Surface velocity of plastic component vs steel component 1

[elogesh]

Hai,

I have a sheet metal component(similar to silencer) of approximately 2 mm thickness. It is elliptic shaped closed one with one inlet and outlet hole.

Earlier it was made up of steel.Later the material changed to plastic. Hot gas passes through the component.
The temperature aren't so high as it is in automotives.

The acoustic cavity is same for both the components.Therfore I felt noise attenuation characteristics are same.

But still the system with plastic component is having 4dB
higher noise level compared to metallic component.
Few mentioned that it may be because of break-out noise(Break-out noise is due to flexing effects of the structure).But I said that the plastic will respond slower to the foreces compared to metals.Therefore plastic may have less break-out noise compared to steel.Whether my interpretation is correct? My interpretation is based rate of change of force,pressure or speed is important for noise.

Please provide your feed backs.

Regards,
elogesh

 

[GregLocock]

I think you are on the right track, in that the skin noise will be different for the two materials.

Generally denser thicker skins radiate less noise than less dense, thinner skins, in fact it is often wise to adjust the measured attenuation of materials by their surface density as a first step in the comparison. Surface density = thickness*density.

BUT

The radiation of noise by a given skin material is frequency dependent, and is also somewhat dependednt on the geometry of the panel or structure. Therefore you cannot simply use the surface density to explain the differences.

The Young's modulus comes into this equation, amongst other things.

If you want to get a good theoretical understanding then read Beranek. You are looking for the general subject 'noise radiation from panels', and a keyword is 'coincidence frequency' or 'coincident frequency'.

If you want to find out whether the plastic skin is responsible, wrap the muffler in glassfibre and then encase that in lead sheet (wired on and screwed together if your health and safety people don't like you welding lead). If that doesn't shut it up nothing will.



Cheers

Greg Locock

 

[MikeyP]

For a panel which is only 2 mm thick, the critical coincident frequency mentioned by Greg is almost certainly above your frequency range of interest (about 6 kHz for steel and aluminium 2mm thick) and the effects of surface density will dominate over any radiation issues. To a reasonable approximation, the sound power reduction is directly proportional to the surface density below the critical frequency.

M