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In Reply to: RE: Rob's room again. posted by rpoort@bigpond.net.au on July 03, 2008 at 22:50:57
I would definitely think your slightly left of centre centre image is due to the reflection difference between left and right sides. If the left side is a plaster board wall and the right side a curtain, the left side will probably be providing a stronger reflection and the image will tend to pull slightly to the side with the stronger reflection. Quick and dirty test: rig up some sort of stand that you can drape a blanket over and place that at the first reflection point on the left side. If the image moves back towards the centre there's every indication that is your problem. The safest and surest cure—short of (heresy) tinkering with the balance knob if you have one—is to put equivalent absorption at the first reflection points on each side to ensure that there is equal absorption/reflection. If you're using free standing panels, check out my post below on the effects of angling panels for an interesting suggestion to trial.
Toe in of one speaker may affect the centre image, but it will affect other things as well. It puts you more off axis with one speaker than with the other and that can change the tonal balance of the speaker which is adjusted, depending on how fast the polar response pattern changes as you move off axis. The obvious suggestion would be to toe the left speaker further in towards you since that moves the reflection further off axis which should weaken the reflection, but it will also likely strengthen the direct sound on that side which may simply recreate the problem with a different cause, perhaps even exacerbating it. Toeing the right speaker in to point directly at you may strengthen the signal from that speaker enough to balance the left side but both speakers are close to pointing at the listening position already so I don't think you'll get a significant benefit either way.
I think your best bet is absorption on both sides at the first reflection point. The quick fix is the balance knob if you have one.
Toe in angle is an oft discussed issue with some people preferring the speakers to cross just in front of the listener as you currently have them, some behind the listener, and some with them pointed at the listener. Having them cross in front is said to produce a slightly larger sweet spot but since your room looks set for a single listener that may not be a critical issue for you. Having them point directly at you will probably yield the most detail and a strong soundstage/image but things may sound a little brighter which could be a problem depending on how you like things to sound. It's a personal adjustment and the best thing to do is to set it to give you a sound that you like.
I don't think the speakers disappearing as you move forward is so much a matter of toe in as the fact that what is changing is the angle the speakers subtend at the listening position. That angle increases as you move forward and decreases as you move back. In the Audio Physic method they suggest that the preferred angle is 72 degrees rather than 60 and my experience in my own system and a couple of others is that as you move forward and increase that angle, the speakers do disappear more completely. I've seen this repeated in several systems now in very different rooms and with very different speakers so I'm starting to wonder whether it is a pretty general rule.
I'd suggest playing around to find the angle/distance from the speaker plane that bests suits your tastes and then experimenting with moving the speakers a little further forward into the room and the listening position in relation to them until you get your best results. I know the speakers are heavy but if you can replace the spikes with casters for a few days, you can move them around easily to play with speaker and listening position placement and then go back to the spikes when you've dialled in the optimum positioning. I think you'd probably benefit from the exercise and it wouldn't be difficult if you can temporarily replace the spikes with casters. It will certainly be a pain if you can't.
Speaker and listening position placement are probably the most important system adjustment you can make in many ways. It affects the whole presentation of your system and it can make significant changes in the sound you get. It's also free, apart from requiring a bit/lot of your time and effort depending on how easy it is to move your speakers though replacing the spikes with casters can simplify that a fair bit for heavier speakers. In my view it is certainly worth doing. Acoustic treatment can always improve the results but the speaker and listening position placement can limit how good things sound, both without and with treatment. Getting things as good as possible prior to treatment is always worth while because it can reduce the amount of treatment required and it gives you the best starting point to build on with treatment if you decide to treat the room. Do that well and the results will be even better than they are without treatment.
David Aiken
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