# General Acoustics Knowledge?



## mrshanes (Mar 3, 2010)

I've been reading through the forums for a while and notice that some take acoustics very seriously, while others... not so much. I'm planning to build a dedicated theater and naturally would like it to look and sound nice, but really don't know or understand anything about acoustics. There are a lot of nice tacked threads around here, but I don't see any related to general acoustics in a build. Some people cover the entire front wall, full bass traps in the front corners, half in the rear, wall panels, insulating a stage, insulating soffits, deflection points... It goes on. I realize every room and space is different, but in general, I would assume there are some things that apply to all spaces. Am I missing a thread on this somewhere? Something like this would really help me during the planning stages.
Thanks,
Shane


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## bpape (Sep 14, 2006)

You're not really missing anything Shane. 

What needs to be done essentially:

Establish what the target decay time curve should be given the room size and usage and what the room should be doing when empty.

Establish what the constructed room and contents are doing.

Bring decay times in line in the bass.

Address destructive reflections that occur early in time or have a skewed response due to poor off axis response (not at all unusual - even in very good speakers). Reflections from the surrounds off the front wall are also destructive to the front soundstage.

That's really the nickel tour of what's going on. Now, how to implement that is a whole 'nother story that can be done in a multitude of ways.

Bryan


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## mrshanes (Mar 3, 2010)

bpape said:


> You're not really missing anything Shane.
> 
> What needs to be done essentially:
> 
> ...


I was hoping for a response in English! :gulp: I guess I've got a lot of research and learning to do, but that will make the journey even more entertaining!


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## bpape (Sep 14, 2006)

LOL - Sorry, thought that was English. What specifically are you unsure about based on what I wrote? Decay time is simply how long it takes a sound of a certain loudness to decrease by a certain decibel level.

Reflections are simply sounds that are not direct to you but bounce off another surface first and then come at you.

Off axis response refers to the difference between how a speaker measures when pointed right at you vs 10, 20, 30, 45, etc. degrees from right at you. Most speakers are pretty different in the higher ranges once you get beyond about 20 degrees off axis.

Bryan


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## mrshanes (Mar 3, 2010)

I'm sure that was a great response, just a little over my head. I guess I was looking for something along the lines of here are the 5 most common acoustic problems in home theaters and here are 5 general fixes. As I said, I understand each room/layout is going to be different, but I would think that in any kind of room or space where you desire good acoustics, there are going to be some general things that would help all of them regardless of the layout.


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## bpape (Sep 14, 2006)

Right. I didn't go there because you basically mentioned them in your initial post.

To list them:

Broadband bass control in the front corners of the room.

Kill the front wall to deal with boundary interactions from close speaker placement and reflections from the surround speakers.

Address side wall reflections - you'll have 1 area from each speaker to each seat on each wall. So, for instance, if you have LCR speakers and 4 seats, you'll have 12 points to address on each wall.

Maintain symmetry as much as possible left to right in the room in terms of seating, speaker, and treatments

Add additional bass control as required to bring the decay time into line (sorry - can't make that one any different). This can be done in soffits, rear wall, etc.

Ceiling treatment will be determined a lot by speaker type, height, ceiling type (drop tile or drywall), etc.

Bryan


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## KalaniP (Dec 17, 2008)

IMO, as a fellow noob to all this acoustics stuff (beyond good sound from your speakers, simple receiver correction, and into the realm of how your room really handles sounds), the first step is get to know your room at a deeper level than your ear can provide.

For me, that means, buying the equipment I need to use the free REW software (available here on HTS), and measuring the actual room response, generating some of those neat charts and graphs you see people posting here.

Once you have the equipment and have generated usable charts and graphs, the experts here can help you learn to analyze it, and see where the problem areas are. Then you can look into what can be done to help correct the problem areas, anything from simply moving your speakers around a bit, to equalization, to full-blown room treatments (acoustic panels, bass traps, etc.).

I have the hardware assembled to run REW but have not had the free time needed to start actually measuring my room and get the next phase going, but hope to before the year ends.

Good luck!


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## mrshanes (Mar 3, 2010)

That's a little easier to understand. From the sounds of it, I don't think I can do too much before the room is actually built. I guess when I have a shell for a room I can then get started on measurements and actually pinpointing what needs done. Thanks. I'll have more questions later.


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## bpape (Sep 14, 2006)

One thing you CAN do while building the room is to make sure you know where your reflections are going to be and keep doors, windows, etc. out of them and out of the front corners - as well as allowing symmetric placement of everything in the room.

Bryan


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## aackthpt (Jan 24, 2011)

bpape said:


> Right. I didn't go there because you basically mentioned them in your initial post.
> To list them:
> A. Broadband bass control in the front corners of the room.
> B. Kill the front wall to deal with boundary interactions from close speaker placement and reflections from the surround speakers.
> ...


Letters for reference to the various points are mine.

Continuing with the suggestions of what you can do:

1. Plan a screen wall such that you can readily hide (and not have to make pretty) the treatments for A and B. The typical kind of implementation of (A) is here, though it may be less expensive (and more effective) to build a similar sized device from fluffy fiberglass, and I would suggest facing it with paper or plastic (faced fluffy makes this easy). For (B) I would only put absorbers (compress fiberglass e.g. owens corning 703 pref. 4" with 4" space behind) at reflection points of LCR, and put diffusers above at the surround reflection points, but different people would do different things depending on their "bent" in the world of acoustics. Also plan what type of screen you will use (assuming front projection) as this likely changes the acoustical treatment plan.

2. Plan for side wall, possibly ceiling and rear wall treatments to address (C). You can use this tool to help get started with that.

3. Design soffits and leave space so that item (E) can be implemented if needed.

4. Identify your planned speaker array and ask questions in the forum to plan for item (F).

5. Procure an REW measuring rig and gain some experience by measuring whatever you have now, and asking questions on how to use and interpret the various different methods of displaying the result data.

6. Read theater build threads. Learn everything you can about soundproofing before starting construction. You need to prevent noise transmission into the room from a variety of sources e.g. other areas of the house, foot stomping upstairs, furnace noise down HVAC ducts etc. You also need to eliminate noise sources in the room such as equipment fans, HVAC vent turbulence. Silence is golden. The more of it you have, the more detail you can hear in the music without having to turn it up to a level that is damaging to your hearing in the long term - and the more you can be sure that foot-stomp is Thor or something rather than your daughter upstairs.

If you're a full-up DIYer and you want a good result there will be a lot of learning involved (depending on how good a result you want of course). The easier way for the casual DIYer would be to hire an acoustics pro to help you create a plan for the theatre. They can provide you with the most cost-effective options of treatments to purchase, or designs for DIY stuff if that is the way you want to go. For most this is probably the most cost-effective, direct way to a good solution. Try to talk to at least three well-known people before choosing one if you go this route.


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## mrshanes (Mar 3, 2010)

aackthpt said:


> Letters for reference to the various points are mine.
> 
> Continuing with the suggestions of what you can do:
> 
> ...



Thanks for taking this even a step further. I've got a much better idea of starting points now. I think I'm going to go with a false screen wall and I'm starting with a blank space, so I can get pretty good ideas of the reflection points. I'm going to start going through the more detailed build threads and go from there.


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## richard7948 (Jan 20, 2010)

All good suggestions here but don't forget the dimensions of the room so the resonances are as evenly spaced as practical. Don't build a cube. Also think about sight lines so those in the front row don't block the view of those behind them.
Richard 7948


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## Owen Bartley (Oct 18, 2006)

Not to step on any toes here, but I thought an image might make some of this easier to grasp. This is just one (unfortunately a small one) that I found illustrating first and second reflection points, which you will usually want to treat. Now imagine these on the floor, ceiling, and opposite wall as well.










Hope that helps a little.


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## SALESEPHOTO (Jul 5, 2009)

I am no expert but what has helped me alot is to think of sound waves as light waves. They bounce off
the walls like light so controlling how and where they reflect is the first job.


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## aackthpt (Jan 24, 2011)

SALESEPHOTO said:


> I am no expert but what has helped me alot is to think of sound waves as light waves. They bounce off
> the walls like light so controlling how and where they reflect is the first job.


True, it's a good starting model for an acoustics beginner, but only above the Schroeder frequency. At the lowest frequencies it's nothing like that. Also, it's not necessarily as simply predictable as mirror reflections even if that model is a good start which is why it is generally important to conduct cycles of test, treat, verify.


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## mtbdudex (Jan 2, 2008)

Bryan and John, great 101 stuff in here, kudos.


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## richard7948 (Jan 20, 2010)

I think the drawing showing the reflections is mislabled. Both of the reflection points are first reflections, just from two different sources. A second reflection would gofor example, first to the side wall ( farther back than the ones shown) then the wall behind the people (the second reflection point) then to the people.
Rich Morrison


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## aackthpt (Jan 24, 2011)

richard7948 said:


> I think the drawing showing the reflections is mislabled. Both of the reflection points are first reflections, just from two different sources. A second reflection would go, for example, first to the side wall ( farther back than the ones shown) then the wall behind the people (the second reflection point) then to the people.
> Rich Morrison


Yes and no. The picture poster meant only for them to be ordinal numbers (first, second, third, etc), which if you care to catalog them, isn't incorrect. However it is indeed confusing with respect to the somewhat-standard acoustical terminology of the class of "first reflection points" as paths containing exactly one reflection between source and listener and "second reflection points" as those containing exactly two reflections between source and listener. These terms probably are unfortunate shortened versions of "first-order reflections" etc. So I'd say that while the image was confusing, it was not necessarily incorrect.

This is directly related to why some (e.g. SAC) rarely talk about first or second reflection points - because really it is immaterial to the problem at hand. Psychoacoustically, what we are really interested in is the timing and strength of acoustic arrivals at the listening position, relative to time and strength of the direct sound, regardless of source (as the source could be diffraction e.g. from a speaker cabinet or false wall framing just as easily as it could be a reflection). It's therefore better to speak of "early reflections" (as they are, admittedly, typically caused by reflections) or "early arrivals" better still.

This thread was being kept on a simpler, more superficial level intentionally. Most acoustical n00bs want the blue pill - they don't really want to know how deep the rabbit hole goes. They really don't need it, and there's really nothing wrong with that.


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## Highside (Jan 4, 2010)

aackthpt said:


> For (B) I would only put absorbers (compress fiberglass e.g. owens corning 703 pref. 4" with 4" space behind) at reflection points of LCR, and put diffusers above at the surround reflection points,.........


In your learned experience, are you less likely to treat the front wall with any type of fiberglass? It sounds like you would diffuse the front and rear wall for the surrounds? I have a set of DiPole/Bipole speakers that I have always left in the DiPole mode.

Are front and rear treatments even necessary when using a pseudo "out of phase" method with DiPole speakers?

Being a nOOb myself, Hopefully I'm understanding your statement on (B)?


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## bpape (Sep 14, 2006)

Front wall should be dead - not diffuse (unless you're using OB, dipole, etc. FRONT speakers). We don't want to scatter those reflections. We want to kill them to create a better front image and proper screen lock.

In the rear, it's your preference. 

Bryan


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## aackthpt (Jan 24, 2011)

Highside said:


> In your learned experience, are you less likely to treat the front wall with any type of fiberglass? It sounds like you would diffuse the front and rear wall for the surrounds? I have a set of DiPole/Bipole speakers that I have always left in the DiPole mode.
> 
> Are front and rear treatments even necessary when using a pseudo "out of phase" method with DiPole speakers?
> 
> Being a nOOb myself, Hopefully I'm understanding your statement on (B)?





Bpape said:


> Front wall should be dead - not diffuse (unless you're using OB, dipole, etc. FRONT speakers). We don't want to scatter those reflections. We want to kill them to create a better front image and proper screen lock.
> 
> In the rear, it's your preference.


I'm probably a n00b too compared to Bryan. But IMO a more correct answer is, as with most things in acoustics, "it depends".

I'd say that treatments for the sound from the surround speakers is always secondary (very secondary) to treatments for the sound from the LCR. Dipole surrounds might (strong might) make it matter even less.

Acoustics is an area where general discussions rarely go well (and there can often be several equally good opinions for how to proceed in a given situation). If there is some area of the sound in your room that you would like to improve, you should probably start a thread to discuss it, giving information you think is pertinent, and answering queries for more background as required. Then you will probably get much better answers.


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