Wednesday, September 1, 2010

Symmetrical vs. Non-symmetrical Layouts

“Symmetrical vs. Non-symmetrical Layouts

Audio has been a serious part of my life for more than 40 years. Having said that, never in my experience has the notion of non-symmetry in audio layout been regarded as a good thing. However, I see it being done frequently these days. Open most any magazine about home theater and you'll see pictures of layouts without a seat centered between the speakers. Where is "the money seat"?  Sadly, there can only be one designated location where all the speakers can converge at the same space, at the same time, and at the same amplitude. To have two seats flank the spot where all the magic lives means a large investment is not paying off to its full potential. Sure, you can adjust levels and delays electronically to any location, but it will still be severely skewed acoustically between left and right if it is not centered in the room. The design hierarchy must be: set-up, calibration, acoustics, and equipment. Even a stereo boom box can't be perceived as stereo unless your head is oriented correctly in front of it.

I have also been seeing dedicated listening rooms where the both the speaker and listener positions are deliberately positioned off-center of the side walls. This is very odd! Why would someone design such a layout? It turns out that the idea is to avoid the width axial room modes. Sounds like a great idea until you realize that even with a relatively small theater width of 16 feet, the f1 axial mode cancellation is 35.3Hz., which means:
a) This frequency is the fifth note up from the bottom of a piano, so except for keyboards, it is not likely that a soundtrack will play 35.3Hz. long enough to cause much of a standing wave issue. 

b) This wave is 32 feet long.  You may ask how far off-center you'll have to move to be out of the cancelled wave? You'll have to shift about 3' to gain only about 5dB. And what about f2, which is a crest at 70Hz. and f3, which is another trough at 105Hz.? And what about the length and height axial modes? Well, room modes are most everywhere, and as you move out of one, you likely move into another. In addition, they become more plentiful as you move towards a boundary.

c) If we also move the speakers, aren't we likely to exasperate the room modes? Yes. 

d) If we move ourselves and the speakers closer to one side, aren't we likely to perceive a difference in low frequency pressure between right and left? Yes, and also hear differences in timbre, spatial ques, imaging, etc. all due to boundary effects. If we were to measure the same signal being fed from both L and R speakers at the skewed listening position, they would be very different. And no, a digital signal processor ain't gonna fix it.

It is true that we want to avoid placing ourselves and the speakers in areas that will exacerbate room modes. It is also true that in an enclosed space, we must make many compromises. The best choice for speaker/listener positions are to avoid the fundamental height and length modes and sacrifice the width. This has to do with human perception and the fact that by design, we are much more sensitive to the horizontal plane than we are to the median or lateral planes. I'll sacrifice a few rare low frequency irregularities for constant linear mid and high frequency tonal balance, and accurate soundstage and imaging any day.

Lastly, a rectangular room is more predictable and easier to treat acoustically because of symmetry. Irregularly shaped rooms are difficult to computer model, and difficult to control acoustically. Symmetry in audio is always a good thing.

No comments:

Post a Comment