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".
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... 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! 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.
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 -