Vent baffling for basement studio

[joshuamorales]

Hey guys,

I've been lurking for a while and learning tons! Thanks to the whole community for being so available and open! With that said, I'm a little lost with a vent problem that I'm having. When I moved into my current studio, I was unaware of a vent that leads directly outside (it was hidden behind a shelf of books and equipment that I've since taken down). To make matters worse the vent leads to a small space between my building and another to which I have little to no access. I really need to reduce the amount of sound leaving my room via that vent. If I want to play drums or have loud amplifiers (which can be 115-120db) I have to wait until my neighbours go home, which is drastically cutting into my work schedule. If I could cut that volume in half by the time it goes out of the vent, I would be thrilled. If I could do better, I'd be even happier! With all that in mind, when I monitor for mixes (at 80-85db) no one ever seems to mind the noise. So maybe I don't need 60 db of reduction???

The info about the vent is as follows: It's at the top of one of my walls, and after the opening there's a brick duct area that goes up about 20 inches that opens to a grate on the back of my building. The opening in my room is about 11" x 8.5", but the bottom of the vent slopes a bit... So, it gets down to about 11" x 6.5" before it changes direction upward. The upward portion is about 11" x 5". I'm not sure of the exact size of the vent on the outside of the building. The vent is on the left wall... in case that matters...

The room dimensions are roughly 17.5' x 10.5' x 7.5' (l x w x h)

I'm looking to build a muffler for the vent, but I think it'll have to be inside my studio as I can't access the back of my building properly. I can either build the muffler on the wall or in the roughly 14" space above my ceiling. Here are my questions:

1. How can I determine the proper size/number of turns to get adequate reduction (is there a calculator or formula?)
2. Is it better to build into the ceiling or on the wall considering the size of my room (ie, will the muffler drastically change the acoustics of the room if it's on the wall)?
3. Should I include a fan in the design?

[joshuamorales]

By the way, I think I may have posted this in the wrong section... Can a moderator move it to studio construction?

[FriFlo]

Are you sure, you want to stay for longer periods in this room? The walls look like they have serious problems with humidity and those can be really nasty and destroy all the work and money, you spent on the built. If you want to seriously reduce the noise going out, it is not really about this vent. It is actually necessary to stay there for fresh air to come in. If you need isolation, you would have to build a room-in-room construction and probably hire someone to install a ventilation/AC. But I actually doubt, it is wise to put any money into that room. When you build drywall before that kind of wall, the humidity might grow behind it bringing mold or even ...

[joshuamorales]

The picture is a little deceiving. There isn't a moisture problem at all. It's just a really old brick wall. I'm not really interested in building a room in a room as the only place I have real sound leakage is via the vent. Apparently, the vent had been plugged in the past, and there weren't any problems. However, somebody at some point realized that fresh air is kind of important, but they didn't do anything to deal with the sound... So, I'm just exploring the best way to baffle the vent properly.

[FriFlo]

If you don't get wet hands touching the walls, it doesn't mean, you have no problems with humidity there. Make sure, you really have no wet walls (which is unfortunately dependent on the season and rainfall rate), before you build anything there. I have my own experience in this field and - as you said yourself - looks can be deceiving, if you are not an expert at this field. My cellar had perfectly painted walls without a stain before I move in, but it was a labour intensive and time/money consuming process to get them ready for a studio build. If I would have known about this, I would probably have looked for another room. Just a well-meaning advice to you!

Apart from that: I would do an easy and inexpensive test: Buy an SPL meter and measure the average background noise level at the neighbor. Then makes some loud noise in your room and measure it and see, how much SPL you reach at the neighbors house. stuff the port with some mineral wool and tightly packed stones, maybe chaulk it air tight and repeat the process before exactly at the same level. If you got no sufficient noise reduction like that (this is what I assume!), you won't be able to build any baffle that will be better. In that case, the only way is a room in room build.

[joshuamorales]

Hey FriFlo, thanks for the input. My wall is completely underground (so there is a ton of mass for isolation), and the only port to the outside is via this vent that travels nearly 2 feet upward and then out. As it has free flowing air, that is my main concern. The vent lets out to a space between my building and the one next to it, and all the sound escaping is bouncing back and forth between the buildings. The neighbors have windows into that small space. Again, this is all well above my room. I am below ground level. By the way, both buildings are full of artists and recording studios. Mine is the only vent that isn't properly sealed. I was informed that when the vent was entirely blocked that there weren't any noise issues. I'd be happy to try what you're proposing, but I'm fairly confident that this vent is the main problem. So, until I'm able to do the test, can you point me in the direction of some formulas or calculators for baffles? The Master Handbook of Acoustics doesn't seem to be giving me any formulas for baffles... instead it gives me an idea for a large plenum or a half sized plenum with baffles... nothing specific enough for me to extrapolate enough information for design specifications. Rod Gervais's book doesn't give any formulas or calculations either...

[joshuamorales]

Also, on the moisture front, this room as well as others around it have been used as studios for decades, and no one seems to have had moisture problems. So, I don't think there should be any issues if I have to build room in room other than potentially prohibitive costs and losing space...

[FriFlo]

Sorry, I do not know of any formulas. It would be a rather complicated one anyway, because it relies on so many factors. What I do know, though, is that sound takes the easiest way, it can find. That means, the weakest link is defining what isolation you get. If blocking this hole with mass and insulation material won't be enough, no baffle inside your studio will be.
Also, you must consider the fresh air coming in! If you have only one opening to the outside (this vent) you already have a pretty bad air exchange rate without a ventilation system of some sort. When you make this input and output opening even smaller and take some turns, the air exchange will be minimized even more. There are some decontrol ventilations, that alternate between blowing air in and sucking it out, that might help you.
But still: You won't get any significant isolation of this leak without a room in room construction, as far as I understand acoustics.

[joshuamorales]

Ok, so I've taken some measurements. When I blast my room with roughly 110db of sound, the neighbours hear about 65 db of that sound through their window. That's the sound traveling through my vent, into a gap between the buildings, and into their window. That's not the worst case that I was expecting. I thought that they're room was shaking or something. It seems that I'm getting a free plenum by the change of direction in the vent and the massive space between the buildings... So, next is to seal the vent temporarily to take some measurements as per FriFlo's suggestion. Then I can determine whether or not a vent baffle would help... Any suggestions that don''t require caulking?

[Soundman2020]

Hi there Joshua, and welcome to the forum!

This is probably a bit more complicated than you'd like to hear, but the good news is that there is a solution.

As FriFlo already pointed out, your room does need ventilation, absolutely (because you need to breathe!), so you cannot just block it and do nothing. Not an option. Therefore, you have two other initial choices: Either keep this vent and install a silencer on it, or block this one off then make another vent some other place.

As FriFlo also pointed out, one single vent by itself is useless for a studio: You need two vents: one that brings fresh air into the room, and the other to remove the stale air. Air will only flow through the room if there is a pressure differential to drive it. One single path between the room and the outside cannot possibly create a pressure differential (unless you open a door or window, or the if the room is not sealed air-tight, in which case it has no business being a studio anyway!) So You must have two vents, and also a fan to create the pressure differential. The fan can go anywhere; either the inlet duct or the exhaust duct, but you do need one. Without it, no air will move.

OK, so lets assume that you decide to keep this vent, since it is already there, and will use it as either the inlet or the exhaust. Great. That can be done, provided that you put a silencer on it and also install the other vent at some other location in the room, also with a silencer on it.

So now comes the math you were asking about. The first thing you need to figure out, is how much air you need to have moving through that vent. That is generally calculated using the strange units of "room changes per hour". In other words, how many times per hour you should replace the entire volume of air in the room. There are tables and recommendations for that, for various different types of room, from libraries to hospital operating theaters, bedrooms to factories, cinemas to bathrooms, etc. But a rough rule of thumb for studios, is about 6 to 8. So assume that you need to replace all the air in your room, between 6 and 8 times every hour. You said the dimensions of your room are 17.5' x 10.5' x 7.5', so the volume is roughly 1,400 cubic feet. Times 6 = 8,400, or times 8 = 11,200, so you need to move about 8,000 to 12,000 cubic feet of air through your room every hour. Fans are normally rated in cubic feet per MINUTE, not per hour, so divide by 60: you need a fan that can move about 130 to 190 cubic feet per minute (CFM). Call it 150 CFM, for the sake of argument. (I'm rounding up and down all over the place here, to make the math simpler).

So far so good: but the air does not flow freely through your room system: there is resistance to the flow of air (which you'll need to calculate at some point) due to the "back pressure" that the fan has to work against. In HVAC jargon, this is called "static pressure", and is expressed in units of "inches of water column". It works sort of like blood pressure. Think of static pressure as being the total resistance that your entire room + ducts + silencers presents to the fan, just like blood pressure is the total resistance the your veins + arteries present to your heart.

Fans are designed to work in a certain static pressure range, so you do have to calculate what it is in your system. The higher the static pressure, the less air the fan can move. So if you bought a fan rated to produce 150 CFM flow volume into a static pressure of 0.5 inches of water column, but the static pressure in your system is actually 0.75, not 0.5, then the fan will not be able to actually move 150 CFM, since the resistance is too high. It will move less. The rating table on the fan data sheet will tell you how much air it can move for each static pressure, which is why you need to know what that static pressure is in your system, so you can buy the right fan. You figure out static pressure by knowing your duct diameter and length, including the silencer boxes. There are tables that show you that for xxx feet of yyy" duct, the static pressure is zzz inWC.

OK, so now you have to figure out the duct sizes and airflow speeds. You know that you need to move 150 CFM of air through your room, but that is NOT a measure of speed! it is a measure of flow rate. Not the same thing at all. Think of water in a garden hose: you might have a volume of 1 liter per minute flowing out the open end of your hose quite slowly, but if you press your thumb part way across the end of the hose, the speed goes up a huge amount (you can now squirt water a long distance), even though the flow RATE is the same: it is still only 1 liter per minute, but you have increased the SPEED a lot by restricting the diameter of the flow: The same happens in your HVAC system. If the duct is narrow, then your 150 CFM must flow through it very fast, which makes a lot of noise. Slower is better. The rule for studios is that the flow speed has to be kept under 300 feet per minute, absolute max, and better still, under 100 fpm. (To give you some perspective, 300 fpm is roughly 3 MPH, and 100 fpm is roughly 1 MPH)

So now you know that you have to get a flow volume of 150 CFM, and you have to allow it to move at a flow velocity of no faster than 300 fpm. See where this is going? You can now figure out what duct size you need to achieve that goal! Bingo!

Simple math: What cross sectional AREA (square feet) will allow a VOLUME (cubic feet per minute) to move at a SPEED (feet per minute). High school math: 150 CFM / 300 fpm = 0.5 square feet. So the cross sectional area of your ducts needs to be 0.5 ft2 or more, which is 72 square inches (it's easier to work in in2, than ft2). Your existing brick duct measures 11 x 5, which is only 55 in2, so the air will be flowing faster than 300 fpm up there. Nothing you can do about that, since the duct is made of brick and you have no access to it anyway, but frankly I'm not concerned about how fast it goes through that duct, since it will be on the other side of your silencer, and therefore any slight airflow noise in that duct won't make it back into the room, and it won't hassle your neighbors either, since it is just the sound of air moving, and will be very faint anyway. What I am worried about, is duct sizes inside your room, and register sizes. You will need registers that are at least 10 x 8 inches (80 in2), and actually more than that, since the vanes in the register block off part of the total cross section. That's often expressed as "percent open area" of the register: So if your register has 70% open area, then you need registers with a total area of 104 square inches, because only 72 of those are actually open to the flow of air, and you need 72 square inches to get the maximum speed. In fact, you'd want registers that have a much larger area, in order to get the speed down as low as possible, because slow moving air makes less noise than fast moving air. A 12 x 12 register would be a good choice for you. And you also want 36" of perfectly straight 12" duct right before the register, to eliminate turbulent flow and the associated noise, before the register (the rule is that the straight run should be at least as long as three times the duct diameter). But that's just the final few feet of duct: What about all the rest?

All of the above will allow you to figure out how big those ducts need to be! You know you need at least 72 square inches of cross section in your duct, and if you use round duct then once again you can use simple high-school math to figure out the cross section: Area = PI x R ^2. But duct sizes are given in diameter, not radius, so divide the diameter by 2 to get the radius, square it, multiply by 3.141. An 8" duct has a 4" radius, and thus an area of 50 in2. Not enough. A 10" duct has a radius of 5" and an area of nearly 79 in2. Perfect! So you need 10" diameter ducts (5" radius).

Finally, with all the above info, you can design your silencer box. Here are some examples of what silencer boxes look like:

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Basically, just a large box made of thick wood, with an entry port on one end, an exit port on the other, several baffle plates in the middle, and the entire interior lined with 1" duct liner.

Great! ... but how BIG to make it? Once again, math and science comes to the rescue:

Silencers work on several principles at once. The first principle is that when sound waves moving through air experience a sudden change in acosutic impedance, part of the wave bounces back, and the other part loses energy. That is called "silencing"! Good stuff! In a duct, you can create a sudden change in acoustic impedance by suddenly changing the cross sectional area of the duct. It turns out that changing it by a factor of 2 is a good thing. So you want to make the box suddenly have twice the area where the duct comes in, then suddenly go back to half the area where it goes out. Cool! So at the point where the 79 in2 duct comes in, then box needs to have an internal free cross sectional area of at least 158 in2. Call it 160, to make the math easy. So it has to measure at least 16" x 10" internally. But wait! I said you need that much OPEN space, but that you also need an inch of duct liner on each side of the box, inside. So the internal size of the wooden box itself needs to be art least 18 x 12, in order that the open area left after you add an inch of duct liner on each side, is 16 x 10. That's the MINIMUM size you need: you could easily make it 20 x 14 on the inside if you wanted (and can afford the space in your room), to get greater impedance mismatch, but even at 18 x 12 you'd be good.

Ok, so that's the cross sectional size of the box... but how LONG do you need to make it? That's determined by how many baffles you want to put in it. Since the baffles need to be far enough apart to keep the same cross sectional area, and they are also covered with an inch of duct liner, in your case you'd have to have them either 18" apart (if the box is 12" deep) or 12" apart (if the box is 18" deep). Your choice. So if you want just two baffles in there spaced at 18", then your box would have to be 3x18 = 54" long (why "3x" for only two baffles? Because 2 baffles splits the box into three parts...). If you wanted three baffles it would have to be 4x18 = 72" long. 4 baffles would make it 5x18 = 90" long.

So how do you figure out how many baffles you need? That depends on how much acoustic isolation you need. Which is why you need to do the test that FriFlo suggestied; meaure levels with a sound level meter, so you can figur out how loud you are (in dB) and how quite you need to be (in dB). More isolation = more baffles (and thicker wood). You need at least one baffle, and two is recommendable. (I have been known to put as many as five in a huge box made from two layers of 3/4" plywood, to get very high levels of isolation....)

Now for the fun part! If you put in more baffles, or if you have narrow ducts, then your static pressure goes up for the entire system, meaning that the flow rate drops, the air velocity goes up (thus making more noise), and the fan has to work harder (thus making more noise). So you need to re-calculate all of the above!

In other words, it's an iterative process: You need to go over it a few times, tweaking numbers here and there, until it all makes sense: The limiting factors are the air speed (never faster than 300 fpm at the register, and preferably more like 100 fpm), and the static pressure (most fans can't handle more than about 0.5 inWC of static pressure). If need be, you could even drop the flow rate (CFM) down a bit below the recommended 6 room changes per hour, so you can get smaller ducts, smaller silencer boxes... but higher static pressure! (HVAC is fun, isn't it!)

So that's how you do it: as FriFlo said: there is no simple formula for this: there's a whole bunch of them, and they all depend on each other, and interact with each other, and it is up to you to fiddle and tweak until you get it right.

But ball-park: You'll need a fan that can move 150 CFM or air, 10" ducts, 100 square inch registers, and a silencer box that measures 18 x 12 on the inside and is at least 52" long. The box would be made of at least 5/8" plywood or MDF. If you want more isolation then make it with 3/4" wood and make it 72" long with 3 baffles in it. If you want fantastic isolation, then make it 90" long with 4" baffles in it, and make if from two layers of 5/8" plywood.

Of course, you need two boxes like that: one on the existing duct (which I'll assume is the exhaust duct) and the other on the intake duct (which doesn't exist yet). The fan should go on the exterior inlet port for the supply duct, so it won't be heard.

Where to put the box? Wall or ceiling? Wherever it will fit best! Yes it will change the room acoustics slightly, but probably not enough to worry about. If this is a control room, then put the box someplace where it does not mess up the left-right symmetry of the room. If it is a live room, then that's not an issue. In your case, you are a bit limited by the fact that one end of the box has to mate with that huge hole in your wall, but from that point on the box can go in any direction: down the wall, across the top of the wall to the left, across the top of the wall to the right, or across the ceiling.

And when all that is in place, you then need to do the math to figure out the sensible heat load of your room, and the latent heat load for your room, so you can buy the right size mini-split system ito take care of your humidity and temperature issues. But that's a totally different subject, not related to your question about the vent....

- Stuart -

[Soundman2020]

When I blast my room with roughly 110db of sound, the neighbours hear about 65 db of that sound through their window...

That's pretty loud. Considering that legal noise limits in many residential areas are around 45 dB (perhaps even lower at night), you are subjecting your neighbors to 100 times more sound intensity than the legal limit!

It also implies that the insertion loss of your silencer box needs to be at least 20 dB.

- Stuart -

[joshuamorales]

Hey Stuart,

Thanks the information!!! That helps so much! I'm really into formulas, and that helps a ton! one question though, can you give me an idea of how much reduction each baffle can do so that I can determine how many baffles are necessary? By the way, I don't actually run at 110db normally. I mix at 80-85db, and most of my work consists of mixing. At that level, the neighbours never hear me. I was just testing extremes today, and it was startling. It was basically like hearing an obnoxious radio outside their window. They're great neighbours, and super patient while I'm trying to sort out the issue. So, I really want to solve this as quickly as possible. Also, the buildings involved are not residential. They're both commercial buildings... That doesn't make the levels any less offensive, but at least I'm not messing with anyone's sleep at night when I track guitars and/or drums!?

[joshuamorales]

I said I had one more question, but I was wrong... Is it possible for the plenum to be shallower but wider? ie, Could it be 5 x 32 and sit really close to the ceiling or wall? My other questions concern the fan. Would I only need one fan for the two vents? Or do I need a fan for each vent. As a final question is it better to have my current vent as an exhaust or intake? Should I be grabbing the fresh air from outside or from inside? The opposite side of my room (where I assume I should build the other vent) is technically an inside portion of the building, but is actually the same temperature as outside. Basically, on the outside of the that side of my room is a stairwell that leads to a large open garage of sorts... It's a commercial building, and the garage area is open all the time... Does it matter which vent is used for intake/exhaust in this situation? Thanks again for all the help!!!

[joshuamorales]

I've temporarily sealed the vent up, and there isn't any measurable sound reaching my neighbours anymore! Basically, the ambient noise level of their room is higher than anything leaving my room. So, I just need to build the mufflers and put them into place... Until then, I'm stuck with a slightly stuffy room without proper ventilation. There are worse things. I think I'll try to get the build together next week or the week after. I'll post pictures of the process!

[FriFlo]

Congrats! Happy to hear, you get your isolation so easily. I have a similar condition, so I can live without a room in room construction, as I don't record drums and do mixing and composing most of the time. Just another well-meant hint from somebody, who started off with a budget solution for air exchange: Don't underestimate the importance of fresh air for your well-being! You are going to invest some money in order to get a proper solution for a sufficient exchange. Don't be afraid to spend that money, as it will be worth it in the end, if you plan to keep that room for a longer time. Just building a muffler won't do it. You have to have a proper ventilation, as the efficiency of that port will be greatly reduced my the turn the air has to take. Do yourself a favor and plan it with ventilation right form the beginning.
Without being an expert in this area, I suppose you will be best of with an external unit, as you only have one narrow vent. But it can become tricky, because those devices freeze in the winter! So maybe an internal unit with two independent external ports (which have to be apart form each other. It might become tricky though, to isolate those devices sonically! Best would be to have a small room next to yours, where the unit is installed.
In our lines of latitude in a cellar you will probably not need an AC (for the cooling part), if you have proper heat source/radiator in place. But, in order to not spend your money to heat the air outside too much, you are probably best of to install a ventilation system with recuperative heat exchanger. This way you will constantly have fresh air with a stable temperature without having to worry about it.
Also consider: If you have no humidity problems right now, you might get some later when you block the air vent or just decrease its efficiency by muffling it.