Sound diffusion below 80 Hz

[Lee Batchelor]

Hi all. It's been a while since I posted.

I was in touch with Dennis Foley at Acoustic Fields. He looked at my HT specs and said I had a major sound absorption issue below 80 Hz and recommended several of his activated carbon absorption panels. The price was way above my budget but I'm sure they would have worked. I can build panels myself but the question is with what materials? What design?

My room is quite large at 22' x 34' x 7'. It has two Altec Lansing Valencia speakers for the mains and two subs - one 15" Eminence and a 12" Lab 12 Eminence. Dennis said that I have a lot of sound pressure going into the room, despite its size because my drivers are all large. He used a great analogy, "You have an 8 oz glass but you're trying to put 12 oz of beer into it." Makes sense. Sadly, most information about sound absorption is either about two-channel listening or for small rectangular rooms. My room is large with a non-rectangular shape. That is, it has an open wall on one side and a hallway.

The system sounds very good but I know I'm missing some definition in the 80 Hz and below range. I've performed several measurements using the REW software. I have a proper measurement mic and a Steinberg UR-44 ASIO sound module. So, my measurements should be quite reliable. I attached a response curve with both subs time aligned. I know there are a few peaks but I have since EQed them out. I'm driving the subs with a Yamaha PX-3 power amp. The subs have no cross-overs. I use the processors within the PX-3 instead.

Any thoughts on how to build these panels? Many thanks....

 

[highfigh]

Activated carbon? What is that supposed to do that other materials can't/don't?

 

[shadyJ]

If I were you, I would look at the kind of bass traps you can get from GIK Acoustics. They might be able to address your issue and I think they would be a lot more affordable than carbon absorption panels.

 

[Lee Batchelor]

Not sure. It's a process Dennis invented. He says it's superior to insulation filled panels. For me, anything that absorbs will do the trick. For absorbing frequencies below 80 Hz, he recommended 12 of his panels at 12" think each. Cost without shipping to the Toronto area, without import duties, was about $30,000 . Obviously, a big ticket.

 

[Lee Batchelor]

Thanks, Shady. I've been to their site. The big issue is not the cost of these panels but the import fees, shipping, ridiculous Canadian taxes (13%), but I'll still take a look.

 

[shadyJ]

To be sure, you can build bass traps pretty easily, once you understand the theory and know what materials work best. But it would take a lot of homework. Might not be worth it, depending on how much you value your time. Do a google search for "DIY bass traps" to see what is involved.

 

[Lee Batchelor]

Thanks, Shady. I will. I'm currently watching a video produced by GIK. He's explaining room modes and which of their products to use for treatment. Interesting stuff.

 

[ski2xblack]

Wow, that's a lot of coin for passive treatments. I think I would lean more toward a properly calibrated multiple sub array than the necessarily huge and outrageously priced bass traps route. Good grief, $30k for activated charcoal traps? Incredible.

 

[Lee Batchelor]

Quite right, Ski...I rejected the idea.
I'm leaning more toward proper subwoofer placement experiments and using the REW software to EQ my subs. I just found a stellar video on how to use the tool. Thanks for the thoughts .

 

[Eppie]

If moving the subs is an option that's the first place to start. Then address the room modes. Depends too whether you need to EQ for one seat or a whole seating area. GIK makes their own product so there may not be import duties. I would look into that more. Shipping would be the concern but they do make nice custom art panels. The HST you're stuck paying whether it's American, Canadian or DIY parts.

 

[William Lemmerhirt]

I would also spend some time experimenting with placement first. Also rescale the graph for limits of 45-105 and 5db increments. What range were the sweeps? Seems like there’s nothing below about 32hz.

 

[Lee Batchelor]

Thanks, Eppie. You're advice is well taken.

William, I used a cutoff of about 30 Hz because I'm not one who needs to "feel" base waves like many do. So, information below 30 Hz is wasted on me . Besides, the recommended driver high pass of the 15" Eminence is 35 Hz. I still get good explosion sounds and retain musicality in the sound track. The system sounds darn good but I want to get the most out it that I can.

 

[lovinthehd]

Curious, what is the room like in general? Is it a dedicated theater or a living room or ? That analogy about all being large drivers doesn't make a lot of sense, as that would simply vary with volume levels.

 

[Lee Batchelor]

This is a basement room. It's a good size (diagram attached). The ceiling is Sonoflex suspended ceiling tiles, the floor is carpeted, and the walls are drywall. It's pretty dead in terms of echo but I suspect the walls are the culprit.

Dennis' comment about the large drivers is probably valid when the volumes are beyond 85dB. You're right about the lower volumes, in that the sound pressure is greatly reduced.

The picture doesn't show the items that are sitting around the perimeter of the room. They are mostly hard objects and there are few wall coverings - if any.

 

[Lee Batchelor]

Tonight I changed my crossover settings. The fronts were crossed at 110 Hz (speakers sets to Small). I dropped them to 90 Hz. The low bass is better defined. This tells me I have some decent wiggle room before resorting to acoustic absorbers.

 

[lovinthehd]

How about measurements at each crossover?

 

[Lee Batchelor]

Good idea. I already have the sweeps at 110 Hz. I can do the 90 Hz sweeps tomorrow. It's about 10:00 p.m. where I live . Thanks for the idea!

 

[JohnnieB]

Activated carbon? What is that supposed to do that other materials can't/don't?

https://www.researchgate.net/publication/5327159_Enhancing_bass_absorbers_using_activated_carbon

Not an easy read but…
If my understanding is correct low frequencies, especially those below 125hz cause huge pressure nodes within a room that muddy the bass and mask the harmonics that exist up through the mid range. This study seems to conclude that activated carbon is superior to rock wool and other medium at reducing those pressures.

 

[Lee Batchelor]

Correct, Johnnie. After further research, I think by using more conventional (and far cheaper) materials like rockwool and messing further with EQ, crossover points, and time alignments, I should be able to achieve better results. Dennis Foley mentioned that not only am I dealing with very long wave lengths but there's a ton of pressure in the room. Activated carbon panels would definitely outclass everything but they are VERY pricy.

 

[Verdinut]

Tonight I changed my crossover settings. The fronts were crossed at 110 Hz (speakers sets to Small). I dropped them to 90 Hz. The low bass is better defined. This tells me I have some decent wiggle room before resorting to acoustic absorbers.

IMO, you don't have that much sound pressure from the Altec woofers and the two Eminence subs. My living room is only 12.5 by 18' and my three front loudspeakers contain a 15" Dayton sub, which replaced Altec 416-8A woofers. No problem with too much bass when I can EQ these front speakers. The L & R speakers are close to corners and I was getting too much bass room gain.

If your left and right Altecs are placed in corners, I suggest that you move them away by at least 3 feet from the surrounding walls. By the way, I think you should set the Altec speakers to large. That will do some change to your overall bass response. It's worth trying it.