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Q: Why do I have to record acoustic guitar twice?
It is common knowledge that recording studios and control rooms need acoustic treatment. But what does acoustic treatment do, and why is it essential?
Acoustic treatment, which is something different entirely from soundproofing, exists to control reverberation time and to lessen the effects of standing waves.
Imagine you start out with a bare room with hard plaster walls. Clap your hands and you will hear the reverberation echoing for quite a long time. The bigger the room, the longer it will go on for.
An untreated room such as this is always too reverberant. It's out of the ordinary (except for bathrooms), so it isn't what the ear expects.
If the room is a standard box-shape with parallel walls, then there will also be strong standing waves.
A standing wave is a sound wave that has a wavelength that fits exactly between two parallel surfaces and reflects back and forth. Since it reflects very easily in this manner, the reverberation time at this frequency is significantly extended. There are also standing waves at whole number multiples of this frequency. (There's a bit more to standing waves than that, but this information is quite enough to get started.)
So because standing waves are particularly troublesome, they need special treatment,
Reverberation time can be controlled by using absorption in the room. Porous absorbers, such as carpet, drapes, acoustic foam and mineral wool are good for controlling high-mid and high frequencies.
Porous absorbers however only work well when they are at least a quarter of a wavelength thick. This means that they are only really practical for high-mid and high frequencies.
If the only acoustic treatment used in a room is porous absorption, then the room will sound incredibly dull and lifeless.
Another type of absorber is the panel or membrane absorber. A flexible wood panel (around 4 mm to 18 mm thick) mounted over a sealed air space (around 100 mm to 300 mm in depth) will resonate at low frequencies, and in flexing will absorb energy.
If damping material (typically mineral wool - 'Rockwool') is added inside, or a flexible membrane is used, then this type of absorber can be effective over a range of low frequencies. Drill some holes in the panel and the absorption becomes wide band.
Panel absorbers with little damping can be tuned to the frequencies of standing waves and control them very effectively.
The other way of dealing with standing waves, and at the same time waving a magic wand and making the room sound really great, is to use diffusion.
Irregular surfaces break up reflections creating a denser pattern of low level reflections than would occur with mirror-like flat surfaces.
The irregularities however have to be comparable in size to the wavelengths you want to diffuse.
Acoustic treatment is an incredibly complex subject. But you can do a lot with a little, and we've just made a start!