Do hood sound absorbers really work ?

[EJ333]

As an offshoot to a posting from the E3 section, do underhood sound absorbers really work to reduce engine noise in the cabin ?

 

[teahead]

Probably a little bit if you have a carburetor.

I question more that soundproofing on the shock towers. Just a magnet for rust to lurk underneath. Those will NOT come back on when I repaint the car. Ugly too.

 

[deQuincey]

As an offshoot to a posting from the E3 section, do underhood sound absorbers really work to reduce engine noise in the cabin ?

if i understand well your description we are talking about the open cell foam that is glued on the internal hood skin

there are many sources for airborne noise under the hood that can only be reduced by absortion and blocked by barrier materials, these barriers can consist of open cell foams that will disipate the energy through those multiple cavities, barriers should be a complete as possible, no gaps or openings are advisable, i.e. firewall

other type of noises, structure borne noises will affect essentially sheet panels due to their size will allow free vibration of long length wave, this type of noise should be atenuated modifying the geometry to alter eiggenmodes i.e. by bending the surface with that sort of cross bends, alternatively to glue heavy bituminous materials will increase slef mass and thus alter the mentiond eiggenmodes

those heavy plates do not need to cover the whole surface and there is no problem to leave open passages

so two different questions, underhood foam and sock towers bituminous materials, radically different causes and effects, i would not remove neither one nor the other, those are not rust traps in a well preserved coupe

hope this helps

 

[rsporsche]

i have always thought that the insulation on the underside of the hood did more to protect the transfer of heat of the metal skin where it is only one layer thick - which protects the paint on the top side ... rather than sound insulation

 

[jmackro]

i have always thought that the insulation on the underside of the hood did more to protect the transfer of heat of the metal skin where it is only one layer thick - which protects the paint on the top side ... rather than sound insulation

No, I don't think heat is the issue - the top of the engine isn't that hot, and there is a significant air gap between the exhaust manifold (the only really hot thing) and the hood.

I think that deQuincey nailed it - the key is preventing the large flat sheet metal hood from drumming.

deQ: Here in the States, we spell eigenmode with one "g".

 

[aearch]

Option

Stock is stock
i think its looks great -every hood has it except MY ole 53 chevy truck

 

[jmackro]

every hood has it except MY ole 53 chevy truck

Well yea, a '53 Chevy truck has bodywork made out of something like 12 gauge steel. The steel is thick enough that its natural vibration frequency is below the audible range.

Also, a '53 Chevy truck hood is fairly rounded. The noise problem comes with large, flat areas of sheet metal, which behave like a flat drumhead.

 

[dave v. in nc]

truck

Sorry to continue the off-thread diversion, but when challenged with the decision, in the midst of a divorce c.1995, dumbass moi sold the 54 Chev pickup, and kept the 914...Loved that big ol round, uninsulated hood.

 

[Cornishman]

Hood foam is essential

I asked a Ford, former Jaguar, design engineer about the noise in my E9, he took a look and we took a ride. He suggested that I replace the decayed hood foam and all the holes in the firewall. He suggested that sound would go up through the bonnet (hood) and then through the windscreen. I did as he advised and it worked, the car is quieter.

He also suggested thick, dense rubber pads to large panels and adding a second box to the single box stainless exhaust. If I wanted to go further, add a mild steel section to the stainless system as that is proven to have a different resonance. I have not done these so far.

The BMW engineers would have been aware of all of this, that is why they put these sound proofing items so in place the first time. door panels and also the panel between the boot (trunk) and back seat often have these dense rubber pads attached, not sure if our E9s have these as standard.

Charles

 

[deQuincey]

No, I don't think heat is the issue - the top of the engine isn't that hot, and there is a significant air gap between the exhaust manifold (the only really hot thing) and the hood.

I think that deQuincey nailed it - the key is preventing the large flat sheet metal hood from drumming.

deQ: Here in the States, we spell eigenmode with one "g".

thank you

 

[pmansson]

Exhaust in mild steel !!!

IF you sant the car to run quiter, then use the original exhaust system in mild steel. I have that on serveral cars. And it sounds better at all rpm