How to know when youve gone too far with room treatments
If you have modified your room with acoustic treatments, listen to the sound of the voices of people that you know well. Have the person talk to you from a point as near to the speakers as is practical. You should be in the listening seat. You should test from every speaker, but especially the fronts in a multichannel installation. If a live voice that you know intimately sounds dead and lifeless, youve probably gone too far in your acoustic treatment regime.
Another test is to have someone sit in a nearby seat in the listening area. Run the same test. How does their voice sound? If its dead and lifeless, you definitely have gone too far.
Ask two or three people to speak from the speaker area at the same time (one left, one center, one right). Can you follow them easily? If not, the room may be too lively.
I find that if a room has acoustics that make conversation feel unnatural (either too lively and reflective or too dead and lifeless), listening to music will not be as effective as it might be in the very same room.
If youve ever noticed the acoustics in theaters, you can recognize the effect. Theaters are designed to maximize vocal articulation. They almost always make unsatisfactory places to listen to music concerts. If you have a multichannel system, this is especially important, as some combo rooms end up with a poor environment for listening to music with maximum impact.
The Top Three most important places for room treatments
This observation comes from over three decades of experience and hundreds (if not thousands) of successful critical installations.
You definitely want to address the first horizontal reflection from each front speaker. Im assuming that in most installations that there is a nearby wall or other object(s) that can reflect sound from the side of your speaker.
Its not so much for correction of tonal balance (although it may be required if your speakers have uneven frequency response off axis), but its mostly to prevent smearing of the sound. The slightly late arrival of reflected sound will muddy your overall sound and affect your imaging.
Think of a stone dropped into a pool. The waves are like sound waves. If you drop a stone simultaneously near the edge of the pool, those waves will merge with and affect the original waves.
Some audiophiles are surprised to find that with proper room treatments, including absorption, recorded reverberation will be increased, not diminished! For example, the sound of a choir singing a cappella in a large space will sound more spacious when you absorb the unwanted speaker/room reflections than it would if played in a live room without treatment. Thats because unwanted room reflections are smearing and even covering the sound of the subtleties of recorded ambience, spaciousness, and acoustic delay.
So my top three are:
(1) The side walls where the sound reflects from the speaker and then arrives at your ear.
(2) The area behind the listening seat.
(3) As many corners as possible.
Note: With panel & open baffle speakers, you will probably need to absorb some of the anti-phase wave-front before it can interfere with your sound. Dispersing this reflected sound is a popular, though questionable practice. Yes, you can ask why :)
Regardless of the type of speaker, if you determine that a secondary reflection from the opposite channel speaker is capable of reflecting off the wall at your seat, Id consider addressing it as well. However, when that is possible, you are somewhat likely to be sitting too far from your speakers.
There are two other places where room treatments can help, but they may not be WAF friendly:
(1) The first reflection on the ceiling from each speaker to your ear.
(2) If you have bare floors, the first reflection on the floor from each speaker to your ear. Of course, carpet or area rugs can take care of the floor-bounce issue in the mid and high frequencies.
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