Where do Acoustic Panels go?
For example: Is Killing Side Wall Audio Reflections a good thing for your home theater? Well, that depends....
Okay, so you got the right number of acoustic absorption panels. See this post for more information about that. Now you need to know where to place them in your room.
The following illustration is a useful guide to the placement of acoustic panels.
This image is from How to Fix Speaker Boundary Interference (SBIR) Issues which is a worthwhile video to watch. Note that when they call them "Bass Traps" what they are actually referring to are "thick broadband absorption panels" in general.
We have done ourselves a disservice by calling such panels "bass traps" because “trapping bass” is a misnomer. They don’t trap bass to an appreciable degree. They are, however, immensely useful for other reasons (like controlling the clarity, and accuracy, of the audio in your room, by creating a consistent decay time / reverberation.)
Similar to these experts, I prioritize the rear wall, first. No matter how good the speakers are, reflections from the rear wall almost always smear the sound in the room.
Absorption behind the listeners is far more useful than a flat wall or even some diffusion back there. Yes, diffusion on the back wall of a dedicated room can be useful, just not at the first reflection points / behind listeners' heads. Please it off axis to create spaciousness without creating imprecise audio reproduction and pay close attention to the manufacturers engineering tests: diffusion needs several feet of distance to do it’s work.
Then, the front wall tends to benefit from panels, as well.
First, there is SBIR, which is important but complicated enough it deserves it's own summary. Second, there are reflections off the front wall from the rear speakers, which are typically spraying their sound toward the front of the room. And, finally, if you are using an acoustically transparent screen, there is comb filtering created by sound existing the front speakers, and bouncing immediately off the screen back toward the speakers themselves, ad nauseum.
Then, often, the ceiling first reflection points can benefit from some panels, since so many great speakers have inconsistent vertical off axis dispersion. (Don’t know what your speaker’s vertical dispersion looks like? Or why it might matter? See this post.)
Those are typically less useful that side wall reflections for creating a sense of space, and even speakers with wide even dispersion often don't have great (as in similar to on axis) off axis response vertically, so killing the ceiling first reflections and floor first reflection makes sense more of the time. (Killing the floor is hard, because getting something thick enough there to act broadband instead of as a tone control can be challenging. If you can build a pit into the floor in front of you, filled with insulation, and topped with a metal grate, that can work well. For most of us, that’s not realistic.)
Then, finally, we get into the thorny discussion about side wall reflections, and especially the first reflection points on the side walls.
For a few decades in the late 20th C, it was a given that killing all the first reflections was "always a good thing." This may have been because almost all speakers had poor off axis performance (in terms of being consistent with their one axis sound) so reflections didn’t sound similar in tone to the speakers’ direct sound... creating a noisy, distorted listening experience.
Whatever the case, eventually researchers evaluated preferences in double blind listening tests. It turns out that side wall reflections were disliked some times, but were pleasing, at other times. Blame Canada and their national research facilities for upending this "truth" of audio reproduction (that side wall reflections were bad) and figuring out exactly what kinds of speakers and what kinds of rooms sounded better with absorption at the side wall reflection points, and which do not.
In summary, having some of your acoustic panels on the side walls is typically a good way to tame the overall reverberation (liveliness, or RT60) of your home theater. Whether to place the treatments evenly but someone randomly around the side walls, or with more deliberate precision at the first reflection points depends on a number of factors.
Whether side wall first reflections are likely to improve or hamper the quality of your music and movies involves three key questions:
1. How far away are the side walls? More than 2 meters = less important to kill the first reflections.
The closer the speakers are to the side walls, the closer in time the reflected sound is to the direct sound, and the more likely it is to confuse things, especially if #2 is an issue.
2. How even (consistent) is the off axis response of the speakers? Very consistent = less important to kill the first reflections.
Consistent off axis sound does not mean the AMOUNT of sound off axis is the same as on axis. Consistent off axis sound means that the shape of the frequency response of the off axis sound is similar to the shape of the on axis frequency response.
The more consistent on and off axis are with one another, the less confusion there is in the brain about what's going on in the music or movie.
3. Do you like a very spacious sound, perhaps more than what is specifically intended by the recording (ie, some two channel audiophiles)? If so and what you are listening to is two channel audio = less side first reflection side wall absorption may be pleasant.
If criteria 1 and 2 are favorable for allowing side wall first reflection points to persist (that is, if you have more than a meter between your speakers and the side walls & the off axis frequency response of your speakers is similar to their on axis frequency response) and especially if you are listening to two channel audio, allowing the side wall first reflection points to persist can create a very nice open spacious sound stage.
On the other hand, are the side walls within 1 meter of the speakers? If so, then absorption can be helpful at the first reflection points.
Is the off axis response of the speakers inconsistent with their on axis response? If so, then it is likely worthwhile to abosrb the first reflections to keep the sound clear. Much of what you hear are actually the reflections of off axis sound in the room:
So if they don't have the same general response curve as the on axis sound, the experience tends of be poor, so absorbing more of those first reflection points can really clean up the sound.
Are you mostly playing movies and not two channel stereo recordings but you still have consistent off axis response and at least a meter between speaker and the side walls? This is the interesting case where folks sometimes split the difference. What does that mean? They may NOT kill the first reflection points closest to the left and right speaker.....but still kill the opposing wall first reflection points (eg, where the left speaker reflects first off the right wall, and vice versa). Or they just place panels at random spacing around the room. And, in general, this is where diffusion in domestic sized rooms begins to make a lot of sense (ie, when there is at least a meter between speakers and side walls, between listeners and walls, and the speakers have consistent off axis dispersion).
Note that this doesn't mean there should be no room treatment (absorption) in your listening space! These days, with better speakers (more consistent off axis response) and EQ, the overall RT60 time is still very important, but killing first reflections completely is not always necessary, nor necessarily preferred (in blind listening tests.)
See: Dial in your RT60 time even if you aren't actually interested in measuring your decay time, since there are also some "rules of thumb" that can be very helpful without much effort.
Finally, ask yourself: Are you in doubt about the nature of your speaker's response? Close to the side walls? Mostly listening to surround sound? The conservative approach is to kill those first reflections, but there is no law against experimentation and adjusting to taste.
FOOTNOTES
If you want to deep dive into this topic, here is some of the key peer reviewed research from one of the main textbooks on this topic, written by Floyd Toole.
Tohyama, M. and Suzuki, A. (1989). “Interaural Cross-Correlation Coefficients in Stereo-Reproduced Sound Fields”, J. Acoust. Soc. Am., vol. 85, pp. 780–786. Allowing for side wall reflections increased the sense of a classical music recording sounding like a real concert hall.
Kishinaga, S., Shimizu, Y., Ando, S. and Yamaguchi, K. (1979). “On the Room Acoustic Design of Listening Rooms”, 64th Convention, Audio Eng. Soc., Preprint 1524. Absorbing side wall reflections made it easier to hear differences between speakers and source content (more clarity) but that when asked what sounded more pleasing, allowing more reflections was preferred.
Toole, F. E. (1985). “Subjective Measurements of Loudspeaker Sound Quality and Listener Preferences”, J. Audio Eng. Soc., vol. 33, pp. 2–31.
Toole, F. E. (1986). “Loudspeaker Measurements and Their Relationship to Listener Preferences”, J. Audio Eng. Soc., vol. 34, pt.1, pp. 227–235, pt. 2, pp. 323–348. Side wall reflections and wide dispersion are often preferred I blind tests.
Choisel, S. (2005). “Effect of Loudspeaker Directivity on the Perceived Direction of Panned Sources”, Reports from the Sound Quality Research Unit, No. 32, Dept. of Acoustics, Aalborg Univ., Denmark, and PhD Thesis “Spatial Aspects of Sound Quality”, Univ. of Aalborg. When the side walls are a few METERS from the speakers, the ability to have a coherent sound space isn't heavily influenced by side wall absorption (presumably because the distance attenuates the sound so much).
Klippel, W. (1990a). “Multidimensional Relationship Between Subjective Listening Impression and Objective Loudspeaker Parameters”, Acustica, vol. 70, pp. 45–54.
Klippel, W. (1990b). “Assessing the Subjectively Perceived Loudspeaker Quality on the Basis of Objective Parameters”, 88th Convention, Audio Eng. Soc., Preprint 2929. Wide, consistent dispersion was preferred by more listeners more of the time in a reflective room.