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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 is...is 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
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carl_eber
1,425 posts
 
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).
bagwell
20 posts
 
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 say...it's the room man, it's the room...and the Sigtech will do the trick.
greysquirrel
62 posts
 
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
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