Bass response & speaker positions problems

[StanleyMuso]

I have an L shaped room roughly shaped as below. My hi-fi and TV are in the middle of the wall top of diagram. This wall is roughly 16ft long, as is the wall directly to the right. The long wall on the left is roughly 29 ft and is all glass window from floor to ceiling, heavily draped. The wall opposite the hi-fi (bottom diagram) is about 26 ft, while the short wall on ther right leg of the L is about about 14 ft and has double sliding doors leading to the kitchen. All walls, apart from the glass wall on the left, are of double brick construction. The ceiling is sloping slightly downwards from right to left and is of wooden planks, and is about 11 feet on the right side of the room and about 9ft at its low end by the windows.. The floors are hardwood parquetry, but mostly covered with a couple of large, thick carpets. There are rugs and paintings on the walls, and several bookshelves. The room appears to be well damped, and not echoey.

Now my problem. I sit about 10 feet away from the TV and Hi-Fi, well within the square area at the top of the diagram. I have experimented with speaker and sub positions, but can't seem to find a satisfying bass balance at the seating position. It is either too boomy, or too weak depending on where I place the speakers. At the moment, the speakers are about a foot away from the wall and at least a couple of feet way from the corners.

To my surprise, I find that the best response seems to be well behind my seating position, almost where the bottom stroke of the L begins. But that is where the pool table stands, so I cannot move my seats that far back. Incidently, even when I think the bass response is weak, members of the family on the other side of the house complain that the base is too loud.

What am I doing wrong? Surely even in a room like this I should be able to arrange my speakers in such a way as to get satisfactory bass response at the seating position, rather than so far back in the room? Would moving the sub to a side wall change things? So far I have resisted moving the sub around because it would mean major furniture moving.

As always, I look forward to hearing about your experiences.

PS The system did not let me include room diag - force to create sepperate message.

[StanleyMuso]

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PS I have no idea how the face and other symbols got into the diagram. The gremlins are bugging me big time today.

[snickelfritz]

One effective rule for finding a good listening position is to locate it at 38% of the room dimensions from the nearest room boundary.
For example, in a 10' x 10' room; the ideal location for the listener would be 3.8' from both the back and side wall. This places the listener in an area that should have neither maximum nulls nor maximum peaks out to approximately the fifth order of the room modes.
11' from the front wall (top of the diagram), and 6' from the left wall might be a good start in your current listening area, but the "L" at the bottom of the diagram complicates things a bit.
http://www.byrographics.com/media/38_position.gif
The disparity you're experiencing with regard to your family's perception of the bass level, is that you are probably sitting in a null, which tends to promote very high subwoofer settings to compensate.
You cannot correct low frequency standing wave nulls very effectively this way; you end up with weak, boomy bass, instead tight, punchy, realistic bass.

You might also consider installing some effective bass traps in the corners of your room to soak up some of the reflections that are causing your standing wave problems.
This should allow you to reduce the subwoofer setting considerably, while providing more balanced overall low frequency reproduction.
You'll need traps that are effective to well below 100hz, since your first order room modes are 19hz, 18hz, 43hz, and 35hz. third order and lower modes will occur well below 100hz, and the longer dimensions will support up to fifth order in that region.

35-40hz and 70-75hz are going to have strong modes in your current listening area, which essentially guts the mid-bass "power" region.

[StanleyMuso]

Good explanation. I will experiment with listening locations as per your suggestion first. Somehow I don't think my wife would be happy if I installed bass traps. This is a family / entertainment room - unfortunately I don't have a dedicated listening room.

[Peter Duminy]

Are you using the B&W 703's in this system by chance? ......

[StanleyMuso]

Yep, that's exactly what I'm using. Do you think my problems are related to this particular speaker?

[Peter Duminy]

No, it is not a problem, it's that we have found here in our testing room here, that the use of the foam plugs (bungs) that are supplied with each speaker really work at tuning the bass from 100Hz down. You can use the plugs in just one speaker or all of them, in various combinations. The same goes for the Tannoys and Dynaudio Studio Monitors here as well. We have one speaker from each Manufacturer in one end of the room fitted with the plugs as a general rule. It is time consuming, but well worth the fine-tuning. Good luck! :)

[StanleyMuso]

Much obliged. It did not occur to me to try the bungs in different combinations. As well as varying sitting positions suggested by snickelfritz, I will be experimenting with the bungs. I'm now hopeful that I will be able to tame my bass problems. I can't stand bloated and boomy bass, especially with music, and on the flip side, its not nice to be lacking in bass while watching a film which should obviously have it in loads.

[Peter Duminy]

You're very welcome Stanley. Here is a CLIO plot showing the effect of one foam plug. As a general rule one can expect about -3dB around the 50Hz mark. Some speakers have 2 ports to work with, so up to -5dB /- 6dB is possible to re-shape the bass response depending on the room and location.


http://img.photobucket.com/albums/v226/voicecoil/plugs.jpg

[Sir Terrence the Terrible]

One effective rule for finding a good listening position is to locate it at 38% of the room dimensions from the nearest room boundary.
For example, in a 10' x 10' room; the ideal location for the listener would be 3.8' from both the back and side wall. This places the listener in an area that should have neither maximum nulls nor maximum peaks out to approximately the fifth order of the room modes.
11' from the front wall (top of the diagram), and 6' from the left wall might be a good start in your current listening area, but the "L" at the bottom of the diagram complicates things a bit.
http://www.byrographics.com/media/38_position.gif
The disparity you're experiencing with regard to your family's perception of the bass level, is that you are probably sitting in a null, which tends to promote very high subwoofer settings to compensate.
You cannot correct low frequency standing wave nulls very effectively this way; you end up with weak, boomy bass, instead tight, punchy, realistic bass.

You might also consider installing some effective bass traps in the corners of your room to soak up some of the reflections that are causing your standing wave problems.
This should allow you to reduce the subwoofer setting considerably, while providing more balanced overall low frequency reproduction.
You'll need traps that are effective to well below 100hz, since your first order room modes are 19hz, 18hz, 43hz, and 35hz. third order and lower modes will occur well below 100hz, and the longer dimensions will support up to fifth order in that region.

35-40hz and 70-75hz are going to have strong modes in your current listening area, which essentially guts the mid-bass "power" region.

You are using models and rules that are based on rectangular rooms. With L shaped rooms, all bets are off. The only things that is absolute in these kinds of rooms is a floor to ceiling mode or node. Ask me how I know, I have a L-shaped room that tool alot of work(measuring and correcting) to get acoustically right.

Some modes can be safely ignored. 19hz and 18hz can safely be ignored since there is not alot down there, and at those frequencies subs could use some help anyway. All depends on whether the sub loads that room. Only rectangular rooms are predictable, L shape rooms are not.

[snickelfritz]

OK, if you insist, although multiples of 18 and 19 are relevant at ten frequencies below 100hz.
I doubt it's going to cost him much time or effort to try my suggestion, especially since there are very few alternatives, due to the position of the pool table in what I would consider prime, and fairly predictable, real estate for a listening position in that space.

[StanleyMuso]

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On the other hand, there is nothing in the corner to absorb sound near the left hand speaker. In addition, the entire left wall is glass, albeit heavily draped. But I suspect this wall may be reflecting sound energy more strongly.

Also, the ceiling over the right hand speaker is just a little higher than over the left due to the slope of the ceiling.

I will wait until the weekend to move furniture around to experiment with seating positions.

Once again, I am grateful for all the advice and knowledge so generously offered.
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