How Low Can You Go?

[Introspector]

I'm trying to address some low frequency acoustics issues in my living room home theater. The room is 23' x 12.5' with 8' ceilings. The walls are standard painted drywall, ceiling is plaster and floor is medium pile carpet with padding on a first story framed floor (basement below). The room has a fairly even distribution of windows and doors along both side walls. The orientation of the home theater is along the long axis of the room with the mains roughly 18" from the front wall and listening position slightly behind the mid point of the room.

As I begin the long journey of raising the capabilities of my home theater to audioholic levels, I am starting where everyone ought to start; with the acoustics. After spending many hours capturing data using a SMAART rig, there are a few issues to be addressed. The first is a series of strong reflections (some more easily located than others) that can be addressed through basic surface absorption treatments. Much more troubling however, are some low frequency anamolies.

Impulse responses indicate that the RT60 time of the space is roughly 0.35 seconds from 20kHz all the way down to 100Hz. At 100Hz the RT60 begins to rise and by 80Hz reaches 0.54 seconds. At 150% of the higher frequency RT60 times, this increase is audibly noticeable.

I believe that the first axial node of the room width and the second axial node of the room length are combining to create a low frequency boost that is audible and detracts from the precision of the system response.

To track down these nodes I took two boundary measurements, one on the side wall and one on the rear wall. These response curves showed strong peaks at 41.5Hz for the side wall and 45.9Hz and 53.3Hz for the rear wall. (I am not allowed to post images to the forum yet, but peaks on the plots for the side and rear wall response curves correlate strongly with the overall response curve taken from the listening position. Email me if you are interested in seeing the plots.)

So the measurements seem to confirm that I am dealing with axial nodes. MY QUESTION IS: Is it possible to address nodes at such low frequencies as these by any means that do not include building permits? These frequencies are below those addressed by even huge absorption corner traps or resonating panel traps by any data I've seen. The entire front wall of the room is currently covered by ceiling to floor drapery for aesthetic purposes. So it is conceivable that I could build a hemholtz slot resonator on that wall. But would meaningful absorption be possible with a resonator 6 to 8 inches deep given a total surface area of less than 100 square feet? Is there anything I can do to correct low frequency nodes below 50Hz without major construction?

 

[Davemcc]

I have to think that a combination of corner traps and active equalization could tame those nodes. I managed to tame a really nasty 40hz spike with home built 7' tall corner traps and the Velodyne SMS-1. I know there are even better solutions than this, and cheaper, which is to say I don't think you're asking for the moon. It's fixable.

 

[Introspector]

Thanks Davemcc for your feedback. The fact that you mentioned both corner traps and the SMS-1 is ironic given some reading I've been doing of some older threads here at Audioholics. I came across a thread from Jan 2006 involving, among other people, Buckle-Meister, WmAx, Ethan Winer and JohnPM, titled "Phase 2" in which John provides a number of compelling waterfall plots to show the effects of EQ. While the plots are clearly theoretical, since they are way too clean to be empirical, they seem to indicate that an EQ is capable of increasing the decay rate of a resonant node. WHAT!?

Everything that I have ever learned in any physics, engineering, and pro audio classes tells me that EQ and bass traps address nodes in two completely different ways. An EQ directly modifies the audio stimulus and does not affect in any direct way the room node. Conversely, a bass trap exists independently of any particular audio stimulus, but damps the resonance of any given room node.

Take for example a discrete broadband audio pulse introduced into a sound space. An EQ can affect the amplitude of that audio pulse at any given frequency, but nothing more. Once you've launched that acoustic energy into a room, the EQ's job is done and for the rest of the story it is powerless. Once in the space, the acoustic energy will begin to reflect, diffuse, and absorb. Should that space have a node at a given frequency, the acoustic energy at the node frequency will be preserved by way of reinforcing reflections. However even those reinforcing reflections are not 100% so in time even that energy is absorbed.

So here is my level of understanding. An EQ can reduce the amplitude of the acoustic stimulus at the same frequency as the node, so as to provide less excitement of said node. However the rate of decay of that node is governed only by the mechanical physics of the space which exists completely beyond the scope of the EQ. However JohnPM's data seems to indicate that an EQ is capable of changing the rate of decay of the node (acoustic under damped system) WITHOUT producing inverse phase cancelling stimuli. This proposal violates the laws of physics as I understand them. How can an EQ change the rate of decay of a node without either changing one of the variables of an under damped system (which all represent physical characteristics of the space) or introducing precise phase cancelling energy? Am I missing something?

 

[Introspector]

Now that I'm able to post images...

Below are the frequency response graphs to which I referred in my initial posting.

The first image shows two boundary measurements taken at the side and rear walls. I believe the yellow trace is the side wall and purple rear.

You can see the coincident nodes just below 30Hz as well as nearly coincident ones at 42Hz and 45Hz. These are made even worse by some need nulls starting at around 100Hz. There is also a nasty peak in the side axis just shy of 250 Hz.

The second image is the system response at the listening position. As you can see, the lowest frequency bump tops out at over +8dB over the nominal SPL.

The more reading I do, the less confident I am that any sort of corner bass trap or even reasonably sized resonator can reach below 50Hz.

-Byron

 

[peerlesser]

How can an EQ change the rate of decay of a node without either changing one of the variables of an under damped system (which all represent physical characteristics of the space) or introducing precise phase cancelling energy? Am I missing something?

As you said, DSPs can affect decay time by introducing anti-phasing sound. There is only one device that tries to truly make anti-phasing sound like that, Anti-Mode 8033. The effect is always quite local but for 16-45 Hz theres not much plausible alternatives. It's' better to have good response in the proximity of the listening area than nowhere. This device is really considerable tool for even professional acoustics, as the client will be more pleased to the result with it than without it. It has been out only for 2 month in Northern Europe, none in US has even tested it yet.

Example before:

and after:

It can't fix and shouldn't fix everything, only the part that can be anti-phased without adding the energy too much everywhere else. Also it does not fix nulls, unless they are wide and global (not infinite and local).

 

[bpape]

This is one of those where we try to address the 3 nulls via treatment and seating/speaker positioning as well as minimizing the ringing below as much as possible.

Then, the proper use of EQ as shown is the finishing touch to bring the broad hump down low inline with the rest.

This is an excellent example of why/how a combination of treatment and GOOD eq is the best solution for the room. Having complete flexibility in the EQ over all the required parameters makes a lot of difference.

Bryan