Noob question: Exterior bass traps

[CaptainAmazing]

Hi all, I'm a new voice artist living in the suburbs near Seattle. I'm loving this forum, I'm learning a ton. Sadly I cannot find any reference to my question anywhere so here it is.

I'm building an iso booth in a small, pre-existing closet, 75"w x 30"d x 84"h. Two of the walls and the floor are exterior, the ceiling and two other walls are interior, all wood frame construction. I've coated all the interior surfaces of the closet with multiple layers and types of carpet pad (I do floors professionally, it was free), furring strips and sound board and have gotten very nearly all the mid and high range signal to negligible levels, but cars and planes still send too much low frequency noise through.

My noob question is this: Can I treat the outside the house immediately adjacent to the iso booth that might effectively block the low frequency signal, especially on the large 75 x 84 exterior wall? I can't really fit too much more inside the booth so I'm looking for a low cost way to lower the noise, but not shrink the space. Bass traps or even a stud wall, any ideas would be welcome. It's my house so the landlord is not an issue, but the bank holds the note, so money IS an issue.

Thanks in advance

[Soundman2020]

Sadly, the acoustic properties of carpet are pretty useless. Using carpets for a studio is a myth, something that everyone "knows" is the right thing to do, but with zero actual basis in acoustic reality, or physics. Lose it. It is doing nothing useful, and is in fact making your room worse.

How many professional studios do you see with carpeted floor or walls? There's a reason for that. Carpet is randomly selective absorption: it absorbs high frequencies reasonably well, mids somewhat, and lows not at all. And does so in random, unknown, unpredictable patterns. Most carpet manufacturers don't even bother getting their carpets tested acoustically, so you can't even find out what the properties are! But basically they are the exact opposite of what you need for a small room, such as an iso booth. Small rooms need lots of bass absorption, not so much in the mids, and almost nothing in the highs. So carpet is the perfect inverse of what you need: It will make your room sound muddy, lifeless, dull, and dead. That's a terrible acoustic environment for recording.

I've coated all the interior surfaces of the closet with multiple layers and types of carpet pad (I do floors professionally, it was free), furring strips and sound board and have gotten very nearly all the mid and high range signal to negligible levels, but cars and planes still send too much low frequency noise through

You seem to have fallen for another common misconception, or "myth". People frequently confuse isolation with treatment, and think that putting acoustic absorption on the wall will somehow stop sound getting in and out: it wont. You cannot stop sound with absorption. You can treat acoustic issues with absorption, but you cannot isolate a room with absorption. Isolation and treatment are two entirely different and exactly opposite things. The ONLY way to isolate ("soundproof") a room is with mass. Heavy, thick, dense, massive, rigid airtight surfaces, such as brick, concrete, drywall, wood, glass, lead, steel, etc. Not carpet. Carpet has very little useful mass, no rigidity, is highly porous (low gas flow resistivity), and the density is too low. It is no use at all for isolating. And no use for treating either.

Can I treat the outside the house immediately adjacent to the iso booth that might effectively block the low frequency signal, especially on the large 75 x 84 exterior wall?

In a word: No. That won't work, for the same reason as above: you need mass, not bass traps. Bass traps do not isolate: they treat. And you need air-tight seals, not carpet.

OK, lets see if I can put this in more simple terms: Trying to isolate a room is the same as trying to build an aquarium for your pet gold fish. The basic goal is simple: With the fish tank, you have to have some way of keeping the water in. With the room, you have to have some way of keeping the sound in. Sound behaves like water in many ways, so the analogy is useful. You don't want any water/sound getting out, so you have to figure out how to keep it in.

So, can you build a fish tank by getting a metal frame, putting glass on one side, paper on some other sides, cardboard on a couple of others, and carpet on the rest? Obviously, no. It won't work. It won't hold water. Water will leak out of all the sides that don't have glass on them, as glass is the best material for stopping water. Same with your room: you have to use the best material for stopping sound, and you have to do it on all sides of the room, equally, including the door. And just like the aquarium, the entire room must be sealed airtight. Even a tiny gap, and the water (sound) will leak out. And just like your goldfish, you also need to breathe, so just like your aquarium has an air pump to keep the fish alive, so too you also need a fresh air supply, to keep YOU alive. Standing in an airtight closet for a few hours is not good for your health....

So you need to line your closet with mass, not carpet, to stop sound getting in and out. Drywall is usually the best option, since it is massive, inexpensive, and available everywhere. So your first order of business is to take out all of that carpet, and install drywall, but do it properly as a fully decoupled acoustic MSM system. More on that later. But basically, throw the carpet away; it has no use, acoustically.

Another point here with the water/sound analogy: You can use a sponge to mop up water in your kitchen that spilled some place you don't want it, but can you put a sponge across the end of the tap, to stop the water coming out? Nope. Even though sponge is great at sucking up unwanted water, it is no use for STOPPING water. Same with absorption (eg, carpet): it can "mop up" sound that spilled in places you don't want it, but it is useless for stopping sound getting in and out.

So that takes care of isolation ("soundproofing" or "waterproofing"). Now for treatment.

If you have enough mass around your room so that sound cannot get out, well than that means the the sound stayed IN! Therefore, it is now bouncing around inside the room, slowly dissipating and eventually dying away. That makes the room sound TERRIBLE! And that's the reason why you need treatment (eg. absorption): to "mop up" that unwanted sound, and help it die out faster, so that the room sounds good again. But here too carpet is useless: it does the wrong thing, as I mentioned before. You need something that absorbs a LOT of low frequencies, and not so much highs. Things like fiberglass and mineral wool are good at sucking up lows, but they also suck up highs, so you can put lots of that on the walls and ceiling, very thick, then partially cover it withe something to reflect the highs back in to the room. Slot walls do that.

So, after you have thrown away the carpet, put in the massive drywall for isolation, then you can treat them room with loads of mineral wool / fiberglass insulation, and put angled slats over it.

Doing this will give you a well-isolated, good sounding iso booth.

I can't really fit too much more inside the booth so I'm looking for a low cost way to lower the noise, but not shrink the space.

By isolating properly (probably with RC-1 plus a layer or two of 5/8" drywall, and insulation in the cavity, you won't be losing much more space then you are already losing with the carpet.

Bass traps or even a stud wall

The stud wall will do the trick, provided that it goes in the INSIDE, and is decoupled, but you don't have the space for that. I'd go with RC and a couple of layers of drywall, properly sealed.

And don't forget the air supply! You will need a couple of really quiet, high-volume fans, and a pair of properly designed and built silencer boxes: One bringing fresh air in, the other taking stale air out.

Also, please check the forum rules for posting: you seem to have missed something important!

- Stuart -

[CaptainAmazing]

Thanks for all the feedback, Stuart, that certainly answers my questions! Sadly, I tried filling out my profile location and got an error message. I just signed on so I'm sure it will clear up soon and I can finish.