Thread: Polyethylene sheet for sound dampening?

Originally Posted by popotla

I am wondering about the usefulness of POLYETHYLENE FOAM SHEET for use as sound dampening material in my Defender.

I've successfully used those foam camping matresses cut to shape in the footwells and over the transmission tunnel in my defender. It has made a noticable difference. Is this the sort of material you had in mind ?

Yes, there's better stuff out there, but the price was right.

Like isuzu110 I have used camping mattresses (in my 2a), so I expect it would have some effect.

You need to consider how soundproofing works.

Noise comes from the engine, transmission and wind and tyre noise, and gets into the passenger compartment by three mechanisms.

1. Transmission though air via holes (including open windows)

2. Transmission through panels

3. Setting panels into motion either by impacting sound waves or actual mechanical transmission of vibration to panels that act as soundboards.

To eliminate noise:-

1. Block holes, particularly near noise generators. Reduce reflection on these routes where the holes can't be blocked. Effective moves include ensuring good seals on the sides and back of the bonnet. Under bonnet insulation reduces the amount of sound reflected down onto the road.

2. Reduce the effect of impinging sound waves by adding insulation (to either or both sides). Sound waves encountering an interface between two materials are transmitted or reflected according to the difference in properties of the materials. The biggest effect is from differences in density (and since there is a large difference between air and any metal, this makes this route normally quite small). But any insulation will be something that is soft and springy compared to metal, and ideally will be a sandwich of a soft material and second layer of metal.

3. Vibrating panels are best treated by adding just about anything to them that is markedly different, and preferably soft and flexible, as this will damp the movement. Heavy materials are best as they will not only damp the movement of the panel but will reduce the resonant frequency, and lower pitched sounds are less obtrusive. Even better, particularly on the inside, is a layer of metal (or other heavy, stiff material) on top of the soft material to make it into a filter for transmitted sound.

For most vehicles, noise comes by routes 1 and 3, and in any case most route 2 problems will be fixed by action on route 2.

Of course, it is always preferable to reduce noise at the source if possible (e.g. fix the rattles) although much of this has to be done at the design stage.

John

Be aware that poly gives off toxic fumes when burnt.

I just put some sheeting under the cardboard behind the pedals, It made that much difference I put more on pasengers side behind the cardboard.