I have had some correspondence with Mercedes GB about the excess of engine noise from my B200 CDI, as I've discovered that you can get soundproofing for the underside of the bonnet lid for a steep £117 plus fitting. I also found out that B-classes used as taxis have this as standard.
The letter back (to his credit, the MD himself answered my letter by return - own signature in blue ink!) said that the kit fitted to taxis was to make them quieter externally (e.g. at night in built-up areas) - the implication being that it wouldn't make much difference to the noise level as heard from inside the car.
I would have thought that the recerberation within the engine compartment caused by the lack of soundproofing contrivutes to the noise. Anyone out there who knows about acoustics care to tell me whether I'm right or wrong?
|
Accoustic material "absorbs*" noise, not seal it in. It therefore stands to reason that if ay noise is absorbed, there would be less intrusion into the cabin.
*of course it doesnt really absorb noise, but it stops sound waves that it hit from bouncing anywhere else.
------------------------------
TourVanMan TM < Ex RF >
|
Insulating the bonnet will cause a reduction in the noise level in the cabin (just don't ask me to quantify it!).
There are many paths for engine noise to enter the cabin. Among them are this non-exhaustive list;
engine vibrations transmitted to the body via the physical connections between powertrain and body - predominantly the engine/gearbox mountings, but also exhaust pipes, wire, cables, and hoses.
Vibration transmitted via clutch and gear linkage mechanisms.
Acoustic radiation passing from the engine, exciting the bulkhead, and thus the cabin.
Acoustic radiation passing from the engine, being reflected from the road, exciting the floor, and thus the cabin.
Acoustic radiation exciting the outside of any cabin panels, and thus the cabin.
Acoustic radiation passing directly through body openings, without the interim excitation of a body panel.
......
The best way to deal with these problems is at source. However, unless you want to re-design the engine this is out of your hands.
For an engineering solution, one might carry out a so-called noise path analysis, where each identified noise path is tested and quantifed over the acoustic frequency range, and the troublesome path is identified, and modified. However, this is neither a quick, or a cheap, unskilled task. It would need access to an acoustics department of a large company to carry out. Doubtlessly MB have these facilities somewhere! Sound re-radiation from vibrating panels happens most strongly at any of the natural frequencies of the panel - reducing the amplitude of these resonances by adding damping (in the vibration sense as opposed to the acoustic sense) is a good mehod of reducing the noise. Vehicle manufacturers sometimes do this to a test vehicle, with damping on evry panel. Then, while testing small panels of damping material are removed, and the effect of damping each panel is thus quantified. In this, labour intensive way, the panel damping and soundproofing can be optimised.
Without this testing and analysis, the best thing to do is to increase the general level of soundproofing/damping in the cabin itself. There are some particularly effective foams which contain a very heavy thin, consolidated layer, and a good, thicker, layer of absorbent material. The heavy layer works by imposing a sudden change in acoustic impedance (it makes the sound reflect, back where it came from!), while the foam layer forms a labyrinth, down which the acoustic wave gets "lost", and eventually destructively inteferes with itself, or dissipates via aerodynamic friction in the small cells.
Number_Cruncher
|
|
As good an explanation as I have heard on this topic! Excellent, NC.
|
In the old days I used to put a layer of thickish old carpet under the vehicle's own interior carpets to help deaden unwanted noise.
- - - - - - - - - - - - - - - - - - - - -
What\'s for you won\'t pass you by
|
Very interesting post, NC. Many thanks. From which:
"There are some particularly effective foams which contain a very heavy thin, consolidated layer, and a good, thicker, layer of absorbent material."
Do you know any brand names and sources for this stuff?
|
>>Do you know any brand names and sources for this stuff?
Sorry Roger - for the past hour or so, I've been rifling through my old notes and the few research papers I still have on this subject (it's 10 years since I was working on the relevant project!), and alas, I couldn't find the info.
I can remember the materials were investigated, modelled, and characterised by French researchers, working under Allard.
Number_Cruncher
|
|
|
|
Many thanks everyone for your help.
I'll check the prices of soundproofing kits from the likes of Noisekiller but I'd need to be careful not to invalidate the warranty. Mercedes' underbonnet kit may be enough, given that you've confirmed my own thoughts about it.
|
just make sure the aftermarket kit is fitted properly.
I saw a car start to go up in flames in a car park the other week. When the bonnet was forced it was found that the soundproofing on the underside of the bonnet was on fire.
I didnt hang around for the post-mortem so I dont know why it was on fire but I assumed it had sagged down onto the exhaust manifold.
|
|
|
|
:no_upscale():format(webp)/https://cdn.uk.prod.group-mobility-trader.com/images/cas/69ac02f2f4a7606201b0e3beccc33c05/3b59c1e4943ebc4489b2ad3787efddef.jpg)
:no_upscale():format(webp)/https://cdn.uk.prod.group-mobility-trader.com/images/cas/e75a50cff09da10878ae7598fb05188a/eccf0b7d9fd810b9446988c2bf8993aa.jpg)
:no_upscale():format(webp)/https://cdn.uk.prod.group-mobility-trader.com/images/cas/c9d57180847d730aa1bd896b932031fe/5cad34f12a213afdc36b1d6d9884337f.jpg)
:no_upscale():format(webp)/https://cdn.uk.prod.group-mobility-trader.com/images/cas/57dd11ee19ba766974f7bdcbc6acb624/cd9f2e3098ad6fb1bd74763d6cd7ecad.jpg)
