Tony Grimani once said "life would be so much easier if people did not insist on having walls in theaters". He was of course kidding, but it does recognize that walls create a fair number of problems.
Your loudspeakers being mounted on the walls will interact with the "Boundary" which will effect the pattern of sound radiating from the loudspeaker. Whether this will be a serious problem depends largely on the design of the loudspeaker. In general big loudspeakers are more of a problem, as they typically are used to lower frequencies. The longer the wave length, the lower the directivity of the loudspeaker, and the more energy wrapping around to interact with the wall. This is somewhat generalized, but if the loudspeakers are small, and you are using a separate sub not on the wall, it is somewhat easier. Otherwise, some fairly thick acoustical materials may be beneficial on the wall surfaces behind and along side the loudspeaker.
There are also issues of cancellation. Cancellation occurs when the direct sound from the loudspeaker arrives one half wave length before reflected energy. One of the arrivals is trying to pressurize the air, while the wavelength a half wave back is in the de-pressurized portion of its cycle. When one adds a positive air pressure to a negative pressure, one gets less pressure, which is to say less sound at that frequency.
If the distance is short, the frequency will be high, and as loudspeakers are fairly directional at high frequencies, the wrap around is not much of a problem. A 1kHz a wave is 1.1' long, so a 6" offset would cause cancellation. The wave must go two directions, back to the wall and then forward, so if the driver was 3" path length from the wall, one would expect that it would cause a dip at 1kHz. A 100Hz wave is 11.3' long, so the distance would be nearly a yard each direction. The bottom line, is that if there is a problem, the answer is acoustical materials to absorb the reflection.
If the loudspeakers are in wall, manufacturers take all of this into their designs, and there is less of a problem. Reality, go ahead an try it. Assuming this is not a million dollar system, it may sound just fine.
As to toe in, you state there is none. That means that the loudspeakers are pointed straight back. The tweeters are very directional, but in modern systems they stack them vertically to get greater horizontal dispersion. This will probably cover your listening area. It is not ideal, as in my experience tweaters sound best when they are around 5-10 degrees off axis as referenced to the central seating area, but once again there are many variables. And, for home theater we often set them wider to get better coverage for side seats.
The Radio Shack SPL meters are a good start. They will allow you to see the relative levels between the loudspeakers. In home theater we are looking for 75dB C weighted SPL as referenced from the central seating location, ear level. I have found them to be wildly inaccurate for subwoofers. Obviously more expensive gear is more accurate, but they are a great starting point.
In addition to the reference levels in your Processor, there are a couple of companies which offer calibration disks. Some of these have 1/3rd octave band limited pink noise. With the RS ignore what it says above about 4kHz or below 100Hz, but in between they can give you a good idea of system frequency response. Alan Parsan has a CD, and Tom Holman and Gold Line have DVD with the sources.
As to the balance between dollars for calibration gear vs. audio gear, I guess the question is how do I balance my budget. This is purely my opinion, so remember I work for a manufacturer, and that some of the audio equipment guys and perhaps some of the people on this site my strongly disagree. Having said that:
1. Good Loudspeakers. I have done demo's using a really cheap CD transport with a non-name amplifier and some plain jane heavy gauge oxygen free copper wire connected to a good loudspeaker, and it sounded surprisingly good. If you can afford the really great loudspeakers wonderful, but if you do not like the sound of the loudspeaker, the game is pretty much over. So, that is number one on my check list.
2. Good Loudspeaker placement. I will take a set of $1,500/ pr loudspeakers and set them in the correct location in a room, and blow away a $100k set of loudspeakers poorly placed. And, big full frequency boxes are the most sensitive to location. One of the worst systems I have heard lately used the lovely B&W 801. Great loudspeaker, utterly wasted.
3. Good Processor- Life is tough, but a good processor still sounds a lot better than the bargain shelf units.
4. Room treatments. Few of us have perfect rooms, or the ability to choose perfect loudspeaker locations. Think of acoustical treatments as passive EQ. Well placed to take care of boundary problems and first reflections, a little material does wonders. Use too much, and the room gets dead.
5. Now I really get into trouble. I would much rather have a $1k analyzer, then $1k of fancy wire. If you can afford both, great, but $ for $ I think calibration is much more important in sound quality than 9x sterling silver conductors with magic connectors. There is nothing wrong with fancy wire, just if one has to decide, I would rather have the tools to set up the system, before I spent big dollars on interconnects.
6. Noise Reduction- This is surprisingly important. Take that battery operated tick tock clock, and give it to the kids. Get small LCD clock that makes no noise for your sound room. Also, when you turn down the lights, there is no distracting lights. I am a bit of a fanatic. When really listening, I take off my noisy mechanical wrist watch. If your watch is electric, might not be a problem. Mine, however, is audible. Hate that.
OK- I've opened the flood gates, just remember this is supposed to be fun, so don't get too offended if you disagree, and glad to discuss differences of opinion. That is what this is all about.
Greg Miller
I'll kick this off with a series of questions. Curious as to how you guys and gals out there in the real world are handling these issues:
