One of the key aspects of my studio plan was trying to achieve a room that was as airtight as possible, in order to maximise the acoustic isolation. This meant sealing up any gaps or cracks in all of the different wall layers. The issue with having an air tight room, however, is pretty obvious - the people inside still need to breathe!
Given that I was designing the room to accommodate a five-person band, potentially in the summer heat, there needed to be fairly significant ventilation. Therefore, I planned to use a powerful extractor fan on an air outtake, and a passive air intake.
Clearly, I didn't want to just cut a large hole all the way through my air-tight layers, effectively compromising all my soundproofing efforts. This is where the acoustic baffle boxes came in: these would force the air to pass down a piece of flexible ducting, snaking through a series of wooden baffles. The direct path for sound to follow from entrance to exit would therefore be blocked by the baffles, and at each reflection the sound leaving the ducting would lose intensity due to rockwool insulation filling the box. A heavy plasterboard lining would also minimise the amount of sound leaking into/out of the box. The diagram below shows the concept:
The idea was then to use one box for the studio's air intake, positioned on the outside wall of the studio, and another for the studio's outtake, positioned up in the rafters, above the highest false ceiling, but still inside the block wall shell. These would then allow fresh air to be drawn into and out of the studio with only a minimal reduction in the overall acoustic isolation. The two specific locations were chosen to try and assist the passage of air, so I opted to have the intake as low as possible to the ground to encourage the absorption of fresh cool air, and the outtake up in the rafters, at the highest point, to encourage the removal of the hottest air from the studio.
In order to work out the size of the ducting to use, I first had to spec up the fan that would be used as the extractor fan to achieve the desired air circulation in the room. I will discuss more on the exact components of the ventilation system later on, but for the fan-size calculation I worked out the following:
Based on data found here, I worked out the desired number of air changes per hour to be around 8: this meant that the full amount of air inside the studio would be exchanged 8 times an hour. Then, by knowing the dimensions of the inner room, I calculated the volume to be roughly 52m^3. By multiplying the volume by the required changes I arrived at a required flow rate of 416m^3/hour.
Searching for a fan with this capacity, I determined that the fan diameter (and thus the ducting diameter) would need to be 6" (150mm). This then informed the dimensions of the baffle box: I went with around 40x90x120 for the bigger outside box and 25x60x90 for the smaller inside box. This one ended up smaller as it didn't need a plywood lining (it would be kept inside so could be just plasterboard), and it also needed to fit into a specific space in the rafters.