Fume Cupboard noise level

[RSTC]

Hi could anybody point me in the right direction to find the design noise level for fume cupboard installations. CIBSE Guild A table 1.5 doesn't go that specific.

Thanks for your help

[Brian_UK]

If you have noise problems then I would suggest that there is something wrong there.

Fan noise or duct damper noise point to poor design.

What are the requirements of the space where the fume cupboards operate in? That is the design point.

[Magoo]

I agree with Brian_UK.
probably an installation problem, fan noise transmitting down duct., dime dark memory it should be in range of 35DBA at 1 metre feild, don't quote me on it though. Had a similar problem in a Lab., straight flexible duct straight up to fan extract, fan over performing so added extra insulated flexible ducting with a dog leg in duct, cut noise level to within standard and within air flow rates. The ducting was perforated to absorb sound pressure levels. I remember the problem well as it was a Lab that was cooking for analysis feacial ( crap ) samples.

[RSTC]

Ew, I wouldn't have used flexible ducting in that application (would have looked for an easily wiped surface!)

I'm not totally sure of the requirements it's not for myself, I would imagine it's for reference for a performance spec (rather than for something that's broken).

NC45 would be what I would work to, but I was looking for something to back that up. Thanks for your help lads.

[RSTC]

BTW Magoo if the fan was over preforming would it not have been easier to put speed control on the fan and leave it running at the right volume? Would have reduced running costs as well (particularly if it was in a heated/conditioned space)

[Magoo]

Hi Rstc,
FYI
roof mounted prop fan extracts air side performances drop like a brick with speed control and do not reduce noise levels, depends alot on total static loses. The sound pressure levels could increase in fact. Axial fans maybe sometimes, depending whether back or forward curve blades, accoustic isolation is generally required. Sound levels are recorded in A range B range and C range pressure levels and are rated at a published average dba at an open feild of 1 metre, then depending where recorded inside or outside. Noise control is abit of a black art, but fun though.
If you have particular issues I seriously recommend engaging an Accoustic Engineer

[RSTC]

Cheers Magoo, probably made an assumption too many there, was thinking out loud (or whatever the internet version of that is!)

I was pretty sure I had a bit more reference data here but can't put my hand on it right now. CIBSE Guide A table 1.5 gives NR45 for similar rooms. Maybe there was more in the ASHRAE guides but I don't have a copy here.

[frank]

CIBSE and other engineering GUIDES are just that....GUIDES.

You have to work to whatever the client wants or Consultant specifies. If there is no specification, then you should quote that your design is based on CIBSE guide or similar.

The fan manufacturer will publish sound spectra for their products, and, if this is above the required induct level, you can select attenuation to reduce it. Almost all attenuator manufacturers will assist in selecting the required induct silencer for the job, as they want to sell their products.

If it's breakout noise from the fan then this can be overcome by siting the fan in a remote position or applying acoustic external attenuation.

At the end of the day, you have to work to what is required by the client.

[RSTC]

Thanks Frank.
As I said, it's not for myself so sorry that I am vague on the details. It's the design criteria within the room that the occupant is subjected to that I was after... actually more specifically I was looking for the reference within the guide to quote to back up the criteria (it was a consultant that was asking!). I couldn't put my hand on it so I thought one of you guys might have come across a spec that mentions it. Thanks for all your help.