Can you correct nulls with acoustic treatments.

I said I would try follow-up with the setbacks and breakthroughs. I do not know how to post the graphs in this format so I will just try to accurately describe the highlights and observations.

I spent 2 days measuring under the most controlled circumstances I could manage. Focusing on speaker position, this is what I found.

Started at my established listening position. Speaker centerlines - 50" from back wall, 83.5" apart, 36.5 from angled (45deg) sidewalls, and 115" from listening postion. Speakers were tilted back 4 degrees (guess, one washer in the bottom of the stand mount, will measure exactly later).

I will descibe the sequential set up for 7 different test runs with the Real Traps Test CD, because the measured results changed very little.

#0.. Baseline
#1.. Changed tilt to upright.
#2.. moved speakers straight back 6".
#3.. moved speakers forward 12".
#4.. moved speakers toward each other 6" each.
#5.. moved speakers back 6".

When I sat down and graphed the numbers...

Steady rise from 53db @ 32hz. to 63bd @ 38hz
6db drop @ 42hz. rising steadily to 76db @ 55hz.
9db dip @64hz rising to 77db @ 70hz.
Drops to 67db @ 86hz.
Rises to a steady 69bd @ 90hz. to 118hz.
Starts to dip steadily @ 118hz. to 53db @ 138hz.
Broad null for 10hz.
Rises steadily to 67db @ 172hz.

Things get real eratic at this point with wide 2,3,4hz wide swings down to < 50db and back up. Does seem to be another deep (<50db) broad null at 230hz. to 270hz.

In general terms, and to my untrained eyes, I have a big boost from 45hz to 82hz with a little dip thrown in the middle of that, another small dip @ 85hz, and 2 serious nulls @ 135hz to 175hz and 230hz. to 300hz.

Experimenting a little more

#6.. Opened laundry room door, checking for suckout.
Virtually no change. (big surprise).

#7.. Pulled single speaker out into middle of room (13' from back wall, facing into open area) and measured @ 2' with mono source. (closed LR door)
Same pattern of boosts and nulls, just a little less dynamic. Who would have thought? The tones really started to audibly oscillate @ 105hz through 135hz. Reinforcement/filtering, who knows?

Really surprised me how little the pattern changed through all 8 tests. I took the time to listen to familiar music in-between each test run and the position changes were easily perceptable.

At this point, I have some acoustic reference material coming for study so, although I can recognize the pattern, I don't know what it really means.

Other interesting (to me) observations. Putting the 1.6's closer together than I remember trying before starts to push the sound stage to the front of the speakers instead of behind and starting to produce the clarity in the vocal registers that I was seeking at the start of this. Despite the nice clarity, it is too bright to listen to anything but female vocals and acoustic instruments in that position and the soundstage is too narrow.

Next step... read a little theory and build some really big DIY bass traps to test.

Please forgive the typos, mispellings, and my grammar.

Regards,
Jim S.

I don't trust readings below 100 Hz for reasons of speaker and instrument inaccuracy. However, the dips centered around 125 and 250 Hz are definately room related. I would suggest focusing on those and hope for ancillary benefits below.

Playing those test tones, you should also be able to find the nodes by walking around the room, probably by ear. I would be willing to bet that there are two nodes parallel to the speaker plane. I mention this because it might lead to the easiest solution, by experimenting with speaker/trap location. Start with traps in the corner.

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. I built some using 1/8" hardboard for the panels. Some basics are covered online through the Sound On Sound articles, "Room for Improvement" (5 parts in DIY section). Geared toward studios and pros. At least, they aren't selling anything.

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.