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bobkoure · 03-28-2005, 09:16 PM

I think there may be some easy/cheap things that could be done.
For instance, much of the annoying noise is higher frequencies. A simple 1" of sound absorbent foam can attenuate frequencies down to 7KHz or so (trick is to have air friction a half wavelength away from a reflecting surface). So... if you're set up in the "typical" pull style, put a baffle over the rad opening, spaced, say, so there's a 1" space between the foam and the rad - and put a 1" foam panel on the baffle. Higher frequencies are pretty "beamy" and won't go around the baffle - and you're attenuating whatever might reflect out.
Then there are the tricks the exhaust guys use for attenuating impulse noise. There might be something we could pick up from there, even though we're not dealing with impulse noise. For instance, if you have a duct, why not make it of perforated pipe (or hardware cloth) surrounded with, say, an inch of fiberglass packing - or the "acoustistuff" the speaker builders use as temp resistance isn't a big issue), and wrap the entire business with duct tape or something to maintain pressure.

Marci says that a good portion of the noise is generated by air passing through a radiator. At low air velocity, I'm not sure this is true - and if most of the noise is from the fan(s), then maybe just putting the fan at right angles to the radiator fins may provide a good deal of attenuation.

Back when I was a bike mechanic, there were mufflers called "Dunstall Decibel Silencers". They didn't really silence, but they were nearly wide open and did provide some silencing. The "magic" seemed to be a pack of short tubes (like a handful of straws, held parallel), about 3/4" ID and maybe 3" long that most of the exhaust passed through. At the time I wondered whether the tubes were providing a barrier to any particular frequencies, and how the ID and length worked together to do this. There was no comment about this system in The Scientific Design of Intake and Exhaust Systems (a must-read if you were putting together race exhaust systems) and I waded through a number of Bolt&Beraneck (yes, the BBN founders - but they started as audio engineers) papers in the library (this was pre-web) but found nothing that might explain it. Might have been a gimmick - but you had to disassemble the "silencer" to see the gimmick, so I dunno...
If tubes of a particular length and ID have audio blocking properties (which they might not - just supposing) then there may be silencing advantages to using a radiator with similar passageway size and depth. Maybe we'll come full circle back to extra-thick heater cores, but for silencing rather than performance reasons. Just a thought, of course...

03-28-2005, 09:16 PM	#194
bobkoure Cooling Savant Join Date: Feb 2003 Location: USA - Boston area Posts: 798	I think there may be some easy/cheap things that could be done. For instance, much of the annoying noise is higher frequencies. A simple 1" of sound absorbent foam can attenuate frequencies down to 7KHz or so (trick is to have air friction a half wavelength away from a reflecting surface). So... if you're set up in the "typical" pull style, put a baffle over the rad opening, spaced, say, so there's a 1" space between the foam and the rad - and put a 1" foam panel on the baffle. Higher frequencies are pretty "beamy" and won't go around the baffle - and you're attenuating whatever might reflect out. Then there are the tricks the exhaust guys use for attenuating impulse noise. There might be something we could pick up from there, even though we're not dealing with impulse noise. For instance, if you have a duct, why not make it of perforated pipe (or hardware cloth) surrounded with, say, an inch of fiberglass packing - or the "acoustistuff" the speaker builders use as temp resistance isn't a big issue), and wrap the entire business with duct tape or something to maintain pressure. Marci says that a good portion of the noise is generated by air passing through a radiator. At low air velocity, I'm not sure this is true - and if most of the noise is from the fan(s), then maybe just putting the fan at right angles to the radiator fins may provide a good deal of attenuation. Back when I was a bike mechanic, there were mufflers called "Dunstall Decibel Silencers". They didn't really silence, but they were nearly wide open and did provide some silencing. The "magic" seemed to be a pack of short tubes (like a handful of straws, held parallel), about 3/4" ID and maybe 3" long that most of the exhaust passed through. At the time I wondered whether the tubes were providing a barrier to any particular frequencies, and how the ID and length worked together to do this. There was no comment about this system in The Scientific Design of Intake and Exhaust Systems (a must-read if you were putting together race exhaust systems) and I waded through a number of Bolt&Beraneck (yes, the BBN founders - but they started as audio engineers) papers in the library (this was pre-web) but found nothing that might explain it. Might have been a gimmick - but you had to disassemble the "silencer" to see the gimmick, so I dunno... If tubes of a particular length and ID have audio blocking properties (which they might not - just supposing) then there may be silencing advantages to using a radiator with similar passageway size and depth. Maybe we'll come full circle back to extra-thick heater cores, but for silencing rather than performance reasons. Just a thought, of course...