Isolation
Quiet rooms: air-conditioning, equipment and background noise
How quiet a theatre, studio or meeting room should be, how that is specified with NC curves, and how air-conditioning, ducts and equipment racks are kept from spoiling it.
8 min read9 sourcesChapter 12 of 24
Schematic
Noise enters a quiet room by more routes than the wall: through a shared duct, a ceiling void, the slab and the door gap. Air-conditioning ducts are among the most common — they connect rooms with an open tube.
Animated illustration of the principle. Not measured data.
A room can be perfectly isolated from its neighbours and still be noisy, because the noise is made inside it: the air-conditioner, the air rushing through a grille, a projector fan, an equipment rack. In a theatre, that noise fills the quiet passages and masks dialogue; in a meeting room the microphones pick it up. Background noise is designed for, like reverberation — with a target and the services detailed to meet it.
How quiet is quiet
Background noise is specified with noise criterion (NC) curves, defined in ANSI/ASA S12.2. A room meets NC 25 only if its measured noise is at or below the NC 25 curve in every octave band — a single dB(A) figure is not enough, because low-frequency rumble can hide under it.
Published targets give the order of magnitude. SMPTE’s recommendation for review rooms and premiere screening is NC 20 to 25, with NC 40 or worse described as leading to dialogue intelligibility problems. US federal design guidance (UFC 3-450-01) gives NC 30 for conference rooms, NC 25 for teleconference rooms and music teaching studios, and NC 20 for concert and recital halls. A dedicated home theatre is reasonably designed to the review-room range; a living room with a projector will not get there without help.
As a rough guide, US federal design guidance pairs NC 25 with about 30 dB(A) and NC 30 with about 35 dB(A).
Split air-conditioners
Most Indian homes cool with split units. Their indoor units publish sound pressure figures by fan speed, and those figures matter. One manufacturer’s range, for example, quotes 19–29 dB(A) at the quietest setting rising to 42–49 dB(A) at top fan speed, depending on capacity. At its lowest speed such a unit is near a theatre target; at high fan it is well above it.
The practical consequence: choose and size the unit so it can hold temperature at its lowest fan speed during a film, mount it where it does not blow across the seats or rattle the structure, and run its condensate and refrigerant lines so they do not open a gap in the isolation shell. A unit chosen only by tonnage will be run on high fan — and heard.
Ducted systems
Ducted air brings two problems: the noise of the fan travelling down the duct, and the noise of the air itself at bends and grilles. The New York City environmental department’s guidance for building HVAC is a useful public checklist: keep air velocities down (below about 2,000 ft/min in ducts and 1,000 ft/min at elbows), use lined ducts and large lined plenums, place silencers as close to the noise source as possible, isolate rotating equipment on springs or pads, use flexible connections at the fan, and prefer round ducts, which are stiffer than rectangular ones. Australian guidance from AIRAH tabulates maximum velocities for quiet rooms — for an NR 30 room, run-outs no faster than about 4 m/s.
Ducts also carry speech and soundtrack between rooms. Canada’s National Research Council warned that “ductwork connecting offices can act as a speaking tube”. Where one duct serves two rooms, a crosstalk silencer or a lined, indirect route between them is needed — otherwise the duct undoes the wall.
Equipment, racks and projectors
Equipment turns almost all its power into heat. One watt is about 3.41 BTU per hour, and an equipment-rack maker’s guidance warns not to use nameplate ratings alone and to assume amplifiers are only around 50 % efficient. That heat has to leave — usually with fans, and fans make noise. In a quiet room, the rack belongs in a ventilated cupboard or an adjacent room, with its fans thermostatically controlled.
Where equipment lives in a plant or machine room next to the theatre, the separating construction needs real isolation: US federal guidance asks for STC 60–65 between a mechanical room and an occupied space. Projectors are the other common source; a projector in the room should be chosen with its published fan noise in mind, or enclosed in a ventilated, isolated housing.
For integrators and architectsThe numbers, the standards, and what to specify
- Criterion
- NC curves per ANSI/ASA S12.2-2019, compliance in every octave band. Review/screening rooms NC 20–25 (SMPTE RP 141); teleconference NC 25, conference NC 30, recital NC 20 (UFC 3-450-01, 2022).
- Split units
- Use the indoor unit’s published sound pressure by fan speed; size so the room holds temperature at low fan. Seal every line penetration through the shell.
- Ducts
- Low velocities (NYC DEP: <2,000 fpm ducts, <1,000 fpm elbows; AIRAH NR 30 run-outs ≤4 m/s), lined ducts and plenums, silencers near the fan, flexible fan connections, vibration isolation, crosstalk silencers between rooms.
- Heat
- 1 W ≈ 3.41 BTU/h (NIST SP 811). Do not size from nameplate ratings alone; Middle Atlantic assumes Class AB amplifiers are about 50 % efficient and advises using the maker’s waste-heat figures.
- Plant rooms
- STC 60–65 between mechanical room and occupied space (UFC 3-450-01, Table 5-1).
What to write into the brief
- A background-noise criterion (NC) for each room in the brief, with the HVAC design, unit selection and duct routes reviewed against it before the services drawings are issued.
Sources
- 01ANSI/ASA S12.2-2019 — Criteria for evaluating room noise
- 02SMPTE RP 141-2005 — Background acoustic noise levels in theaters and review rooms
- 03UFC 3-450-01 (2022) — Noise and vibration control (US Department of Defense)Background noise and STC targets by room type.
- 04NYC Department of Environmental Protection — Noise control for building interior HVAC equipment
- 05AIRAH — Managing noise in ductwork systems (2019)Maximum duct velocities by room noise rating.
- 06Warnock, NRC Canada Building Digest CBD-186 — Office partitions: acoustical requirementsDucts as speaking tubes; leaks limit isolation.
- 07Mitsubishi Electric — MSZ-LN split system data (indoor unit sound pressure by fan speed)
- 08NIST SP 811 — Guide for the use of the SI, Appendix B.9 (heat flow rate)
- 09Middle Atlantic Products — Controlling the temperature inside equipment racksRack-maker’s white paper, hosted by AVI-SPL.
External links open in a new tab. Figures quoted in this chapter are taken from these sources; a Science of Sound measurement is only ever presented as one, with its file, date and instrument.
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