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There are two aspects to soundproofing. Firstly, it is important that sounds coming in from outside of your studio do not affect your recording. Secondly, it is equally important that sounds emanating from your studio do not annoy your neighbors. Irritated neighbors will reflect every bit of irritation they suffer back towards you, and probably more!
But let's look at incoming noise. What effect does it have on the recording process?
Incoming noise has two effects. One is that it can enter your microphones and get onto your recording. The other is that it will interfere with the monitoring process so that you can't listen clearly to the sounds you are recording and mixing.
Some types of music are more problematical than others...
The type of music that suffers most from incoming noise isn't even music - it is speech. Speech has the unfortunate property of having gaps between the words. Any background noise will be clearly audible in the gaps because there is nothing to mask it.
(If you work in audio for picture then so-called Foley effects are even more vulnerable. Foley effects include very quiet sounds such as the rustle of clothing. Plainly, absolute silence is required.)
But getting back to music - what type of music is most sensitive to noise?
The answer is classical music, because it alternates between loud and quiet passages. The quiet parts of the music won't be loud enough to cover the background noise.
Next up is any kind of acoustic music, such as folk music. Quiet instruments once again cannot mask the noise.
If you have a heavy rock band however, the guitars and drums will be so much louder than the background noise that you will hardly notice a problem.
The maximum resilience against noise is to be found in electronic music, including hip hop. Any sounds that are purely electronically generated or sampled do not require microphones, hence there is no possibility of picking up background noise.
With regard to monitoring, the greater the dynamic range of the music you record (that is the difference in level between the quiet and the loud sections) the more you will be affected by background noise. Any style of music that is loud all the time will be pretty much OK, unless you live next door to an airport or train station!
So the quality of soundproofing you need depends on the kind of sounds you record. If you record Foley effects for film or TV, then you need an absolutely silent studio. A desert location would be good. If you record speech, then you will have to take very great care over your sound insulation.
But with other styles of music, there is some leeway. You don't always need to have a completely soundproof room.
Which is just as well since complete soundproofing is almost impossible to attain. And of course it is very expensive.