Noise from a neighbour's air conditioner: what the law says and how to fix it

The letter rarely arrives in July. It arrives in late September, just as the air conditioner switches back to heating mode and the compressor runs through the night while the windows are still open: a registered letter, or a note slipped into the letterbox, complaining of "constant noise pollution". In the vast majority of cases, the unit isn't faulty and the neighbour isn't acting in bad faith. The culprit is the positioning of the outdoor unit, and the applicable legal framework is far stricter than most homeowners imagine.

Noise from an air conditioner or an air-to-air heat pump is not judged in absolute decibels. It is judged in terms of emergence: the difference between ambient noise with and without the machine running. That's the whole trap. A very quiet unit installed on a peaceful suburban cul-de-sac can be ruled unlawful, while a noisy model sitting alongside a busy boulevard will pass without issue.

The governing text: emergence, not level

The reference is Article R. 1336-5 of the Code de la santé publique (French Public Health Code), which applies to behavioural noise and activity noise, including individual equipment such as an air conditioner, a heat pump or a mechanical ventilation extractor. The principle is simple to state:

An offence is established when the overall emergence of the noise perceived at the neighbour's property exceeds 5 dB(A) between 7 a.m. and 10 p.m. and 3 dB(A) between 10 p.m. and 7 a.m., after applying a corrective term that depends on how long the noise occurs.

Three points that almost nobody grasps the first time round:

1. The measurement is taken at the complainant's property, not yours. In the main room with the window open, or with the window closed, and also outdoors at the property boundary. The level recorded on your own terrace carries no legal weight.
2. The residual noise is measured with the unit switched off, at the same spot, under the same conditions. The two readings are then compared.
3. The corrective term makes night-time even stricter: a machine running continuously for more than 8 hours gets no allowance at all, whereas highly intermittent noise does.

In practical terms, in a suburban area where residual night-time noise drops to 28 dB(A), your unit must not exceed 31 dB(A) at the neighbour's window. That is extremely low. A standard wall-mounted split rated at 48 dB(A) sound power at 1 metre easily reaches 35 to 38 dB(A) at 3 metres. The gap is simple arithmetic.

Sound power and sound pressure: don't confuse the two

Two figures coexist on technical datasheets, and salespeople cheerfully mix them up:

A unit advertised at "62 dB(A)" is not necessarily louder than one at "48 dB(A)": the first figure is often a power level, the second a pressure level at 1 m. Always check which of the two is quoted. The mandatory European energy label for air-to-air heat pumps, for its part, states the outdoor sound power — the only figure that is genuinely comparable from one manufacturer to another.

A useful rule of thumb: in a free field, sound pressure drops by roughly 6 dB every time the distance doubles. A unit at 38 dB(A) at 3 m falls to 32 dB(A) at 6 m, and 26 dB(A) at 12 m. Moving the unit a few metres often does more than any accessory.

Why things nearly always go wrong in the same places

Reviewing the actual causes of disputes, a small number of configurations keep coming up.

• The unit installed in a corner between two walls. Two perpendicular surfaces reflect the sound and easily add 5 to 6 dB compared with a free-field installation. The corner between two semi-detached houses is the worst possible location.
• Wall mounting without decoupling. Compressor vibrations travel through the brackets, into the wall, and re-emerge as structure-borne noise in the neighbouring bedroom — a dull, low-frequency rumble that windows don't filter out.
• The fan pointing at the façade opposite. The airflow is directional. A 90° rotation radically changes how it is perceived.
• Heating mode and defrost cycles. In winter, the heat pump reverses its cycle: the fan stops abruptly, the compressor changes speed, and you hear the four-way valve clunk. It is this event-based noise, not the continuous hum, that wakes people at 4 a.m.
• Ageing anti-vibration mounts. After 7 to 10 years, the original rubber mounts harden. A machine that caused no complaints for years can become a nuisance without anything having changed.

One point needs stating plainly: night mode (variously called silent mode, quiet or night mode depending on the brand) exists on virtually every recent unit and is disabled by default. It caps fan speed and limits compressor frequency, at a cost of 10 to 20 % of output. In a properly sized home, that is more than enough at night. Plenty of disputes are settled with five minutes on the remote control.

Measure before you argue

Before taking any action — whether you're the homeowner being blamed or the complainant — you need figures.

An official measurement must be carried out by an acoustic consultant or the local council's public health department, using a class 1 sound level meter and producing a report admissible in court. Budget €400 to €900 for a private assessment. But for a first indication, a portable digital sound level meter costing around fifty euros gives perfectly usable readings, provided you follow the method:

• measure at night, between 1 a.m. and 4 a.m., when residual noise is at its lowest;
• take two sets of readings: unit running, then unit switched off at the breaker, at the same point and within the same minute if possible;
• stand 1 m from the complainant's façade, microphone 1.5 m above the ground, away from corners;
• use A-weighting, slow response, and record the LAeq over at least 10 minutes, not the peak level.

If the gap between the two sets exceeds 3 dB(A) at night, you are almost certainly in breach and there is no point contesting it: go straight to the remedies. Recording your readings in an acoustic measurement logbook, dated, with the outdoor temperature and operating mode, is enormously helpful if the case ends up in court — for either side.