Vibration / Noise Damping

[John P]

Hi,

Last year I asked if anyone could suggest how I could improve the sound of my Ulsteroid which has a Brooklands style “silencer”.

Although it is quite loud what I was actually trying to achieve was not so much to reduce the volume but to make it sound less rattly.

Since then I’ve tried a few of the suggestions but have concluded that what’s possibly happening is that the silencer, which is in effect an empty box with flat sides, is resonating as the exhaust pulses expand inside it. The flat sides transmit this noise to the bonnet side which is immediately behind it and this acts as a sounding board amplifying the noise further.

If I run the engine without the bonnet sides in position it certainly sounds a lot better so I’m looking for a low cost method of damping this panel.

I know you can buy sound deadening material but it looks like you have to buy much more than I need which would be quite expensive and it also needs to be non-flammable as it’s very close to the exhaust.

I did consider bonding a 4 inch square piece of aluminum (or even lead because it’s acoustically dead) to the inside of the panel with a structural adhesive to see if this would help but I’m not totally happy with this and removing it would probably be an issue if it didn’t work.

Any further suggestions?

John.

[Reckless Rat]

Earplugs.

[Howard Wright]

Hi John

I’ve used stuff like this on my bulkhead.

https://funkmotorsport.com/product/adhes...gLhb_D_BwE

I think I bought similar stuff of eBay much cheaper.

Cheers

Howard

[Alan]

Turn the radio up!

[Bob Culver]

Do tempoaray clamps ( even of wood, g calmps or whatever )reduce the silencer radiated noise? If not already there could fit tie bolts or rivetted ties across the silencer, Only coachbolt style heads need be obvious .Does the bonnet seat on some material or direct on metal?

[John P]

I did try clamping across the silencer but once the bonnet side panel is in place there's not really any clearance for a clamp.

I had also considered passing a coach bolt right through the silencer but I'm not absolutely sure what's inside and am reluctant to damage it.

As the noise is much less pronounced with the bonnet sides off I think reducing the resonating effect of the side panel is the first thing to try.   The panel bears up against the radiator surround at the front and has a rubber moulding which it rests on but at the back it simply sits quite loosely in a hooked bracket at the bottom and then is secured with a single bolt at the top so there's not much to damp out the vibration.

Really I was hoping to come up with a simple method of making the panel a bit less flexible without having to weld or rivet anything to the panel.   I've seen this on e-bay so may give it a try.

https://www.ebay.co.uk/itm/Sound-Deadeni...1196.m2219

John.

[Andy Bennett]

If it is responding to a resonant frequency from the silencer then how about dampening the source rather than the panel. Wrap the silencer?
However, if it is just a rattling bonnet vibrating against the bulkhead from engine vibration rather than a higher frequency resonance then surely it just needs more secure fastening at the bulkhead end to stop it rattling.
Try wedging a bit of material or spare rubber in there and see if it works then make permanent solution? Am not sure adding deadening sheet to the bonnet will stop a rattle unless you put it between the bits rattling and then its just a piece of 'rubber' anyway.
Andy

[Paul N-M]

Bob's suggestion would be my first course of action. Tackle problem at source first.

Paul N-M

[Terrytuned]

John having made a couple of Brooklands style silencers you need to put anti- resonate spacers between the sides of the box section, I also had to put some in the fishtail to get it past the noise test for hillclimbs.  Terry,

[Reckless Rat]

Less flippantly, the reason you're getting so much noise/rattling is because the "silencer" isn't doing what it is supposed to do. Ideally, if you can, it should be opened up and the incoming pipe joined to the outgoing pipe by a section of perforated tube, with the void around it packed with a heavy grade fibreglass wadding. Once re-closed you will find the exhaust note nicely muted but still sporty. You may even find that bottom end torque is improved slightly. While the silencer is off the car being sorted you could also affix some sort of flexible connection to the bonnet side, for example Triumph bonnevilles used to have flexible engine mounts to reduce vibration and they may be suitable.