Take three positions, log for at least twelve minutes in each so a compressor cycle is captured, and repeat the whole thing with the air handling off; the pair of numbers is the measurement.

Measure a room's noise floor in about twenty minutes

checked 2026-09-16responsible Room Tone Register (a named register, not a person)

What you need

• A meter with A-weighting and slow averaging, calibrated at the start and the end of the session with a 94 dB, 1 kHz calibrator. Uncalibrated readings are relative, not absolute.
• Or a phone with a sound level app, understood as indicative: the microphone and its housing are not flat, and the app cannot be more accurate than the hardware. Use it for comparing two rooms, not for quoting a figure.
• A note of the HVAC state, the time of day, and whether the building is occupied. A floor without those three is not reproducible.

The three positions

1. Where the microphone will actually be, at the height it will actually be. This is the number that matters.
2. The centre of the room. This is the number other people will quote back at you, and it is usually the quietest.
3. Near the noisiest boundary — the wall shared with the plant room, the window on the street. This is the number that explains why position one failed.

The twelve-minute log

Twelve minutes is chosen to exceed the duty cycle of a domestic refrigerator and a small air handler. Take a reading every thirty seconds and write them down, or use the meter's Leq function over the whole window. What you are looking for is the shape: a flat trace means a steady floor, a stepped trace means a cycling source, and a stepped trace is the more common result.

Do not report the minimum. The minimum is the floor with the compressor off, and you cannot record in a room with the compressor off unless you can actually switch it off for the take.

Three errors that ruin the measurement

1. Holding the meter. Your hand reflects and your body absorbs; hold it at arm's length or put it on a stand. This alone can move the reading by 2 dB.
2. Measuring near a wall. Within about half a metre of a large surface the reflected energy adds to the direct, and the reading is no longer the free-field level.
3. Quoting the meter's own accuracy figure as the uncertainty of the result. The meter's tolerance is one term; positioning, calibration drift and the room's own variation are usually larger.

Leave this page Close this page when the numbers are written down. The decision they feed belongs to the noise floor bands.