Thread: Sound Mat Orientation.

Offender90 is the acoustics engineer on here. However the high density bit stops sound transmission and the foam stops sound reverberation/reflection. So my guess would be heavy side down...

I should add this isn't stuck down, it's simply laid on the sheet-metal. I figure a heavy layer stuck down would cetainly be better, but if it ever got wet would become a perfect rust habitat.

Yes on that aside I have been looking for something for sound/heat insulation and need something that will NOT retain water. Anyone know of such a product ?

Originally Posted by VladTepes

Yes on that aside I have been looking for something for sound/heat insulation and need something that will NOT retain water. Anyone know of such a product ?

Lead.

On of the sound deadeners is a rubber sheet that has metal or something similarly heavy cast into it. I used it on the 120", great sound insulation and didn't hold water. Didn't stop water from collecting underneath.

Originally Posted by Dougal

A quick question for those who know about sound insulation.

Like many sound mats the one I have has a dense layer of mass-loaded foam on one side and about 13mm of open cell foam on the other. When this mat is used as noise reduction on a car floor (steel below, carpet & rubber mats above) which way should it lie? Dense side down or foam side down?

I doubt it will make a lot of difference in practice - but would be better with the dense layer away from the metal. The way these products work is to layer together two properties with widely different acoustic properties, so that the interface between the two transmits acoustic energy poorly. If the dense layer is on top, you have two such acoustic barriers - floor to foam and foam to dense layer. If the dense layer is on the floor, to the extent that the dense layer and the metal have the same acoustic properties, there will only be one interface with poor transmission. Look at it this way - if the dense layer is in contact with the floor, and the floor vibrates, so will the dense layer. If the foam is in contact with the floor, and the floor vibrates, the dense layer will not, as its inertia makes it hard to accelerate, so the sound energy is reflected back and ultimately absorbed by the foam (along with the water that leaked in!).

John

Foam on the bottom - the purpose of the thin foam is to dampen vibration in the metal panel and decoupe it from the loaded vinyl. - What John said - minus the foam absorbing reflected sound bit at the end.

Ben, you may have been thinking of an explanation I provided in a previous post (here) for a similar three-layer material. It has a thin foam on the panel side (to decouple vibration), loaded vinyl in the middle (to provide mass for low frequency sound transmission), and thick (50mm) open cell foam on the outside (for reverberent noise absorption within the engine bay). In order for the absoprtion layer to work at low frequencies it needs to be thick - the thicker the better, (an air cavity behind it helps, but is not practical in an engine bay).

Vlad - you may want to consider a spray on liner, I can't think of much else that would do what you want it to. The choice of insulation is unfortunately a compromise of performance vs practicality. The trouble with most materials that are not spray on is that they can trap water underneath. If you're worried about the open cell foam in the material that Dougal describes absorbing /retaining water - it doesn't need to be open cell to work. It would still absorb some water, but I wouldn't imagine it being any worse than carpet on counties. Just make sure it's not glued down so you can take it out to dry if it gets a real good soak.

Cheers

Bojan