Baffle - A physical object that absorbs or otherwise reduces the volume of sound which passes through it, or is reflected by it.In other words, It absorbs or blocks sound. Those are the 2 main ways this effect can be achieved:
Gobo - see Baffle.
- Absorbing the sound (converting it to heat via friction) - this is what foam, cloth, and other porous materials do.
- Reflecting the sound (bouncing it back where it came from) - this is what concrete, and other non-porous materials do.
You can also use a gobo between 2 microphones to keep the two sounds from bleeding into each other's microphones. For example if you're recording two guitar players in the same room together.
And if that isn't enough uses for you here's another! You can use large gobos to simulate a wall making the room effectively a different size (or shape). Which can come in handy if there's a note that seems to resonate in the room and sounds bad.
So how do we know what types of acoustical properties things have? Well the long answer involves measuring and anechoic chambers, but luckily physicists have invented something called an absorption coefficient. This coefficient is basically just a proportion of the energy which is absorbed by a given material.
For instance, a coefficient of 0.5 would absorb half of the energy thereby reducing the volume of the sound by 3 dB (and a coefficient of .9 would absorb 90% of the sound reducing the volume by 10dB). Unfortunately the absorption coefficient changes with frequency. So here's a few charts to help you out.