The Dead Vent
My Problem:
I have been struggling with putting vent holes in the walls of this airtight sound-tight room. I wanted to see if there was a way to power ventilate the room while significantly reducing the risk of sound leaking in or out. This is a challenge since air movement of any significance could only occur with at least a 6" duct in and a similar return out. Holes that big will leak sound quite effectively.
The Sound Scenario:
Thinking about sound, there are really two types, bass and everything else. Bass is powerful, like a bullet, everything else is more like a ping-pong ball. Thinking of those two items, and more directly how to stop them, I found it helpful to think of how these two items bounce off surfaces and come to a halt.
A bullet requires mass, especially limp mass. Too hard and rigid a surface won't absorb the bullet, it simply reflects and bounces off. A limp mass like a tree will allow the bullet to penetrate and come to a stop.
A ping-pong ball requires much less mass to stop. A ping-pong ball will bounce off a tree, and continue on but a ping-pong ball thrown into the grass will stop. So there are two distinct material requirements to stop both the bullet and the ping-pong ball.
Similarly two different materials are required to stop bass and non-base. A limp mass like loose sheet lead is very effective at stopping bass that is directed toward it, but non-bass will simply bounce off. Fiberglass is very effective at stopping non-base, but bass simply passed through. A mix of both is required for the task.
If the goal were to simply terminate a 6" pipe and not allow sound (or air) to pass this would be simple. If you have a system requirement to require air to travel through unrestricted this poses a different problem altogether.
Create a scenario where the course of the sound traveling through the vent pipe encourages the interaction of that sound with materials that we know reduce the likelihood of that sound reflecting and continuing on. For example a tube aimed at a limp sheet of lead will discourage the reflection and propagation of a bass wave exiting the tube.
If the course of the tube were serpentine, as the attached picture illustrates, it would force the sound wave to constantly contact the absorbtive surfaces as it traveled. Theoretically, the sound wave would be significantly diminished as it exited the tube.
The trick would be to have a material inside the assembly that kept the fiberglass clear to maintain the 6" conduit. This material must allow high frequency sound to easily pass and be absorbed by the fiberglass, though, perhaps a starched Guilford fabric.
This entire serpentine assembly would be housed in an MDF box. The pipe feeding it would be lead-covered PVC to prevent sound loss prior to entering the box. The fan that is either pushing or pulling air would be located on the far end of the assembly.
