Generic Advice on Accoustics - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 24 Old 09-20-06, 08:38 PM Thread Starter
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Generic Advice on Accoustics

I thought I'd throw this out there for the experts and pro's in accoustics..

Assume we have a rectangular room with a carpeted floor and a typical height ceiling. I'd like to keep the advice as generic as possible, but if you need a dimension for the room, how about 10x15.

If I want to address the accoustics in the room properly, please answer the following:

1) Where would you put the front speakers, on the short or long wall?
2) What treatments would you use first and where would they be placed?
3) What treatments would you use second and where would they be placed?
4) What treatments would you use third and where would they be placed?
5) Etc
6) Would any of the answers change if it were simply a 2 channel system?

Mostly what I'd like to get at is if someone were on a limited budget, how could they get the best accoustics from a room.

JCD
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post #2 of 24 Old 09-21-06, 06:35 AM
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Re: Generic Advice on Accoustics

1. Speakers should be on the short wall firing down the long dimension. Exact placement would depend on factors including screen size, screen type, # and placement of seating, etc.

2,3,4. Don't know that you can really say any one thing is a priority over another. I think it depends on budget, quantity discounts, DIY or not, etc. If I had to get pinned down, I'd say:
- Side wall refleciton points and in between speakers on the front wall
- Bass control in the room corners, probably front corners first to do some double duty for SBIR and front wall reflection control
- Balance of Front Wall
- Scattered targeted absorbtion throughout the rest of the space.

6. Yes. In a pure 2 channel room, you'd not have the front wall as dead nor would you have as much in the rear half of the room. Target decay times for 2 channel duties are higher for the same room than they are for HT or multi-channel music duties.

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post #3 of 24 Old 09-21-06, 05:48 PM
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Re: Generic Advice on Accoustics

I've seen Bryan give very good advice on many occasions, so I would listen to him. My thoughts would be:
01) Do the mirror trick and use absorption on the first reflection points on the side walls.
02) Look into some form of diffusion for the rear wall. (A bookcase with different sized books can some times work effectively).
03) Take care of the first reflection points on the ceiling.
04) Bass traps. You can rarely have enough. I mean true bass traps that will absorb the deep stuff, usually very expensive and bulky even doing it DIY.

All of these suggestions do not include any WAF problems, but in reality you usually have to compromise. If you are serious about it, measure, measure, measure, both before and after and at different listening positions. What may sound good at the sweet spot might not sound very good 2 seats over or behind. Back in my day, everyone strove to build an LEDE (Live End Dead End) room and as time has gone on the buzz words are now RFZ (Reflection Free Zone). These are types of environments you would find in studios more than home theaters but the concepts hold true in general. Do some googling on DIY absorption or acoustics and start getting some ideas. It can be done quite effectively and relatively inexpensively if you put your mind to it.

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post #4 of 24 Old 09-21-06, 06:30 PM
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Re: Generic Advice on Accoustics

Very true on the seat to seat variations. That's one of the things that properly treating a room will help to overcome. You'll find after appropriate decay control and sufficient bass absorbtion, your response variation from seat to seat will be much less than it was before.

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post #5 of 24 Old 09-21-06, 09:43 PM Thread Starter
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Re: Generic Advice on Accoustics

Thanks for the insights.

I have a basic understanding of how to treat a room. I assumed the side reflections (using mirror trick) was the place to start, but was unsure what location/type would be next best step. I knew about the LEDE concept, but wasn't familiar with the RFZ.

In my particular situation, I have a small garage that I will partition a listening room in. Backround, it's a tiny single car garage that I plan on cutting off the back area for storage and leaving the center as a listening room and the front as a work area. When I built my speakers, I bought 6 (8?) sheets of an OC703 equivelent. I'm trying to think of a way to best use those sheets in my room. Eventually, I'll probably get more, but thought I'd go slow for various reasons.

Oh, and the garage is mine, so no WAF to consider.

JCD
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post #6 of 24 Old 09-21-06, 10:34 PM
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Re: Generic Advice on Accoustics

I think you’re going for 2-channel in there, right?

Yes, the mirror trick has its uses, but I think most people don’t use it correctly. The problem is that sound from speakers is not narrowly focused like what you’ll visually see with the mirror. Once you find the exact reflection point, I don’t see how it can do much good to simply throw some absorption up on the wall there a foot or two square. Speakers generate considerable high frequency energy 20 degrees or more off-axis. So basically, at the so-called reflection point, that’s merely the “epicenter” of the radiant soundfield. I would think you’d need to treat at least several feet in all directions from that point if it’s going to have any effect at all. Maybe Bryan can comment...


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post #7 of 24 Old 09-22-06, 04:25 AM
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Re: Generic Advice on Accoustics

I agree with Wayne. The mirror trick is just a good starting or localizing point. And it can extend left & right, and up & down!
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post #8 of 24 Old 09-22-06, 09:50 PM
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Re: Generic Advice on Accoustics

JCD , Do not get confused about the general acoustic treatment of a room nightmare. It is very simple to understand.The following statement may upset a lot of readers due to their misinformed knowledge.The best acoustics a person can wish , Is out doors in a open field ,no reverberation,no room distortions . Seeing that is not practical to listen to music etc. in that perfect infinite sized room we have to compromise.The solution is to absorb ALL reflections from the speakers as the sound is bounced around the room. To come even close to this one needs to have heavy couches carpet with heavy underfelt, thick scatter rugs ,ALL walls coated with sound absorbancy,heavy drapes and the whole ceiling also treated. The only reverberation that you will hear is the engineered ones or the natural ones ie, reverberation from the hall where the recording was made.So, getting back to your enquiry. First , heavy scatter rugs.Second, heavy window drapes.Third ,heavy drapes around walls etc. good luck , kind regards alan.
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post #9 of 24 Old 09-23-06, 04:46 PM
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Re: Generic Advice on Accoustics

Sorry Alan but I'd disagree on how to get there. Your assertion of being outside being a perfect environment is close - but unfortunately almost all speakers are designed with room gain in mind and accounted for. If you have ever listened in an anechoic chamber which is pretty close to no room gain, you'd realize quickly that it's not a pleasant thing to listen to. I'm not 'upset' with your comments but feel I need to clarify a few things.

If one treated a room as you suggested, it would be very overly dead and bordering on uncomfortable to be in. I've been in several rooms that were treated this way and they were in fact uncomfortable. And more importantly, the reason I was there was because after doing this, the owner had a mess on his hands and didn't like the sound at all so he hired me to come 'fix it'. The first thing we normally do is pull down all the curtains, all the treatment on the wall, all the treatment on the ceiling and then measure the room and see what it's actually doing based on a mic position in the appropriate seating position(s).

A studio control room is relatively dead and is about as close as you'll get to what you describe. However, those rooms are also set up to listen in the nearfield which is almost never what is done in a home 2 channel or home theater room. Treating a room for farfield listening is a whole different game. And just to be clear, treating for 2 channel vs multi-channel is another whole different set of design goals.

Also, using tons of drapes and carpet and having all the walls covered with relatively thin material will unbalance the absorbtion curve by giving you tons of absorbtion from 1kHz up and almost zero below that. It also ignores treating things like SBIR issues. In fact, hanging thin absorbers spaced from the wall can actually act like a very narrow filter absorber based on the wavelength related to the distance from the wall. If you really want to closely replicate the great outdoors, you'd build a series of tuned bass absorbers and cover the rest of the room in diffusion (which would be huge to diffuse lower frequencies) - not to mention the fact that the absorbtion of people and furniture compared to the room volume vs. being outside makes this impossible. 1 single person in 1 stuffed chair emulates the absorbtion outdoor absorbtion of air volume of of approximately 1 square mile!

Every room has a target decay time curve based on usage (HT, Multi-channel music, 2 channel music, studio live room, studio control room, classroom, etc.) and volume. Covering ALL wall surfaces and the whole ceiling as well as having carpet will yield a very skewed frequency AND decay time response curve that will be too dead in relation to the target up high yet still uncontrolled on the bottom end. Leaving a room uncontrolled on the bottom ignores trying to smooth bass response from seat to seat and also leaves symptoms such as dialog intelligibility issues in a home theater environment.

As for reflection points, yes. There is considerable energy off axis. However, most of that will bounce at least a couple of times before it would hit your ear and is therefore outside the definition of 'early' reflections. However, if you map out the points from 3 front speakers to 2-3 front seats, you'll find that you'll have 6-9 points on each side wall - some of which are relatively close in proximity. Also, you don't want to lock your listener's head into a vice so treating an area can certainly help and is generally called for.

Bryan

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post #10 of 24 Old 09-24-06, 06:04 AM
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Re: Generic Advice on Accoustics

Brien, In response to your reply regarding JVC's query and my unbiased advice to him. First of all you say " most speakers are designed with room gain accounted for' By room gain you mean multiple echo,s That's a new one on me.To every ones understanding Primary' Speakers are designed for the flattest frequency response ,lowest distortion,highest power and to a price.The exception is for example Klipshhorns that are designed to sit in corners and use the adjacent walls as an extension of their Basshorn and as such increase the bass efficiancy.For the life of me i can not say being outside "with no room gain" is unpleasant,that is exactly as being in a room that is "dead"(no echo's).Lets face it 'all we want to hear from speakers are :-the frequencies coming from them thats all. NO ECHO'S no distortions from the room.Kind regards alan. Have to go Brien , Thirsty work wrighting this ,time for a beer

Last edited by alan monro; 09-24-06 at 06:12 AM. Reason: spelt Bryan brien
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