# Improving my home theater acoustics



## jerome

Hi all,

I am in the process of improving both my equipment and my HT room down in my basement. I have started another thread when I describe my room and try to solve some calibration problems:
http://www.hometheatershack.com/forums/rew-forum/8811-calibration-rew.html

But as you can see in the following picture, I basically have naked walls at the moment:










The room is around 4.6m (L) x 3.5m (W) x 2.05m (H). I write around because the side walls are not completely parallel. There is maybe 8-10cm more on one side than on the other. It's a mistake I made when I covered the concrete walls but I thought that it would help acoustically. Does it?

Before I started to renovate this room, it was a standard basement with thick concrete walls, floor and ceiling:














The roof when installing isolation:








There is an air gap of 1-2cm between the concrete and the isolation (which is 5cm thick).
As shown on the first picture, the floor is now covered with standard carpet. Walls and ceiling are covered with 13mm-thick sheets/plates of plaster that I painted with a standard water-based paint (you probably have those in the US but I don't know how it's called:huh

I have added 2 pieces of treatment on the back of the front speakers and 4 on the back of my couch to see if it would improve the sound and decrease the feeling of echo that I experience without them.

You can find my first REW measurements in the thread linked below.

I find the sound stage quite satisfying but I know that I can improve it. The 3d feeling in the entire room is very good (when playing stereo of course). I have some recordings where I can hear things happening on my back and on my side but I don't get the feeling that the sound stage is very deep between the front speakers. Everything happens on a restricted plane. I have problems placing musicians and singers sometimes. A bit hard to explain ...:scratch:

What would you do to improve my room acoustics ? I am currently thinking of covering the front wall with some kind of carpet (300g/m2) and many using diffraction on the back wall (1m back the sofa).

I was also thinking of adding a few posters with built-in isolation plates in order to reduce the first reflection from the main speakers. Is it a good idea?

As you can see, I'm just at the beginning of this process. So any advice or help is warmly welcomed !!! :jump:


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## bpape

Hi Jerome. The non-parallel walls aren't different enough to really make a lot of difference. Normally, we need 1" per foot of length per wall to really be effective. What you have just makes it harder to pinpoint things as there will be small variations - sorry.

Diffusion behind your head is not likely a good option if it's only 1m away. That said, your seating is a bit far back in that room. Ideally, you'd be about 2.9m from the front wall to your ears. 

Some of the issue you're having with imaging is that you're toeing in the speakers a lot and firing into a corner. Try them more straight ahead. Something on the front wall that's absorbant would help for 2 channel but obviously not be allowed when viewing videos.

Treatments: Some of the masking is due to bass ringing in the room - lack of broadband bass control. I'd also recommend a couple of panels on the front half of the side walls. If you want to try some diffusion, the rear half of the side walls would be a good choice.

Bryan


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## jerome

Got a question from Wayne in another thread:


jerome said:


> Wayne A. Pflughaupt said:
> 
> 
> 
> Your waterfall looks pretty impressive. I’m guessing you’re using bass traps?
> 
> 
> 
> Thanks! No, I don't use any bass traps. I only have a standard 3-seat sofa in the middle of the room.
> But the walls around are in concrete and the right wall is next to the ground outside the house. Could that be the explanation?:huh:
Click to expand...

Does someone has an idea ?


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## jerome

bpape said:


> The non-parallel walls aren't different enough to really make a lot of difference. Normally, we need 1" per foot of length per wall to really be effective. What you have just makes it harder to pinpoint things as there will be small variations - sorry.


If only I knew that before ... :thud:




bpape said:


> Diffusion behind your head is not likely a good option if it's only 1m away. That said, your seating is a bit far back in that room. Ideally, you'd be about 2.9m from the front wall to your ears.


That's around half a meter forward. I always thought that the most important was the distance from the seating position to the speakers, not to the front wall.
I placed my speakers that far from the front wall to get the most satisfying bass response. Then I moved my seating position a bit to get the best sound stage/imaging I could.

I'll try to move my sweet spot tonight when I get home. Maybe I should start over again by sitting at 2.9m from the front wall and adjusting speaker placement afterwards... :scratchhead:




bpape said:


> Some of the issue you're having with imaging is that you're toeing in the speakers a lot and firing into a corner. Try them more straight ahead. Something on the front wall that's absorbent would help for 2 channel but obviously not be allowed when viewing videos.


You mean firing into a corner on the back wall, right?
I had in mind to attach an absorbent material to the front wall around and maybe behind the screen, so that won't be a problem when viewing videos. What I have now is a temporary solution to test different configurations.




bpape said:


> Treatments: Some of the masking is due to bass ringing in the room - lack of broadband bass control. I'd also recommend a couple of panels on the front half of the side walls. If you want to try some diffusion, the rear half of the side walls would be a good choice.


Ok, the first one is pretty easy to try. Thanks !


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## bpape

Yeah - that was a bit backward - sorry. You start with the seating position in basically the right place. Then you move the speakers to fit the situation. You may have to tweak the seating position a little bit (a few inches) but nothing like 1/2 meter.

The other issue is that the screen is awfully big for that room and those speakers as it pushes things pretty far outside so any toe in has their backwave shooting right into a corner. We'll do what we can.

Absorption behind the screen will help a little in the upper bass thru the mids but the screen obviously will be naturally reflective in the highs. Again, a little less toe in will help.

Bryan


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## jerome

bpape said:


> Yeah - that was a bit backward - sorry. You start with the seating position in basically the right place. Then you move the speakers to fit the situation. You may have to tweak the seating position a little bit (a few inches) but nothing like 1/2 meter.


I have now moved my seating position forward to fit your recommendation. How do you compute the theoretical sitting position btw?
I actually hear clearly that it becomes easier to place instruments/vocals in the sound field and I might have a bit more defined 3d sound too, even if it feels like I am too close to the speakers. I also have toed out the speakers a little.
I'll wait a few days before doing any other modifications, like moving the speakers.




bpape said:


> The other issue is that the screen is awfully big for that room and those speakers as it pushes things pretty far outside so any toe in has their backwave shooting right into a corner. We'll do what we can.


The reason for the 100" screen is that I had in mind to sit at around 4m from the front wall at first. Many months later, I know better an see that the screen is fairly large from 3m. I want to setup the 2-channel first without thinking too much about the screen size. Now that I sit closer, I see that I have some margin on both sides.
If it's too big then I will have to switch it for a smaller one later on.




bpape said:


> Absorption behind the screen will help a little in the upper bass thru the mids but the screen obviously will be naturally reflective in the highs. Again, a little less toe in will help.


Noted :T


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## bpape

General rule of thumb for a good starting position for seating is 62-66% of the room length. This can be from the front or rear wall of the room. That's usually the range that gives you the smoothest bass response which is the hardest to deal with and it will minimize the need for EQ in the bottom end.

You can also play with the distance from the front of the speaker to the wall behind it and change bass response that way. With a panel speaker, that's harder to do as it's much more sensitive to not having space behind it but sometimes it can still work well.

Bryan


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## Doctor X

I thought the 38% rule was the optimum distance from either back wall or front wall. I guess things have changed.

--Regards,


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## jerome

Vaughan100 said:


> I thought the 38% rule was the optimum distance from either back wall or front wall. I guess things have changed.





bpape said:


> General rule of thumb for a good starting position for seating is 62-66% of the room length. This can be from the front or rear wall of the room.


38% from back/front wall is the same as 62% from front/back wall. Looks to me that things are still the same ... :rubeyes:


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## bpape

Yup - same thing. Sorry - should have put the 38%

Bryan


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## jerome

Just read a list of advices from Bryan. One of them is:
_Don't put a sub in a corner if you can help it. Almost never is this even close to the best place for a sub._ Strange that so many people put them there and are told to do so if it so bad... :scratch:

How far from one the corner should it be placed then? Is there a rule of thumb or is the best location found using the trial&fail method?


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## bpape

This is not always a hard and fast rule but more often than not, it's true. If you have one sub, try this:

Find a prime number such as 7. Put the sub say 2 or 3/7 of the room width and say 1/7 of the room length. This may not be perfect but it's usually a decent spot. Tweak as necessary making sure to account for phase changes/interactions with the mains.

If you have 2 subs, a guaranteed good spot (but sometimes hard to accomplish realistically) is 1 dead center on the front wall and 1 dead center on the back wall.

Bryan


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## Ethan Winer

jerome said:


> How far from one the corner should it be placed then?


The _only_ way to know where to put your sub for the flattest response is to measure the bass response at high resolution as you move it around. Personally, I've found right in the corner to be best for my living room, but I have a lot of bass trapping and that makes all the difference.

--Ethan


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## bpape

That's very true. As I said, that was just a starting point that USUALLY gets you out of the nasties.

Every room is different.

Bryan


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## jerome

Thanks guys for your advices. I have some space between my sofa and the rear wall. I will try what you suggest and check out how I can improve my measurements.

I'm also reading the excellent guidelines that Ethan wrote on his site. I also have to start reading the 'Master Handbook of Acoustics' that I bought a few months ago (but never got the time to read).

I'm learning slowing but surely


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## atledreier

My sub ended up almost centered in the width and behind the listening position. Get me a semi-nearfield to the sub as well, and I love it there. I'm within +-3dB of my target with 1 filter applied in the BFD.


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## bpape

Good deal. Glad you found a usable spot that works for you. Don't forget to verify your phase adjustment now that you've moved things around. Sometimes you can fudge a little more to improve things by playing with xover frequency, slope and phase. Since you're pretty close now, make sure you write down and mark where you're starting from so you can go back if needed.

Bryan


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## jerome

I've now spent my commuting time reading some very well written papers and info from Ethan, Bryan and Wayne. I realize now that the answers to many of my previous questions are there...

Anyway, I have spent some time trying some new places for my sub but it only seems to make matter worse. I can't find a relation between the theory and the results in practice.
The following figure shows 3 measurements:
1) Green one: sub placed behind my front right main speaker. Around 1 foot from the room corner
2) Blue one: 3/7 room width and 1/7 room length from right back corner
3) Red one: 1 feet from right back corner
In all cases, my sub is tuned to 15Hz and with the same gain, no PEQ. Measurements are made with RS on top of center seat.








For info, I also tried a few other spots in the right back corners without being able to get rid of the 25Hz null.

So I really don't see why I get this null around 25Hz:huh: According to ModeCalc, my lowest room mode is around 36Hz but there's definitively some bad standing waves in this area.
Do you guys have an explanation?

If I decide to keep my sub in the front right corner (green curve), what kind and how much bass trapping would you recommend?


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## bpape

Well, you get the null both times the sub is in the back but not when it's in the front. That should tell you something. Did you try the 3/7, 1/7 of the front? I'd also try 2/7 to see if that dip up high is caused by the width relationship.

35-70Hz the trend is the same for all of them, which likely points to a seating related issue.

Bryan


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## jerome

bpape said:


> Well, you get the null both times the sub is in the back but not when it's in the front. That should tell you something.


The only thing I can find is standing waves at this spot and that I need some bass traps on the back of my room to get rid of them. Am I correct?




bpape said:


> Did you try the 3/7, 1/7 of the front? I'd also try 2/7 to see if that dip up high is caused by the width relationship.


No, I didn't. The reason is that I can't place my sub there because I have my equipment rack. These positions would also mean to move the sub in front of the screen, something I obviously can't do. I have only two available positions on the front and that's behind my main speakers (with something like one foot around the sub)
But I'll will give a try, just to see what kind of curves I get and learn a bit more ...




bpape said:


> 35-70Hz the trend is the same for all of them, which likely points to a seating related issue.


Last week I moved my sofa (a lot) and my speakers (lightly) to suit the 38% rule. The best measurement I got was on the right seat. But I can't sit there to watch a movie or listen to music, that would be odd to sit on the side :no:

Are you saying that I should not follow this rule and move my sofa in order to get a better sub response?


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## jerome

I'm back to this project after a few weeks busy with work...

Got a new mic (ECM8000) and sound card (EMU 0404 USB). Upgraded to REW 4.1 last week too. Bought some more damping materials (10cm/4in thick) too. So everything is there for me to start working !

I calibrated the new sound card, used the new calibration file for the ECM8000 and made some measurements last evening. As I found before with my sub located in the back of my right front main speaker, the right seat is the best:







Unfortunately, the response in the middle seat (with no bass traps) looks like this:








Then I decided to add some bass traps in the two back corners. The bass traps are not really nice, just raw isolation material (Rockwool) that I packed vertically. I am not going to let them like this, it's only to check that it works. Space between it and the wall is probably around 2-4in. But it should work, isn't it?
That's my results with some bass trapping (from floor to half way up):







And with everything I had (from floor to ceiling):








Now the problem area is 50Hz-100Hz. I can't see any big change, except for the deep 60Hz valley which is gone. I was hoping for some better results :sad2:

As noted before by Bryan, I have a _seating related issue_. What can I do to address it? My sweet spot is correct I believe. Imaging is better than before, so I'm happy there.

There's something which is not right here but I can't find what to do to fix it. To me, it looks like standing waves but how do I reduce them if bass trapping doesn't help?


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## bpape

You need to move the seat. If the right one doesn't show what the center does, then it's not length related, it's width related (or a combination of width and one of the 2nd or 3rd order modes, tangential or oblique). 

Is the entire setup centered left to right in the room? If so, try shifting it about 6-8" off to one side, speakers and seat but leave the sub where it is for now.

Bryan


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## jerome

bpape said:


> You need to move the seat. If the right one doesn't show what the center does, then it's not length related, it's width related (or a combination of width and one of the 2nd or 3rd order modes, tangential or oblique).


That's what I was afraid of. Not good ... :no:



bpape said:


> Is the entire setup centered left to right in the room? If so, try shifting it about 6-8" off to one side, speakers and seat but leave the sub where it is for now.


Yes, my entire setup is centered left to right in the room. I am in the sweet spot (middle of the room and mains and screen) when I seat in the middle of the sofa.
I will see what I can do but my room is not very wide (see my first post in this thread) so it may be a problem for me to move everything to one side. The right front speaker would then almost be against the side wall, which is not acceptable for 2-channel listening.

I think that I will try to move my sub around in my room and measure the responses once more. Adding some bass trapping in the opposite corners for one position may help this time ?


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## jerome

*Improving my home theater acoustics: adding a new sub!*

Long time since I updated this thread. Nothing big happened until I got a new sub last week, an SVS 25-31 

With only one sub (SVS Ultra), I found that the best place was on the right side, around (but not exactly in) the middle or the room length. Unfortunately, this is not a good place for stereo imaging.

Now come the problem to recalibrate my system with the two subs. I made a new detailed diagram of my room:







Note: I still have to buy several panels to attenuate the front speaker reflections.

The sub which is located on this diagram is the small SVS 25-31. I found that it is quite nice to have a sub close to my back, acting as a sub as well as a kind of bodyshacker :bigsmile: Not sure I will keep it there...

Here is the curve I got with the 2 subs (Ultra located into the right back corner):








SVS Ultra alone:








SVS 25-31 alone:








I also tried closer to the door, with worse results:









I have now moved the Ultra close to the front right corner, right back the front speaker. That's where it would be nice to hide it. The curve becomes:







Not very good IMO...

With the 2 subs:







I tried to play a bit with the sub phases but it takes quite some time to measure and try every combination. Any idea on how to optimize this process?

One problem I have is that the curves are not very good. The other problem with the two subs located on the back is that I can more easily hear where they are placed, and I don't like that either....
Because my room is not huge, I don't have so many places for the subs: behind the front speakers, on the right side of the room and behind the sofa.
Any recommendation on what I should try?

I am planing on buying an FBQ2496 to fix the room gain problems, I'm more concerned about the 60Hz-90Hz range.

I'm also thinking that it would be nice if I could have two subs on the front and one on my back:
1) Buy a new Ultra. Place one Ultra behind each front speaker and the 25-31 as on the diagram
2) Buy a new 25-31. Place one 25-31 behind each front speaker and the Ultra as on the diagram

This would suppress the 'sub location' problem and give me more bass. What do you think?


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## bpape

So you can't leave it how you had it in the first graph? That's actually pretty darned good!

With the new version of REW, there is a real time function that lets you monitor as you move things around the room.

Bryan


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## jerome

bpape said:


> So you can't leave it how you had it in the first graph? That's actually pretty darned good!


Yes, the measurement is good but, as I wrote, I can often hear where the Ultra is placed (in this case on my right side, behind me). Do you know how I could fix that?



bpape said:


> With the new version of REW, there is a real time function that lets you monitor as you move things around the room.


I tried this function quickly one evening but had some troubles. Earlier today I found this method from Bruce:
http://www.hometheatershack.com/forums/rew-forum/13178-how-start-measurements-when-you-have-multiple-subs.html#post117869
I this that I will try it, looks like what you are suggesting too.


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## bpape

What is the xover point you're using? If the xover is too high on that sub it will be localizable. Even if they're the same, since your issues are not around 80Hz generally, you could likely dial it down to maybe 60-70Hz and reduce the issue somewhat.

Bryan


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## jerome

bpape said:


> What is the xover point you're using? If the xover is too high on that sub it will be localizable. Even if they're the same, since your issues are not around 80Hz generally, you could likely dial it down to maybe 60-70Hz and reduce the issue somewhat.


I'm using 80Hz on my receiver and I disabled it on both subs. Enabling it on the subs would make the curve fall down at 40dB/decade, which is too fast compared to the reference curve.

But now that you're asking, I'm wondering: is disabling the crossover on the subs a mistake? Since I got a correct shape for the frequency curve, I actually didn't think about modifying the crossover frequency. I will try that and see the effects.


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## bpape

Usually, that's the correct thing to do. However, in this case, the response is SO much better that potentially, for just that one sub, you might try it.

Bryan


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## jerome

Found a few hours this week-end to try to improve the sub measurements. I could not get anything better than using xover=80Hz on my receiver and around 80Hz on both subs.







Adjusting the gain on the smallest sub (right behind the seat) helps a lot in suppressing standing waves problems. However, I could not find a configuration where the largest sub's gain is higher, which means that I am limited by the smallest sub 

I have been recommended by several people here in Norway to always place the subs in/behind the front row. The reason exposed was that the subs integrates better with the rest of the sound field, especially with the fron speakers. I find that it does not work so well in my room. What do you think of this guys?


Just took a few full-range measurements with the subs+main speakers.
First, with 1/1 octave smoothing:







Then 1/3 octave smoothing:







Then 1/12 octave smoothing:








Looks like I will have to increase the general gain on the subs to get a better house curve. Looks also like I have to add more acoustic panels to flatten the curve, am I right?
My impulse curve looks so bad that I don't want to post it until I have fixed the main problems :hide:


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## atledreier

That looks pretty good, Jerome. The non-standard positioning works well in my room too. I have onesub in each rear corner.


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## jerome

Bryan, I was wondering about something:
I'm planning on buying some new acoustic panels that I will install on my front wall. In Norway, we can get panels from Rockwool and Rockfon.
Rockwool (30mm or 50mm):















Rockfon (40mm)








_Tykkelse_ means thickness
_Konstruksjonshøde/nedhengt_ represents the distance between panel and ceiling.


I was initially thinking of buying Rockwool 50mm and attach them directly on my front wall. You can see how my walls are built early in this thread.

But it's obvious that the panels will function better (ie higher absorption coef ) if I leave some space between them and the wall.

So now I'm thinking of buying the 30mm panels and have around 5cm space. So the final pan cake will be (from outside to inside):
- Around 30cm concrete wall
- Around 2cm air gap
- 5cm rockwool isolation
- 13mm plaster sheet (1 layer)
- 5cm air gap
- 30mm Rockwool acoustic panel

So do you think that I should go on with my project or am I wrong? Will the 5cm air gap help as much as it will make a noticeable difference in the lower bass range?

Thanks!


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## bpape

Front wall - 99x out of 100, 2" is the best solution. The only time I'd do 4" there is if you have no other options for bottom end control. Skip the air gap - not worth it in this application.

If you want bass control, consider thicker on the rear wall with a facing and/or in the corners of the room.

Bryan


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## jerome

Thanks for your answer Bryan!

I have plans for bass control but, since I don't have a very large room, I first want to check out if I really need it.


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## bpape

Smaller rooms generally require proportionately more broadband bass control than larger ones. Good idea to see for yourself and learn along the way.

Bryan


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