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JON RISCH .versus. DECWARE Room treatments - A Comparative Review

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Jon Risch .versus. Decware Room Treatments

A Comparative Review


Introduction

This article summarises the results of 18 months of listening and measuring with two substantially different room treatments installed at various times and for various intervals. These were the Decware (http://www.decware.com) CWALS and DWALS combination and the Jon Risch acoustic treatments as described by Jon at XOOM.COM. All sound pressure levels referred to herein were measured with a Type 2 meter. The description following avoids all but the most basic technicality needed to describe various processes, in an attempt to make it both easily readable and comprehensible.


Active Equipment

The active equipment in use for the listening tests remained constant throughout. Therefore, other than to say that the equipment was kept in top-notch working order, it is irrelevant to describe it further. This is one simple controllable parameter taken care of.


The Room

The physical dimensions of the listening space are 7.8 (25.5) x 4.5 (14.8) x 2.4 (7.9) metres (feet). The listening environment was fully controlled for all tests, in so far as is practical in a domestic situation (i.e. these were not laboratory tests). A floor plan was drawn, and all furnishings present were noted and their positions recorded on the drawing to within 1cm. An inventory of 'portable' items in the room was taken, but I can't be sure that a extra video cassette was/was not left on a shelf at the opposite end of the room now and then. The room, sans room treatments was therefore removed as an influence vis-à-vis any sound field altering devices being present - other than those under examination.


Listeners and Listening

The usual gang of idiots - all experienced listeners. Each listener was allowed into the 'hot' seat (this was not a mass panel test), so that all the features of the sound stage were apparent to the listener, and even the smallest alterations to the sound field could be fully assessed, if any. Two people were present in the room at any given time, one sitting on the floor, 'out of the way'. The testing was not "blinded" in any way, i.e. the listeners could clearly see the room treatment in use in the room, or not. So, yes, we were "expecting" to hear something. Program material was varied, selected by the listener, and very familiar to the listener. Jazz, Pop, Rock, Symphonic works, New Age and Monadic Chant(!) covers it pretty well. The same CDs were used repeatedly, not a similar copy from the same company.


The Acoustic Science

The modes of the room were calculated. This is at best an unsatisfactory procedure involving the use of simple mathematical tools, which are based on some very thin theory. Also, the calculations assume that the room walls are 'ideal' reflectors, and that the walls do not flex, and a hundred other factors that we can neither account for, nor measure in this environment. The speed of sound was assumed to be 345 m/s on average, and we did not trot out the barometer to assess the air density every day. Given all the above we applied the following simple model:
Room Modes

Classical Formula : f = c/2 * SQRT((Nx/2)^2) + (Ny/2)^2 + (Nz/Lz)^2)

where

f : frequency of the Nth mode
Nx,Ny,Nz:Integer or Integer pair/triplet indicating
the mode under examination.
Lx,Ly,Lz :Dimensions of the room in metres
C:Speed of Sound in air at 25 degrees Celsius.


Axial Modes

It is clear that for the axial modes, this simple function reduces to:

F = c / 2L

where L is the length of the axis in question. An axis being the Length, Breadth or Height of the room.

So, for the listening room described above, we have:

Length:f(1,0,0)=22.1 Hz
Breadthf(0,1,0) = 38.3 Hz
Heightf(0,0,1) =71.8 Hz


Tangential Modes

Clearly, for the above function, for 2-D resonances, the expression reduces to the form:

f(x,y) = c/2 * SQRT ((Nx/Lx)^2 + (Ny/Ly)^2)

and similar expressions for the (x,z) and (y,z) planes.

So, we then have:

f(1,1,0)=44.2 Hz (!!!!!!){ X - Y }
f(1,0,1)=75.3 Hz{ X - Z }
f(0,1,1)=81.5 Hz{ Y- Z }

Hence the following major modes are predictable for this room:

Axial (1st order)22.1,38.3,71.8
(2nd order)44.2,76.6,143.6
(3rd order)66.3,114.9,215.4

Tangential (1st order)44.2,75.3, 81.5
(2nd order)88.4,150.6,163.0
(3rd order)132.6,225.9,244.5

It is apparent that some nasty re-enforcements in the lowest 3 audio octaves could be occurring in this room, if left bare, (i.e. no furniture, no carpet, no curtains, etc.) and driven with a speaker system that activates all the room modes (it isn't).

The Real World

The room, however, contains two large leather upholstered chairs, curtains of medium weight, carpet, and 15 square metres of glass doors and windows. Measurement showed that a 10 to 12 dB emphasis was occurring in the 20Hz to 80Hz region as "expected", tapering to 'only' 5 to 7dB emphasis at 200Hz to 1000Hz. Various other anomalies were observed. The room did not have a 'flat' response by any stretch of the imagination. I said we were "expecting" to hear something, and given these sorts of bumps and grinds, we were even more expectant. If we couldn't hear differences of 10dB or so, well, we would turn in our hearing aids forthwith!


The DECWARE CWALS / DWALS

The pictorial details can be viewed at http://www.decware.com. Plans were purchased and the devices professionally constructed - too many complex cuts and joins for a carpentry tyro like myself.


The CWAL

Briefly, a C.W.A.L. is a 60 inch high sealed MDF enclosure with an irregular pentagonal cross section uniform throughout its length. It isa designed to fit in a corner and not quite touch the walls. Internally, it is filled longitudinally with alternating layers of THERMAFIBER (tm) (mineral wool) and layers of bubble wrap. The bubble wrap, it is presumed, is there to ensure a defined air space between the mineral wool layers.

The DWAL

A D.W.A.L. is basically a 60 inch high by 22.5 inch wide by 12 inch deep MDF box in appearance. The rear 4 inches of the 'box' is sealed with a layer of Thermafiber and two layers of bubble wrap enclosed in it. The bottom 12 inches of the box is also sealed and filled with three layers of Thermafiber, alternating with bubble wrap. The front 8 inches of the device is faced with an assembly of rectangular MDF baffles. These create 7 inch by 3 inch, 3.5 x 3 inch and 2 x 3 inch wells across the front of the device. Unpacked 'dead' air is trapped in a 48 inch high resonant column behind the left and right baffles and each of the two central baffles. There is no attempt in the design to damp this latter air space.


The CWAL is claimed to provide "significant absorption" (unspecified) down to 40 Hz. The DWAL is claimed to "make the front wall disappear": OK - it's somewhat less than quantitative, but you could clearly hear what it does, if it does it(!) That's the point I keep making here. The front of this device appears to be a pretty uniform reflector of all energy down to about 580 Hz or so.

The front of the DWAL must reflect above 4 600 Hz since its 'well' width is 3 inches (75mm). Measurement confirms this. A well depth of 7 inches (~175mm) implies some absorption/diffusion down to 1971 Hz. However, the diaphragmatic absorbers at the rear and bottom would appear to come into play at 300 Hz and lower. Theory predicts that this is going to be a fairly non-linear device, in more ways than one - don't forget those 4 foot 'organ pipes' at the back of the baffles (!)


Listening to the CWALS / DWALS

Two CWALS were placed in the 'front' corners of the listening room, 'behind' the speakers. In all listening tests, listeners were asked to give a description of the "sound" of a familiar piece of music, and to mark on a diagram of the room, where they perceived the performers / instruments to be on the sound stage. Most of the listening material used displayed excellent 3-D sound staging when listened to in the untreated room.


Summary of Comments (Listening with CWALS only present, no DWALS)

"...marginal emphases in the lower mid band....'chesty' male voices ..."
"... voices of massed singers too prominent...pushed forward..."
"... an odd decay character..."(???)
"...bass strings light in body and lacking in 'stringiness'..."
"... electric bass a bit lumpy, but well-defined compared to standard room..."
"...sound stage flatter, less of the third dimension..."
"...width of sound stage seems slightly constricted on light operatic material..."
....and so on. Quite a lot more was accumulated in the 50 listening sessions.

The last comment was re-enforced by the listener being able to plot the perceived positions of the performers on the sound stage with good repeatability in tests separated by several months(!) Perhaps he has superior aural memory?

The stunning realization is that the CWALS have minimal effect in the bass spectrum in the regions claimed! They appear to absorb at a specific set of frequencies in the low bass and then act as substantial specular reflectors of most of the energy above 500 Hz. Hence, we note the comments about shifting images and "odd decay character". Mid-range is emphasized it would seem, and I am sure that quite a large number of listeners / constructors would find this attractive, in the short term. It is not however 'correct' in the documentary sense of reproducing the sound stage event, as we might perceive it live. The sound stage is (as a number of listeners noted) "pushed forward" or "flattened".


Summary of Comments (2 x CWALS + 3 x DWALS)

The DWALS were placed centrally between the CWALS and the speakers; slightly off the 'front' wall and behind the speakers, as is recommended by the designer.

"..the rear wall doesn't disappear ...hasn't disappeared yet...(unprintable bits here).." (unanimous observation)
"...some differences in ambience reproduction, like bucket-brigade echo..."
"... some improvement in diction and placement of singers..."
"...massed voices still 'in your face' too much...these people are at the back of the orchestra(!)..."
"...violins a bit shrill, male voice seems oddly pitched..."
"...not a dramatic effect as claimed, some differences in lower mid-range, but some very odd things happening up and down the spectrum...program dependent..."
"...the bass drum shouldn't be down there..." (don't ask me!)


CWALS / DWALS Overall.

After a very long series of listening sessions with these devices in and out of the room, we observed that they produce easily discernable alterations to the sound field. If they were extended from floor to ceiling, they might show more dramatic effects. However, I have had to conclude (interim) that what they do to a sound field, although detectable, and definable, is not particularly useful in the reproduction part of the audio chain. The devices display odd "tuning" characteristics, and may even be of more use in the recording studio, placed around or near specific instruments. I am giving the set I had made to a friend with a recording studio, in fact. The CWALS / DWALS do nothing to 'correctly' flatten the room response, and produce sound staging 'effects' which will be more pronounced in some rooms than others. Some listeners will find this attractive in the short term, but it probably will become wearying after some time, not unlike a pair of loudspeakers with a certain frequency emphasis - can be impressive at first - but... These combined effects are no doubt the 'improvements' that some have noted in listening conditions on a short-term basis. I would also suggest that a high sound pressure level during replay of bass-emphasized material could be superficially improved by using these (rather dense) devices.


The JON RISCH Tube Traps and Wall Panels

Some conferencing with Jon, via AA, was required before settling on the final materials for construction. The designs as proposed by Jon at XOOM.COM were implemented with his guidance where we were unsure of the exact instruction or wished to change a material. The traps and panels were built using R2.5 polyester insulation batts with an average density of 0.7 lbs / cu.ft. A less dense bonded acrylic fibre (0.3 lb / cu. ft.) was used to face the panels and traps as per Jon's instructions. Tube traps, six in all, of 16-inch diameter were constructed of wire and chipboard to Jon's published assembly instructions. Large wall panels, 7 feet by 3 feet by 14 inches thick were made on relatively light wooden frames. The front of each panel was compressed at the long edges with industrial staples such that the thickness of each panel was 12 inches at the edges with a uniformly convex front to a maximum thickness of 14 inches at the panel centre. These thicknesses include the two layers of lower density polyester batting affixed to the front and sides of the panels. No part of the wooden frame was left exposed but for the "feet". The frames are self-supporting and free-standing, lending themselves well to being maneuvered around the room.

Jon claims that the devices absorb in a linear fashion producing audible improvements in the overall 'response' of the room, and consequently 'cleaning up' of the sound stage, depth, perspective and so on.


Listening to the JON RISCH Tube Trap

1. Initially, two 3 feet high and 16 inch diameter tube traps were introduced to EACH of the 'front' corners of the room, behind the speakers; in the exact places where the CWALS had been, but hard against the wall, as the designer specified. So, we have 4 tube traps in operation.

Summary of Comments ( 4 tube traps in front corners)

"...dramatic improvement in apparent ambience and depth of sound stage..."
"...instruments deep in the mix are clearly defined and easily heard, though not prominent or emphasised in any way..."
"...deep and mid-bass exceptionally clean and well-defined...notes easily being followed...you could learn a part from this..."
"...spectacular rendition of choral pieces...singers are moving air...not shouting in your face..."
"...male voice sounds correct...I imagine this was a velocity microphone and not a pressure microphone..."
"...subtle changes in rhythm very easy to follow and 'pick up on'...percussion is effortless and conveys power well..."

Since I am used to "clean and well-defined" bass from the Quads, this was a shock for a start. Others confirm the result though, so I wasn't hearing things. In the main, I can only agree with the above observations.

2. The JR tube traps were extended with another trap added to the top of the stack such that the top of the stack was within 1 inch of the ceiling.
Bass control improved further (there is a sub-woofer in the system BTW), and there was far more than the 'slight' improvement we were "expecting". So it appears that a room treatment must be able to be extended to the ceiling for optimum results.

3. The JR wall panels were added to the room, in the same positions between the speakers on the 'front' wall as previously used for the DWALS. The panels were "listened to" at varying distances from the wall. - progressively, 4 inches, 8 inches, and 12 inches to the rear of the panel.


Summary of Comments ( 6 tube traps, 3 panels)

"...subtle parts of the mix, such as in softly played massed violin are now effortlessly appreciated...no strain...very 'light' as in the real thing..."
"...bass drum is up the back, and it sounds like a big bass drum...tympani likewise..."
"...close-miked voice is now very intimate without the rest of the stage being pulled forward with the soloist..."
"...light opera works show precise positioning in the sound stage...did not think even more apparent depth of staging would be possible..."

Quite a lot more. The differences in sound staging the factors were most commented upon in the finish. It seems that these panels helped further tame the LF axial modes we predicted and observed very uniformly. No weird emphases were commented upon, and the listener's were unanimous in their comments about dramatic improvements between these devices and the DECWARE units. I don't like saying it but - "no comparison" and "chalk and cheese" - were the commonest aural-reference clichés used, by people who don't like to express themselves that way normally.


Conclusions

We have to conclude that both the JON RISCH and DECWARE devices make significant subjective differences to the listening environment. It is very clear that the listeners (20 in all) when seated at an optimum position in the room, listening to speakers that have very low colourations, and low levels of room interaction to start with (cosine polar characteristics); unanimously and by a very wide margin preferred the JON RISCH room treatments for both their quantity and quality of performance as claimed by the designer. The neutrality that these devices confer on the listening environment must be heard to be believed, really. It is probable that the design of the DECWARE devices introduces odd resonances and tuned structures to the room that further complicate the listening environment in a very non-linear fashion. In our opinion, in the case of the DWALS the device does not perform as claimed by the designer.

The Jon Risch designs are also clearly superior in engineering terms, costing less than one fifth of the materials outlay than for the Decware devices, taking into account the use of the more expensive polyester insulation batts in the panels and traps throughout. Maneuverability and general portability of the JR devices is another factor that cannot be ignored - of course, none of that matters if they don't work acoustically. However, I must say it again - they work extremely well, offering the better of these two alternatives in all respects and by a wide margin.

Copyright (c) Gary Jacobson, June, 2000
(WORD 2000 document available)


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Topic - JON RISCH .versus. DECWARE Room treatments - A Comparative Review - Gary Jacobson 03:31:03 06/05/00 (22)


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