Myth: "My studio does not need HVAC"

Maybe you've heard of "Fake News"? The same happens in the world of acoustics! Here's a place for discussing acoustic myths, legends, mysteries, "questionable" treatment, scams, hoaxes, and just plain old bad information about acoustics, sound, and audio.
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Myth: "My studio does not need HVAC"

#1

Postby Soundman2020 » Sun, 2020-Jan-19, 23:05

Many first time studio builders think that their home studio will not need HVAC (the acronym for "Heating, Ventilation and Air Conditioning"), for any of several reasons. Perhaps because they live in a moderate climate with no extremes in temperature, humidity, or rainfall. Or maybe because they plan to just open the door. Or because they look at the other rooms in their house, which are fine without HVAC, and assume that the studio will be the same.

This is a myth!

You do, in fact, need HVAC in your studio. Even if it is a very small, one-man studio, it still needs at least ventilation, and very probably also needs another part of HVAC: the "AC" part. In some climates it might be possible to have a studio with no heating, but all of them need ventilation, and the vast majority also need "Air Conditioning".

Why?

Basically because studios are isolated from the rest of the world. Studios almost always need some form of acoustic isolation, to stop the loud sounds from getting out (more about how to do that in other articles here on the forum), and also often to stop exterior sounds from getting in. Acoustic isolation implies two things: 1) Two complete hermetic seals around the entire room. 2) Very large amounts of thermal insulation material inside the walls and ceiling.

The room must be sealed hermetically because that's part of how isolation works. Hermetic seals imply that no air can get in or out of the room. None at all. This is very different from typical rooms in a house, office, shop, etc. where there are numerous small gaps all over the place, and some air can still circulate, even with the doors and windows closed. But in a studio, no air can get in or out. Because if air could get in or out, then so could sound! "Sound" is just rapid changes in air pressure, so if there are gaps in the room boundaries where air can move, then sound WILL get through those gaps too. Thus, a big part of isolating your room, is ensuring that there are no gaps. And in fact, you need to do that twice: once for the "inner-leaf" of your isolation walls, and again for the "outer-leaf". You also have to take great care to seal the gaps around the edges of your doors, with several independent seals on each door if you need high isolation (see this article on how to build a door for your studio). And you also have to do the same for your electrical system, since air could leak through the switches, outlets, light fittings, and conduits.

So your studio is air-tight. It must be, if you want to isolate it so sound can't get in or out.

But that's a problem! Because we humans have a bad habit: We like to breathe. In fact, we like it so much that we do it several times per minute! And with each breath we suck in a large volume of air, extract Oxygen (O2) from it, dump Carbon Dioxide (CO2) and other gasses into it, as well as moisture, then exhale it again. Several times per minute. Clearly, if you do that inside a sealed room, then O2 levels in the room air will slowly drop, the CO2 levels will slowly rise, and the humidity in the room air will also rise.

At the same time, your body is also emitting moisture from your skin (both sweat and ordinary skin evaporation), and possibly other gasses from different orifices... (!)

You can see that, in a small room, it won't take long before the room air gets unpleasant: Oxygen depletes, CO2 rises, along with other gasses and humidity. All of that "bad stuff" has no place to go, because the room is sealed, air-tight. And if your drummer ate backed beans and drank beer before he arrived for the session, while the bass player forget to shower and put on deodorant, the keyboard player had garlic bread for breakfast, and the singer forgot to brush her teeth... :shock: Well, you get the picture! All of that is trapped inside, because the room is sealed.

Thus, you need ventilation! You need to suck out that nasty air, and replace it with fresh air, brought in from the outside world.

Now, some people then say: "But I'll just open the door! That will fix it!". Actually, no: it won't. Because the air in the room won't go out through that door unless you make it go, and the air outside won't come in unless you make it go in. Air will not move by itself unless there is "pressure differential" between two points. Expecting the air in the room to change all by itself because you opened a door, is about the same as putting a jug full of water next to an empty glass and expecting the water to somehow pour itself out of the jug, into the glass, just because you took the lid off the jug. In reality: nothing happens.

So, you need to make the air move: you need to increase the pressure at one point, and/or decrease the pressure at another point, to make air move. Thus, you need a fan. Fans decrease air pressure behind them, and increase it in front of them, so the air moves through. And since you are trying to get air to go through the doorway, that's where you'd have to put a fan: just inside or just outside the doorway... or right in the doorway itself.

So think about that: You built a studio with isolation to stop noise getting in and out, but in order to make it work you have to open the door, which lets noise in and out! :shock: :roll: 8-) Ummmm.... doesn't seem like a good way to build a studio. But you also built it so you can have a quiet, comfortable place to work, yet you now have to have a noisy fan in the door way, blowing air all over you...

The solution, clearly, is to just ventilate the room properly: put in a pair of ducts: one to bring in fresh air, the other to remove stale air. Put the fan in one of the ducts. Bingo! You have ventilation.

Buuuut! Those ducts are basically huge holes in the isolation of your studio, which means two things. 1) the isolation is gone, because sound can get out through those ducts and holes. 2) The hermetic seals are gone, because you cut two large holes in them. The solution here is something called "silencer boxes" or "baffle boxes". These are just large wooden boxes, lined inside with duct liner, and with a series of baffles and other features. Basically, what they do is to let the air get through, while stopping the sound. They also completely "plug the holes" in the wall, restoring isolation and the hermetic seals.

So you need ventilation. Your studio MUST have ventilation, and "I'll just open the door" is not a valid solution.

Now for the other part: In addition to being sealed twice over hermetically, your room is also wrapped with a very thick layer of thermal insulation. It has to be, because that is part of the isolation system. If you want high isolation, then you have to completely fill your wall cavities with acosutic insulation, and that turns out to be excellent thermal insulation. Basically, this is like wrapping up your studio in blankets. But! Inside the room you have lots of things that get hot: your DAW, console, speakers, outboard audio gear, instrument amps, lights, perhaps a small fridge, maybe a kettle to make coffee, etc. You also have... people! In addition to emitting nasty gasses and humidity, we humans also emit heat. Even when we are just sitting still, each person emits about as much heat as a 75 watt incandescent electric light bulb, and your drummer going crazy in a hard jamming session might be emitting as much as a small space heater (in addition to emitting the baked beans from lunch....). So, to summarize; you have a bunch of hot things inside a room that is wrapped in very thick thermal blankets.... The conclusion is obvious: the temperature inside the room will rise. There's no place for the heat to go! So it stays inside.

The ventilation system MIGHT be of some help here, assuming that the outside air is cooler than the inside air... But if you live in a climate with temperature extremes, then outside air is no use for controlling inside temperature. If it is 35°C outside, and 30°C inside, then the ventilation system is going to make it HOTTER in there, not cooler. And if it is very cold outside, say below 0°C, then you'll have an icy blast of air coming into your room, spraying all over the people and equipment. Unpleasant.

The solution, then, is the "AC" part of HVAC: Air Conditioning. You need a small air conditioner to cool the air in the room, and keep the temperature under control. It's that simple. The air conditioner also has another effect that you need: it dehumidifies the room air. Remember that I said that people exhale moisture with every breath? And they also sweat? That ends up as humidity in the air. Once again, the ventilation system can help to control humidity... assuming that you live in a mild climate where the humidity outside is a bit lower than the humidity inside. But if you live in a humid climate, then the ventilation system might bring in even MORE humidity, instead of getting rid of it, and if you live in a very dry climate, the incoming air might be so dry that it is unpleasant, and can even cause instruments to change their tone.... and some types of microphone too.

An air conditioner deals with that. It removes humidity as part of the cooling process.

CONCLUSION:

You need HVAC. It's a myth that your room won't need that. You need to bring in fresh air, dump out the stale air, cool the hot air, and dry the damp air. You cannot do that by opening the door, and even if you could that brings up the question: Why bother building a studio to provide good acoustic isolation, good acoustic response, quiet, and comfort, but then open the door, which trashes the isolation, trashes the acoustic response, lets noise in and out, and trashes your comfort!

To finish: Here's some points on the process for designing the HVAC system for your room. The basic concept is:

  • Calculate the volume of air in the room (multiply length x width x height), and assume you need to circulate that volume 6 times per hour (minimum, preferably 8 )
  • Of that circulating volume, you need to exhaust somewhere between 20% and 40% to the outside world, and replace it with the same volume of fresh air. The actual amount depends mostly on how many people are in the room, and the size of the room.
  • If the room is high occupancy (eg, small live room for tracking an orchestra or choir), you might need to increase both of those, to ensure that you are removing enough CO2, and replacing it with enough O2 to keep everyone happy, healthy, and (worst case) alive. If it is low occupancy (eg, just one person in a very large control room), then you might be able to relax those numbers a little.
  • Once you know the total volume of air that you will be moving (air flow rate, usually measured in CFM [Cubic Feet per Minute], M3/m [cubic meters per minute], or l/S [liters per second] ), you need to figure out how big the registers need to be, to keep the air flow velocity below 300 fpm. (If the air velocity is higher than that, it creates its own noise as it moves through the register, in the form of both hissing and rumbling sounds).
  • With all of that in mind, you can calculate the sizes of your ducts and silencer boxes (sometimes also called "baffle boxes").
  • The silencers need to be designed with a certain "insertion loss" in mind, which means "how much sound do they stop?" The insertion loss of each silencer needs to be similar to the design isolation level of the room it is attached too.
  • You can increase insertion loss by making the box heavier (more mass in the building materials used to make it), longer, putting more baffles in it, thicker baffles, having several changes in air flow direction, and changing the cross-sectional area suddenly at several places, by a factor of at least two (preferably much more).
  • All of that increases the static pressure of your air flow path, so you need to take care to keep it within the range of the fans and/or AHU that you intend to use to drive the air through it. "Static pressure" is a complex concept, but it basically refers to how the duct system resists the flow of air through it. High static pressure means that the system resists air flow a lot, so the fans have to work harder to make the air move). The term "AHU" means "Air Handler Unit", and refers to the actual "Air Conditioner" thingy that cools the air and circulates it through the room.
  • If your static pressure is too high, the fan won't be able to move the air correctly, the fan blades will stall / create turbulence / be noisy, the motor will work too hard and burn out too soon. Etc.
  • To dimension the actual cooling capacity of the AHU, figure out the total heat that will be produced in the room from things like people, lights, equipment, hot food, musical instruments, etc, plus take into account the climate (hot air coming in through the fresh air duct from outside), plus possible heat coming in through windows, walls, roof, etc (usually not an issue in studios). Calculate all that in watts, convert to BTU or tons of cooling required.
  • That's your "sensible heat load", but you also need to consider the "latent heat load": due to humidity in the air. The AHU cannot actually cool the air much if it is very humid, until the humidity is first removed... which happens by condensation on the cooling coils inside the AHU. So you need to factor in the latent heat load in the studio, from the climate itself, and from the moist air exhaled by the people, as well as from any other sources (if there's half a dozen pizzas in the room, and several beer mugs, coffee cups, and glasses of Coke, that's extra moisture... ). Convert the latent heat load to "BTU" (British Thermal Units) or "tons" (a measure of cooling capacity, based on one ton of ice), as above.
  • The capacity of the AHU needs to be sufficient to handle both the latent heat load and the sensible heat load for the worst case scenario: Room fully occupied by a bunch of hot, sweat, smelly musicians on the hottest, most humid day in mid summer, with all of them eating and drinking copiously, while they jam hard and fast, getting hotter and sweatier, producing a lot of body heat, and exhaling lots of moisture, with all the lights, equipment and instruments going full bore.
  • It also needs to be able to deal with the other "worst case scenario": one single person in the room, sitting quietly, doing nothing at all, with most of the gear and lights off, and no food or drink.
  • The AHU needs to be able to handle both of those while maintaining RH at around 40% or so, and air temperature in the room at around 21° or so, and also supplying enough fresh air to replace the O2 being used by the people in the room, and exhaust the CO2 and other gasses that they are emitting.
  • If you have multiple rooms, then do the above for each of the rooms separately. Assuming you have one single central AHU for all of them, then you'll also need to design a control system that automatically adjusts the flow to each room as needed, sensing changes in the conditions inside the room (temperature, humidity, CO2 level, etc)
  • In all cases, try to keep the AHU unit outside the acoustically isolated area of the studio. Even very quiet AHU's are still too loud for most studios. We normally design home studios to have internal background noise levels of NC-20 - NC-25, or so, and professional studios for NC-15 or less.

That's the basic procedure. In reality, it's more complicated....

That's about it!




- Stuart -




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Re: Myth: "My studio does not need HVAC"

#2

Postby endorka » Mon, 2020-Jan-20, 11:17

A very useful post, and something of a wake up call especially for some of us living in places where air conditioning in domestic properties is virtually unheard of.

- The silencers need to be designed with a certain "insertion loss" in mind, which means "how much sound do they stop?" The insertion loss of each silencer needs to be similar to the design isolation level of the room it is attached too.


Most of the other aspects of HVAC systems have a well understood theoretical basis on which to design a system, easily found in reference texts. I've not come across as many equations describing the behaviour of these "twisty" silencer boxes though. There is a method of estimating static pressure using "90 degree bends into an equivalent length of straight duct". Also, Rod Gervais' book says (I paraphrase): The "outside" duct should have a cross sectional area such that air velocity there is no more than twice the air velocity at the room registers. This gives a nice implementation of your requirement to at least double the cross sectional area.

From these you have sufficient information to design the silencer, but I've not found any equations predicting the insertion loss based on the other factors you mention;

- You can increase insertion loss by making the box heavier (more mass in the building materials used to make it), longer, putting more baffles in it, thicker baffles, having several changes in air flow direction, and changing the cross-sectional area suddenly at several places, by a factor of at least two (preferably much more).


Empirical evidence and logic show this to be true, but it would be incredibly useful to quantify the effect of these to help avoid over or under engineering the silencers. I've yet to find anything that predicts this though. At the moment I'm using the "design as large as possible" approach :-)

Cheers,
Jennifer



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Re: Myth: "My studio does not need HVAC"

#3

Postby Soundman2020 » Mon, 2020-Jan-20, 20:57

I wish there was a simple formula for figuring out insertion loss, Jennifer, but it isn't a simple thing at all. There are several processes going on at once to cause that, and each plays a part. There's the obvious one, of course: the mass of the panels of which the box is made. If you keep that similar to the mass of the "leaf" that the box is serving, then you should at least get useful isolation from that. However, there's also the fact that air is moving through the box, so the mass alone is not the entire story.

The next process at work, is "impedance mismatch", and this is a bit harder to explain, and to calculate. Basically, it works like this. If sound is traveling down an enclosed space (such as a duct), and it sudden gets into an open space, part of it will be reflected back up the way it came, and only part of carries on traveling the way it was going. Why? Because there are two difference "impedances" here: the air inside the duct has one characteristic impedance, and the open air beyond it has another characteristic impedance. You can think of "impedance" as being how much the air itself attenuates sound waves that are moving through it (that's not actually accurate, but it's a good mental image to help you get your head around it). Here's another way of thinking about it: Imagine a bunch of traffic moving along a six-lane highway, that suddenly narrows down to one single lane: the traffic sees a sudden change in "impedance" as it hits the bottle neck. But don't think of the SPEED of the traffic here: think of the density, to get a better mental picture. If you count the number of cars per square mile just after the bottle-neck, vs. just before, there's a lot more of them before because what was spread out over six lanes is now all bunched up in one lane. So the "density" suddenly changes because there is greater "impedance" to the flow of traffic. And if the road then widens back out to 6 lanes again a bit further down, if you measure the density just after that widening, you'll find that it is lower again! Because the bunched-up cars coming out of the single lane, can suddenly spread out across six lanes.... Basically, the impedance to the flow of traffic changed in both cases. It increased suddenly at the start of the bottleneck, then decreased again at the end.

OK, so that's not really a great analogy, but it's the best one I could come up with! In terms of physics, cars are "particles" not waves, and sound is "waves", not particles. But there's at least something there that might help understand what's going on with "impedance". In fact, what I described with the "traffic" analogy is more like resistance than impedance, but resistance and impedance are related.

So, whenever a sound wave encounters a sudden change in the cross sectional area of the medium it is moving through, it experiences a change in impedance: Some of it "bounces back" off that change in impedance, and returns up the duct (maybe think of some impatient drivers in the traffic as they reach the bottle-neck, doing a U-turn and going back they way they came? [OK, now I'm REALLY reaching to make that analogy work!]). This is sometimes called "end reflection". In other words, you lose some of the sound! Not all of it carries on across the "interface": some is reflected back from where it came. You need a really large change in cross-section to get a useful effect (at least double or half the area), but if you have SEVERAL such changes throughout the path, then it can add up. A single change of double/half the area increases the insertion loss by about 1/2 dB. If the change in area is 8 times (eg, you have a 4" duct [area = 12.6in2] going into a box with dimensions of 10" x 10" [area=100in2], then the insertion loss of about 4 dB. That doesn't sound like a lot, but if you can make that happen three or four times inside the box, then it all adds up.

And don't forget that where the register is at the end of the duct, there's a huge change in area: duct cross section, to room cross section! And the effects here are mostly in the low frequencies. For example, a 6" duct that terminates directly into a room, will have 17 dB of "end reflection" loss for the 63 Hz frequency band, 12 dB for 125 Hz, and 8 dB for 250 Hz. Those are very useful effects!

Here's a graph that shows how much attenuation you get from end reflection from various sizes of duct ending in the empty space inside a room:
HVAC-sound-attenuation-from-end-reflection-FXD.png
HVAC-sound-attenuation-from-end-reflection-FXD.png (14.26 KiB) Viewed 5775 times
HVAC-sound-attenuation-from-end-reflection-FXD.png
HVAC-sound-attenuation-from-end-reflection-FXD.png (14.26 KiB) Viewed 5775 times


And here's a graph that shows how much insertion loss you get from each change in cross sectional area:
HVAC-Insertion-loss-from-cross-section-change--Engineering-Acoustics.png


But it also depends on frequency, because this is impedance, not resistance. In other words, each wavelength is affected differently. The effect is much greater for longer waves (lower frequency), than for shorter waves (higher frequencies). But I won't go into that... it's a bit more complex. The above graph is plenty.

Or if you want the actual equation for calculating how this works:

Sound attenuation when changing duct areas:

Il = 10 log( ( 1 + A1 / A2 )2 / 4 (A1 / A2) )

Where:
Il = insertion loss (dB)
A1 = inlet area (m2)
A2 = outlet area (m2)

But there's also other stuff going on: the walls of the box and the ducts will bend and flex from the sound waves moving through them, and that also absorbs some of the energy. And of course lining the box with proper duct liner also has an effect, since the duct liner is an acoustic absorber. The general form of the equation for calculating the insertion loss from the duct liner, is:

Il = 3.5 α * (1.4 P / S) dB per meter length.

Where:
α = the coefficient of absorption of the duct liner
P = the perimeter of the duct, in meters
S = the cross-sectional area of the duct

But once again, frequency dependent: α is different for each frequency, of course.

Also, the above is for a straight duct, with no changes in cross-section, but silencers DO have changes in cross section, so you need to calculate it for each different cross section, and each frequency... Like I said, it gets complex! :)

Then there's the baffles themselves: The baffles inside a silencer box force the air to make multiple 90° turns as it goes through the box. The air can do that, but sound cannot. Sound waves want to propagate in straight lines, and they do not like going around corners: next time you encounter a jack-hammer out in the street, find a nearby building and step around the corner.... notice how much quieter it gets! Now, sound CAN get around corners, of course (edge diffraction helps it do that, for example), but there's a large reduction in level. So several such 90° bends make it really hard for the sound to get through the box.

Then there's the loss just from the total distance that sound would have to travel to get through.

And a few other minor thing too.

So, if you really wanted to predict the total insertion loss for any given silencer box design, you'd have to take all of that into account, calculating each effect separately, for each frequency band, and each part of the box, add it all up, and arrive at a final conclusion.

Like I said, it's complex. And I don't do it that way in any case! :) Rather, I calculate some things that are easy to do, and estimate the rest from experience.

It would be wonderful if there was one simple equation where you could just plug in a bunch of variables and it would produce the answer, but unfortunately, there isn't. At least, none that I'm aware of.

By the way, you might want to check the "Short Book List" thread: Andre just added a really excellent primer on HVAC, put out by Daikin: (HVAC acoustic fundamentals) (Update: I also put that same document in the document library). Well worth downloading and reading!

- Stuart -



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Re: Myth: "My studio does not need HVAC"

#4

Postby endorka » Wed, 2020-Jan-22, 20:49

That's really helpful Stuart, thank you. Sounds like a calculator for these silencer boxes would make an amazing PhD project for someone. Many would be grateful, I am sure. I'm reading the primer document now.

Just to check: do the impedance changes within the silencer happen as it goes round the corners? I've attached a clip of Gregwor's silencer box design to illustrate. If the cross section is initially proportional to X, then as it goes round the corner will it expand proportional to sqrt(2)X, then back to X again as it goes into the next straight section? And it is the cumulative effect of all these, the initial increase from duct to silencer, and the final increase from silencer to room that add up to something significant in terms of impedance change?
Gregwor's Silencer Box.png


[quote]And don't forget that where the register is at the end of the duct, there's a huge change in area: duct cross section, to room cross section! And the effects here are mostly in the low frequencies. For example, an 8" duct that terminates directly into a room, will have 18 dB of "end reflection" loss for the 63 Hz frequency band, 12 dB for 125 Hz, and 7 dB for 250 Hz. Those are very useful effects![quote]

Very useful indeed. Might be a daft question, but would the effect still be present if the silencer duct did not terminate directly into the room, but instead was routed through some metres of normal duct in the room with the register at the end of that?

Cheers,
Jennifer



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Myth: "My studio does not need HVAC"

#5

Postby Soundman2020 » Wed, 2020-Jan-22, 22:24

endorka wrote:That's really helpful Stuart, thank you. Sounds like a calculator for these silencer boxes would make an amazing PhD project for someone. Many would be grateful, I am sure. I'm reading the primer document now.
Actually, I already did that... :) Not gonna give away too much, but over the years I have developed a silencer box model in SketchUp, using Dynamic Components, to do most of the hard work for me. I only need to input things like the room dimensions, the air flow velocity at the AHU and the registers, the materials I want to use for the box, and things like that, then I let it crunch the numbers for a short time, and it spits out a complete silencer box, along with all of the calculations for cross sectional area and air flow velocity at each point in the box, the percentage change in flow, and stuff like that. I can then tweak more stuff within the model, and let it crunch some more numbers for me.

It doesn't do everything yet: no final estimate for insertion loss, for example (only mass-based TL so far). But I keep on adding to it as I have time. I'll probably put some basic estimating equations in there soon. However, it does do the basic boring stuff, such as figuring out the room volume then recommending the best duct size, and dimensioning the entire box for me for that duct size, to produce the air flow velocity that I want at the registers.

It even produces dimensioned drawings of the box for me: plan views, elevations, sections, details... (That's the most recent addition I did to it).

I've been tempted to release parts of it into the public domain... at least the bit that does the basic math, since everyone seems to get hung up on that.... Maybe one day....

In the meantime, it makes my life a lot easier on studio design projects for my clients: I just used it on Tom's "very high isolation" studio, for example, to help design his silencers... which is why they got to be so huge! That's what the model says we will need to get into the range of isolation that Tom is aiming for. And that's why his silences are going to weigh over 300 kg each! (Oh, right! I forget to mention that the model also tells me how much the box will weigh....)

Just to check: do the impedance changes within the silencer happen as it goes round the corners?
The box you show is a rather simple one, with no useful changes in cross-sectional area. That design would work if you only need low to moderate isolation in the HVAC system. I do things a bit differently. If you follow Tom's thread, he's about to start building his huge silencers, so he'll probably post some photos of that. Here's the basic concept that I came up with a few years back, that I use in most of my studios, with variations:

SOUNDMAN2020--Recording-studio-design--HVAC-split-flow-silencer--bot--S0134.jpg
You can't see the details (on purpose!), but the air flow enters the box from the duct into in an expansion chamber, which is several times the cross section (and volume) of the duct. The air flow is then split in two, branching off down the two legs at right angles to the entry. If you look at the equations for TL in plenums, you'll see why I did that: the trig function "COS" comes into play here, and the angle is 90°... :). Then the air flows down each leg, around the baffles (where the side cross section is not the same as the end cross section), before finally entering a second expansion chamber right above the "sleeve" at the end, which is the exit. The sleeve is what penetrates the actual isolation leaf, into the room.

Here's part of the data entry page for my semi-automated tool, so you can see what sort of things go into it:
SOUNDMAN--HVAC-silencer-box-tool--V4.177--data-entry-page.jpg

That's just part of it: there's a lot more than that.

OK, I probably already gave away more than I should have there! It's taken many hundreds of hours to perfect this design, over many years, so I think I won't go into any more details than that. ... :)

Just to check: do the impedance changes within the silencer happen as it goes round the corners?
Not really, no. There's some effect there, but you need a very large, sudden change in cross sectional area to get a decent impedance mismatch. At least twice or half the area. With the design you show, that doesn't happen. There's only minimal changes in area, because the cross section alongside each baffle is the same as the cross section at the end of the baffle, with only slight variation as the air flow is forced to turn the corner.

And it is the cumulative effect of all these, the initial increase from duct to silencer, and the final increase from silencer to room that add up to something significant in terms of impedance change?
Exactly, yes. But there's more to it than that. Plus, don't forget that there are no flow direction vanes in these boxes, so the load imposed by each corner is brutal for static pressure... which is one reason why I have two "legs" on the silencer in my design. The air flow is in parallel down those two legs, which has a positive effect on static pressure. Much like resistors in parallel in an electrical circuit: the total resistance is less than either of the resistors on its own... (I used every trick I could think of with this design!).

Might be a daft question, but would the effect still be present if the silencer duct did not terminate directly into the room, but instead was routed through some metres of normal duct in the room with the register at the end of that?
Correct! It's the "interface" that matters: where the end of the duct meets the room. And it's different for a register that is flush with the ceiling, vs. one that hangs down a bit below the ceiling. Which is why my designs do not have the registers flush with the ceiling... :) It's the difference between opening out into half-space or full-space (sort of like Q=1, 2 or 4 for a sound source against a wall)



Avare
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Re: Myth: "My studio does not need HVAC"

#6

Postby Avare » Wed, 2020-Jan-22, 23:54

Nice Stuart!


Good studio building is 90% design and 10% construction.

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Soundman2020
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Re: Myth: "My studio does not need HVAC"

#7

Postby Soundman2020 » Thu, 2020-Jan-23, 02:08

Avare wrote:Nice Stuart!

Thanks Andre! :thu:
Your vote of confidence is very much appreciated!

- Stuart -




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