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RE: Here's my challenge to you.

Posted by KlausR. (A) on January 24, 2008 at 04:50:25

In Reply to: RE: Here's my challenge to you. posted by David Aiken on January 23, 2008 at 06:45:04:

>You've offered no proof that cube shaped rooms aren't more problematic than others…<

Well, since a cube is, at least in theory, the worst possible case the approach is very much the same as for room with optimum dimensions: the only manner to have the worst possible conditions is to place the loudspeakers and yourself in a corner. Here you excite and hear all modes.

There is one question I’m asking myself: does it matter where on the room mode pressure curve you place the loudspeaker? If you place it in a maximum, you fully excite the mode, if you place it in a minimum, you don’t excite it at all (at least in theory). What happens if you place the loudspeaker somewhere between maximum and minimum? Do get full excitement, partial excitement, or what? I did not yet start investigating the relevant literature, so I don’t know the answer. Everest says that the modes are driven proportionally, but what exactly does that mean?

>Of course it is just that, but the main contributor to that difference will be the first reflection which is the strongest reflection from the area and which arrives first so there's your evidence that, in this case, we have a first reflection problem.<

Not necessarily, the quality of the loudspeakers may be such that pair matching is bad, in which case the louder speaker would dominate apparent source location and image width.

Then there is the fact that not everyone hears the same: in-ear measurements have shown that there are substantial differences between individuals because of different shapes of head, torso, outer ear, different hair, etc. What the questioner is perceiving may be different from what you or I would perceive. I’m just reading Newell, “Recording studio design” and the author mentions personal anecdotal evidence. See also

Shaw, “Ear canal pressure generated by a free-field sound”, J. of Acoust. Soc. America 1966, p.465

>Yes, a balance control might do the trick but a lot of equipment no longer has one. While that will fix the location of the centre image it won't control the sideways shift of the apparent source from the speaker to the reflection point on the wall. Toe-in is unlikely to fix any shifts of the centre image or the sideways shift of the apparent source nor will moving the speaker though all of these suggestions will certainly change the size of the apparent shift.<

Balance control will lower the SPL of both the speaker and the reflection. The apparent source, the phantom source, is located between the speakers, ideally it is centred, but that depends on program material. With speech mono recordings the phantom source is dead centre, if not, you have a problem.

Toe-in will change the reflection’s SPL, depending on the speaker’s off-axis behaviour. Changing the distance speaker-listener will change SPL directly, both of direct sound and reflection. All these measures might be sufficient and they are for free. One measure I forgot is to shift both speakers to the left side, this increases the SPL from the left while decreasing SPL from the right.

When you consider the setup with the dimensions and distances as indicated by the questioner, especially the 2.5' from side walls, this means that the reflection is delayed by about 2.8 ms. The lower limit of the precedence effect for artificial signals is 0,8 ms, for music I could not find any data. It may well be that the figure of 2-3 ms is critical in this respect in which case a location and soundstage issue would be obvious. Anyway, Toole actually says that the reflections falling within this range are least beneficial, evidence provided by Ando.

>You carefully ignore the room dimensions when you talk about speaker movement. In a 12' x 12' floor area there isn't much in the way of scope available to move the speakers without putting them too close together to generate a wide soundstage and one of the original poster's requests was for a solution with a wide soundstage with good spread to the outside of the speakers.<

Agreed, a room of this size limits the possibilities. However, without any treatment the questioner states that he “hears the soundstage often extending on the right side”. So apparently the bare wall on the right does something he likes. Yet what you are proposing would destroy this. What is more, here we have anecdotal evidence for the beneficial effect of early reflections. And checkmate!

Instead of treating the reflections in the usual manner, it would seem more appropriate to take the logical step and provide the same conditions on the left side, the soundstage logically will extend also beyond the left speaker.

>You demand measurements in proof of a problem but there's more than sufficient practical experience around in my view to support the claim that room modes do cause problems in most small rooms and that dealing with them in some way is beneficial<

I’m not demanding measurements, I’m saying that measurements will not prove anything unless you consider the mechanisms our hearing is using such as shown in the curves of equal loudness. Most people, I guess, get themselves a SPL meter, measure and find that there are huge differences, whilst blissfully ignoring or forgetting that our hearing will smooth out a great deal.

>There isn't much room to vary speaker or listening position placement to minimise problems in this room yet that's your only suggestion.<

The questioner hasn’ even reported a mode problem yet you suggest treatment right away. In my room I haven’t any mode treatment whatsoever except for the floor-to-ceiling modes, I did not pay any attention whatsoever w.r.t placement of speakers and sofa, yet so far I discovered only one mode that is actualy excited, the hazard has it that the responsible track is called “Pressure points”

Who says that under all circumstances there are mode problems, even in a cubus? General agreement again?

>Bass traps will certainly reduce the reverberation time associated with any modes and reverberation time is likely to be longer in a room where each dimension has the same modal frequency because the three modes will reinforce each other.<

You seem to assume that when one of the axial modes is driven by the speaker because of unlucky placement, all three modes are automatically driven. Let’s consider the case where the speakers are against the front wall, 2.5’ from the side walls. The first front-to-rear wall mode will be driven because the speaker is in the pressure maximum of that mode. With floorstanding speakers the woofer is low but not on the floor. Will it drive the first floor-to-ceiling mode? The speakers are 2.5’ from the side wall, hence not in a pressure maximum of the first right-to-left mode? Everest says that modes are driven proportionally. Which clarly means that there are loudspeaker positions where the three axial modes are not driven fully. Determine these points and then place the chair close to a null.

>it's usually taken that response will be smoothing out by the time there are 5 modes per octave<

The Schroeder frequency is a concept that does not work in small rooms due to the very different nature of the sound field. It has further been shown that the transition from the mode region to the non-mode region is at frequencies much higher than the calculated Schroder frequency:

Baskind et al., “Sound power radiated by sources in diffuse field”, Audio Eng. Soc. preprint 5146 (2000)

> What I said was that it would smooth out more quickly in a room with 3 different axial modes rather than a single mode replicated in all 3 axes. I stand by that.<

What you said was “that behaviour will also be problematic over a wider range”. Wider range is commonly understood as frequency range, not time span.

>Finally we come to the section on detail, imaging, and breadth of soundstage and your response which was: "Since I could not find any research at all on this issue I would not know what actions have which effect." I would strongly suggest to you that all of those anecdotal reports you choose to reject, plus some of the material in Everest who you criticise for not having done any research himself, would strongly suggest that treating the first reflections would assist in meeting the original poster's needs.<

Everest is misinterpreting Olive’s results and statements. There is no reliable research on how reflections treatment changes detail, imaging, image width etc. Your anecdotal evidence certainly classifies the changes as improvement. For good reasons Everest addresses this issue in vague, general hence imprecise terms. What Everest does say is that interaural cross-correlation is a factor which affects sense of spaciousness, the weaker it is the better the effect. This is fully confirmed by psychoacoustic research yet nobody takes this into account when making treatment suggestion. The (wrong) general agreement is that the setup has to be acoustically symmetric.


> the suggestions you're getting upset about, ie treatment of lateral reflections, are often intended to address those areas where you said you were unaware of any research.<

There is research re: early reflections and the conclusions are that these are not a problem and hence don’t need treatment. As long as I’m convinced that this conclusion is true I will give advice accordingly.

Summarizing it up: the questioner has not reported any mode problem, yet you advise treatment. He says that he likes the soundstage extending beyond the speakers, this is what happens on the right side because of the bare wall, yet you advise treatment.

My advice would be: since there is no apparent mode problem, leave things as they are. For the soundstage extending beyond the speakers, first check if that is not a system problem (bad pair matching: swap speakers; amplifier output: swap speaker cables), then try and move the speakers in order to decrease SPL on the right side/increase SPL on the left side, if that doesn’t do it, what works on the right side should logically also work on the left side so try to have a similar surface at the left reflection point.

Klaus

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