Take off 6dB (or more) from a noisy source
Take off 6dB (or more) from a noisy source
(OP)
I have a 0.5 liter volume available to take off 6dB (or more) from a noisy source (motor+fan)with the following constraints/environnement.
-Noise to be taken off at air inlet
-Pressure drop < 2hPa for 1L/s
-noisy source : 42 dBA of broadband noise + F0 + 2xF0 (not dominant)
-F0 can be anywhere from 250Hz up to 450 Hz
-The small available space doesnot easily allow any Hellmoltz or 1/4 wavelength based design.
So far,I have tried a perforated panel design with only a 4dB absorbtion coefficient.
Any tips or litterature that could help will be welcomed.
Many thanks.
-Noise to be taken off at air inlet
-Pressure drop < 2hPa for 1L/s
-noisy source : 42 dBA of broadband noise + F0 + 2xF0 (not dominant)
-F0 can be anywhere from 250Hz up to 450 Hz
-The small available space doesnot easily allow any Hellmoltz or 1/4 wavelength based design.
So far,I have tried a perforated panel design with only a 4dB absorbtion coefficient.
Any tips or litterature that could help will be welcomed.
Many thanks.





RE: Take off 6dB (or more) from a noisy source
Failing that try a labyrinth of semiporous foam. At least make sure that there is no direct path from outlet to inlet.
Cheers
Greg Locock
RE: Take off 6dB (or more) from a noisy source
How can i post a spectrum plot easily?
I'd like to avoid classical semi-porous foam as this device can be used in high RH environements
are you aware of any material that can have some sound absorbing propery and still be waterproof?
Cheers
RE: Take off 6dB (or more) from a noisy source
I don't know much about specific absorbent materials, it has been a long time since I had the dubious pleasure of playing with that sort of thing.
Cheers
Greg Locock
RE: Take off 6dB (or more) from a noisy source
RE: Take off 6dB (or more) from a noisy source
RE: Take off 6dB (or more) from a noisy source
-Classical silencer (expansion chamber) + few damping materials ... 4dB
-Helmholtz : 4 dB
-Active noise cancellation coming from an old design : 7-10 dBs
As there is some cost involved I am hoping to have one of the 2 first guys work (2dB's left...they are the toughest)
In parralel the noise source is also worked out.
Cheers
RE: Take off 6dB (or more) from a noisy source
BTW, what are you making?
RE: Take off 6dB (or more) from a noisy source
Currently in the enclosure it's doing 27dBA noise @ 1m (not that bad though)
The deal is to go down to 25 dbA where 80% of the people and 99% of the patients will not hear it.
The other part of the deal is not to have any open cell foam in the air path.
The expansion chamber after the Helmoltz is hard to fit due to the tubing size requiring at least 0.2L or I'll have to give up a few hPa in pressure drop thus efficiency...
Cheers
RE: Take off 6dB (or more) from a noisy source
RE: Take off 6dB (or more) from a noisy source
BUT I am trying not to have any acoustic foam in the air path inside the device and that is where the challenge is....
RE: Take off 6dB (or more) from a noisy source
Cheers
Greg Locock
RE: Take off 6dB (or more) from a noisy source
optimized air gap ... more volume ... more efficiency in theory ..
And suspension + a sealed housing is a lot better than trying to calm down the silencer case with foam as this one is loosing efficiency.
RE: Take off 6dB (or more) from a noisy source
You cannot tell when it is on.
I did find a really great enclosure damper somewhat "magic" as opposed to what I tested before and wanted to let you know:
Manufatured by TMAT it's named P15/D05.
Very usefull for plastic enclosure
RE: Take off 6dB (or more) from a noisy source
RE: Take off 6dB (or more) from a noisy source
many texts don't mention that there's a minimum volume requirement for a Helmholtz tuner to work:
in Imperial units, this minimum volume must be such that the product of the tuning frequency and the tuner volume is at least 18,000 "inch-Hz".
So for the 250 Hz firing order noise in "Highspeed's" case, the minimum volume requirement would be 72 cubic inches, or just about 1.2 liter.
Just something to keep in mind.