Speakers typically have a suckout between 200 and 300 Hz as well as other modal problems. Placing the woofers at a room boundary - as in a down firing sub is one way to deal with the problem. Another approach is to soffit mount the speakers
. With free standing speakers you can move the speakers around to find a good spot but unless you soffit mount them then you are really exchanging one suckout for another - although with a bit of luck you can at least even out the response (which is what you appear to have achieved).
Pretty much all the serious setups are soffit mounted - here is one that was installed recently and appeared in ProsoundWeb
today - you'll see that all three of the principal control rooms were outfitted with soffit-mounted speakers...
See also this interesting interview with Roy Allison
about the "Allison Effect".
I decided to find out what was going on with loudspeakers and room interaction. I'd had a hint of it while doing some papers at AR. There was an unexplained phenomenonnobody could tell me why it happened: a suckout in the middle bass range in almost every loudspeaker, almost every room transmission curve that we measured. That got my curiosity aroused. I wanted to find out what was causing it.
I did a great deal of empirical testing of my own and racked my brain, trying to figure out how to avoid this problemand it was indeed a problem. Reflections from room surfaces can increase or decrease the power output of a woofer. Reflected energy increases the instantaneous density of the air in front of the woofer at very low frequencies. This provides an improved impedance match, and the efficiency of the woofer is thereby increased, along with the woofer's power output. At some higher frequency that depends on the distance or distances from the room surface or surfaces, the reflected energy goes out of phase with the woofer cone motion. That decreases the instantaneous density, and the woofer efficiency decreases. That's what causes the dip.
Now if the woofer is fairly close to one room surface and distant from others, in most home listening systems, power output in the range between 100 and 300Hz will drop about 1dB below what it would be without the nearby reflecting surface. At very low frequencies, there would be a 3dB increase in power output. That means, given maximum increase and maximum decrease, there's a total variation of 4dB. With the woofer equidistant from two intersecting surfaces, the dip is 3dB; factor in the maximum rise, in this case 6dB, and you have a 9dB variation. If it's equidistant from three surfaces that intersect at right angles, the dip would be a devastating 11dB and the maximum rise 9dBa 20dB change over the bottom octaves. If the woofer is not on the line of symmetry, which is to say the same distance from all three surfaces, the dip is less severe but can still be significant. In home listening situations, I've found this reflected impedance typically causes variations from 5 to 12dB. If a tuner or receiver exhibited variations like this, it would be rejected out of hand.
Roy holds a patent for down firing woofers that help eliminate or reduce the in room dips that occur in the bass with conventional speaker woofers. The idea is to get the woofer as close to a surface as possible and push the dips up and out of the critical bass range. Soffit mounting is the same idea only the speaker cabinet is now in the wall.
I have only been involved in Audiogon for about a year but have appreciated your input on many of the technical threads, especially your practical approach to expensive "tweaks". With some engineering background (more in digital technology) and also as a retired professional musician, I am generally very skeptical about audio tweaks that don't seem to make sense from a physics perspective. Anyway, thanks for your input.
I can see the sense in the idea of an infinite baffle for speakers and feel this would be an ideal solution with the green mountains. I am wondering if the baffle, rather than being a complete soffit, could be a large standalone baffle that surrounds the speaker to the floor and then 2 or 3 feet in all other directions. The speaker would be flush with the baffle. I am envisioning a plywood construction with solid foam insulation (1 or 2 inch) to absorb vibration.
I rent and cannot make permanent mods. So I am thinking of alternatives. This would take the place of numerous and complicated corner and wall sound absorbing materials.
What do think?
I am wondering if the baffle, rather than being a complete soffit, could be a large standalone baffle that surrounds the speaker to the floor and then 2 or 3 feet in all other directions.
You can build half walls or 3/4 walls and they will still improve things (the arger the wall the more ideal), however, you have to kill all the rear energy. Green Mountains use solid cabinets - so that is already a good start but you really want to enclose the speakers soffits with acoustic damping and then heavily brace your construction to make the wall solid and ensure it does act as a "sound board". In practice this means plenty of MDF and it means filling empty spaces (cavities) with wadding. The John Sayers website I linked to explains what is involved - I would not recommend it in a rental as you are talking significant expense and time.
Genelec sell soffit mount kits for their 8050A monitors - you can look up these too - but again it is not something suited to a rental property.
FWIW: Improvements from soffit mounting are definitely audible (why else would studios go to so much trouble) however it will not be night and day. 5 - 10% improvement max...
First I will say that correct speaker height is essential.
Now I will add that I think you unknowingly resolved an issue with your stands. I suspect that they were not mass loaded and your reducing their height by 6" provided for a stiffer connection to the floor, one that sucked up less of your sound. If you can, try adding sand or BBs to your speaker stands and see if what results.
Do you use blu-tac under your speakers to secure them to your stands?
I have always had the stands filled with sand. I also, according to the suggestion of Roy Johnson of GMA, use blue-tak to secure the speakers to the stands.
I had assumed that it had to do with wave cancellation but you're right, s.hortening the stands would cause them to be stiffer. GMA's are made of epoxy-concrete and are pretty impervious to cabinet resonance.
The late Peter Garland Snell has patents from the early 70's on the shape and design of baffles and their relationship to loading to room surfaces. Some may look to the United States Patent Office for guidance, number 3,964,571 issued June 22nd 1976 for these always contemporary disclosures. Mr Snell's designs also lend themselves to the acoustical design and enhancement of one's listening room. This is only possible if your mind is not filled with convoluted foam. Tom