Measuring Noise Outdoors: What the Standards Require

Outdoors, a sound field is shaped by the ground, by nearby walls, and by whether the source behaves as a point or a line. Move a microphone a metre and the level can change measurably; move it against a hard façade and reflection changes it again.

This is why standards specify positioning rather than leaving it to judgement. Two people measuring the same road from the same garden can legitimately disagree by several decibels purely on where they stood and how high they held the microphone.

For an idealised point source radiating freely, level falls by about 6 dB for each doubling of distance, because the same energy spreads over four times the area. For an idealised line source such as a steady stream of traffic, the fall is closer to 3 dB per doubling.

Real situations sit between and beyond these idealisations. Ground absorption, barriers, buildings and atmospheric effects all intervene, so the geometric rules are a starting point for reasoning rather than a prediction.

Wind produces noise directly at the microphone, which is why outdoor measurement uses a windscreen and why standards restrict the conditions under which a measurement is valid. Temperature gradients and wind direction also refract sound, which can change a downwind level substantially relative to upwind.

For anything consequential — a complaint, an assessment — the weather at the time is part of the record, not an aside.

Environmental noise varies. A single instantaneous reading of a road is nearly meaningless; what is normally reported is an equivalent continuous level over a defined period, which describes the same total energy spread evenly across it.

Choosing that period is a substantive decision. A five-minute average during a lull and a five-minute average during the school run describe the same road and answer different questions.

A phone cannot meet IEC 61672, cannot be verified against a class 1 instrument's tolerances, and has no windscreen worth the name. Its microphone is also optimised for speech, with processing that may not be fully defeatable.

What it can do is compare: this side of the house against that side, window open against closed, before a barrier against after, morning against evening. Those comparisons hold even when the absolute figure does not, and they are often exactly what a person actually wants to know.

Uncalibrated, our own meter reports an A-weighted digital signal level in dBFS rather than sound pressure; a one-time local calibration produces an estimated dBA figure. Neither is a standards-compliant environmental measurement, and the tool says so on screen.