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.

[Thor]

It would be typical for these tanks to be made from tin or lead coated steel.

In the UK, a range of sound proofing materials is available from 'Noise-Killer Acoustics', but it is not cheap.

A relatively cheap way to achieve quite a lot (but not necessarily eleiminating sloshing noises) is to use the foil and bituminous product which builders use for 'flashing' (that is sealing) around things like a vent pipe passing through a roof. This used to be called Flash-Banding, but I am not sure what the current brand name is now. It is available in rolls of various widths. Unfortunately you will get the best effect by using a rather wide piece, carefuly stuck on, eliminating air bubbles, etc. accros the middle of a panel. There is no need to accurately cover the panel right to its edges, but it is worth having a large area covered in one continuous piece.

The mechanical theory is that if the panel is driven to vibrate the small bending of the panel which this entails causes the apparent relative lengths of the panel and the foil to change. Since the foil is relatively stiff in tension, the vibrations cause the panel and foil together to try to squash the soft bituminous material in between. This requires a huge amount of energy, so the panel vibration is very effectivly damped.

I hope that helps.

Thor.

[Kevin H]

Originally Posted by Thor

A relatively cheap way to achieve quite a lot (but not necessarily eleiminating sloshing noises) is to use the foil and bituminous product which builders use for 'flashing' (that is sealing) ...

There's a brand here called Flash-Tac, I have some on my garage roof.

However, I thought mass was an important factor. Being that the heavier the panel the greater amount of energy is required to cause it to vibrate, and that flash tac stuff isn't very heavy.

That's why I came up with the idea of sticking lead flashing to the tank. Maybe I should bite the bullet and give it a go.

Incidentally, a word of warning to anyone who decides to remove the original horse hair. The bulk of it is easy to remove but I tried removing the remnants of it stuck to the rear bulkhead on my car. I used a Xylene based product, which is brilliant for removing the old brown rubbery glue which was widely used on British cars of that era, but there was also some black stuff on my bulkhead, possibly some sort of underseal, and mixed with Xylene and horse hair makes a horrible black sticky mess!

[412usa]

While I think that the ideas for soundproofing are ...sound, I know that the
tank on the 412USA is aluminium and have just checked a spare tank I have
for the 405's and there is no magnetic attraction. It seems to Aluminium. I
don't see why they aren't all the same. Not that that solves the sloshing
problem.

Peter

[Kevin H]

Just did the magnet test on mine and it is most definitely iron/steel.

If they made them out of different material it begs the question - did they make them with differing internal baffle design?

[Thor]

Yes, mass also helps, but the concept of two relatively stiff sheets sandwiching a softer layer does create good damping, so do try it because it is a really easy thing to do. You have to use a product with a good tough outer foil layer, not some cheapo stuff. The materials used by builders should be tough, being left outside for years in use, but the stiffness of that layer is what makes the product work in sound deadening situations.

I know the tank of my 409 is steel.

Thor.

[Kevin H]

Originally Posted by Thor

Yes, mass also helps, but the concept of two relatively stiff sheets sandwiching a softer layer does create good damping, so do try it because it is a really easy thing to do. You have to use a product with a good tough outer foil layer, not some cheapo stuff.

But isn't adding mass directly to the panel that is vibrating a different approach to suspending a mass parallel to that panel ?

In the first instance you are reducing the noise at the source, because it takes more energy to vibrate a heavier mass (I believe that's why they mount building air conditioning compressors on concrete slabs).

Whereas the second instance - a suspended mass (with a decoupling layer as Philip said) - is good for stopping sound waves travelling further once they left the first panel.

I would have thought the best solution would be both.

As for the Flash-Tac product, it doesn't really meet the criteria to my mind. Firstly the bitumen isn't very thick and isn't very soft as the decoupling layer, not like some sort of foam rubber for example. Also the metal foil layer doesn't seem very substantial, at least not on the product I am thinking of.

Anyway, I've decided to go down the lead route. I already went out and bought some lead sheet last night (15kg/sq m) plus some high strength building adhesive.

I shall report back!