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Just like the good program said an 18db suck out at 200 and 20db at 320.I took the ratshack meter and HP generator and swept around a bit and decided to use 100hz for a reference point. found some suck out at 70 and 80hz but only 3db down at 30hz. The Titanic has a parametric that i could fuss with to help this a little.
For the most part the room looks as good as could be expected until 200hz and 320hz then it goes to crap very quickly and comes back just as quickly. Moving the meter 1-2 foot in any direction at those frequencies causes huge variations in readings.
Looks like a call for heavy diffusion, I moved around some 2'X4'X1" fiberglass panels with no luck what so ever.
Also moving the meter into the rest of the basement showed a lot of attenuation until below 50hz. So I think any absorption outside of the room will have little effect. For bass it looks like a room made of 2X4s with 3.5” insulation between each cavity does a pretty good job at evening out bass. Maybe ¼” plywood on the other side will help a little.
What can a poor boy do at 200 and 320hz outside of re-dimensioning the room? I guess I should find a better seating position, maybe put the coach on rollers for easy repositioning.
Follow Ups:
You need to beware trying to draw very much in the way of conclusions from using a SLM and measuring the room for bass response. As you have already noted, the response changes signficantly with mic position.Room modes have a distinct and regular pattern, but one thing is for sure: the maximum pressure for ALL modes is in the corners. This is why pressure type bass traps are placed in the corners, so they can damp ALL room modes, including the higher frequencies.
You mentioned experimentation with a sound absorbing panel of 1" thickness. This has virtually no capability to absorb bass, and not very much at 200 or even 320 Hz. Even if you used a panel with adequate thickness to do some good with absorbing some bass, such as a 8" thick panel, a single 2 X 4 foot panel is not going to do much, certainly not something that could be readily measured, or even heard.
In fact, measuring bass problems in a room, and the results of using bass traps can be very difficult, even for experienced folks.
See:
http://www.audioasylum.com/forums/rives/messages/1041.htmlAs contrary as it may seem, it is better to rely on your ears, and try some proven techniques to control the room modes, see below.
For info on DIY bass traps that are VERY effective, see:
http://www.geocities.com/jonrisch/a1.htm
AND
http://www.AudioAsylum.com/audio/tweaks/messages/17498.html
PeteW's directions of building classic bass traps w/wire.
Andrew Chasin's description of his build-up:
http://www.AudioAsylum.com/audio/tweaks/messages/110285.htmlFor easy and quick traps, see:
the original post where I reveal the latest Super Quick & Dirty easy bass traps recipe:
http://www.audioasylum.com/audio/tweaks/messages/15737.html
and petew's post where he goes into detail on what he did.
http://www.AudioAsylum.com/audio/general/messages/70817.htmlFor a geenral room treatment recipe that is valid for the typical rectangular room, see my original DIY acoustics note, toward the very end:
http://www.geocities.com/jonrisch/a1.htm
which outlines a copmbination of bass traps, wall panels, and diffusion.HT is a little different, yu have to alow for the extra speakers, but if you place them where they are recommended, they will not be adversely affected by the recommended positining of the room treatments, and they will augment the overall sound field.
The biggest single factor for deep bass room modes when using a subwoofer, is the placement of the sub, which wil very strongly afect the voerall low bass response depending on where it is placed.
There are numerous posts on this subject, and various HT sites with recomendations for sub placement.
BTW, it is not likely that you want to go the Helmholtz resonator route, as these need to be tuned ever so precisely, or they do more harm than good. In order to be effective, they also ned a lot of sheer space/length, and thus are hard to place inside the room. Outboard Helmholtz resonators would still need to be long enough to work at low frequencies, and would be akward to position relative to the room.
Placing a "membrane trap" or any other type of trap physically behind a solid wall of dry wall will do little, and be a waste of time, effort and resources.
You can 'build-in' a bass trap IF you know what you are doing, and can have the details absolutely correct, but it is not easy to do, and can also end up being be a waste of your efforts.
Jon Risch
I'm learning that 1" panels are doing very little even with flutter. I think I have to rethink my aproach. I'm looking at high WAF larger difusion irregular shelves that are a little deeper than most to try to breakup the flat surfaces more and add some deeper panel absorbtion around, above and below. Maybe some sculpture like structures as well.The big mistake was makeing the room rectangular in the first place. I made best use of the space where if I made two walls angled and lost a foot or two at one end the sound would have been better without any fuss.
Thanks for the good information. I'm sure there will be more postings later.
I think you'll find that some trapping (absorption) will help more than anything. As you're aware, there are many ways to do this.To experiment at no $$ risk, you could try the super quick and dirty version of a tube trap - buy as many rolls of fiberglass insulation as you can fit in your front corners, and stack them there in their bags (you may even choose to put another bag over them to make sure they don't leak and fibers). You should hear a distinct improvement across the board, and your suck outs should lessen. You can return the rolls for refund when you're done as the bags will still be sealed.
One of the reasons absorption makes improvements to a room that may measure flat is that a flat response does not indicate how much room-induced sustain any single sonic event may encounter. This sustain can mask a lot of detail. It's very good for a room to be flat, but it's even better for the room's decay to behave nicely.
Also note that you may be able to even things out with some coupling/isolation techniques, particularly with the speakers. If they're not spiked to the concrete already, try doing that. If they are spiked, try isolating them from the floor. There are a million ways you can affect the sound of your system with these techniques, but you just have to play around with them to find the sound you like best.
-Pete
Thanks for the advice.I'll be trying a few different things over the weekend. The first will be some higher frequency flutter echo treatments and then some more measurements with repositioning speakers and coach.
and one i shall be trying within the next few days.. thanks!
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