Can you correct nulls with acoustic treatments.

Stilljd,

I am glad this helped. It is an idea straight from recording engineer Bob Katz and his book on Mastering.

The single biggest benefit is that this track gives you a clear idea of just how room modes affect real music.

What I didn't expect was 10 to 15 hz. wide nulls (-10,-15,even -20 db.) at several other frequencies.

No need to panic. This is quite normal. Do as Plato suggests first (try speaker positioning).

You will need very serious room treatments for these null problem areas if they are below 60 HZ (and I doubt you can cure them entirely without turning your room into something like a set from Flash Gordon). Fortunately you don't need to cure it entirely. For starters, you can easily reduce the worst bumps with a PEQ. (Bumps are more of a problem due to their masking effect). Although nulls cannot be boosted (can't boost what is not there)....smoothing out the peaks will really help.

Try listening to Rebecca Pidgeon The Raven, "Spanish Harlem" and adjust so that you can hear the bass notes evenly and with roughly equal loudness. Good luck...

I'm with Onhwy61....you probably have typical room bass boost. You need to tone down those bass bumps rather than boost 80 to 300 Hz.

Secondly, above 200 Hz...you are dealing with wavelengths of roughly 5 feet...don't expect any kind of smooth response here as movements of a couple of feet of the SPL meter will give you nulls when playing pure test frequencies. (for example quarter wavelength cancellations off the back wall behind the speakers is typically evident with a series of nulls from 40 up to 500 Hz depending on speaker placement)

See this link Quarter Wave Cancellations Explained

The above is why I have soffit mounted my speakers into a heavy braced wall cabinet. This is most important for larger three way full range speakers and is why you see soffit mounting of studio main speakers in almost all studios. (The difference is very audible as you can imagine by looking at the frequency response curves shown in the URL example I gave you above)

Finally, I suspect a 1 Hz pure single frequency test tone with an SPL meter is likely to lead to frustration unless you are an expert like Rives and know how to interprete it.

For example, imagine (in theory) that your speakers are perfect ( zero distortion ) then you will get enormous peaks and nulls with single pure frequency test tones (as the ONLY frequency in the room is exactly what you input to the speaker).

Alternatively, imagine your speakers are more normal or typical (with 50 % distortion in bass frequencies when played at louder SPL levels ) then you will get a beautifully smooth balanced SPL response when playing pure single frequency test tones (because not all the harmonic distortion will be affected by the room in the same way as the fundamental - and SPL meter will pick up the average)

How's that for counter-intuitive.....two different speakers placed in the same position in the same room and the better speaker SHOULD measure far far worse!

How's that for counter intuitive....an amplifier with higher amounts of harmonc distortion may actually sound more balanced compared to a perfect amplifier with zero distortion, as it is less ruthless on room modes.

All this is because nulls will be their very worst when you have pure single frequencies....

So be very careful interpreting 1 Hz pure frequency test tones! (It is unlikely that kind of purity is actually coming out of your speakers unless they are incredibly good and you have an amplifier with very low harmonic distortion)

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.

Jim,

The Maggies should still respond to the same physics in the bass as Genelec describes, however, they radiate front and back in ALL frequencies (not just the LF below 500 Hz) and, therefore it will be recommended to have them as far out as possible - say in the middle of the room. (Soffit mount is not an option and will kill that beautiful atmospheric/ambient sound from the higher frequencies going in all directions). I have not owned these kind of speakers, however, the best demo I have heard was with these type speakers towards the middle of the room.