Quiet exhaust duct?

I am installing an exhaust fan in my bathroom, which will vent through the roof about 2.5 feet above the unit. I have an exhaust hood in my kitchen, and when I stand by the stove I can hear the traffic out back because the outside vent on the roof faces the traffic and acts like a collector to bring the noise into the house.
My bathroom vent will also face the road, and I don't want to carry that noise into the house. What kind of duct will be the quietest? I think the thin, ridged metal tubing would be the loudest. I'm thinking the flexible stuff (like dryer vent hose) will be quieter? I know it has more resistance but its a short run and the fan is plenty powerful for the room its in. But I was definately going to insulate the pipe, is the flexible stuff insulated? It felt pretty thin when I was looking at it in the store.
Also I was planning on getting a plastic vent for the roof, I think that should be quieter than a metal one?
Thanks for any help. -Ryan
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On 12 Sep 2006 07:58:42 -0700, snipped-for-privacy@yahoo.com wrote:

You are on the right track.
You should check for the one way flapper if you hear traffic while the fan is off. Since you have a powerful fan, you can add another. Modify the roof appliance and shield the side facing the traffic.
And the flexible duct for hvac will also dampen the sound substantially.
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snipped-for-privacy@yahoo.com wrote:

The quietest would likely be some flexible duct, but that is also not very efficient. I think I would try spraying the inside of metal duct with the type of stuff used for undercoating cars. This should work well, be cheap and may even reduce condensation inside the pipe. You might also check out: http://www.soundproofing.org/ and see if they have any ideas there.
Note: do not use the undercoating trick for a range hood vent. That would be a fire hazard.
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Joseph Meehan

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On 12 Sep 2006 07:58:42 -0700, snipped-for-privacy@yahoo.com wrote:

Connect the duct to a muffler from a tractor-trailer, and mount the fan on the far side, just below the rain-flap.
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