Sound proofing / deadening

[Kevin H]

I'm looking to improve the soundproofing of the petrol tank on my 411. I currently do not have any insulation between the bulkhead and the rear seat in my 411 and I can clearly hear fuel sloshing around in the tank.

While some insulation would surely improve this I was thinking about doing some additional sound proofing / deadening on the tank.

I have looked at aftermarket sound deadening sheet, which appears to be a thin sheet of metal (probably aluminium) sandwiched between two layers of bitumen. This is stuck to any panels which transmit sound, but considering what it is, it's very expensive.

The theory I believe, is to add mass to the panel so that it takes a greater level of sound energy to make the panel vibrate, which is how it transmits the sound.

I was thinking it might be more effective and more economical to simply stick pieces of lead flashing to the panel, such as the walls of the petrol tank.

Has anyone tried anything like this or other 'home made' alternative methods of sound deadening?

[Philip]

There are two distinct things to consider - the stick-on pads (Dynamat-style polymer sheets down to bitumen roof flashing) are just to stop panels resonating, but the key to blocking sound is to put mass, in the form of a barrier (like a mineral-impregnated vinyl sheet, lead or heavy rubber), between the source and your ears, leaving as few gaps as possible.

You should "float" the barrier sheet over a decoupling-layer of closed-cell foam to obtain the best benefits.

Philip

[UK6]

Pb SOUND DAMPING

Kevin/Phil,
I don't know much about sound deadening - however, the floating
lead sheet idea sounds good. Watch out for corrosion between lead and
aluminium - as you recall, you can unwittingly generate a redox reaction
between these two dissimilar metals. Fishermen know all about this
phenomenon and the long term result of lost lead sinkers in the depths of
their aluminium dinghies! I guess a non conducting barrier between the
metals would negate the problem. Do the 411s have steel tanks?

Regards,

Brett

[412usa]

I find the notion of hearing the fuel sloshing around a little surprising. I
have never heard it in the 412 and assume that the construction is similar.
The aluminium fuel tanks in the V8's (and also my 405's) are fitted with
internal baffles, much like airplane tanks, to slow the weight transfer when
cornering. I have heard that remade tanks and in some cars with fuel
injection have had that baffles removed and this might create the problem
described. Is this the problem here?


Peter

[Kevin H]

Mine is an original tank. I've had it out of the car and peered inside too and the baffles run front to back. The time when I really hear the fuel sloshing around is when I stop, so the baffles wouldn't stop that fuel movement (they would only reduce sideways movement).

I should emphasise the point that all of the horse hair insulation has been removed from my car and I haven't gotten around to replacing it yet. Maybe I should do that first and see if I can still hear it before I go to the expense of trying to deaden the sound!

As for the tank material, I don't think it's aluminium - it seems much too hard - but I don't think it is steel either because it doesn't appear to rust, although my did have a few spots of surface corrosion on the outside, which resulted in white powdery deposits (see poor photo attached). Perhaps it's an aluminium alloy such as used in the aircraft industry?

[David Olivant]

I removed my tank [408] and used an old endoscope to look inside. The tank appears to be made from tinned steel, hence no rust. I sprayed my tank at the same time as the whole of the inside of the with a sound deadening compound. I then used 25mm chipped high density foam/rubber sheet. When I get the car on the road, I'll let you know if it has worked!

David

[Sam410]

Originally Posted by Kevin Howard

Mine is an original tank. I've had it out of the car and peered inside too and the baffles run front to back. The time when I really hear the fuel sloshing around is when I stop, so the baffles wouldn't stop that fuel movement (they would only reduce sideways movement).

I should emphasise the point that all of the horse hair insulation has been removed from my car and I haven't gotten around to replacing it yet. Maybe I should do that first and see if I can still hear it before I go to the expense of trying to deaden the sound!

I hear the sloshing in the 410 too, and mine still has it's hairy bits, so to speak, so I'm not sure how much it will help you.