Can you correct nulls with acoustic treatments.

Ngjockey,

Thanks for the advice and sources. I found a couple of DIY bass traps recipes on AA posted by Jon Risch. I can afford to put together a couple of 18-20" tube replicas to test. They seem to be rather narrow range in absorption and if big enough can work down into the double digit frequencies.

I also have my eye on a corner, angled just off (4-5') the left speaker, that houses a fireplace. Not sure that is the problem, but I never did like that fireplace anyway.

I can't knowledgably comment on the meter accuracy or my testing accuracy. One thing that gives me some confidence in the measurements, are the repeatability I am observing. And the fact that the 70-80hz boost correlates to Shadornes recording reference. If the bass note progression is described correctly (and I believe it is), the third note in the beginning progression is consistantly much louder.

Do I just not understand all that is involved? Certainly possible/probable!

Regards,
Jim S.

"If you're ambitious, you can try DIY panel traps. There isn't much in the way of designs and plans available on the net but you'll get the vague idea with research."

All you ever wanted to know about bass trapping and acoustics is available at Ethan Weiners site Real Traps. Designs for bass traps or buy his product. The best money you will spend in audio and a real ear opener. Dont wait, we all should have done this from the very beginning. Why didnt anyone tell me this years ago? I dont know how anyone can be serious witout them, you will finally hear your equipment for what it is.
Bob

Jim,

Check out this Standing Waves

If the speakers are 53" from the back wall then you will get a first dip at around 64 Hz followed by another at 128 Hz and another at 256 Hz...

This is a common problem for all freestanding speakers. Of course, your ACTUAL room response will include all kinds of other standing waves; but rear wall quarter wavelengh cancellations will dominate the omnidirectional upper bass and lower mid range frequencies up to roughly 500 Hz, as it is the closest wall to the speakers, the surface is roughly equidistant to the listener, and therefore the wall produces the strongest and broadest coherent signal that aligns and either reinforces or cancels the primary speaker signal reaching the listener across the room.

This effect is well known. Being a detrimental first order effect it is worth worrying about as it dominates. Therefore most studios (who can afford and need to do it right or the mix will not transalate) will mount main speakers into a wall and completely eliminate this first order problem and leaving only third order effects from side walls/ceiling and a second order effect from rear wall (behind the listener). So the rear wall behind the listener is the NEXT biggest problem after fixing the quarter wave front wall nulls (Studios often put plenty of rear wall absorption to counter the effect or they try to ensure the listening postion is far enough away from the rear wall for this effect to remain small enough, or they will mix in a nearfield configuration far from all walls and where primary signal is very strong due to the proximity of the speaker to the listener).

You can ignore sharp eratic 2,3 or 4 Hz nulls above 172 Hz. The half wavelength of 172 Hz is roughly 3 feet so moving your microphone a foot or so will make likely make a significant difference at these frequencies (as you approach one side reflection or another the null shifts around...so trying to fix these kind of nulls may only shift the problem somewhere else by a foot or two or several hertz).

This is all high school acoutsic physics - no rocket science. I use "order" in a liberal fashion first order is the worst detrimental effect, second order is the next bad effect, third order is even less of a problem etc.

It often makes me wonder why megabuck systems make little effort to deal with this well known, easily understood, and well documented problem.

The above is not the same as Room Modes. Room Modes is a similar problem of standing waves but is much more complex. Room modes tend to dominate the ultra LF below 100 Hz but odd things can sometimes happen at higher frequencies in peculiar circumstances where dimensions happen to couple with eachother at certain frequencies at the listener position.

BTW: 128+64 = 192 HZ may also be another problem null area with speakers at 53" from rear wall...of course given other room modes/rear wall effects you may not clearly see every one of these dips, as the rear wall quarter wave cancellation effect, as seen by your microphone/SPL meter, is already combined with lots of other reverberant effects in the room.

Disclaimer: Fixing rear wall quarter wavelength may then lead you to see other problems more distinctly. So it is no cure all. For example it won't eliminate the problem of very bad room dimensions or lack of absorptive surfaces/furnture and acoustic treatment. In my experience it improves imaging and clarity in the lower midrange but does nothing for what are often huge room modal bumps below 60 HZ.

Acoustat6,
I have read through the Real Traps site and a lot of Ethan Winer's posts on AA. Learning a lot.

Thanks,
Jim S.