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Discussion Starter · #1 ·
Sorry if this has been discusses before but I could not find it....


As we all search for the holy grail in terms of sound, I have decided that my money should now be spent on my room...more specifically acoustic treatments. My dedicated space was not built specifically for my theater but it became my theater and as such changes in structure and or shape cannot be done, so acoustic treatment (I believe) is my first alternative.


I have begun doing research and have come across room modes or resonance. In plotting my dimensions in the various spreadsheets or formula's, I have determined that along the lenght axis, I have a resonsnce at 24hz. My room dimensions are:


23.6' long

11.9' wide

9.7' high.


I should point out that along the length, I do have an opening that goes from floor to ceiling and is 6' wide that is covered with black draperies. This was a WAF decision as she did not want a door placed here for aesthetic reasons. The draperies do look very nice but I'm sure the opening has effects on the overall acoustics of my space but I have to work with what I have.


I also know, these are not perfect dimensions for proper spacing of room modes but (again) I have to work with what I have.


So my questions are:


1.) In determining how to properly bass trap my theater would it be true that the traps would have to be able to absorb down to 24hz to be effective in my room, since this is where my resonance is along the length axis?

2.) If I can effectively trap that low will it also improve the bass across the other two dimensions of my room since my resonance at the width is 47hz and at the height is 58hz? This is assuming I place four traps in the 4 corners of my room.

3.) Does it really matter because is it really possible to get usable bass that low in a room my size? If I get a trap that's rated down to 50hz would it be just as effective or a waste of money?


I'd appreciate any input (or questions) on this as I continue to delve into the realm of room acoustics.


Thanks,
 

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Just so you don't think nobody is answering...


You asked a lot of questions and there is a lot of ground to cover. I'll have more time later to come back and write some more (if others already haven't.)


In short, yes. You can get bass that deep or deeper in your room.


Yes. The opening in the room effects the volume of the space and thereby the bass pressure and the RT60 calculations.


Yes. You CAN trap down that low but may not want to.


You MAY want to put traps in all 4 corners, you may not. It all depends on what the room is really doing.


Your best idea at this point is to measure the room's response at the seating positions and see where you have issues. SOME bass trapping is likely needed - it usually is. However, you can make things worse if you don't know:

a) what the problems are

b) how to treat them without messing up something else.


More later.
 

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Nabs,


I wouldn't worry so much about trying to trap down to 24 Hz. If a room mode gives you a little extra oomph down there, it's likely to be welcome. Where bass trapping really helps is from 50-80 Hz and higher. This is where the worst of the peaks and deep nulls occur, and where bass instrument clarity is harmed the most by ringing and all those peaks and nulls.


> If I can effectively trap that low will it also improve the bass across the other two dimensions of my room
 

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Discussion Starter · #4 ·
Thanks for the info guys.


I have done some rudimentary measurement at all 5 seating position with respect to the subwoofer channel. First let me state that I have 2 rows of seats with 2 recliners in front and a couch in the back on an 8" riser. Playing pink noise (through my pocessor...Lexicon MC-8) I've got a pretty steady 75db reading at 4 of the 5 seats. The middle of the couch, however, has a -5db null compared to the other 4 seats. It's like the bass disappeared at this one spot. This is primarily what got me to thinking that bass trapping might help smooth out my bass response. I don't think it will completely fix this null but I'm sure it will help bass response in general.


I've got 2 subs..one in the front at the halfway (width-wise) point and one in the back at the 3/4 (width-wise) point. I have not done to much experimenting with moving the subs around but I believe I should do that as well.


At least I won't worry about trapping at 24hz but now will focus on as smooth bass response as I can get given my room.


Any other input will be greatly accepted.


Thanks,
 

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I gotta say, -5 dB null is not bad at all. It may not be worth the trouble.
 

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Depending on where the chair is, you might not fix it.


Playing pink noise is a VERY ROUGH indicator. The room really needs to be measured AT LEAST every 5 Hz from 20 through 200 (80 if just doing the sub or whatever your xover frequency is). It is very tedious but I think you'll find the answers quite different than just pink noise. Just do your primary seat first and check it out.


A constant measurement just says you have pretty much the same problems at all the seats (or at least they average out the same). This is not a bad thing! It is certainly easier than 5 different sets of problems... In addition, this says NOTHING about the curve and relationship between the sub and the mains.


Try moving the front sub slightly to the side - like 3/7 of the width and the opposite side of the one in the back. Might move that one a bit too (3/5). Try to say out of 1/2, 1/4 placement. Prime numbers work quite nicely and 7 gives you some flexibility. If possible, you might want to put one of the subs along the side wall (1/7?). These are just starting points based on rules of thumb. Your mileage may vary.
 

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Discussion Starter · #7 ·
I will experiment with different sub positions today...I did try to keep my seating away from the 1/2 and 1/4 positions in my room but did not do this for my subs.


How can I get an accurate measurement from 20-80hz? Yes I have my crossover set at 80hz. Is this something I can do myself with some test DVD(s) or does it require hiring a professional?


If I can do it myself what type of equipment is required?


Thanks,
 

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Test CD's will work out fine. Rives Audio makes a nice one with a wide variety of bass test tones as well as some compensated for Rat Shack meter (very reasonable price).


If you want something a bit more in depth, you can use a computer, test mic, and ETF5 though that is considerably more.


Moving the subs won't eliminate room modes but it can help smooth things out a bit. You will probably still have the same issues in the same seat - just hopefully to a lesser extent. Just out of curiosity, where is that seat located?


Just for giggles, you could put the sub where the 'bad' seat is and the walk around the room to see where you get the smoothest bass response (test cd or music with variable bass. When you find the spot, put the sub there (just do 1 sub at a time). You might find that moving 1 sub can help that seat without messing up the others. On the other hand, it might make the others worse.


Really though, without knowing what things look like at individual frequencies, in relation to each other and in relation to the mains, it's really tough to judge if you're doing harm or good. The null is an obvious issue - you just don't know why or where.
 

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Discussion Starter · #9 ·
bpape,


Rather than tell you where my null seat is, how about if I show you. Look at my attachment and the couch in the second row with the pillow with the letter 'N'...that is my null seat. The other 4, as Ive mentioned are pretty close in level.


I have heard of ETF5 but have not done any research on it. I'll also look at Rives Audio and see what they have.


Please feel free to offer any other advise/suggestions.


Thanks,
 

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The null seat is in the center of the room which will put you in the null of the primary axial width mode and all even harmonics of that mode. When just doing your sub, 47Hz is almost smack in the center of the 20-80 band. That's why it sounds like 'the bass disappeared.'


Ethan is correct. Finer measurements will give more information. He is also reiterating my contention that you need to really deal with the whole spectrum including your mains. 5Hz increments was just to get an idea of balance. Just pink noise really doesn't say much other than overall average level.


Just understand that when you treat these things, it gets increasingly difficult to treat say a 5Hz range and not affect a half octave or so. This gets more and more difficult as you move up in the bass range. Move the subs around and listen for smoothness of the bass, lack of 'one note bass', minimal changes in intensity when the same instrument is moving through an ascending or descending series of notes, etc.


Once you find where they work best in the room as a whole, then the measurements will tell you a lot more. If you don't have a mic, computer, and don't want to spend the bucks on ETF, the Radio Shack meter and the Rives CD will get you close. It is just more tedious to do by hand.


One last set of questions.


Have you calibrated the levels for each of the speakers using a meter? If not, you should.


If you haven't, did you find yourself having to turn up the center to 'hear the dialog' more clearly?


Those are common things if you have mid-upper bass issues in the room that end up causing cancellations and general muddiness in the vocal region that is most easily apparent by a lack of dialog clarity.
 

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Discussion Starter · #12 ·
Yes, I have calibrated my theater with a Radio Shack SPL meter and test tones from my MC-8. Since I have 2 rows of seats I approached calibration by choosing a "sweet spot" in the absolute center of the seating area. I dialed that spot to 75db and went with that. Obviously not all seats in the seating area will be in perfect balance but there is only one true sweet spot.


I don't have any dialog intelligibility issues nor have I ever had to adjust the center level differently than any other channel but what took me down this path was I decided to play the subwoofer pink noise and move my SPL meter to each seat (out of curiosity) and that is when I noticed the bass was less there. I don't sit in that seat but I did put on a movie (with lots of bass...the lobby shooting spree in the Matrix) and confirmed the SPL meter readings. I also listened to dialog in that seat and it was as clear as any of the other seats...I confirmed this by continually switching between the 5 seats.


I do understand that my null seat is on the 1/2 axis for width (can't do anything about that....except maybe a chainsaw) but what is interesting is if I move the SPL meter back...about a foot..directly behind the null seat...the 5db returns and this spot is in balance with the others. So at that exact spot (at the 1/2 width axis) there is definitely a null but at the 1/2 width axis minus 1ft on the length axis there is no null. Hope that makes sense.


It all makes sense now...the "N" on the pillow stands for null and it's appropriate that it is on the couch in that exact spot.


Thanks,
 

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When you move it back, you are proabably entering a peak area for the length dimension that is 'compensating' for the null from the width. I bet if you go a foot back from the 'other' seats, you'll get a higher reading.


The N is too funny. Sometimes you can't see the forest for the trees....
 

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Discussion Starter · #15 ·
Thanks guys for all the information that you have given. I am going to continue to try to understand acoustics enough to apply the information to my space. What I think I'm going to do in conjunction with reading anything I can is get the test CD from Rives and see what is going on.


This should be one incredible journey.


Thanks again.
 
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