Buy Rod Gervais’ book on home studio construction. Pretty much everything you need to know is in there in plain terms. The bible of the amateur studio building community.
You should buy soundproof foam from Guitar Center. Foam only damps high frequency reflections. Making the room sound deader but not reducing noise on the outside at all. Most of what gets out (and is most difficult to stop even with construction), and annoys neighbors, is low frequency noise. Particularly bass drums.
Real acoustic foam is fairly expensive because it is flame retardant. Hanging mattress pads on the walls is an invitation to another Great White fire trap. Better is rigid fiberglass insulation. Which can burn but is much harder to ignite. Those shapes you see on the walls of movie theaters are made of this.
Isolation of noise takes exactly that. Physical isolation or decoupling of the inner surfaces of the room from the structure or outer surfaces of the room. Lots of ways to do this with varying degrees of effectiveness. From resilient channel behind the drywall (you have to remove the existing drywall to do this) which is what home theaters and hotels do, to a full on independent floating structure inside the existing space. Which is what real recording studios do. The first gets maybe 10 dB of isolation and much less in the bass. A correctly done full on studio can get up to 50-60 dB of isolation. A band or decently loud drumming can be between 95 and 115 dB.
Now think about what you really want to do. Do some research and costing out alternatives.
I have recently realized what an excellent set of percussion instruments are available with the standard K5 library but cannot find any midi files that demonstrate the capabilities to full extent. I am not a percussionist but realize that with ~40 articulations for Congas alone there is huge potential for realistic percussion instruments performances if someone with the appropriate knowledge is able to capture some midi performances. A friend advised me to invest in a pad controller (e.g. Korg nanoPad, all the way up to the bigger units like Korg’s padKontrol or the equivalent Akai ones, or NI’s Maschine) as you’ll find percussion is much more playable fromaa pad controller compared to a keyboard.
(With a keyboard you are always waiting for the key to rebound before you can strike it again, which isn’t authentic to how percussion normally works. With a pad controller, it’s more “true to life.”)
You will still spend a little time mapping the pad controller.
Also here are some everyday objects used by musicians.
1) Striking a propane tank. (make SURE that it is empty first)
2) Striking brake drums (not while they are on the car)
3) Samples taken while striking a large stainless steel salad bowl.
4) Striking a brass flywheel from a tape recorder.
5) The sound of a basketball hitting the floor in a gym. This is sampled tuned and triggered.
Here are a few that I know are cool but never used.
1) The bell housing from an engine/transmission. Many are aluminum and ring real loud.
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People used to say that they’re expensive but actually they’re not – pretty much competitive with any supplier and often beat them price-wise. The main thing is they pretty much always have it, whether it’s latches and catches, sliding door gear for apartments, shelf supports, abrasives, wood veneers, corner braces, egdebanding, cabinet hinges. etc. They also have great customer service! Attentive and friendly.