Saturday, September 16, 2023

Sound proof Drum Isolation Booth in my basement!

    So aside from my day job in special education, and my love of anime and manga (and art, and books, etc...) I used to be a recording engineer/producer. I recently took up drums as a way to keep myself in music. So naturally, I needed to build a soundproof drum room in my basement. Here's a blog post detailing the basics. Perhaps most important is the air handling system I designed (and which is working great in practice).

    I originally designed the room with some added sound proofing features (such as using rockwool) but had to cut some costs when the total materials cost came in (I do all the construction myself). So I ended up with pink fiberglass instead. But after doing much research, it turns out rockwool doesn't make quite as much difference as people say.

    The basic construction including the room having an air gap between the outside foundation walls and the drum room walls. Then there were two layers of drywall on the outside of the wall, pink fiberglass insulation, and two more layers of drywall. For the soffits around the existing HVAC duct work, I did four layers of drywall on the inside of the room, then the insulation, but no drywall on the other side of the soffit. For the ceiling, given the low height of the basement, I went with pink fiberglass between the ceiling joists (first floor floor joists), then a layer of 5/8 OSB, then 2 layers of drywall. Might not seem like much, but that extra half inch of ceiling height helps, especially with the reflection foam on the ceiling after that (remember folks, the wall foam is not sound proofing, it is to limit reflections!).

    The floor plates for the walls were pressure treated in case of moisture from the floor. I kept the cement floor of the basement and just put area rugs instead of a finished floor. Cement slab floors don't transmit sound, so there was no need to build a floating floor and given the low ceiling height, this helped a lot. The floor plates also were caulked under the pressure treated wood, and along each edge, inside and out, to prevent any unevenness in the floor or board from creating a sound gap.

    The door is an exterior steel door with internal insulation. This has good weather stripping for sound, and is better than a wooden door for sound isolation. The challenge was that I had to cut it down due to the low ceiling height. I cut down the bottom edge, reinforced it with additional wood insert, filed down the rough steel edge, and it worked perfectly! 

Here are some random construction pics:

Love google sketchup, I still use the old free one.

Framing, notice on the left, there is an alcove to get access to the central stack snake port (lower left corner)

Insulation and starting drywall.

Cut down the steel door. Then replaced the blocking at the bottom, then glued it all tight and clamped it. After this, I filed down the rough steel edge and primed. Worked perfect.

    But now to the fun part! You can't have a soundproof room, with all the joints caulked, and weather proofed steel doors, without ventilation. You'll roast in there! BUT, if you just run the regular ventilation from the house HVAC system into the room, the sound of the drums (or whatever) will just travel through the vents into the rest of the house, which means you might as well not have even built the room.

    The solution is to build a ventilation system that is NOT connected to the house, but makes use of the fact that the room is in the basement. Basically, I built two baffled duct systems, one with an intake fan, and one with an output fan. The intake fan is located at the front bottom of the room, it draws in cool air from the basement floor. The output fan is located near the ceiling of the drum room, and pulls hot air (which rises) out of the room and spits it back out into the basement. You thus have cool air coming in, warm air going out, and none of this is connected to the house HVAC system. Below I will detail how I built these as well as some acoustic measurements I did showing the sound isolation properties of the baffled vent system.

    The basic premise is to have the sound from the room travel through a twisting and turning path (each right angle slows the sound down, and slow sound is quieter sound). The path should also be absorptive. I went with foam at first and not soft insulation or fabric because I didn't want dust build-up, however, it wasn't quite enough, so when I added a ribbed duct, I added pink insulation in any gaps. So the sound actually goes through the duct, but there's plenty of absorptive material as well. I made the decision to add the duct and foam after doing acoustical tests. You'll see those below too. 

    And then it's also important that your fans don't add noise. The fans were the fun part. I basically bought two 8" super quiet computer case fans, connected them to a cheap 12 volt power adapter, and put that into a switched wall outlet to turn them on and off.

    Lastly, you need to get audio and in and out of the room if you want to record. So I did a multi elbowed PVC conduit into and out of the room, the ends stuffed with foam. It has multiple curves, because each change in direction of sound decreases speed which decreases volume. Works perfect.

Here are pictures with descriptions:

Plywood frame. The box jutting out on the lower left is what will go through the wall into the drum room and have a wall grate placed overtop. The round hole on the bottom right is for the fan.

Here it is with the first layer of foam. 

Added a rigid and ribbed duct with extra pink foam in the gaps after doing acoustical analysis and finding that the hard foam cavity on it's own wasn't enough. Now there's more insulation for the outside, and the ribbed nature of the duct further disrupts and slows the sound.

Here's an early test mounting without the fan for acoustical tests. You can see the test mics laying on top of it.

Here are the two mounted. The bottom one is the cold air intake, the top one in the back is the warm air output from the booth

Side view

Lots to unpack with this one, these are the acoustic measurements with no baffle (just an opening from the drum room into the basement (approx 8"x10"), with the baffle (but without any duct in it), and with the baffle with the duct. The mics were in two positions. "On axis" means directly in front of the opening from the drum room into the basement. "Fan opening" means in front of the part of the baffle where the fan would be (the in/out side of the baffle). For the one with no baffle, I left the mic in the place it would be had the baffle been mounted. Omni directional measurement mics were used. 

As you can see, starting with the on-axis (top row), we get typical white noise profile with no baffle, the mic is simply picking it up straight from the speaker. With the baffle (but no duct in the baffle) we get significant high end and mid-range attenuation (YAY!) but the low end is still not great. Once we add the vent with the added pink foam (top, far right), the low end drops as well. This is because there is more between the mic and the inside of the baffle. The sound goes through the side wall of the metal duct, then pink foam, the rigid foam, then the plywood (instead of just the rigid foam and plywood).

The bottom row is at the fan side. You can see from the left bottom, that high end is a bit attenuated as you would expect since the speaker is in the drum room, projecting out the vent opening, and is perpendicular to the mic placement. The middle bottom shows that with the baffle (with no duct) there is the expected additional high end/mid attenuation. And the bottom right shows the additional modest low end attenuation with the duct and pink foam added. 

All in all, with the baffle, duct, and two types of insulation, we're getting upwards of 20 db attenuation at some frequencies, and at least modest attenuation at the low frequencies (the hardest to cut down). In practice, when walking around the drum room while someone was playing, I was blown away by just how quiet the sound leakage was from these vent systems. Worked better than I hoped.

For fun, I did the same tests using music (Thanks vintage Paramore!):

You can basically see all the same stuff as before. Highs and mids are dramatically cut from the no baffle tests (on the far left). Lows are still hard to cut down, but there's at least some drop.

Here's the audio conduit using PVC:

Here are some pics of the finished room. Enjoy!

You can see one soffit around an existing HVAC line. You can also see the hot air outtake vent in the upper right corner of the picture. There are power outlets on the three walls without the door.

Picture from the door, looking in.


Please legitimately purchase or borrow manga and anime. Never read scanlations or watch fansubs. Those rob the creators of the income they need to survive and reduce the chance of manga and anime being legitimately released in English.

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