Originally Posted by
Muttz
expanding foam in a box works very well, I second this idea. However there is a lot you can do to quiet the exhaust sound.
If you realise that air in its self makes no noise, it only carries soundwaves or it rattles what it hits. Exhaust noise is a combination of a few things.
if your ducting is short you'll hear the fan through the ducting. use a oversize fan with speed control and longer run of ducting to give the noise chance to be deadened inside the ducting.
Ducting can deaden noise but can also cause it. if your using the wire spiral flexi ducting the inside will be very uneven and will cause turbulence and therefore noise, any bends will also cause turbulence and noise. if you can use the solid metal pipe style ducting, firmly fixed and wrapped in some kind of acoustic blanket this is best. If not then acoustic ducting which is the spiral stuff wrapped in a kind of fiberglass is also very good. if using either type of spiral then try and keep it pulled tight so as to smooth out the inside. not have it all saggy and flopping all over the place. plan its run to minimise bends and corners. if you cant use hard ducting for the entire run consider getting a few bits for needed bends and long runs etc.
The most important part tho is the last few foot of the run. its your chance to smooth out the airflow and kill any turbulence. just having the end hang lose or sticking proud is really bad because as the air exits it will attempt to flow back on its self and this will cause turbulence and noise. invest in a flange and flush mount it. try and have it blow into a large empty area, not onto something that will cause noise or a echo.
the other thing that really really makes a difference is the use of a baffle to smooth out the airflow. I'm not really sure how best to describe this bit. if you do a search for ducting baffles it'll show things to restrict airflow not smooth it out. the principle is more like the baffles you get on office florescent tube lighting to direct the light down or under the rear bumper of supercars to smooth out airflow. its a crisscross of flat blades like a chipcutter that forces the air to flow in the intended direction and not be bouncing around inside the ducting like a pinball. If done right you can walk upto your vent and feel the wind off it before you hear it.
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