Over-Engineering for Infrasonics / Anti-Vibration Techniques - AVS Forum
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post #1 of 14 Old 08-14-2012, 08:28 PM - Thread Starter
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I recently browsed through a paper titled “Hearing at Low and Infrasonic Frequencies”. In it, they cite studies which have shown that people can perceive sound all the way down to 2Hz. (See Figure 2 at the bottom of the article) Many studies have shown that 5Hz at roughly 110db is audible. And 10Hz at roughly 95-100db is audible, even for people in their 30s and 40s (Figure 9). The thing is, according to equal loudness curves (Figures 16, 17, 18) 10Hz at 110db is about as loud as 100Hz at 80db.

The problem with all this is the room noise. Generally, 10Hz at 110db+ is going to cause the room to make excessive noise.

If one were to finish a basement into a home theater, and wanted to design a room that would generate as little noise as possible while being excited with something like 10Hz at 110db, (entirely possible if watching a movie at -10 from reference, with the subs running 5db hot) what sort of things would need to be done when designing and constructing the room?

My initial thoughts would be heavier framing. Is it a violation of building codes to have studs closer together? To avoid future squeaking, I would guess that screws would be preferable over nails so they ideally won’t loosen up. Is there some sort of adhesive which could be pre-applied to a tap hole so that a screw will not back out over time? Like a kind of Loc-Tite for wood? Or how about going completely overboard and making a room entirely out of laminated 2x4s? (0” stud gap) You’d have to run wires from the outside in, or have all the wiring internal, if there is a way to do that to code.
Also, probably two or even three layers of the thicker sheetrock, green glue inbetween, and secured with screws.

Still, the problem with a wood frame room would be it would still flex not unlike a subwoofer enclosure. I’d imagine you could brace the room with arches which could look neat at the same time. But here a subfloor would be necessary so that the bottom can also be braced. And to do it right, the braces would be in the way of side walkways, so the aisle would have to be in the center - right down all the prime listening positions.

What about alternative construction methods which might work better or cost less? Cinder block? Concrete? Unobtanium? Can a room be built to be vibration-proof without costing more than the house it’s in?
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post #2 of 14 Old 08-15-2012, 09:42 AM
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Not a practical consideration, unfortunately. To avoid the vibration, you'd have to build a structure that had a significantly lower resonance point than the offending frequencies. If you're looking to stop 5Hz sound waves, the resonance point of a wall would need to be below 3Hz. That's just not going to happen.

You're talking a dozen sheets of drywall on either side of a very large air cavity, with decoupled framing. Solid poured concrete walls would need to be a couple feet thick. You would also have to suspend all of this in the air with giant springs, again tuned for an oscillation below 3Hz.

Take a couple of pics while you're at it. wink.gif

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post #3 of 14 Old 08-15-2012, 12:30 PM - Thread Starter
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Ok, so making the room vibration proof is not feasible, but how about mitigating generated noise? For example, reducing the likelyhood that a joint will squeak or that the sheetrock would drum against the framing? My guess would be going to extra measures to assure the construction is as solid as possible. Not only screwing, but gluing all the studs in place. Would it be better to use wood glue, or something more flexible? And how about lining all the studs with a line of green glue before screwing the shetrock to it?

In-wall wiring would also have to be considered, I would think. But then what would one do if they had wiring conduit? Can anybody think of a way to keep the wires inside from slapping around while still being able to pull new wire? The only thing I could think of is to run the conduit to the outside of the wall. When I do get a house with an unfinished basement, I'll ideally want to do a "room within a room" where the framing for the theater room is mechanically isolated from the unfinished sides and ceiling. In that case, there should be gap enough to run conduit on the outside so that the wall itself would isolate noise from vibrating cables. Another option I can think of would be to run a cable trough through a subfloor, and simply pad the cable trough.

What other room related sources of noise would there be to consider? The only other thing I could think of would be light fixtures.
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post #4 of 14 Old 08-15-2012, 12:33 PM
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Nailing the studs is sufficient. No need to screw, much less glue.Your slaps and squeaks won't be heard through a lot of (damped) decoupled mass.

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post #5 of 14 Old 08-15-2012, 12:40 PM
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At these frequencies, is distortion even an issue? The experience is primarily visceral rather than audible. In other words, any sound that the wires flapping around in your wall might make is going to be severely drowned out smile.gif

In the $20K forum there are some people who have installed the Thigpen Rotary woofer, which is something like a fan where the blade pitch is controlled by the audio signal, and reproduces from DC to 20Hz. You could solicit some information there about structural considerations.
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post #6 of 14 Old 08-15-2012, 12:40 PM
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Quote:
Originally Posted by Ted White View Post

Nailing the studs is sufficient. No need to screw, much less glue.Your slaps and squeaks won't be heard through a lot of (damped) decoupled mass.

*Nailing* studs? Ted, for shame! biggrin.gif
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post #7 of 14 Old 08-15-2012, 01:20 PM
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What can I say? I'm old school!

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post #8 of 14 Old 08-19-2012, 06:29 PM
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Just a snippy point. Sound does not exist at 5Hz or 10Hz. Your kinesthetic senses may allow you to perceive the vibration; but, since the human auditory mechanisms are not involved it is (a) not sound and (b) not heard.

Back to your regular programming.

Ted is exactly correct, the energy level and frequencies you're talking about are not feasibly eliminated (except in a magnetically levitated room inside a vacuum chamber or in space. smile.gif

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post #9 of 14 Old 08-20-2012, 08:48 PM
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Dont waste your time like I did! I put ten times the amount of screws in the drywall, I glued and screwed every sheet of wood that was used on the soffits, baffle wall, and columns. Every place that I could think of that would vibrate I did everything that I could to negate it. Trust me when I say I over glued and over screwed everything possible. I have more subs than the average bear and thought I would be rattle free but hundreds of hours of gluing has left me with several places that still rattle. I even put multiple troughs spanning the entire length of the room in the ceiling during construction to eliminate ceiling flex but there is nothing that will stop the flex other than what was stated above.

I'd put forth the effort to make sure everything is nice and snugg while gluing where its convenient and call it a day.
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post #10 of 14 Old 08-21-2012, 11:52 AM - Thread Starter
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Quote:
Originally Posted by Dennis Erskine View Post

Just a snippy point. Sound does not exist at 5Hz or 10Hz.

Just a snippy reply.

http://www.noiseandhealth.org/article.asp?issn=1463-1741;year=2004;volume=6;issue=23;spage=37;epage=57;aulast=Moller
Quote:
As a surprise to most people (even to many acousticians), the threshold curve continues below 20 and even 16 Hz, and - as it will be seen in the following sections - humans can perceive sound at least down to a few Hertz. This applies to all humans with a normal hearing organ, and not just to a few persons.

snip

Do we sense with our ears?

Connected to the issue of the perception pathway is the question, whether the same thresholds are obtained if the whole body or only the ears are exposed. Yeowart and Evans (1974) measured thresholds in a whole-body chamber and with a binaural earphone. The number of subjects was not the same (12 and five respectively), and it is not stated whether there is overlap between the groups. Nevertheless, psychometric method and conditions in general were probably very similar. The data are seen in [Figure - 8]. It is seen that the agreement between the two data sets is very good. This supports the assumption that also these low frequencies are actually sensed by the ears.

Further, the following paper:

http://www.scribd.com/doc/56798241/4/The-low-frequency-hearing-threshold-and-loudness

On page 12, several studies were done on low frequency hearing. Three were done with headphones so as to eliminate the tactile effect. 1.5-5Hz was the lower limit of these studies.

So yes, sound audible to humans does exist that low.
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Dont waste your time like I did! I put ten times the amount of screws in the drywall, I glued and screwed every sheet of wood that was used on the soffits, baffle wall, and columns. Every place that I could think of that would vibrate I did everything that I could to negate it. Trust me when I say I over glued and over screwed everything possible. I have more subs than the average bear and thought I would be rattle free but hundreds of hours of gluing has left me with several places that still rattle. I even put multiple troughs spanning the entire length of the room in the ceiling during construction to eliminate ceiling flex but there is nothing that will stop the flex other than what was stated above.

Well, that's discouraging. Are the rattles structural, or are they from places you didn't consider? And would you say your room is less rattley than others, or is it about the same? I suppose even in an imaginary room made out of 3' of concrete, once you run wires, lighting, and fixtures, something is going to rattle...
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post #11 of 14 Old 08-21-2012, 12:55 PM
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All very interesting, Dan. I am aware of both articles and the contributory research. We can debate, as is still debated, what, if any, direct role the auditory mechanisms are involved vs the kinesthetic in this "perception". That aside, how many movies/music recordings have intended content below 20Hz? 10Hz? (I know of exactly one music recording with intended 10Hz content ... that was only because we could record to 0DC.) Next, to achieve 120dB at 10Hz requires what speaker and what wattage?

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post #12 of 14 Old 08-21-2012, 04:46 PM
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I'd have to guess that it has less vibrations/rattles than 99% of the standard wood construction theaters that produce insane spl levels but I also spent many many hours with tubes of glue and buckets of screws trying to solve this problem.

There are a few buzzes and rattles that are clearly heard when a scene has sub 20hz content that I can't explain. There are just places that rattle even though I know glue and screws were used in that area. 120+db in the ULFs will cause rattles no matter what with stick construction, the pressure is like water and will find an area that can move.

Just go overboard with screws and glue areas that obviously would need to be glued would be my advice.

There just becomes the diminishing returns like with everything else and I found out the hard way. 5+ cases of pl premium, 100lbs of drywall screws, and a ton of wood screws along with 2x6 troughs in the ceiling still hasnt completely eliminated the rattles.

I did run all my wires in the soffits and used the nail ties that electricians use to secure them. Then I stuffed insulation around them and I dont have any rattles from the wires. The rattles are just in places that the pressure has found a place to flex is all I can come up with. I can hear them but there isnt an explanation as to why I have them in some of the places they occur. I will be covering all my walls in fabric along with acoustic materials after I get the courage to have Dennis send me a set of plans for the entire ceiling, and I will try to isolate as many of the rattles before covering the walls way one more attempt can be made with screws. I would say that out of all my glue and screws the most important thing was the troughs in the ceiling. My framer put them in during construction and the ceiling barely flexes at all. I would imagine the ceiling would move 3-4" without the troughs.
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post #13 of 14 Old 08-22-2012, 04:29 PM - Thread Starter
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Ok, lots of good information there. So while it may be unrealistic to expect to eliminate all vibrations, extra scews, glue, and other considerations towards eliminating vibration does indeed help to create a room with a grater S/N ratio, if I may call it that. I know for sure, it would be comforting to be in a "solid" sounding room that doesn't sound like it's all going to cave in evey time things get intense.

I do have an idea to combat ceiling/wall flex, but I don't think it will be within my budget to implement it. Take a look at the 2nd picture in the page below, the one with the arches in the great room.

http://www.vermonttimberworks.com/home/projects/breed/index.html

I'm thinking arches similar to the ones on the top tier would provide a lot of structural rigidity to help dampen vibrations. It would also look pretty neat with the right theme. But it also looks like it would be really expensive. That and the arches would probably interfere with the surround soundfield, especially if dipoles were in use.

A couple good side effect of such a room, if constructed with multiple layers of sheetrock, is it would also be good at keeping the sound inside the room, as well as keeping unwanted external noise out. Of course, one would have to insert (electronically) a ~20-30Hz highpass filter if somebody outside the theater was trying to sleep while daddy is watching a movie!
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post #14 of 14 Old 08-22-2012, 05:01 PM
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You would probably want to decouple the slab from the foundation at those frequencies, if this were in a basement, to prevent flanking through the floor. I did that on three sides...because of drainage and waterproofing issues though.

Why don't you just strap on a transducer for direct energy transfer and eliminate the inefficient, indirect transfer of energy through air?

Seems like the latest rage is infrasonics and listening...feeling... dubstep. In the DIY speaker forum lots of posts about getting to crazy low frequencies.
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