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/foru...ation-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:
Kjeller 002.jpg
Kjeller 004.jpg
The roof when installing isolation:
Kjeller 018.jpg

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)

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 ...

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 !!!

[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

[jerome]

Got a question from Wayne in another thread:

jerome wrote:

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?

Does someone has an idea ?

[jerome]

bpape wrote:

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 ...

bpape wrote:

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...

bpape wrote:

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 wrote:

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 !

[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

[jerome]

bpape wrote:

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 wrote:

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 wrote:

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

[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

[Vaughan100]

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

--Regards,

[jerome]

Vaughan100 wrote:

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

bpape wrote:

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 ...

[bpape]

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

Bryan

[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...

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?

[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

[Ethan Winer]

jerome wrote:

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

[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

[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

[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.

[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

[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.
sub_15hz_right.jpg

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 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?

[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

[jerome]

bpape wrote:

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 wrote:

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 wrote:

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

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