The first question people ask is not about sound. It is about air: does it get stuffy in there? A fair question — in a closed space with a person in it, carbon dioxide rises quickly.
How fast the air is actually refreshed
In a one-person unit the ventilation runs at 300 m³/h, which refreshes the air every 2–3 minutes. In two- and four-person models it runs at 550–600 m³/h and the air is refreshed every 6–8 minutes: more volume, more people, same result.
It starts from an occupancy sensor — nothing to switch on and nothing to remember.
Why CO₂ matters at all
Fresh outdoor air sits at roughly 400 ppm of CO₂. A full meeting room passes 1500 ppm within an hour, and that is where decision-making and concentration start to drop. The familiar heavy head after a long meeting is not fatigue from information — it is the air.
A small enclosure holds little air, so without ventilation CO₂ would climb faster there than in a meeting room. Ventilation is therefore not an optional extra but a core component.
The fan must not be audible — and it must not mask speech either
There is a contradiction here that has to be solved in engineering. Powerful ventilation means airflow, airflow means noise — and the unit was bought for quiet. The answer is acoustic silencers in the ducts: they let air through and stop sound.
The subtler failure is a fan that is not loud but sits in the wrong part of the spectrum. Broadband fan noise overlapping the 250–4000 Hz speech band does not just annoy you; it competes directly with the voice coming out of your headset, and you compensate by turning the volume up. A well-designed duct pushes its residual noise below that band, where the ear reads it as neutral background rather than interference.
A practical test before buying: sit inside, close the door and listen. If you can clearly hear the fan, so will the person on your video call.
Two types of ventilation
The simplest approach is one fan blowing air in. The air leaves wherever it can, usually through the door gap. The result is uneven and often audible.
Supply-and-extract ventilation means two separate ducts: fresh air in through one, used air out through the other. The exchange is controlled and even.
What procurement should put in the tender
Buying teams tend to specify a dB figure and stop there, which leaves the air side entirely to the supplier's discretion. Four lines are worth writing in: the airflow rating in m³/h per occupancy class, the stated air-change interval, whether the ventilation is automatic or user-operated, and whether the fan is serviceable without removing wall panels. That last one decides whether cleaning is a ten-minute job or a service call.
It is also worth asking for the specification sheet rather than the brochure. “There is a fan in it” is not an answer. See the Duet specification as an example of what those numbers should look like, and the Quartet for the four-person class.
Why an occupancy sensor changes the outcome
Ventilation that has to be switched on manually is ventilation that runs about half the time it should. People step in for a two-minute call, never touch the control, and conclude the unit is stuffy. A sensor removes the decision: the fan is already running before the door closes, and it keeps running long enough after you leave to clear the volume for the next person.
The full range and technical documentation are published by the Silentbox factory.