Decibels explained

Decibels turn up on specification sheets, energy labels and planning noise calculations, and they don't behave like ordinary numbers. Two units that each measure 45 dB(A) don't add up to 90, and a difference that looks small on paper can be easy to hear. A few basic rules make the figures much easier to read.

Why the scale works the way it does

Human hearing copes with an enormous range, from the faintest rustle to a jet engine taking off, so sound is measured on a logarithmic scale that squeezes that range into manageable numbers. Each step up means multiplying the sound energy, not adding to it.

This explains some results that surprise people:

• Two identical sources add 3 dB. Two outdoor units each giving 45 dB(A) at a point give about 48 dB(A) together.
• A much quieter source barely adds anything. If one sound is 10 dB quieter than another, the two together are less than half a decibel louder than the louder one alone.

What the A in dB(A) means

Hearing is less sensitive to very low and very high pitches than to the middle range where speech sits. An A-weighted measurement, written dB(A) or dBA, turns down the parts of a sound that people hear less well, so the figure is closer to how loud the sound seems rather than its raw physical energy. Almost all air conditioner noise figures are A-weighted, as is the 37 dB(A) limit in the MCS 020(a) noise standard.

A-weighting has a known weakness. It can understate low, droning sounds such as hum and vibration, which some people find more intrusive than the number suggests.

Sound power and sound pressure

Specification sheets can give two different kinds of decibel figure for the same unit.

• Sound power level is the total sound energy the unit gives off, whatever its surroundings. It's a property of the unit, like the total light a bulb gives out.
• Sound pressure level is what reaches a particular spot, such as 1 metre from the unit. It depends on distance and surroundings, like how brightly the same bulb lights a page across the room.

Sound power figures are higher than sound pressure figures for the same unit. Energy labels show sound power, and planning noise calculations start from it to work out the sound pressure at a neighbour's window. Brochures more often quote sound pressure. Before comparing two units, check that you're comparing the same kind of figure at the same fan speed. Typical figures for both units are in how loud air conditioning is.

How distance and surroundings change the level

Sound pressure falls as sound spreads out. In open space, away from walls, each doubling of distance lowers the level by about 6 dB, so a sound measured at 1 metre is about 6 dB quieter at 2 metres and about 12 dB quieter at 4 metres. Gardens and streets aren't open space, so in practice the drop can be smaller.

Hard surfaces near a unit reflect sound and raise the level. A unit on the ground against a wall is louder at a neighbour's window than the same unit standing in the open, and one in a corner, with the ground and two walls around it, is louder again. That's why noise calculations count the hard reflecting surfaces close to the unit.

A solid barrier between the unit and a window reduces what gets through. In the MCS 020(a) calculation, a solid barrier that completely hides the unit from the window takes 10 dB off, and one that partly hides it takes off 5 dB. Noise rules and your neighbours explains how the whole calculation fits together, and where the outdoor unit can go covers the practical side.

Background noise matters too. The same sound is more noticeable on a still night in a quiet street than on a busy afternoon, because there's less to mask it.

Everyday sounds for comparison

These are rough, typical levels. Real sounds vary a lot with distance and circumstances.