How Flat is Flat?

I have been trying to optimize speaker placement in my small, dedicated room for the past couple of weeks. I started out by calculating all the theoretical "best" positions following the advice on the Cardas Webstie, the Rule of Thirds (and fifths, and ninths) AND multiple iterations with the RPG Acoustics software. For each possible speaker/listener location, I measured Frequency Response using the Radio Shack meter and several test CD's, and conducted extended listening tests with a variety of music. Leaving questions of soundstagingf aside, I can't get very flat response regardless of where I place the speakers/seat. Response is flat (+/- 3 dB) above 100 HZ and from ~ 40-45 Hz, but plagued with problems in the 50-80 Hz range. The "dip" is in the range of 6 to 15 dB, with the worst frequency being ~ 60 Hz. I have used the SGHT Excel spreadhseet to calculate the room peaks/nodes, and there shouldn't be any nodes at this frequency at any of the seating positions I have tried. I am pretty certain it's not the speakers (SF Guarneri Homage), since they measure flat in an anechoic environment. The room is 14 ft long, by 12.75 ft wide by 7ft9inches long (dimensions chosen using the SGHT spreadsheet based on the fixed height of my basement ceiling). I have minimal sound treatment at this stage (RPG foam at the reflections points, RPG Diffusers behind the listening position, Corner Tunes at the top and bottom corners behind the speakers). My questions it desirable to do better, or am I just being REALLY anal? If desirable, is it possible to do better? How (short of getting a Sigtech or similar)? Would also appreciate any comments on the best scheme for speaker positioning you've tried. Anyone have experienve with the RPG software? The Cardas-based set-ups sound good..but the bect sounding one is VERY awkward even in a dedicated room! AlexC
The AES standard room assumes ALL first reflection points are damped, but it doesn't say how to damp them on the Cardas site. You need to damp them with ASC tubetraps, as well as putting them at least in the corners behind the speakers. If you have a dedicated room, there should be no problem doing this.................I use both RPG profoam, and ASC traps. I feel that you'll never completely get rid of such a severe dip in the response, but you should be able to remove all but 2 to 4 dB of it, with enough trial and error. When you've done enough of this, you'll realize that computer programs that model room acoustics are ALWAYS inadequate. Sometimes, the software might save some time, sometimes not, but it'll always come down to trial and error with placement (of both speakers and absorber/diffusors).
Read the most recent TAS and you will find 3 or more references to the Sigtech. My experience suggests that you will not be able to reach a flat response without the Sigtech. Its a remarkable component that I would not be without. No system is complete without a room correction device. And I's the room man, it's the room...and the Sigtech will do the trick.
I think Carl is right. Just moving the speaker/listening position and absorbing/diffusing first reflection points will not smooth out the low frequency response. For a given room dimension there are optimal locations to reduce the peaks and valleys, but you'll still have peaks and valleys. You smooth out those peaks and valleys by absorbing low frequency standing waves (most effective in the corners of the rooms since this is a common node for all frequencies and foam won't absorb deep enough nor flat enough). This really doesn't get rid of standing waves, it just reduces the amplitude/decay of those waves, hence smoothing out the peaks and valleys you measured. Tube traps are excellent, though a little salty. I'm building my own, based on F. Alton Everest book, "Master Handbook of Acoustics". I'll let you all know if they are effective when I'm finished. Good luck, Greysquirrel
Real world rooms get into our audiophile ways of getting the best sound for us to enjoy. The closer you are to ideal ratios in room dimensions the better !!! Depending on cost (and if you are allowed depending on where you live) or how skilled you are you can modify one of the dimensions with sheetrock to get better ratios according to accoustics (using closer supports than normally recommended for added rigidity). This way you won´t need that much accoustic treatment and in the end if you need to add it, the task will be simpler, this is best if you own the dwelling. Regards
Yes you're being anal, but hey, who visiting this site isn't. A suggestion for you -- place your speakers/listening chair asymetrically. Most text will preach a symetric setup. Without it tonal balance and imaging will suffer. However, sometimes an off center setup will actually improve the tonal balance by eliminating overlapping standing waves. One word of caution, don't overdamp your room. The first (and maybe the second) reflection point(s) are critical, but in general live rooms sound more natural than damped down rooms. Good Luck!
Personally, if I had to use no room treatments, I wouldn't set up asymetrically, but would diagonally (going from a 45 degree angle out from a corner in the room). I've done that many times in different rooms, and it always works pretty well (with the necessary experimenting with placement). Both imaging and bass are terrific this way, especially in a setup that is closer to "nearfield". In the end, the symetrical arrangement with first reflections damped, and bass traps in the corners, is always best, however. And the second best way is in the nearfield, out in the middle of the room, with no walls near the listening seat, or the speakers.
Carl What you said is a mind opener. I have a room where diagonally I´m missing two corners (transit to other areas no doors) and would really like to try your suggestion.I thought on this posibility but having found only standard placings referred didn´t look further into it. Could you be provide further details like speaker and listening position
All Thank you very much for your replies. I meant to post a follow up much sooner...but work interfered (again!) with my hobby. I have however had a chance to try a few other positions, including a diagonal set up. I've gone back to one the "cardas" set-ups since it gave me the best (for me( imaging and nothing else I tried really smoothed the response. Nearfield listening was excellent for imaging, but felt a bit claustrophobic. Net, I am going to embark on the ASC journey (I am not as bold/brave as Greysquirrel!). Any suggestions on which Tube Traps to use in my room? Or should I follow the recommendation from ASC based on a MATT Test recording? Is it best to buy directly from ASC? I had a qestion for Bagwell on his Sigtech. Does it do analog? My LP collection is large (and still growing), and I have a lot of $$ sunk into my analog front-end. If the Sigtech won't "do" analog, are there other "equalizers" that will w/out completely degrading the sound? Has anyone tried professional parametic equalizers? Thanks!
Sol, all I can say is experiment. Alex, if your timing is right, it's better to buy direct from ASC, although they might not sell to you if you have a dealer in your area...I don't know. I feel that the 16 inch full round traps would be adequate for the corners, and the 16x8 rectangular ones for side and back walls.
The only way to use a Sigtech with vinyl is to to use an analog to digital converter since the Sigtech only works with a digital signal. Until next year the Sigtech unit will not accept a 96khz or 192khz digital signal but it does accept a true 24 bit word. Until there is a higher sample rate you will give up too much from a high end vinyl to live with. I own a Sigtech and wouldn't give it up but have basic room treatment to make vinyl listenable. I did a lengthy post a while back if you are really interested. The RPG software, I felt, was a good start. I also found hiring an audio engineer to do in room measurements quite helpful.