Q: How thick should acoustic treatment be?
If your recording room is too reverberant, then this reverberation will get onto your recordings. Reverberation can be added to a dry recording, but it can never be removed.
If your control room is too reverberant, then the sound that is bouncing around will color your judgment and you won't be able to monitor and mix accurately.
If you record and mix in the same room then the problem is going to hit you twice.
The solution in all cases is to apply acoustic treatment.
There are two common types of acoustic treatment. One is the porous absorber, the other is the panel absorber, also known as the membrane absorber.
Any material that is soft and full of air-pockets will function as a porous absorber. Examples include curtains (drapes), carpet, glass fiber loft insulation and mineral wool. Of these, mineral wool is normally preferred for its effectiveness and low cost.
Mineral wool is often supplied in slabs that are around 50 mm deep.
You will find that a surface covered with such slabs will absorb high frequencies almost completely. However it will not absorb low frequencies well at all.
This has to do with the wavelength of audible sounds.
The wavelength of the highest frequency that is normally considered to be in the audible range, 20 kHz or 20,000 Hz, is around 17 mm.
However the wavelength of the lowest frequency that is normally considered to be in the audible range, 20 Hz, is 17 meters!
Clearly, absorption that is a mere 5 centimeters deep will have hardly any effect on a sound wave that is measured in meters.
As a rule of thumb, a porous absorber is effective for a wavelength that is four times its thickness, and all shorter wavelengths
So mineral wool that is 5 cm thick will absorb frequencies of 1700 Hz and above. Below 1700 Hz, it will be less and less effective.
In practical terms, it is effective to use porous absorption up to around 10 cm thick. For frequencies below that, panel absorbers are easier to find space for because they don't have to be so thick. Panel absorbers will be the topic on another occasion.
Bear in mind that if you use only porous absorption, your room will become bass-heavy because of the low frequencies that are still bouncing around.
By the way, the photo shows an anechoic chamber where virtually all reverberation is absorbed.