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Expanding Foam in Scoop
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odc04r View Drop Down
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Points: 5482 		Post Options Post Options   Thanks (0) Thanks(0)   Quote odc04r Quote  Post ReplyReply Direct Link To This Post Posted: 17 November 2017 at 10:36am
Originally posted by Tonskulus Tonskulus wrote:

Originally posted by nickyburnell nickyburnell wrote:

Builder friend says it actually conducts sound and makes things worse when trying to stop noise.

Make of that what you will, I'm not sure

So your friend thinks.. foam conducts sound better than pure air? :D



I could see his point of view. As I recall best wall sound insulation is an air gap followed by dense material.

Solids have a higher speed of sound transmission than air, the stiffer the solid the faster sound will travel. It also travels as a shear wave vs compressive. So I wouldn't be so quick to write off the idea of filling a void with foam being able to enhance propagation. But at the same time it will deaden cabinet resonances, the effect of which will largely depend on how well built it was in the first place.
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Points: 425 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Tonskulus Quote  Post ReplyReply Direct Link To This Post Posted: 17 November 2017 at 11:00am
however, inside foam.. it's mostly "insulated" air anyway. There is not much solid material. 
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Quote I wouldn't be so quick to write off the idea of filling a void with foam being able to enhance propagation
I would - at least in this context anyway. You acknowledge the effect of the foam deadening panel resonances, but seem to imply it may only be marginal. Filling the cavity with foam will have a huge impact on the stiffness of the scoop section. The 'impact' of the soundwave will be so widely distributed by the time it makes it through the panel that the foam may as well be concrete. 

This foam will typically be rated in the 100s of kPa, which works out to kilograms per square cm of surface area. Of course it will propagate the soundwaves in isolation, but in composite sections like this the strength of the skin is what really counts, and the entire composite section takes on the behaviour of a solid block of material. The foam might be 99% air, but it has such a high compressive strength that it's air content is totally irrelevant.

As you know, when sound travels through a panel it travels through vibrations in the panel, and (in the case of a scoop), then through the air, and if there's any energy left over after that (and there won't be much), through the next panel (or back through the panel it went through initially).

So whether it's able to travel through the air (or the foam) is mostly irrelevant - it's already lost almost all of it's energy while travelling through the panel to get to the air (or foam as the case may be). The foam will prevent it from even getting that far.

In any case the actual noise we're trying to prevent isn't soundwaves travelling through the panel and through the air inside, but the noise of the panel clattering - in effect becoming a very high Mms passive radiator in the horn mouth.


Edited by Hemisphere - 17 November 2017 at 12:04pm
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PU foam has variation of density dependent on the pressures applied to it as it cures, so it does not conduct vibration well at all, it is very good for sound deadening. 
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Points: 59 		Post Options Post Options   Thanks (0) Thanks(0)   Quote MINTEVENTRESOURCES Quote  Post ReplyReply Direct Link To This Post Posted: 17 November 2017 at 4:02pm
Hello All,

This is all wonderful information. With that said, as we all know specifically with scoops what is "theoretically" stated and modeled is not always what our real world experience demonstrates. Has anyone actually filled the gap behind the scoop with PU foam and experienced improved results? If so, what were the improvements?   
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Points: 1781 		Post Options Post Options   Thanks (0) Thanks(0)   Quote valve head777 Quote  Post ReplyReply Direct Link To This Post Posted: 17 November 2017 at 4:26pm
well it worked in my application, no need to write an essay about it. But hey, what do I know.
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I was in the middle of writing an essay on it but then my computer crashed lol

The long and the short of it (or at least the short of it) is there really isn't much need for guesswork here. The theory is about as solid as a foam filled scoop section.

If the panel has a space to vibrate into (and isn't optimally braced to begin with), then it will vibrate into it. If it doesn't then it won't.

Think of the foam as an extremely dense bracing structure, because that's literally what it is. The braces are all extremely tiny but there are millions of them. Think of the way insects make their shells strong enough to fight but light enough to fly, or the honeycomb structure of a beehive. It's like that, basically. 

If you really want to take a belt and braces approach, sand the inner panel of the scoop section before injecting the foam. That way you guarantee as strong a level of adhesion of the foam to the inside edge of the panel as possible. Make sure the section filled with foam is completely (100%) airtight and the foam should hold it's form for at least as long as the rest of the cabinet will last.


Edited by Hemisphere - 17 November 2017 at 4:47pm
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I built some very large TLs a few years ago, the first one had open voids the second I filled with mixable foam, at low volume they sounded the same but at high volume there was a difference, not much but you could here it, the foam had stopped the panels resonating.
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Points: 4410 		Post Options Post Options   Thanks (0) Thanks(0)   Quote nickyburnell Quote  Post ReplyReply Direct Link To This Post Posted: 17 November 2017 at 5:57pm

I could see his point of view. As I recall best wall sound insulation is an air gap followed by dense material.

Solids have a higher speed of sound transmission than air, the stiffer the solid the faster sound will travel. It also travels as a shear wave vs compressive. So I wouldn't be so quick to write off the idea of filling a void with foam being able to enhance propagation.

I agree. I once filled a hole with it and a rodent was trying to scratch through (shed). Damn loud, amplified even.
 
 Anyhow, for deadening resonances seems cool.


Edited by nickyburnell - 17 November 2017 at 5:57pm
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Points: 1781 		Post Options Post Options   Thanks (0) Thanks(0)   Quote valve head777 Quote  Post ReplyReply Direct Link To This Post Posted: 17 November 2017 at 10:38pm
I think I've read about your rodent adventures in a different thread (highly amusing).... Thumbs Up
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Points: 1710 		Post Options Post Options   Thanks (0) Thanks(0)   Quote jazomir Quote  Post ReplyReply Direct Link To This Post Posted: 18 November 2017 at 10:58am
Boston Whaler boats use something called Unibond Construction in which the two halves of the hull in the mould are injected with a closed cell foam under pressure which then bonds to the hull making it a monocoque structure. In addition to the unsinkability this give to the boat (probably not required in a scoop) this gives extra rigidity to the structure and noticeably deadens sound transmission. I cannot see how this would not work the same way in a wooden structure.

Edited by jazomir - 18 November 2017 at 10:59am
For sidefills, can we have two enormous things of a type that might be venerated as Gods by the inhabitants of Easter Island, capable of reaching volumes that would make Beelzebub soil his pants.
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odc04r View Drop Down
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Points: 5482 		Post Options Post Options   Thanks (0) Thanks(0)   Quote odc04r Quote  Post ReplyReply Direct Link To This Post Posted: 20 November 2017 at 8:04am
I think we all mostly agree here. Foam is good for deadening resonances, but you fill a void with it and it will act to propagate sound from one side to the other. The same properties that make it great at damping resonances when used in voids also make it good at transferring sound from one side to the other. When dried it is extremely rigid and does not have a lot of mass. Imagine making a laminate PU foam sandwich with outsides of 3mm ply with it, and then the same structure in 12 or 18mm ply. It is the mass of the wood that will primarily damp an impulse if you knock on one side of it.

I'm not knocking the stuff, I use it quite a lot. Was brilliant insulating the garage to fill up voids, filled in an old fire place once with it and plastered over - did a great job. Used small amounts in the internal corners of windows where plaster/render typically suffers damp over the years and works well there too.

I would definitely use it to fill voids in cabs. There is no real disadvantage imo unless you built it so badly it pushes all the panels apart when it expands :)
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