Speaker Room Reenforcement.

It's a wavelength-related effect. At low frequencies, a speaker effectively radiates omnidirectionally. If the speaker is close to a boundary, say a rear wall, the rearward sound radiation is reflected and adds to the sound radiated forward (ignoring phase for a moment); theoretically this gives 3dB gain. Put a speaker in the corner of the room and you get 3dB from each of the floor, and side walls, or 9dB gain. Boom.

In reality things are not quite as straightforward. The actual amount of gain at any frequency depends on the stiffness of the boundary, so you get less than the full 3dB in practice. Also, at frequencies where the distance speaker - to - boundary equates to a quarter wavelength you get cancellation at the speaker. The result is series of ripples in the overall response, and in practice moving toward the room boundaries not only boosts the bass but affects the midrange tonality as well.

M.

Example image of effect of moving my speakers away from rear wall. Note the smooth LF, because the variations are minimal given the long wavelengths at low frequencies; the hump 59-74Hz is due to two axial modes I can't control. Notice also the large ripples in the low midrange seen by moving the speakers just 10cm at a time (legend is along bottom of graph). I settled on using the position which gave the thick red line, which made a large subjective improvement in the sound.

Here is the PFM thread describing the ropey method used to capture the data...

M.

[This message was edited by Martin Clark on Tue 16 March 2004 at 11:16.]

quote:

---

Having spent several months moving my speakers around gradually to try to eliminate a Room mode (boom) problem (unsuccessfully), I started thinking about the effect of the wall/speaker interface

---

Greeny

I sympathise with you. I messed around for ages trying to get the best location for my speakers. One of the added challenges for a lot of so called "free space" speaker designs is that they have a rearward firing port (or ports) about which our "James" of DIY speaker fame has much to say. The output from these ports can be quite substantial in terms of air pressure firing at the back wall and refelecting back into the room when you turn the volume up. This is also part of the problem Martin summarises and illustrates so well.

However without recourse to the absolute measurements it comes down to trial and error and listening over time. I was getting nowhere with that until I went to this site and tried out the reasoning described here.

http://www.immediasound.com/Speaker_set-up.htm

By the use of an excel spreadsheet I sub-divdied my room into "odd" and "even" increments as recommended. I did'nt go crazy, I took it down to increments with intervals of about 10 cms between them.

Once you have fixed your listening position, bearing in mind comments about "reinforcement" when sitting up against a wall, you can use your XL "grid" in combination with the "odd" & "even" rules on bass reinforcement to put the speakers at a location which is within + or minus 5 cms of a theoretical "good" position. You can then move within that box in small increments and listen. A few repeats of that sequence got me to a position that had a minimum of "boom"

Over time I have moved around starting at different points on the same grid to find the best compromise between sidewall and rearwall effects on not just Bass but also mid range. The "grid" remainds the bible in these experiments and has worked for me.

regards
GEOFF

Martin.

Interesting graphs, What size is your room?

Geoff. My problem is I have SBL's which should be right up against the wall so in theory I only have 1 dimension to play with. However I have tried moving the speakers into the room, and even 1 foot from the backwall (a long way for SBL's) I still get a boom problem (though reduced) and the mid band is poorer than when against the wall.

The speakers are firing down the room in a 21ft * 14.5ft * 7.5ft room. I had the speakers initially firing across the room and had no such problems. (they were moved for non-audio related reasons, though in current position do allow integration into AV setup)

Greeny -the room is 5.7 x 4.8 x 2.4m (19' x 15' 10" x 8') speakers 1040mm off the longwall (to front of baffle) and about 1500mm from the side walls.

Generally I've had much a better time of setting speakers up on the long wall of rooms - not entirely sure why. Note that if you listen halfway across the room - typical in your setup - you could have you head close to the first few axial nulls which can make the response sound screwy.


SBLS are designed to be located on the boundary and the room gain is a factor in their response as designed; they belong against the walls. Have you tried varying the spacing between them? It could be that their present position relative to the side walls effectively drives the second mode across the room (78hz from the 14'6" dim). That puts the nulls at 3' 8" from the side walls - you could try moving the SBLS towards that location in the hope it excites things less.

M.

quote:

---

Generally I've had much a better time of setting speakers up on the long wall of rooms - not entirely sure why

---

Yeh, As I say above I had no such problems firing across the room.

quote:

---

Note that if you listen halfway across the room - typical in your setup - you could have you head close to the first few axial nulls which can make the response sound screwy.

---

I am sat approx 16ft from the speakers but pretty much in the middle of the room width (i.e the 14'6" dimension).

quote:

---

SBLS are designed to be located on the boundary and the room gain is a factor in their response as designed; they belong against the walls. Have you tried varying the spacing between them? It could be that their present position relative to the side walls effectively drives the second mode across the room (78hz from the 14'6" dim). That puts the nulls at 3' 8" from the side walls - you could try moving the SBLS towards that location in the hope it excites things less.

---

I currently have them 2'9" from each side wall (measured to the tweeters). However I have a slight problem moving them much closer as I have a radiator and some curtains to contend with, which would force the speakers into the room, and I don't think it's a good idea for SBL's to be sat by a radiator anyway (or in fact any Naim equipment)

Hi Greeney

You are not alone with booming SBL's my friend.

You may have seen other postings by me relating to room acoustic problems. I have a modern house (i.e. not solidly constructed!) with a 29ft long room. It's wider in the end pictured, at about 12ft.

The picture shows the SBL's standing against a plasterboard wall I had built to aesthetically 'break-up' the long room, and hopefully help the acoustics.

Ideally; I would like to use the speakers on the wall to the left of the picture, which is a brick cavity external wall. The speakers on that wall boom like buggery around 40/50hz (before and after the PB wall).

I've also tried acoustic damping panels, and whilst they are very effective in damping an aggressive mid and top, they do not sort out boomy/excessive bass.

I had no intention of using the speakers against the PB wall, but tried it in desperation and hey-presto! - it sounds bloody good. Deep clean bass - no boom.

I specifically had the PB wall densely packed with sound insulation material to stop it acting as a sound box.

My experiences have taught me that rooms are so acoustically complicated, no one can give you answers from dimensions etc:

It just works where it works. It's unfortunate of course, if this does not meet practical necessity.

Good luck.

Greeny

Since you can't move the speakers can you move yourself?

Try moving the listening chair especially if you are against a wall. I am out in the room by about 4'. back against the rear wall I had terrible boom with all the worst sort of reinforcement.

regards
GEOFF

quote:

---

I note that Greeny's room has dimensions that are almost whole multiples of 7.5ft. This would not help the elimination of boom.

---

Yes, There lies the problem, A significant reenforcement, especially around the 75-80 Hz range.

I have looked at the link that Geoff posted (thanks Geoff) And this seems to be a good strategy for getting a good position, so I will have a go at this (of course I don't really like moving SBL's very often), The strategy is pretty much in line with James's thinking w.r.t. Null points etc.

"Note that Greeny's room has dimensions that are almost whole multiples of 7.5ft. This would not help the elimination of boom"

Acousticians would advocate this on the basis that multiples of 7.5ft leads to only one room boom rather than 3 hence the requirement for only one set of bass traps

James -

"Walk around the room and listen to a piece of music with decent bass content and ascertain how the room is pressurised in the corners vs centre. Eventually, you will find a location where the bass levels is as even as the room will allow irrespective of your listening position. Therein lies your optimum loudspeaker position."

Very interesting! I had not heard this before. So...

...the bass will be less uneven with respect to listening position when the speakers are optimally placed. Does this method of positioning have any disadvantages, such as more nodes in other parts of the spectrum?

- GregB

Insert Witty Signature Line Here

James -

Thank you for your informative, as always, reply.

- GregB

Insert Witty Signature Line Here

Geoff,

Thanks for that - I'm going to have a go tomorrow.

Martin

Martin

Let us know how you get on. I may just have hit the right position by luck anyway BUt I do think it worked for me.

regards
GEOFF