Billiards concepts plied to position acoustic panels

If you know your way around a pool table you should be able to apply those skills to improving the sound of your home theater. [Eric Wolfram] put together a post that discusses the issues caused by unwanted sound reflections and shows how to position acoustic tiles to solve the problem.

This is a companion post to his guide on building your own acoustic tiles. Don’t worry if you haven’t gotten around to doing that yet. With just a wood frame, dense fiberglass, and some fabric they’re simple to build. They’re also easy to hang but until now you might have just guessed on where they should go.

Once you have all of your speakers and seats in position grab a mirror and some post-it notes. Take a seat as the viewer and have a friend operate the mirror as seen above. With it flat against the wall, mark each spot with a sticky-note where you can see a reflection of one of the speakers. Finding the reflection points is just like lining up a bank shot in Billiards. With five speakers (5.1 Surround Sound) and six surfaces (walls, ceiling, and floor) you should be able to mark 30 reflections points. Now decide how wild you plan to go with the project. The best result will address all 30 reflection points, but you can get by with just the front marks if you’re a bit more conservative.

Comments

  1. kaye says:

    I’ll certainly be using this idea when living room remodeling time comes.

    Also, it’s applied, not plied (in title). :)

  2. jim says:

    I’m confused. I thought sound radiated like ripples in water. I would have thought you needed to line a proportion or all of the room to achieve good results. Using line of sight seems to imply that sound travels in a cone shaped the same as your field of vision when bounced off a small mirror…it doesn’t seem correct to me.

    if you mirrored all the surfaces, surely you would see an infinite number of speakers decreasing in size. If you treated at least the wall in front of you, would this not substantially cut down on the amount of sound reflected towards your ears and radiated from the back of the speakers?

    • McMajik says:

      Higher frequencies are more directional – this approach will be very effective in the top end, gradually becomming less effective as you get lower (due to how thin the tiles are as well as how sound behaves). This approach would be better than nothing, but would really need some bass traps in the corners too.

      • Eric Wolfram says:

        Check out the AcousticsFREQ article. The “tiles” Mike refers to are 2″ rockwool panels, fabric wrapped. There is an option for building 4″ panels (2 layers of rockwool) if you have serious low frequency issues. Bass traps can also be helpful if you have a bass problem, but a lot of people will get very good results with just the 2″ panel.

    • McMajik says:

      This would work much better with a layer of dense fiberglass behind it, 50-100mm of rockwool or something. I’ve been working in a startup studio for a while now, they tried using acoustic tiles for the vocal booth. It was probably the worst room I’ve ever recorded in, we took some left over rockwool panels and put them behind the tiles, fixed everything.

      Or, my listening/mixing room at home has 100mm dense rockwool from floor to ceiling around the entire room, with diagonal panels in the corners as bass traps, the room’s dead down to about 100hz. Everything’s covered by thin king sized bedsheets cut to size and stapled on. We also made 4 large portable panels out of the left over wood/rockwool/bedsheets, which are brilliant for recording in less permenant environments (done the home studio thing, can’t permenantly convert the living room though)

    • fartface says:

      You are correct. the “tip” is wrong.

    • David S says:

      As someone who studies elastic waves for a living… I confirm your suspicion. You could focus the energy though, but I don’t think you’re likely to do that in a home.

    • Eric Wolfram says:

      You are correct, the energy dispersed in all directions. However, you will receive specular reflections off of wall surfaces where the geometry is right for it. These first order reflections have the highest level of energy compared to the direct sound and cause the most problems. Have you ever listened to a whispering gallery? The concave surface focuses the reflections to one single point. Ever hear the reflections of fireworks off a building?

    • Eric Wolfram says:

      Jim,

      Also, it is precisely *because* sound propagates in all directions that you need to treat all of the first-order reflection points. If the sound propagation was like a laser, then you would only have to absorb at one point on the wall (opposite the face of the speaker). Adding this treatment will also reduce the “decay time” which is a function of the proportion of absorptive to reflective surfaces.

    • Stripe says:

      Waves do reflect just like light. The higher the frequency, the more they move in straight lines and bounce off surfaces.

      • Stripe says:

        If you’re in a room lined up with mirrors that absorb on the order of 90% of the incident light, all you have to do is mask the primary reflections from the point of view where you’re sitting at, and the secondary reflections are too dim for you to see.

        Same thing with sound, really. The wall may still reflect the sound towards your general direction, and the soundwave may spread and diffuse in air, but because the reflected line of sight is blocked by an acoustic panel, less energy is reflected towards your exact position and you hear less of an echo without covering the entire wall.

        You eliminate 90% of the echo that you percieve by eliminating 10% of the reflecting surface, like covering the sun with your hand when looking up in the sky.

    • Kent says:

      Think the thought is to get ride of all each speakers first reflections that target the listener, but this must be done for all walls and all speaker. Also placing the speaker right up to the wall or corners is not a good idea. You will have a uneven low-end response since low frequencies tend to resonate more at theses locations. I still like the idea, and can see it being very useful. Also doing listening test where a listener sits in the sweet spot while a second person holds up a panel to A/B the difference is also a good idea.

  3. McMajik says:

    This would work much better with a layer of dense fiberglass behind it, 50-100mm of rockwool or something. I’ve been working in a startup studio for a while now, they tried using acoustic tiles for the vocal booth. It was probably the worst room I’ve ever recorded in, we took some left over rockwool panels and put them behind the tiles, fixed everything.

    Or, my listening/mixing room at home has 100mm dense rockwool from floor to ceiling around the entire room, with diagonal panels in the corners as bass traps, the room’s dead down to about 100hz. Everything’s covered by thin king sized bedsheets cut to size and stapled on. We also made 4 large portable panels out of the left over wood/rockwool/bedsheets, which are brilliant for recording in less permenant environments (done the home studio thing, can’t permenantly convert the living room though)

  4. Hackerspacer says:

    With just a wood frame, dense fiberglass, and some fabric they’re simple to build.

    With just a wood frame, dense fiberglass, and some fabric they’re simple to catch fire and burn your house down.

    Fixed it for you.

  5. fartface says:

    Or do it the right way and use one of the 60,000 room reflection and node calculators on the internet.

    The “mirror trick” is lightly effective at best.

    http://www.bobgolds.com/Mode/RoomModes.htm for the very first one after you search google for “room mode calculator”

    Do it right and get better results.

    • Eric Wolfram says:

      First order reflections have the most energy as compared to the direct sound are therefore the most problematic. This method finds ALL of them. Room modes are a separate issue entirely, and are determined by the size and proportions of the room.

    • Eric Wolfram says:

      Also -believe it or not- this method is going to be much more precise than building an acoustic model, and will take a heck of a lot less time to execute.

    • Ben says:

      You don’t know what you are talking about. Dampening the room modes is an entirely different problem and will not help with the goal of this project, which is improving channel separation by blocking primary reflections to the listening position.

      Given the (admittedly fascinating) output of the tool you have linked to, just exactly where are you going to put what to solve which problem in this hypothetical room?

      There are calculator tools (online and otherwise) for placement of the but not worth it unless you are designing a room from scratch that is not built yet. Even then, confirm with the mirror. Putting in all the measurements perfectly is a pain, and you won’t get better results anyway. Especially if you missed your guess about just where your butt lands on the sofa and where exactly your ears are while you sit there. The mirror method is more accurate if you don’t have those measurements (guesses) exactly right and no less accurate if you do. Also using a mirror is a lot faster than measuring everything in the room’s exact position (don’t forget height). If you’re trying to treat a room in an older house where everything may not be exactly level, square, and plumb, the mirror is your best chance.

      But I’m arguing with a troll anyway, since you’ve already said you don’t think this will work.

  6. Bob says:

    Those are some expensive B&W’s.

  7. bryn says:

    Just go movie theater style and hang curtains everywhere.

    • Eric Wolfram says:

      Yes, I am of the school of thought that a movie theater or home theater can not have too much absorption! The thickness is important, however, and drapery will not have enough low frequency absorption to give you a balanced frequency response. Insufficient low-frequency absorption will leave things sounding “boomy” or “muddy.”

      There is a benefit to some reflectivity, or “room sound” when listening to music, however.

  8. Etunior says:

    This mirror trick is not a bad idea at all… I dont know much of acoustics, but as far as i know, this trick is focused to solve the first reflection problem of a room.

    Using the idea that high frequency waves will behave like light in a room, imagine like this:
    - A sound “ray” comes to your ear from the speaker
    - A second sound “ray” comes from the same sound source, but now it is reflected from some wall
    - A third “ray” (reflecting two times), a fourth “ray” (reflecting three times), and so on, also comes to your ear. But since it has been reflected more then once (in general, the sound will lose a lot of energy being reflected from a common wall surface), and the space travelled is much bigger than the first reflection (inverse square law is applied here for the sound energy), their sound energy is much smaller than the original one …

    Ok, now considering just the original and the first reflection waves… since their energy are not that different (assuming an untreated home room) and they are not in phase (remember, they have travelled thru a diferent amount of space), when they get mixed at your ear’s “door”, the sound will get some color… meaning that the wave that your brain will interpret will be very different from the original (and intended) sound produced by your speakers…

    Adding some absorbent material at these first reflection spots is a very simple (and very efficient) way to improve the room response to coloration… at least in the (medium-high) high frequency spectre of sound…

    Oh, and i think fire would have a hard time eating thru that fiberglass (wich is what most of the sounf absorbent if made of)…

    A fun and easy introdutory book in acoustics is:
    Master Handbook of Acoustics, F. Alton Everest

    Well, sorry about my english… i know it sux…

  9. Michael says:

    I wonder if this info came from the CX roadshow acoustics seminar (held v. recently discussing exactly this technique). It does work. As for “infinite reflections”, you said it yourself, the reflections are *smaller*. You can make a massive difference to a room by treating the primary reflections.

    And sound is certainly directional. That’s why mics have documented pickup patterns and why line array design is so complex.

  10. Eric Wolfram says:

    Thanks for posting this Mike. One thing I would add is that where you said “but you can get by with just the front marks if you’re a bit more conservative,” I would add that means the reflection images from the front left and right loudspeakers. These will be on each of the wall, floor and ceiling surfaces..

  11. Ben says:

    This is how the pros do it. Well, I used a bigger mirror.

    The biggest reflections will be the primary reflections. If you have a limited area of acoustic tile to use and you want to place the tiles in the most effective way possible, use this method. Does it cut out all reverberation in the room? No.

    I used to design high-end custom home theater. This is how we placed tiles if we were not treating the whole surface of a wall. Also, it’s how we designated the “no zones” where the interior designer was not allowed to hang art.

    The most important of these 30 spots are the 2 that reflect the opposite front channel off the side wall to the listening “sweet spot”. The rest hardly matter in comparison. Since the author of this very good had a window that was already somewhat treated with a drape on the left side and an open arch on the right, there was nothing to treat in these spots.

    If you treat nothing else, treat these spots. Channel separation is not as big a deal in the side and rear channels at all. They mostly carry ambient noises anyway. Preventing the rear channels from reflecting off the front of the room is a good goal, but not nearly as important as treating those side walls, since you won’t perceive the rear channels as coming from the front.

    Treating the ceiling where the front channels reflect to the listener is useful, but a pain. A good rug in front of the viewing seats is good for lots of reasons. The rest of the locations give diminishing returns, but are still good to do.

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