First reflections
Tom Martin -- Sun, 09/17/2006 - 20:05
I know a lot of texts suggest that first reflections should be absorbed or diffused. I have done that, using RPG B.A.D. panels. It makes a big difference. But how do I determine how much additional absorption/diffusion to use? Guys who work on studios suggest that much of the remaining wall surface be covered. Other, audiophile types, seem to reject this notion.
TM,
> It makes a big difference. <
It sure does! The real improvement is from avoiding comb filtering caused by those reflections. The drawing below shows a test I did with the first reflection points treated versus untreated. As you can clearly see, adding absorption at the first reflection points make a huge improvement in the response, especially at mid and high frequencies.
> But how do I determine how much additional absorption/diffusion to use? Guys who work on studios suggest that much of the remaining wall surface be covered. Other, audiophile types, seem to reject this notion. <
I'd say that's a matter of taste. Once the individual reflections are absorbed (or diffused - useful mainly in larger rooms), the remaining surfaces can either be treated or not. It really depends on how much ambience you want in the rest of the room. To my way of thinking, small room ambience is always bad ambience. But larger rooms can get away with more, especially if the walls are at least ten feet from your ears.
Personally I prefer rooms on the dead side. The more ambience in the room, the more the room colors the sound and deviates from what the mix engineers intended. Also, in a surround room there are first reflection points for all the other speakers, and they need treatment too. Likewise, for both surround and two-channel, first reflections occur not only at the side walls but off the ceiling too. And the floor if it's reflective.
Ethan Winer
www.realtraps.com
The acoustic treatment experts
I appreciate the idea that in larger rooms, more reflections can be tolerated. You suggest that this is particularly true if the walls are more than 10 ft. away. Is that just because of level (i.e. a second or third reflection will be low level if it is far away)?
tmartin wrote:I appreciate the idea that in larger rooms, more reflections can be tolerated. You suggest that this is particularly true if the walls are more than 10 ft. away. Is that just because of level (i.e. a second or third reflection will be low level if it is far away)?
Both. When a reflecting surface is 10 or more feet away, the time delay is long enough that the reflections are no longer considered "early." Sound travels at about 1 foot per millisecond, so a 20 foot round trip makes the delay at least 20 milliseconds. But that distance also attenuates the reflections just from distance alone.
Recording engineers use what's called the "three to one rule" for microphone placement, and that's related here. When one microphone can pick up two performers - let's say an acoustic guitar and a banjo player side by side on a stage - the unintended sound source should be at least three times farther from the microphone than the intended source. And vice versa. This makes the unintended source at least 10 dB softer than the intended source, and that's enough to keep the comb filtering peaks and nulls at an acceptable level.
--Ethan
www.realtraps.com
The acoustic treatment experts
Large reflective surfaces are big problems in contempory architechture. Glass, smooth stone, tile, plaster, and hardwood all look sleek, but they are acoustic nightmares.
At the very minimum you need a thick carpet in front of your speakers. Houseplants and decorative objects can break up refective surfaces. Paintings are also a great solution--the rough texture helps, and sound absorbing panels can be hidden behind the canvas. The best diffuser/absorber room treatment is bookshelves loaded with books.
Barry Willis
Barry,
Right on, however:
> The best diffuser/absorber room treatment is bookshelves loaded with books. <
Actually, the best diffusor is a genuine QRD type well or skyline design. A bookshelf could do an okay job if the books were aligned to follow a QRD sequence, or at least not be too regular. But I don't think it could ever be particularly good because book edges are not reflective enough at high frequencies. I'm not saying a book shelf won't help a little, but calling it "best" seems a bit optimistic to me. :)
--Ethan
www.realtraps.com
The acoustic treatment experts
I'm thinking bookshelves fully loaded with the books and other objects randomized. That would approximate a pro diffuser/absorber. Plus the mass of the books themselves acts as a sound isolation barrier to the next room.
BW
I was trying to communicate that I already have pro binary amplitude diffusers/absorbers and QRD type diffusers. I don't question that properly designed products are both relatively inexpensive for a high end system and acoustically superior. The question is about the amount and location of such things. Trial and error is one solution, but I was wondering if there are any other rules of thumb besides a) cover all wall surfaces, front and side, with abfusors (the studio approach) and b) just put an absorber or diffuser at each first reflection point and stop there (the approach used by most serious audiophiles -- I've checked out lots of reviewers rooms and high end store set ups). My point is that a) and b) are very different rules of thumb which makes it unlikely that both are approximately correct. They are also so extreme it makes you wonder if there isn't a better approach.
I have a combo music-movies room and use a projector. My question is this: the screen, a DaLite HighPower, is always down as it's a manual. Do you think this is actually helping, or in fact hindering, the sound? I know that some folks put panels in the middle on the wall between the speakers. Is my screen fulfilling that duty somehow?
I have absorption panels behind my speakers (dipoles), tube traps in the front corners, and abfusors in the rear corners.
Thanks!
My impression is that the problem with screens, and in fact lots of untreated surfaces is that reflectivity varies a lot with frequency. That means that wall surfaces and screens and windows are equalizing the reflected energy. This fact doesn't make your screen necessarily worse than an untreated wall behind it, but might make it worse than a treated wall.
Do you know if Da-Lite publishes acoustic reflectivity information?
tmartin, your helpful reply made me work on treating the front of the screen for audio. i've put diffusors that slide in and out of the way, and they help a lot.
so, yes, i think your thinking about a projector screen being (unhelpfully) reflective is right, at least as far as the specific screen i am using is concerned. thanks!
Glad to be of service. Acoustics seems to be almost the final frontier -- it doesn't take much knowledge (certainly my situation) to know "a lot". Of course, the real final frontier is probably recording quality.
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