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Date: September 21, 2003 at 03:29:14
From: Adrian Mack, [dialup-196-212.wasp.net.au]
Subject: Good post! Heres some more useful information

URL: My website, Peerless XLS 12 Dual PR Sub Project, 18LW1400 vented sub, Horns, etc.....



Hi Richard!

Great post, definitly some good tips there for placing subwoofers.

I personally believe room treatment is most often only minimally required, or in most cases, not needed at all for good sound reproduction.

Standing wave modes are impossible to rid of in any room, and occur across the entire range. There's always nodes and anti-nodes. So it affects not just the subwoofer, but full range speakers too, but the problem's are greatest in the vincinity of the subwoofers operation range. Omni-directional speakers, like subwoofers, tend to excite the most amount of room resonances too. One way to minimize the anti-effects is just place it away from a corner or wall, but that needn't be the case in all situations.

The reflectivity of the rooms walls also plays a factor. Rooms with hard walls, like brick will always excite more pressure changes at its modal frequencies. Ones made from drywall and that have insulation behind them are better in this aspect, and the standing wave modes are usually pretty broken up because of this. Room reverberation time depends on absorbtion, and the more absorbtion there is, the shorter the RT60 time and the wider the mode bandwidth. I believe these rooms made of drywall with the insulation behind them absorb standing waves more than reflecting back and fourth again and again. I guess thats pretty much what a bass trap does, it absorbs the energy, and shortens the revererbation time which adds to the accuracy of the information provided. And of course, damps the standing waves resulting in a smoother in-room response.





Room shape should also have an impact too. Using multiple subwoofers is actually supposed to aid in getting around the problem of room resonances. In an AES article "Subwoofer Placement in NonRectangular Rooms" it discusses subwoofer placement in different shaped rooms, and more importantly, the effects of using two subwoofers to kind of "average out" the other. It involves some pretty good graphical analysis using a software package SoundEasy. Two correctly placed subwoofers gave better smoothness and more SPL output than the single subwoofer could from this document.

Raised hardwood floors also tend to cause some pretty nasty problems. They create large spikes and other unwanted vibrations and anomalies in the response. Its just like another huge resonator to deal with, and are almost impossible to solve. Filling the area underneath your house with some sort of damping can partially solve the problem, but is usually not something people want to do.

I agree with your comment on a little equalization for the bottom octave. Except on program material which has huge amounts of low end information, a rising slope from about 40Hz and downward generally adds a real sense of weight and "presense" to the bass.

Bass traps generally need to be pretty large to really make an effect, plus they usually look rather large and ugly. Thats where the nagging from the female person you live with would cause the most problems. William Cowan has done some measurements of his living room and acoustical treatment's forming some sort's of bass traps have been used with some pretty good results on his DIY Audio Home Page.

Chapter 5 of the "JBL Sound System Design Manual" has some really good information on things like reflected energies, the reverberant field and room equalization, as well as other useful things on indoor room acoustics.

My opinion is that room placement of subwoofers certainly is critical, and so is the construction of the room and the items within it. Use of bass traps is good, but they really need to be quite large to have the desired effect. I would reccomend using these for acoustically poor rooms, or rooms with hard reflective walls.

Some other useful links on the subject:
Room Acoustics by Siegfried Linkwitz
Computerized Loudspeaker Placement Part I by Bohdan Raczynski
Computerized Loudspeaker Placement Part II by Bohdan Raczynski
Room Acoustics and Reverberation by Daniel Russel

Adrian


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