Building an Isolation/Listening/Recording Booth

[Boots Deville]

I have some space available in my basement and I'm thinking about building a (semi)sound proof, "dead" booth to crank up amps in.  Currently I do this in my shop, and I end up hearing all the tools/supplies rattling on the walls and wonder if it's coming from the amp or not.  It would also lessen the impact on the rest of the family,    and be nice for recording.

I'm thinking 8' x 8' might be an appropriate footprint.

Anybody have any experience with this, helpful hints, sources of good info, etc?

-John

[PRR]

You do NOT want to get in a small box with a big amp.

We fought this for years, with brass bands in rehearsal spaces: there is no replacement for a large room. No amount of fuzz will make it tolerable.

Also: the key to family-impact is NO LEAKS. In theory you want massy wall; in practice it always comes down to leaks. But you will suffocate in an 8x8 room even with "normal leaks", much less the sealed construction needed to significantly reduce family impact. That is if ear-bleed doesn't get you first.

I can see putting an amp in a box and you stay outside. Two layers drywall on studs can get close to 26dB attenuation, IF you can seal ALL the leaks. That will make a 100W amp into a "1W sound", a signifiant reduction. Add fuzz (fiberglass attic insulation) on most inside walls. You'll still hear it, and know if it is grossly unhappy. Ofviously this is not suitable for fine-tweaking or small-problem finding.

You want a 200-seat beer-hall, and a hundred dummies (fuzz-filled sacks), to approximate the natural environment for BIG amps.

[RicharD]

>the key to family-impact is NO LEAKS.

The 3 fundamentals of sound proofing in order of importance are density, dead air space, and absorption.  Absorption plays more into acoustics but it does have some impact on sound proofing whereas a reflective service bounces more of the sound around which means you need to increase density.  Obviously you cannot have dead air space without 2 walls.  Dead air space decouples any resonant transfer between the 2 walls.  This is more important than you think.  The less dense the isolation walls are, the more important dead air space becomes.  A cinder block wall filled with sand is actually dense enough to almost not need dead air space.  Hang a nice thick curtain on either side and that will not only absorb but it will dampen much of the resonance, enough to where you can have side by side THX theaters.  Obviously any leaks will slip around the densest wall.

Chances are you probably don't want to build a concrete sound room.  I've constructed a studio or 2 and this is a wall equation that works.  A picture is worth 1000 words.....  see attachment. 

The double insulated wall should use pink stuff w/o paper.  Quite absorbent, very dead room.  Technically you're not supposed to have parallel surfaces but if you go with 99% absorbent walls, standing waves become less of an issue.   Double wall means double door jambs and double solid core doors with weather stripping on both sides.  The ceiling should be constructed similarly although the outer wall will probably be existing ceiling.  If you can build a bigger room, you'll want some reflective surfaces but that's getting into real acoustics which I don't know enough about to be dangerous.

I agree 8' x 8' with a 30 watt amp is gonna hurt, especially if you're successful in sealing up the leaks.  Sound pressure will climb really fast.  In a very dead  very small room, sound pressure can easily climb to ear damaging levels and because of the absorption, you may not realize it.  The bigger the room, the less pressure.  Think about outdoor gigs with a whimpy PA.  Now think about that same PA in your bed room.  An 8 x 8 room with an 8' ceiling is roughly only 250 cubic feet.  The trick is to increase the volume.  How do they do this in a real studio?  Tall ass ceilings.  Build your room as big as you possibly can.  If you can achieve 2000 cu ft, you're golden. 

Air conditioning can be a huge source of leaks.

[Boots Deville]

Excellent info, thanks guys!

It seems I'd be much better off building an iso-box just for the amp and a mic. ...although a 200 seat beer hall sounds kinda fun too!

-John

[PRR]

> The 3 fundamentals of sound proofing in order of importance are density, dead air space, and absorption.

In a real world, with normal ad-hoc construction, it is always about the LEAKS.

Spend a few hours stuffing foam and tissue-paper in cracks. Makes a big difference.

Then you can go back to the theory: MASS and dual-wall.

But in almost all small-room cases, stagger-stud hardly helps until you get a SEAL. Builders don't seal the wall to the floor, maybe drywall-tape at the ceiling. That 5-drywall plan is somewhat overkill; though adding layers does diffuse the leaks. (That's specifically why it uses double-drywall: to cover the drywall sheet-joints with the middle of another sheet.)

Outlet boxes? I quickly realized I had to foam-in several of my boxes just to stop the cold breeze. Sound would have waltzed right through.

Then there is the DOOR. If you have to spec it out, you want fire-rated and weather-stripped and final-inspected. Fire-rate doors (even 20-minute) will be massy enough. But the threshold leaks like a pipe, and if you fix that nearly as much comes around the other three sides. You need the full weather-stripping like to stop the Maine Winter, and you gotta inspect it really seals.

In DIY you do something with 2 sheets 3/4" ply and studs packed with fuzz, let the face lap over the wall and apply sponge-foam and a gate-latch to pull it tight. Put a boom-box inside, stand outside with a rubber hose in your ear, and find where the leaks are worst. Fix, repeat.

A door in a 20'x28' studio is a small thing. A door in an 8'x8' box is a large part of the envelope and will surely leak a large part of the total bleed-through.

If you seal an 8'x8' box well, a person inside will suffocate in 20 minutes. So you have to bring in air. BIG LEAK. You can put mufflers in the ducts: a box far larger than the duct, probably several in series unless you can duct-out so far away nobody complains (the concert hall vented from the roof 50 feet up).

> a 200 seat beer hall sounds kinda fun too!

I say that's best. What you are asking is a variant of the school's rehearsal hall problem. You may have several bands meeting several times a week, they can't all use the Main Hall. Or have a space as big as the main hall. But if you jam an ensemble scaled to fill a 70x70x40' 600-seat space into a 25x35x15' bare space, it's not just different, it is brutal. And there isn't any "good" answer.

[RicharD]

When I've built sound rooms, we built each wall face down on the slab then stood them up on a layer (or 2) of peal and seal, which is glorified double sided tape.  The ceiling is what tied the walls together.  That's the HARD part.  Basically each room is a free standing box inside of an existing shell.  Sheet rock and homasote layers should be staggered as to overlap the seams.  Lots -n- lots of caulk.  All electrical should be run surface mount to eliminate wall penetrations.

You have to have 2 leaks.  Supply & return air.  The duct work is tricky.  I like PRR's DIY duct muffler idea.  Regardless this is always an Achilles heel as are doors.  If you can swing the double door thang, it helps immensely, especially if the doors are gasketed.

[PRR]

> PRR's DIY duct muffler

I didn't get into that here?

Any "muffler" like we need, which will pass a steady stream of air yet attenuates a broad band of sounds, pretty much works on one principle. Long pipe broken up by huge chambers.

Car muffler has to pass exhaust but reduce exhaust noise. Figure a pipe diameter to pass the flow. Say 1.5". Run a length of that. Then open-up dramatically, 10X-20X the area. 6" can. For engine exhaust, make this can at least 7 times the displacement of one piston. You usually want to go down to 1.5" and open up again, usually three chambers.

For John's box, say he needs a 3" muffin-fan to breath. 3" PVC waste pipe is convenient. Open up into a box. Ideally we want the "jug resonance" of the pipe and box to be below the audio band. That means BIG. Think what size of PA speaker would use a 3" vent. More than 3 cubic feet. So it looks like a couple/three large subwoofer boxes linked with good runs of 3" pipe.

Inlet should not face outlet or high frequency sounds will shoot right through. Baffle for long path. Line with fuzz, wool or fiberglass.

I did do something like that here. My hot-air heat system had too-small ducts, and one had to move, so I re-rigged the whole air-return.

I was mainly concerned with getting a good 200 square inches of flow so the furnace fan didn't strain to suck. But there's some noise reduction.

Photo is looking up at the cellar joists. I boxed between two joists. The box is 42"x9"x22". Four 8" ducts run down to furnace. On top there is a 20"x12" floor-grate. It lays perpendicular to the row of duct entries.

Ducts coming in are 200 sq.in. The box volume is 8,300 cubic inches, equivalent to 42" of duct. Since the mean path through the box is 20 inches, and the duct cross-section is 200 sq.in, this is not a "large chamber". Because the flow is skewed, the left-end duct's noise is reduced, the right-end duct's noise shoots right up through the grate. I figure, and hear, that I have a little less noise than the one duct which used to serve this area, but I have more than twice the total flow in the house. I also hear less strain and oil-canning when the blower starts-up. And it is running less for the same heat.

[HotBluePlates]

Building a true recording-worthy room sucks.

I was unskilled labor in a build of a professional studio towards the end of high school. The studio was located in one suite wihin an office park. Fortunately, the office park was several seperate buildings with several suite per building, each taking the whole space from front to back of the building. So we really only had to worry about the folks on either side.

Anyway, inside the building was setup to be 2 story. The outer structure was cinder block and a corrugated steel roof, with a cement floor. Naturally, each office space was finished out as normal.

First job was to demolish everything inside the space to the cement floor, steel roof and cinder block walls. Then, we had about a hundred cans of expanding foam, and went through filling every crack, crevice and gap in the entire space, much of which was between the roof and the walls. I spent hours upon hours upon hours doing that.

When everything was sealed, fiberglass boards were attached to the cinder block walls, each panel being about 3 inches thick. There were 2 complete layers of those panels, floor to cieling, covering the entire inside of the space.

Next, framing was done, which was attached to the cement floor. This completely framed out the main space, a couple of iso booths and the control room, but was done primarily to give structural support and to attach some of the basic wiring to be done. Within this framework, a second frame was built for each individual room, maybe 8-12 inches seperated from the first framework. Each of these was floated off the cement floor on rubber discs. I know that before the actual subfloor and finsihed flooring were installed for each room that sand was used to fill the dead space, but I'm not positive what they used between the seperate frames. I wasn't there for a good bit of the finish work, cause I didn't already know how to do it.

I can attest that they had bands in there with screaming stacks all hours of the day and night, and never had a single complaint from the business neighbors. They were concerned about that, and also about hearing a firetruck rumbling down the road outside the studio.

I dunno what all was done with HVAC stuff, but I never could detect and a/c related noises at any time in there.

[RicharD]

>I did do something like that here. My hot-air heat system had too-small ducts, and one had to move, so I re-rigged the whole air-return.

I was mainly concerned with getting a good 200 square inches of flow so the furnace fan didn't strain to suck. But there's some noise reduction.

I wish "Joe Average Ductmann" thought this way.  Instead it's just fling an 8" flex into each room and call it good.  My bedroom is far from the return air.  It's door is off the breakfast room, which is off the kitche, which is off the living room, which is where the return air grill is.  I get almost no air flow which in August is a bummer.  My rat-rig fix was to attach a small fan to the ceiling in front of the supply grill which encourages air flow.  Unfortunately ye olde fan got full of dirt and has given up on the task.  There is a critter that is actually a duct fan which might be a better bandage fix.  The correct solution would be to run a return air duct but... it's downstairs in a 2 story house which sits on a slab.  The prospect of running another duct will require the removal of a considerable amount of ceiling.  Something I'm not looking forward too.

It looks like you're having a lot of fun fixing up your house.

[PRR]

> 8" flex into each room

Eight INCH? Each ROOM??

Ah. You are south of Kennebunkport. Your HVAC is less about Hvac and more about hvAC. I can put my hot-air 50 deg above room temp; you can hardly run your coolth more than 20 deg below the room.

FWIW, this is now the ONLY air return for the house.

The house is pretty open and we don't close room doors. I figure the air-flow in the front hallway is 0.25 MPH, there's no draft. It may approach 0.7 MPH in the back hall where the return grate is. That's not a "draft" until you are right at the grate (~~6MPH face-flow).

It started with one 8" duct (50 sq.in.) feeding a 20x20" (400 sq.in.) plenum and filter to a 10x10 (100 sq.in.) fan. They lived that way; the second duct was added only when the house was split to two appartments and (duh!) one of them wouldn't heat.

Even with 100 sq.in. return, it just didn't sound right to me.

While I did some entropy computations and such, the caveman analysis asks why the furnace maker used a 100 sq.in. fan? And is a 50, or even two 50 sq.in., returns "enough" for this furnace? Clearly 400 sq.in. would be ample (for any reasonable run), probably over-ample. If we assume the lowest velocity should be the filter (so dirt drops out) and the highest velocity at the fan (or fire-pot), then 400 > X > 100. Hence a 240 sq.in. grate to 200 sq.in. ducting.

> I wish "Joe Average Ductmann" thought this way.

Electricity is so much simpler. You can run 12 gauge to a trivial load (wi-fi router). "Trivial" loads can become critical (electric heater on wi-fi circuit). Code can simply require a worst-case solution. Heating loads seem simpler because "they won't change" (I can't instantly add a room or subtract insulation). But you can't put a Watt-Meter on a room's BTU (especially before it is buit) and duct-bulk forces compromises.

My estimate of this as-rennovated house say I need 25,000-30,000 BTU and the furnace is rated 80,000 BTU. That's in-line with how it runs on the coldest day. And in-line with how heat is usually installed (vast excess for quick pick-up).

Moved the living room furniture and the over-accessorized PC today. Too tired to type.