Wisconsin basement mix room

paulmitch · Post by **paulmitch** » Fri Nov 20, 2015 4:54 am

I'm a recording and mix engineer in Wisconsin. I've been a member on the board for a few years, and find it to be a great reference for all things DIY acoustic construction.

My wife and I recently moved into a new home and we added onto the back of the house including the basement. I had the basement dug down a bit more in order to build a mix studio with higher ceilings than are typically found in the older homes in our neighborhood. The room is essentially 16x18 bare poured concrete, with approximately 9' ceilings. Because this is the foundation of the two floors above it, there is HVAC running through the space, and directly above the space is the new kitchen and powder room. Three of the walls are against dirt, and one wall is shared with the old foundation. On the other side of this wall is the furnace, laundry, water softener, etc.

In terms of HVAC in this space, I've completely isolated it from the main system, and won't actually be feeding the room any heat or air conditioning, eliminating the need to make large diffusors, or have a separate system entirely. Given our climate and the fact that the room is as deep in the ground as it is, it maintains a moderate temperature in summer and winter without the need for control.

Given these parameters, I feel I will be able to isolate the room from the rest of the house and vice versa quite effectively.

I built a sketchup model mainly as a guide to try to figure out what I was going to be able to do with the space. The measurements are not exact, but I wanted to go through the exercise to get a feel for what I was building. I'll post the renderings in a following post.

I have begun framing the space, and intend to do heavy insulation (a combination of mineral wool and 703), resilient channel, drywall, green glue, and drywall. I'll have some sort of wood or laminate floor at the end of things. I'm almost finished framing and will move on to run electrical next. I plan to employ putty pads on all the boxes.

I'm open to discussing any facet of the project if people want to talk about it. Otherwise, hopefully this helps others out there looking to do the same sorts of things. Pics to follow.

Soundman2020 · Post by **Soundman2020** » Fri Nov 20, 2015 5:35 am

Hi Paul, and welcome!

Because this is the foundation of the two floors above it, there is HVAC running through the space,

How will you be dealing with that? Are you going to soffit around it, then build your inner-leaf ceiling below that? Or are you going to move it so that it does not interfere with the room?

In terms of HVAC in this space, I've completely isolated it from the main system, and won't actually be feeding the room any heat or air conditioning, eliminating the need to make large diffusors, or have a separate system entirely.

Actually, you have created the need to have a separate system entirely! If you aren't getting your fresh air through the house HVAC system, then you will have to install another system that feeds only the studio, all by itself.

Given our climate and the fact that the room is as deep in the ground as it is, it maintains a moderate temperature in summer and winter without the need for control.

Well, that would be fine if you did not ever plan to have people in the studio, and did not plan to have equipment and lights and instruments in there... The problem is that people need to breath in order to stay happy, conscious, and alive, and they also give off a lot of heat and moisture while they are doing that (staying alive). Equipment and lights also give off heat. A studio is massively isolated, thermally, so no heat will be getting out, like it would if that was just a normal basement. So heat will build up fairly quickly, especially if you have a bunch of musicians in there jamming hard... A studio is also sealed absolutely hermetically, twice over at least, so no stale air is leaving, and no fresh air is arriving, like it would if that where just a normal basement. CO2 and other exhaled gasses will build up, and Oxygen will be depleted...

In other words, you will need to design and install a new HVAC system that supplies the correct amount of fresh air, and removes the same amount of stale air, as well as providing enough cooling capacity to remove the excess heat that will be building up in there, and also to provide the humidity controlling capacity needed to remove the excess moisture that will be building up in there, due to exhaled breath. So you need an entirely new, independent HVAV system for your room.

If you look around the forum you will find guidelines and examples of how other people have accomplished that, as well as all the calculations you need to do in order to correctly dimension the HVAC system, in terms of duct sizes, air exchange rates, air flow volumes, air flow speeds, static pressure, silencer box insertion loss, sensible heat load, latent heat load, and all the other fun stuff that goes into HVAC design for hoe studios.

Given these parameters, I feel I will be able to isolate the room from the rest of the house and vice versa quite effectively.

How much isolation do you need, in decibels? What is your plan for achieving that? How much mass will you have on each leaf, and what size will the MSM gap be between them? What target MSM resonant frequency are you aiming for?

I built a sketchup model mainly as a guide to try to figure out what I was going to be able to do with the space. The measurements are not exact, but I wanted to go through the exercise to get a feel for what I was building. I'll post the renderings in a following post.

Great! But you really should make sure that the dimensions on your design model are very accurate: if not, how will you be sure that the room ratio is working correctly? Especially when you have the situation that your ceiling height is exactly twice the length of the room, implying pretty poor modal response for those two axials, and the associated tangentials. Precision is critical for good isolation, and also for good acoustics in the room. Are you following ITU specs for your interior acoustics, rations and dimensions, or are you going with AES specs? Or EBU? Or maybe with something else? What design philosphy are you using? LEDE, RFZ, NER, CID, MR? Something else?

I have begun framing the space

Maybe I'm not understand this too well, but you don't have an accurate design, and you have made no provision for HVAC, so how can you be framing already? That doesn't make a lot of sense....

and intend to do heavy insulation (a combination of mineral wool and 703), resilient channel, drywall, green glue, and drywall

If you are framing this properly as a decoupled room-in-a-room, then you do not need resilient channel. The decoupled frame is all that you need. Decoupling twice has no useful effect. If you are sitting at the bottom of a swimming pool, then you are already as wet as you possibly can be, so taking a hosepipe with you to get "even wetter" won't be noticeably useful. The same applies here: decoupled framing is sufficient to get you totally "wet", and attempting to decouple again with RC won't make you any "wetter". It will just cost you extra money, and extra time.

I plan to employ putty pads on all the boxes.

On the other hand, if you follow the recommended surface-mount concept for your electrical work, then you would not need to chop large holes in your carefully built and sealed isolation walls, to poke the boxes through ...

I'm open to discussing any facet of the project if people want to talk about it.

Please post a link to your SketchUp model, as well as a few photos of the room as it is right now, so we can see where you are, and what needs to be done to fix that so you can carry on correctly.

- Stuart -

paulmitch · Post by **paulmitch** » Sat Nov 21, 2015 2:44 am

Hey Stuart,

Thanks for the Pro Tips. I honestly hadn't thought about making a completely acoustically isolated space was also totally air-tight and suffocating. That doesn't sound all that great. Pictures coming soon, I just need to edit them so they're the appropriate size to post in the forum. The other points you make, I typically had answers for. While my Sketchup is pretty accurate, it's not down-to-the-inch accurate. Given that it's a poured basement foundation, some of the walls aren't perfectly square, but for the most-part, the dimensions are pretty good.

How will you be dealing with that? Are you going to soffit around it, then build your inner-leaf ceiling below that? Or are you going to move it so that it does not interfere with the room?

I am planning soffits around all of the duct work, and an inner leaf ceiling there-in.

How much isolation do you need, in decibels? What is your plan for achieving that? How much mass will you have on each leaf, and what size will the MSM gap be between them? What target MSM resonant frequency are you aiming for?

So it will primarily be a mix space, but I'll occasionally do vocal overdubs, acoustic guitar, and electronic drums. I typically mix at or below (often below) 80 dBA. Next time I'm wailing on the e-drums, I'll let you know what happens there. I'll also listen in the space to the appliances when they kick on, but I would think being careful about how much SPL gets out of the room would cover those items as well. As for MSM, I couldn't find a specific definition on the site as it's mentioned quite a bit, but not called out on it's own... From what I can infer, it's MASS SPRING MASS... I'm not entirely sure how I calculate that, or determine the resonant frequency of the various cavities I have. On the three sides of the room that are against the earth outside, outside to in is as follows:
dirt
2-inch exterior foam board
6-inch concrete foundation wall
2x4 stud wall (with 2" mineral wool insulation leaving a 1.5" air gap)
resilient channel (1/2 spacing from stud to drywall)
1/2" drywall
Green Glue Layer
1/2" drywall

Wall against existing original foundation:
6" Poured concrete
8" air gap
2x4 stud wall with 2" 703 insulation
resilient channel (1/2 air gap to drywall)
1/2" drywall
Green Glue
1/2" drywall

Ceiling
Wood floor
Subfloor
12" manufactured floor joists
2" mineral wool insulation between joists
resilient channel (1/2 air gap to drywall)
1/2" drywall
Green Glue
1/2" drywall

Are you following ITU specs for your interior acoustics, rations and dimensions, or are you going with AES specs? Or EBU? Or maybe with something else? What design philosphy are you using? LEDE, RFZ, NER, CID, MR? Something else?

I can't say I'm using a specific design philosophy for the space. I'm designing it based on a studio in which I work a lot. There are various diffusive and absorptive elements in the room, and I intend to create a suspended cloud system as well.

If you are framing this properly as a decoupled room-in-a-room, then you do not need resilient channel.

I understand your point, and don't intend to get any wetter. Given the way I framed the walls, the studs are not actually decoupled from the floor joists above; hence the channel.

On the other hand, if you follow the recommended surface-mount concept for your electrical work, then you would not need to chop large holes in your carefully built and sealed isolation walls, to poke the boxes through

I'm not in love with that aesthetic, and am trying to keep home re-sale value in mind as well. Do you think I'm shooting myself in the foot by using putty pads rather than a surface mounted box of some sort?

Honestly, the HVAC thing is the only thing you've really scared me on to this point. I'm not against a separate system. Some of the ideas that have been put forward in terms of a split system sounded interesting if it indeed actually gets me the circulation needed. The utility area of the basement is pretty small, and adding an additional air-handling system sounds like it would take a fair amount of space. We had initially looked at a new furnace with some zoning capabilities, but it was cost prohibitive at the time.

Pics will be in my next post.

paulmitch · Post by **paulmitch** » Sat Nov 21, 2015 2:56 am

Question on surface mounting the outlets. If I were to go this way, how do I secure the box to the wall without coupling the box and drywall to a stud? Do I not need to anchor the box to a stud? Or perhaps I just anchor the box through the drywall to the resilient channel?

paulmitch · Post by **paulmitch** » Mon Nov 23, 2015 7:05 am

Here are some photos of my sketchup plan as well as where I'm currently in the process.

You'll notice the ~8-inch gap between the existing foundation wall, and back of the new wall. The rest of the walls are against poured concreted foundation.

paulmitch · Post by **paulmitch** » Sun Nov 29, 2015 6:08 am

So in trying to heed Soundman 2020's advice, I'm researching trying to build a silencer to feed the room with the house HVAC and vent it out of a different part of the basement. If anyone's got any advice on doing this sort of thing, I'd love to hear it.

In looking at some other similar studio builds on this site, it looks like my dimensions fit Allen's (amac2673) studio, and if you've looked at his thread, it's really inspiring. here His construction was so sound he can play drums at 2am and won't even wake his family. Pretty awesome. I'll try to reach out to him on silencer as well, but if anyone has anything to point me to, I'd sure appreciate it.

Soundman2020 · Post by **Soundman2020** » Sun Nov 29, 2015 9:46 am

I honestly hadn't thought about making a completely acoustically isolated space was also totally air-tight and suffocating.

Yup! That's a common oversight, but it is necessary: if your room is not air-tight, then it is also not "sound-tight". Since sound travels through air, if there is any connection at all between the air inside and the air outside, sound can and will take that path.

I am planning soffits around all of the duct work, and an inner leaf ceiling there-in

Excellent! that will work.

I typically mix at or below (often below) 80 dBA

You should measure dBC, not dBA. "A" weighting is insensitive to low frequencies, while "C" weighting more accurately models the human perception of loud sounds.

I would think being careful about how much SPL gets out of the room would cover those items as well.

Right. Isolation is largely bi-directional. If you have good isolation for out-bound sound, then you should also have good isolation for in-coming sound, assuming they are in the same part of the spectrum. Of course, if they are in different parts of the spectrum, then that does not apply.

As for MSM, I couldn't find a specific definition on the site as it's mentioned quite a bit, but not called out on it's own... From what I can infer, it's MASS SPRING MASS

Correct. That refers to the principle of physics on which low frequency wall isolation is based. Consider a mass hanging from a spring that is in turn suspended from another mass (such as a ceiling). There is a set of equations that describe how that system will resonate. a two-leaf wall, where there is a heavy panel on one side, an air gap, then a heavy panel on the other side is an exact equivalent, since the air acts just like a spring would, and the weight of each panel is the mass.

I'm not entirely sure how I calculate that

Here's the equation:

MSM-resonance-equation.jpg

or determine the resonant frequency of the various cavities I have.

You can only have one cavity, not several. There's no way you can have a true "room-in-a-room" studio with MSM isolation, that has more than one cavity. Even if there are multiple rooms, the wall cavities are all interconnected with each other. The cavity that has the least isolation defines the isolation or the entire studio.

I can't say I'm using a specific design philosophy for the space. I'm designing it based on a studio in which I work a lot.

That's probably a mistake. Rooms are not scale-able. What works perfectly in one specific room will very likely nor work at all in a room that has different dimensions. Even worse, it might work against the room acoustics, instead of in favor of them. For example, if the room you are working in right now has a specific modal resonance that is being treated by on or more tuned devices, but the dimensions of your room are different, then if you duplicated those devices. not only would you BOT be treating the real modal issues in your room, you would also be treating frequencies that don't need any treatment: you would be damaging the acoustics like that. All rooms are different, and each needs its own treatment that is specific to that room.

There are various diffusive and absorptive elements in the room,

Diffusers are commonly overused in home studios, and used unnecessarily. They are often also used even in rooms where they should never be used, and are diffusion is not even applicable.

I intend to create a suspended cloud system as well

That probably will be needed, but what reason did you have for doing that? Will if be hard-backed or not? Will it be angled, or not?

Given the way I framed the walls, the studs are not actually decoupled from the floor joists above; hence the channel.

If the studs are connected to the joists, then how are you going to de-couple the ceiling from the floor above?

Do you think I'm shooting myself in the foot by using putty pads rather than a surface mounted box of some sort?

Not really. You can do it that way of you want. It's just a lot more work, harder to get right, takes longer to do, and provides much less flexibility down the line, if you ever need to change anything. But you certainly can do it that way, if you prefer, and don't mind the extra work.

Honestly, the HVAC thing is the only thing you've really scared me on to this point

Good!

It's a critical issue, and is very often overlooked by first-time studio builders, until it is too late to do it properly.

Some of the ideas that have been put forward in terms of a split system sounded interesting if it indeed actually gets me the circulation needed.

Most split-systems provide only heating/cooling/dehumidifying, but not ventilation. Only a very small number of mini-split systems also include a fresh-air intake. The vast majority do not. Even with those systems, the supply is very low volume, and not meant for a studio.

And you still need to exhaust the stale air, even then.

The utility area of the basement is pretty small, and adding an additional air-handling system sounds like it would take a fair amount of space.

If you use a small mini-split system for the cooling and dehumidifying, then all you need in addition to that is ducts and silencer boxes, plus a fan. Nothing else.

Or perhaps I just anchor the box through the drywall to the resilient channel?

Correct!

You'll notice the ~8-inch gap between the existing foundation wall, and back of the new wall.

maybe I'm not understanding that photo correctly, but it seems to me that the gap is only about 5-3/4", not 8".

I'm researching trying to build a silencer to feed the room with the house HVAC and vent it out of a different part of the basement. If anyone's got any advice on doing this sort of thing, I'd love to hear it.

In looking at some other similar studio builds on this site, it looks like my dimensions fit Allen's (amac2673) studio, and if you've looked at his thread, it's really inspiring

He does show how he did his silencers, on pages 4 and 5 of his thread.

- Stuart -

paulmitch · Post by **paulmitch** » Tue Dec 01, 2015 3:26 am

Hey Stuart,

Thanks for your help here. You appear to be quite the guru on this board.

We all appreciate your help and advice.

So I tried to fumble through the equation below, and came up with a number, but I'm not sure what it means. I'll sort of 'show my work' here.
p = .0762
(65 degrees, 30.22 air pressure, 34 degree dew point currently)

Cc = 765mph at 65 degrees

d = .33ft or 4inches
at the smallest point in the room, the gap from the concrete foundation to the nearest point in the drywall is only the depth of a 2x4 plus the resilient channel. A 2x4 is 1.5x3.5, and the channel adds another .5".

ps = 3.3756
or the density of two .5" layers of drywall, assuming the green glue layer does nothing for the density. Here I was not sure I could use 2 times the 1.6878 knightfly references here since that layer will be double drywall.

Assuming the above is all true, and I can use the numbers I did, I came out with 63.6. Now what can I do with that?

You mentioned in the beeros05 thread on page 3 when talking about silencers that with a 6-inch duct, your cross section is 180cm^2. What do you mean by cross-section here? I'm planning a 6-inch duct into a silencer, and I'm trying to plan my silencer size. In the picture I posted of the cavity next to the old poured foundation with the measuring tape, I get what you thought you were looking at. If you look near the bottom of the pic, you can see where the stud actually begins. The anomaly you're seeing where it appears I only have a 5 3/4" inch gap is actually the third or fourth stud. It's a bad angle. There is actually 7 3/4" at the point where the tape is. Further down the line, I can fit an 8" deep silencer. I'm just trying to figure out how large I need to make it.

I totally understand why it may be shortsighted to plan off of a room with different dimensions than the one I'm trying to build. There is a lot going on in these types of rooms for sure. The main reason I wanted to model it, is that the walls are all perpendicular, but with treatment, the room actually sounds pretty good. I'll upload those plans today for folks to take a look at as well.

As for the ceiling, I intend to use resilient channel there as well. I'll have it on all four walls, and all the soffits. It seems it's the only way to really maximize the interior of the room, and not end up building a true room-within a room. This way, I didn't really have to worry about coupling my framed walls to the floor joists above.

Do you think it's necessary to beef up the area between the floor joists for the ceiling above? I notice Allen did, and everyone says it's a pretty big pain in the ass. I wonder if I'm saving myself the trouble by decoupling with the channel for the ceiling drywall, or if I really do want that additional mass around all the ductwork going through the joists...

As for my mix levels, I don't typically go much higher than 90 dBc. I wouldn't see why I would need to do that at 2am, but it would sure be fun

I have two studios that I work in outside my home. This will be a space for mixing side projects, and recording my own music at times.

Soundman2020 · Post by **Soundman2020** » Tue Dec 01, 2015 4:30 am

Assuming the above is all true, and I can use the numbers I did, I came out with 63.6. Now what can I do with that?

If all your math is correct, then 63.6 is the MSM resonant frequency of your wall. That is the frequency where it does not isolate at all, and might even amplify the sound. You get decent isolation starting one octave higher than the MSM resonance, so you'd be getting isolation above about 130 Hz. That's why the general rule of thumb is to tune your wall such that the resonant frequency is one octave below the lowest frequency that you need to isolate.

There's a simplified version of the equation that assumes a lot of things to be true that might not be, but it does make it a bit easier to check:

f0 = 75 / SQRT (d) * SQRT ( (1/m1) + (1/m2) )

Another way of writing that is:

f0 = 75 [(m1 + m2)]^.5 / [(m1 x m2 x d)]^.5

Which might be easier to calculate.

In both cases "m1" and "m2" are the surface densities of leaf 1 and leaf 2, measured in kg/m2m, and "d" is the depth of the cavity between the leaves, measured in meters.

You mentioned in the beeros05 thread on page 3 when talking about silencers that with a 6-inch duct, your cross section is 180cm^2. What do you mean by cross-section here?

If you take a piece of that duct and look straight at it from one end, you will see it as a circle. The area of that circle is 180 square centimeters. That's what the air flow "sees" as it tries to move through the duct. That is what controls the flow velocity on the duct. Since your fan is pumping in a certain volume of air every second, measured in "cubic centimeters per second", then dividing that by the f cross sectional area of the duct, in "square centimeters " gives you the speed of the airflow, in "centimeters per second". For a wider duct, the air can move slower, thus making less noise. For a smaller duct, it has to move faster, thus making more noise. The flow RATE does not change: that is given by the fan. The flow SPEED changes in direct proportion to the cross sectional area.

I'm planning a 6-inch duct into a silencer, and I'm trying to plan my silencer size.

The internal cross section of your silencer has to be at least twice the area of the duct itself. so if you have 6" duct, with 180cm2 area, your silencer needs to have 360 cm2 cross sectional area. Since it is rectangular, that could be 36cm x 10cm, for example. Or it could be 18cm x 20cm. Or any other combination that gives you 360.

Do you think it's necessary to beef up the area between the floor joists for the ceiling above? I notice Allen did, and everyone says it's a pretty big pain in the ass

Do the math using the equations for MSM!

If you can get a good frequency without beefing it up, and you don't need a lot of isolation, then you are OK. However, there's also the issue of seals. Is that floor sealed 100% air-tight? No gaps, cracks, penetrations, etc? Even a very tiny gap can trash your isolation.... Beefing it up adds another layer of "safety net" there, helping to ensure an even better seal. Yeah, it's a pain to do, but if you want a well isolated room, then....

I wonder if I'm saving myself the trouble by decoupling with the channel for the ceiling drywall,

Nope! You need both. You need both the extra mass AND the decoupling. If you don't decouple, then you only have a single leaf wall... A fully coupled two-leaf wall acts mostly like a single leaf wall...

- Stuart -

paulmitch · Post by **paulmitch** » Tue Dec 01, 2015 5:30 am

Here are the layouts for the work mix room. As I look at the dimensions they're obviously totally different than what I require at home. It would still be good to hear what you have to say about it.

21'L x 16'W x 11.5'D with a ceiling soffit at 10ft that's 2 ft wide around the perimiter. The cloud structures hang at 9.5' near the front of the room and 8.5 for the rear cloud.

paulmitch · Post by **paulmitch** » Tue Dec 01, 2015 6:23 am

f0 = 75 [(m1 + m2)]^.5 / [(m1 x m2 x d)]^.5

So for the surface density, do I use both sheets of drywall as one leaf with a density of 3.3756 (1.6875 per sheet), then the density of the subfloor plus the hardwood above? Meaning m1 is (drywall sheet 1 + drywall sheet 2), and m2 is (subfloor + hardwood)? The figure I presented earlier was essentially the number not counting the concrete side walls. I'm not as worried on the side walls as I am with the ceiling, so that's probably my weak point for isolation, so my MSM calculation should really be for that.

The internal cross section of your silencer has to be at least twice the area of the duct itself. so if you have 6" duct, with 180cm2 area, your silencer needs to have 360 cm2 cross sectional area. Since it is rectangular, that could be 36cm x 10cm, for example. Or it could be 18cm x 20cm. Or any other combination that gives you 360.

So bringing it to inches, I need to maintain at least 56in^2 of area at a time (or 8"x7"). Does oversizing one's silencer do anything on the reverse side of things? I am more concerned with sound getting out than some fan noise from the HVAC unit. I was planning a 2'x4' silencer which would maintain more like 120in^2. I'll sketch up the silencer this evening hopefully.

Yeah, it's a pain to do, but if you want a well isolated room, then....

I guess I'll be starting to beef up between the joists

I need to figure out the best way to mount the drywall up there between the joists. For this I'll probably get 5/8" drywall, and caulk the hell out of it. I'll start to go through all the other ceiling protrusion with acoustic caulk to really seal 'er up. Then the beef. Then the channel.

paulmitch · Post by **paulmitch** » Tue Dec 01, 2015 1:08 pm

How does beefing up the subfloor above in terms of adding drywall around ductwork not constitute adding another leaf in the layer?

Soundman2020 · Post by **Soundman2020** » Thu Dec 03, 2015 10:28 am

How does beefing up the subfloor above in terms of adding drywall around ductwork not constitute adding another leaf in the layer?

Actually, it's the other way around! It doesn't add a leaf in the layer: it adds a layer to the leaf!

A "leaf" is just a bunch of mass that is next to a bunch of air, that has another leaf on the other side. Each leaf might be made up of several layers. For example, a leaf might consist of two layers of drywall, or a layer of plywood plus a layer of drywall, etc. As long as they are right on top of each other, with no air gaps in between, then they are just "layers in a leaf". In order for something to be a separate leaf, it needs to have an air gap next to it. It's actually the air gap that creates the resonant system, since it acts like a spring in between the two leaves of mass. Hence the name "MSM" Mass-Spring-Mass.

- Stuart -

paulmitch · Post by **paulmitch** » Fri Dec 04, 2015 2:56 am

I found some duct liner, and damn, that stuff is expensive. I found it on https://Zoro.com for those of you in the US. I couldn't find under 100sqft, so that's what I had to get. This is for my silencers.

Soundman2020 · Post by **Soundman2020** » Fri Dec 04, 2015 3:12 am

I couldn't find under 100sqft, so that's what I had to get. This is for my silencers.

You might be short with only 100 square feet. A typical silencer box can easily use 25 ft2 or more. Call it 30 ft2 to be safe. Two silencer boxes for the supply duct, plus two for the exhaust duct adds up to 120 ft2. And that's without even considering the ducts themselves: just the silencer boxes. The ducts also need lining.

It's surprising how much surface area there is inside a silencer box...

- Stuart -

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