There are two commonly used traps, both of which are easy to build. They are
the panel Resonator and the Helmholtz resonator. However, to be successful you
first have to know precisely where your problem frequencies are and then you
have to build these traps accurately to ensure that they work at the right
frequency.
Both types of Resonator take up a large area, but have the advantage of being
only a few cm deep. Even so, you must bear in mind that these are tuned
traps and so are normally used to reduce specific resonances. They are not
suitable for use as broad-band Resonators, with the exception of a panel trap
constructed with a highly damped, limp membrane.
Disclaimer
Please keep in mind that I am not an expert or an authority on
acoustics, and the information presented here is just my opinion. I do however
have many years of practical experience and my opinion is based on that. Please
take this information for what it's worth and hopefully you will find it
useful.
Absorbers & Diffusers
