# Basic Questions about acoustics



## JCD (Apr 20, 2006)

I thought I'd start a new thread that we could address some of the basic questions about acoustics that came up in the "mirror trick" thread as well as some other questions that followed on the originals.


Why have the panels mounted Portrait vs Landscape?
Why can't you have a small square of absorption wherever the mirror trick indicates instead of those big 2'x4' panels?
How much longer after the initial sound wave is heard will a reflection be treated as a separate sound (i.e., so the brain doesn't combine the two)
Using the mirror trick with the tweeter, where would you mount the panel? Centered on the tweeter reflection? or off center?
Assuming the side reflections are the most important, what would be the next issue to be addressed?
Is there some sort of "decay" ideal? Not sure if I'm using the right term there.
Is a dead front live back the preferred method for treating a room?
When should we use OC703 vs OC705?
What is the preferred method of treating the bass in a room?
Here's a loaded question, assuming your average room, what would it cost to get a room OK, Good and Perfect?
Why bother with acoustics in the first place?


Thanks.

JCD


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## bpape (Sep 14, 2006)

Wow, where to start. That's quite a list! :sweat:



> Why have the panels mounted Portrait vs Landscape?


Many people find that more aesthetically pleasing looking more like the window and door shapes rather than a blob on the wall at an odd height when not seated. Also, when having 2 rows or more and a riser, the height of the refletion points also changes.



> Why can't you have a small square of absorption wherever the mirror trick indicates instead of those big 2'x4' panels?


To a point you can. But, remember that many points will be very close to each other for different speaker/seat combinations. Also, doing refleciton control is more of an area exercise rather than a specific point treatment. Lastly, if you had 3 speakers and 8 seats, do you really want 24 small oddly placed panels (which would be a PITA to build) scattered on the wall? Talk about non-wife friendly.



> How much longer after the initial sound wave is heard will a reflection be treated as a separate sound (i.e., so the brain doesn't combine the two)


It really depends on what we're trying to do. For a conference room or classroom, the time we consider is different than it is for music or for a surround experience. Longer away seems to be an echo or spaciousness depending on the situation. The ones closer manifest themselves more as a muddiness or lack of clarity or what some of the pundits will call smearing.



> Using the mirror trick with the tweeter, where would you mount the panel? Centered on the tweeter reflection? or off center?


Again, it's more of an area thing. If you mark where you see it, you still must remember that the sound is (for simplicity) coming at that point from an angle. So, you can hit it a bit before the actual point even to the point of hitting the wall first and then bouncing back through the absorption or hitting a frame and bouncing back toward the dead front wall. Either way, it's not getting to my ears.



> Assuming the side reflections are the most important, what would be the next issue to be addressed?


Hard to say - depends on the situation and what you're aiming for. 

- Rear wall to kill a null in the bottom end.
- Behind the speakers to minimize SBIR
- Over your seated head position to minimize the huge bass hump in the 70-90Hz range in most standard height rooms since you can't move seating in the vertical dimension to avoid this.



> Is there some sort of "decay" ideal? Not sure if I'm using the right term there.


What kind of room? A 2 channel room, a studio live room, a studio vocal booth, a home theater, a concert hall, and a classroom all have different target decay curves. Those curves are also then dependent on the volume of the space.



> Is a dead front live back the preferred method for treating a room?


It's a way that works somewhat for a variety of things. Again, depending on the type of room and the usage(s), there are a variety of preferred methods. Even with the same type, there is always more than one way to skin a cat.



> When should we use OC703 vs OC705?


Whenever 703 will do just a good or better :bigsmile: Seriously though, for reflection duties where there are shallow angles of attack, 703 is preferred (assuming you're using a framed panel - for an unframed panel, 703 will just not hold it's shape and nice edges). 

When you can do bass absorbtion and do 6" or more of thickness, 703 will do just as good a job as 705 for half the money. If you can only do 4", then 705 is worth the extra.



> What is the preferred method of treating the bass in a room?


That's a loaded question... There are a variety of ways to do so. Again, it depends on what else is going on. How big is the room? If it's a large room and you have the luxury, membrane absorbers can go deeper into the bass with less depth required than more broadband "soft" absorbtion. In smaller rooms (most home rooms will fit this description) you need most treatments do deal with more of the spectrum.

Corners are a good place to start as they're at the end of at least 2 of the room dimensions. Tri corners are at the end of all 3. There are however other places where bass needs to be dealt with as I described above (rear wall to kill the null, behind speakers to avoid SBIR issues, etc.)



> Here's a loaded question, assuming your average room, what would it cost to get a room OK, Good and Perfect


Yes it is. Who's average? What is the desired look? Is there DIY involved? What is the use of the room? Will you do any physical construction or physical changes to the space itself? Just ignore all of those and look at the 2nd question. Who's product are you using? To do the same basic thing with 3 different company's products, it could vary by an order of magnitude very easily.



> Why bother with acoustics in the first place


Ah Hah! A question that can actually be answered without a caveat :jump::jump:

Because $ for $, it's the biggest change/improvement you'll make in the sound and performance level of your system. Some would argue that speakers are the same, just ahead, or just behind. Personally I'll take a $500 pair of speakers in a proper room over a pair of $5k speakers in a bad room.

Next?

Bryan


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## Bob_99 (May 8, 2006)

Good thought Jacen. I should have also started a new thread with my question but this one seems more appropriate now so here it is again:

In regard to the first reflection point, I have read that any reflected sound that arrives within 15 to 30ms (depending on whose writing) is not resolved by the brain (normal brain). I'm by no means suggesting not treating first reflections but what are your thoughts on this. Specifically, in a small room, if a wall is close, does it pay to treat it? Also, how much lower in dB level does a reflection have to be before it's not important. Again, I've read from 15 to 30 dB.

I hope that Bryan or Ethan will comment but if anyone else has thoughts about this, please jump in.

Bob

NOTE: Ethan has answered this question on this thread:

http://www.hometheatershack.com/for...-first-reflection-mirror-trick.html#post47079

My apologies for getting this in two places.

Bob


--------------------------------------------------------------------------------
www.realtraps.com
The acoustic treatment experts!


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## JCD (Apr 20, 2006)

bpape said:


> Re: minimum time delay between sound waves
> It really depends on what we're trying to do. For a conference room or classroom, the time we consider is different than it is for music or for a surround experience. Longer away seems to be an echo or spaciousness depending on the situation. The ones closer manifest themselves more as a muddiness or lack of clarity or what some of the pundits will call smearing.


Assume we're in a 2 channel listening environment. And would it change if we were in a HT?



bpape said:


> Re: Orientation of panel
> Again, it's more of an area thing. If you mark where you see it, you still must remember that the sound is (for simplicity) coming at that point from an angle. So, you can hit it a bit before the actual point even to the point of hitting the wall first and then bouncing back through the absorption or hitting a frame and bouncing back toward the dead front wall. Either way, it's not getting to my ears.


So, to clarify, as long as the tweeter's reflection is SOMEWHERE within the area where the panel is mounted, you'd be OK with it? What about the mids or woofers?




bpape said:


> Re: ideal decay
> What kind of room? A 2 channel room, a studio live room, a studio vocal booth, a home theater, a concert hall, and a classroom all have different target decay curves. Those curves are also then dependent on the volume of the space.


Let's assume a 2 channel room. And would it change for a HT? 
How would the volume affect the desired curve?




bpape said:


> Re: Loaded question about cost
> Yes it is. Who's average? What is the desired look? Is there DIY involved? What is the use of the room? Will you do any physical construction or physical changes to the space itself? Just ignore all of those and look at the 2nd question. Who's product are you using? To do the same basic thing with 3 different company's products, it could vary by an order of magnitude very easily.


I guess what I was looking for here was if Joe Audio calls you up and says I want to treat my room, just your average living room, what would it ballpark at? Assume we have a forgiving wife. And I knew this one would be a loaded question :demon: but I was hoping we could get some ballpark numbers to know what it would cost to treat a room.

Oh, and assume no DIY, and products coming from your company. 





bpape said:


> Ah Hah! A question that can actually be answered without a caveat :jump::jump:
> 
> Because $ for $, it's the biggest change/improvement you'll make in the sound and performance level of your system. Some would argue that speakers are the same, just ahead, or just behind. Personally I'll take a $500 pair of speakers in a proper room over a pair of $5k speakers in a bad room.


That was for the benefit of those who haven't been in a well treated room.. I've been in one and it really was a revelation.

Thank you for taking the time to answer these questions.

JCD


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## Bob_99 (May 8, 2006)

> ... I've been in one and it really was a revelation.


It really is and I'm amazed that most places that sell speakers (including the high end stores) don't emphasize it more and in a majority of cases, don't even mention it.

Bob


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## JCD (Apr 20, 2006)

Bob_99 said:


> It really is and I'm amazed that most places that sell speakers (including the high end stores) don't emphasize it more and in a majority of cases, don't even mention it.
> 
> Bob


That's an excellent point -- I've been in a handful of hi-end audio stores -- at best, I've seen 2 panels that were hanging on the walls, but weren't really taken advantage of. It seems like an easy sell to add those to the cost of some of these hi-end systems they're trying to sell.

JCD


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## bpape (Sep 14, 2006)

I'll have to answer this later - just swamped right now and have to head out to a studio I'm working on. If I don't post back, it's because I've spaced it- bug me...

Bryan


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## JCD (Apr 20, 2006)

No prob -- will do -- and thank you!

JCD


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## Kal Rubinson (Aug 3, 2006)

JCD said:


> That's an excellent point -- I've been in a handful of hi-end audio stores -- at best, I've seen 2 panels that were hanging on the walls, but weren't really taken advantage of. It seems like an easy sell to add those to the cost of some of these hi-end systems they're trying to sell.JCD


I don't think so. 

First, in order to convince the naive buyer of the effectiveness of the treatment/panels, the store needs to let him compare the sound with and without them. This is either tedious (remove and replace) or extravagant (have two rooms, with and without). Rives has done the latter at several audio shows and those demos probably convinced more consumers than all the talk.

Second, even if such were done, getting the panels past the significant other will be a problem. In fact, the latter is usually the reason cited by those convinced of the need for acoustic treatment and why many think of room EQ as their holy grail. Would that it were so easy.

Even though my wife has acknowledged that (1) she hears the improvements wrought by room treatments and (2) that I am certified/committed(!) to optimizing the sound, it has been an uphill battle. I think it always will be unless one has a custom and dedicated room.

Kal


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## JCD (Apr 20, 2006)

I guess I'm looking at this slightly differently. I agree 100% when discussing your average big box store, but hi-end boutiques I would expect better from. At the very least, I think they'd set up their auditioning rooms to best take advatage of the speakers. And maybe it's only because I know of it now, but I would think that someone who's willing to fork over ~$16k for a pair of Dynaudio C4's is going to have a certain amount of sophistication with these kinds of issues. Not that there won't be the poser with too much $$ that just wants "the best", but generally speaking, this is someone who has been around the hobby for a while and done some research.

I do think you make some good points -- the before and after test is going to be hard to do. Altough, I think it'd be fairly easy to have something like this to play with:









Also, these are the same stores that stock the uber-expensive wires and interconnects which I haven't been offered an a/b test with.

I do think you hit the nail on the head though when you get into the WAF angle. That is the biggest reason why Bose is so popular -- (in a high pitched voice) they're so small and unobtrusive, you can have those.

Although, another reason might simply be the profit margins. They're unlikely to be as high as the cables mentioned, and they're a lot smaller so they're cheaper to store.

Still sad, regardless of the reason.

JCD


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## Kal Rubinson (Aug 3, 2006)

JCD said:


> I guess I'm looking at this slightly differently. I agree 100% when discussing your average big box store, but hi-end boutiques I would expect better from. At the very least, I think they'd set up their auditioning rooms to best take advatage of the speakers.


Yes, the rooms at the better shops (as my mother used to say) are acoustically designed.



> And maybe it's only because I know of it now, but I would think that someone who's willing to fork over ~$16k for a pair of Dynaudio C4's is going to have a certain amount of sophistication with these kinds of issues. Not that there won't be the poser with too much $$ that just wants "the best", but generally speaking, this is someone who has been around the hobby for a while and done some research.


I doubt it. The people here on AVS are not representative and most purchasers would rather throw money at new electronics and gadgets than at something really effective but obtrusive.



> I do think you make some good points -- the before and after test is going to be hard to do. Altough, I think it'd be fairly easy to have something like this to play with:


Sure but that's only one item. What is needed is a general awareness of room acoustics and related setup. That requires a more drastic demo.



> Also, these are the same stores that stock the uber-expensive wires and interconnects which I haven't been offered an a/b test with.


Could they sell them if they did?:hide:



> Although, another reason might simply be the profit margins. They're unlikely to be as high as the cables mentioned, and they're a lot smaller so they're cheaper to store.


Charge more. :devil:

Kal


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## JCD (Apr 20, 2006)

Those are some excellent counterpoints.. although I might quibble a couple of points. For example, the hi-end shops that I've been to recently haven't been set up well. Either over or under kill.

Everything else, I may have to agree with you on.

In either case, I do wish there was more emphasis put on this stuff.

JCD


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## Kal Rubinson (Aug 3, 2006)

JCD said:


> Those are some excellent counterpoints.. although I might quibble a couple of points. For example, the hi-end shops that I've been to recently haven't been set up well. Either over or under kill.
> 
> Everything else, I may have to agree with you on.
> 
> ...


Agreed. We keep trying. :jump:

Kal


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## Sonnie (Apr 11, 2006)

Kal Rubinson said:


> I doubt it. The people here on AVS ...


:scratch: :flex: :boxer:


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## lcaillo (May 2, 2006)

OK, Sonnie, don't get too bent...think of it as a compliment...if you can.

I suspect he meant here AND:bigsmile: at ... you know where.


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## Bob_99 (May 8, 2006)

There have been some very good points made regarding treatment and I don't wish to sidetrack this thread any more than need be, but I have 2 cents burning a hole in my pocket. I speak only from personal experience which is not a whole lot but it seems to me that 

1) at some point of making the sale or after the sale, a simple statement to the effect that "You should look into room treatment to enhance the sound of these great new speakers that you are looking at or just bought" 

2) it's easier to convince the customer that better sound comes from more expensive speakers.



> Because $ for $, it's the biggest change/improvement you'll make in the sound and performance level of your system. Some would argue that speakers are the same, just ahead, or just behind. Personally I'll take a $500 pair of speakers in a proper room over a pair of $5k speakers in a bad room.


Bryan hit the nail on head, as he usually does.

Bob


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## Kal Rubinson (Aug 3, 2006)

lcaillo said:


> OK, Sonnie, don't get too bent...think of it as a compliment...if you can.
> 
> I suspect he meant here AND:bigsmile: at ... you know where.


Ummm. Yeah, that's what I meant.:daydream:

Kal


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## JCD (Apr 20, 2006)

bpape said:


> I'll have to answer this later - just swamped right now and have to head out to a studio I'm working on. If I don't post back, it's because I've spaced it- bug me...
> 
> Bryan


As requested.. 
























JCD


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## Wayne A. Pflughaupt (Apr 13, 2006)

Kal Rubinson said:


> First, in order to convince the naive buyer of the effectiveness of the treatment/panels, the store needs to let him compare the sound with and without them. This is either tedious (remove and replace) or extravagant (have two rooms, with and without).


Oh, I dunno... It seems to me that if they treated the room right, the “naïve buyer” would notice how lacking things were when he got home. When he came back in inquiring as to why, that’s when they could nail him. :banana:

Regarding the LEDE room, I seem to recall seeing somewhere that they had been discredited in recent years. Something about their only purpose was to make poorly designed speakers listenable. Not finding anything Googling it, though...

Regards,
Wayne


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## Kal Rubinson (Aug 3, 2006)

Wayne A. Pflughaupt said:


> Oh, I dunno... It seems to me that if they treated the room right, the “naïve buyer” would notice how lacking things were when he got home. When he came back in inquiring as to why, that’s when they could nail him. :banana:


That happens now and all they do is sell him more components and connectors.


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## Wayne A. Pflughaupt (Apr 13, 2006)

Um, okay. I take it these places don’t sell treatments?

Regards,
Wayne


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## Kal Rubinson (Aug 3, 2006)

Wayne A. Pflughaupt said:


> Um, okay. I take it these places don’t sell treatments?


I guess I have never seen a retail shop that does. They sure use them in their demo rooms and probably sell them as part of the package if they do custom installations. 

Come to think of it, my son-in-law did a middle 6-figure a/v installation some years back and none of the proposals included acoustic treatments. 

Kal


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## JCD (Apr 20, 2006)

Kal Rubinson said:


> Come to think of it, my son-in-law did a middle 6-figure a/v installation some years back and none of the proposals included acoustic treatments.
> 
> Kal


That's actually a little sad. Of course, I've been in a dedicated room, top of the line equipment and the builder went way overboard on the absorption, put the back chairs right up against the wall, etc. It just sounded bad. It would have been a lot bette with no treatments I think.

JCD


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## Ethan Winer (Jul 21, 2006)

Kal,



> That happens now and all they do is sell him more components and connectors.


Exactly. :hissyfit:

It is my sincere wish that one day we'll see dozens of full page ads in the hi-fi mags for acoustic treatment, and all the wire and other tweaks will be relegated to tiny ads in the classifieds section. :jump:

--Ethan


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## Kal Rubinson (Aug 3, 2006)

Ethan Winer said:


> Kal,
> 
> 
> 
> ...


Here's an idea. Set up a closely-spaced pair of microphones in an untreated room and record a variety of program material. Install the traps/treatments and re-record the entire program. Make it downloadable or offer a CD so that customers can actually HEAR the difference. I recommend making a binaural recording for headphone listening (much as I abhor headphones) in order to eliminate the customer's own room acoustics from the equation and to give an appropriate recorded perspective. Mebbe you and Richard could collaborate on this the next time you do a dual-room demo.

Kal


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## Kal Rubinson (Aug 3, 2006)

JCD said:


> That's actually a little sad. Of course, I've been in a dedicated room, top of the line equipment and the builder went way overboard on the absorption, put the back chairs right up against the wall, etc. It just sounded bad. It would have been a lot bette with no treatments I think.
> JCD


Well, in the interests of domestic tranquility, there is no uniquely dedicated room in this apartment and my daughter,and her decorator, would have represented major impediments. 

Kal


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## JRace (Aug 24, 2006)

Kal Rubinson said:


> Here's an idea. Set up a closely-spaced pair of microphones in an untreated room and record a variety of program material. Install the traps/treatments and re-record the entire program. Make it downloadable or offer a CD so that customers can actually HEAR the difference. I recommend making a binaural recording for headphone listening (much as I abhor headphones) in order to eliminate the customer's own room acoustics from the equation and to give an appropriate recorded perspective. Mebbe you and Richard could collaborate on this the next time you do a dual-room demo.
> 
> Kal


That would be great...this software comes to mind:
http://www.libinst.com/Audio DiffMaker.htm


> Audio DiffMaker is a freeware tool set intended to help determine the audibility of things intended (or expected) to change sound quality in an audio playback system. It finds the absolute difference between two audio recordings.
> 
> The difference recording that results is only what has changed between the two recordings. If anything - a change of component, a treatment, mechanical damping, etc. - is having any audible effect on the audio signal in a system, the difference recording will have audible content. The end result is primarily intended to be evaluated by ear.


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