If you read the review in detail (you probably have already) then you will see that the listening room has very noticeable re-inforcement at around 40Hz (44.2Hz, in theory). We measured a 10 to 12 dB 'hump' in the 'room response' in this general area. The tangential and axial modes re-inforced in that particular band.

This 'went away' - down to 2 dB. We DID measure it, and it DID disappear (down to 2 DB, an 8 to 10dB reduction), so that is your answer in bald numbers. I didn't put this measurement in, because I have no satisfying theoretical explanation for it, i.e. I can't write you an equation for it.

Except possibly... that when the entire column of traps is in each corner then you DO have a very large absorber in fact, with a height of about 94 inches in this case. This represents a very large VOLUME of trapped air (about 10 cu. ft. per corner - quick calculation in my head - please double check that). Add the fact that this trapped air is surrounded by a denser, less compliant material equivalent to many metres of air in the form of compressed polyester. This second factor, I am sure contributes significant pseudo-isothermal loss of energy at the frequencies under discussion.

Let us consider it then in terms of ENERGY and just ignore the equations that tell us about wavelength, etc., for a moment, as this is only significant in a partial description of resonance, and does not model this situation at all (ibid). These devices are clearly NOT resonators, and this is where we can quickly go astray when thinking about them.

Energy dissipation is what we are trying to achieve. We do not want this reflected energy that is normally in the room to come back to our ears and annoy us, basically.

So, we put a couple of very large "squeeze tubes" in the corners of the room, up to the ceiling (VERY important for deep LF, BTW). It takes quite a lot of energy to compress that tube at low frequencies, and energy is lost not only in the compression phase but in the elastic restoration of the tube to its null position. Hooke's Law would broadly describe the flexing, and then I guess you could get the energy loss (very roughly) from the acceleration and velocity of restoration, and the "notional mass" that is in motion (displaced) at any given time. There's a catch - "notional mass" - we'd need to get some good high speed photos, model the surface flexure, and... mmmm... or then we could just treat it like engineers (very reasonably) do, and call it a compressed tube/column...but that's not quite right either, since the load is not axial. Anyway, it could be modelled in energy terms, no doubt.

This is not unlike what happens with a diaphragmatic absorber. However, the Decware devices are so massive that they are, in fact, hard to activate, and do not succeed in doing much at normal replay levels (85 to 105 dB).

Although this is (above) a very appealing operational intuition, I cannot give you a 'nice' mechanism, and that is why my review refers to "thin theory". Rest assured the measurements coincide with the listening in large part. Pure frequency measurement (all that we did) tells us that we are not "frequency deaf", but, not much else.

Conversely, the CWALS did not produce a measured reduction at 40Hz or anywhere else exactly. They produced large reflections in the sound field, causing a confused sound stage. The diaphragms that are meant to be vibrating internally, to isobarically (one presumes) reduce the resonances of the room will only do so at low frequencies and HIGH SPLs. This is why I refer to using them in a studio as being more appropriate than a replay system/room. They are only 'activated' by high levels of environmental (sound) energy, and come to think of it, structurally borne vibration; which the large, but lighter tube trap is not so prone to, since it does not "couple" well with the larger mass of the structure (room/house) that it is in.

I would leave any further explanation to the designer himself. I do recall Jon pointing out to someone on this board in the past that the linear measurements were not as significant as the VOLUME of the trap. So, an 80 inch diameter by 6 inch high trap may well be the equivalent.

I also wouldn't be giving away $AUD 1500-00 of CWALS / DWALS if I thought they were really great, either.


Regards,
Gary Jacobson


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