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In Reply to: RE: New Room Measurements posted by bhjazz on May 13, 2009 at 21:43:50
Your room response at bass frequencies is determined by the room's dimensions and the stiffness of the walls, floor and ceiling. The basic room performance isn't going to change with speaker movement except that changes in the speaker distance to the front wall will affect the frequency of the quarter wave cancellation to some degree and placement across the width of the room will cause some degree of cancellation or reinforcement of some of the modes associated with the room width. You didn't mention the room dimensions but I've watched a friend measuring his room while moving speakers and his plots at different locations had the same kind of broad similarity to each other as yours have.
You would probably get bigger differences if you left the speakers in the one location and took a series of plots moving the mic to different positions along the listening axis. You'll see more change then because at some locations you will be measuring a peak and at others a null in the various modes. I've been in a room where you could hear a bass note strongly in one location and you could move a pace or two forward and be totally unable to hear the same note. The locations of the peaks and troughs are fixed by the room dimensions and don't change when you move the speakers so if the mic stays in the one place the measurements won't change anywhere near as much as you would expect when you move the speakers, but they will change a lot more if you try moving the mic.
Every room has room modes which will cause peaks and troughs in the frequency response. You can't avoid that and bass traps will help smooth them out. Floyd Toole, an acoustics researcher and speaker designer, comments in his recent book "Sound Reproduction" that bass traps will be almost universally beneficial so you can basically take it as read that they will produce some benefits in your room. How much benefit depends on your room and how bad its problems are, and on the effectiveness of the trap design and how many you install.
It's worth reading up on acoustics. Toole's book is good but if you want a better explanation of how room modes work, I think Everest's "Master Handbook of Acoustics" may be a better choice. Both are very good books with different technical emphases but their basic treatment recommendations are actually very similar. Everest provides a lot of useful info on the construction of treatments so if you're planning on going the DIY route I'd incline more to his book than Toole's. Toole has a lot more on psychoacoustics and how we process/interpret what we hear plus a fair bit on surround sound matters while Everest devotes little time to surround sound. I'm glad I have both.
David Aiken
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