The Control of Noise at Work Regulations 2005 set two action values and one limit, and a compressor running near a workbench for a full shift can put a workshop across all three without anyone noticing until a survey turns up the number.

The two action values

The lower exposure action value is 80 dB(A). Once average daily or weekly exposure reaches this, an employer has to make hearing protection available to anyone who asks for it and provide information and training on the risk.

The upper exposure action value is 85 dB(A). At this level, hearing protection becomes mandatory rather than optional, and the area normally needs to be marked as a hearing protection zone.

The exposure limit

The exposure limit value is 87 dB(A), measured with any hearing protection worn taken into account. This one cannot be exceeded under any circumstances.

These figures are averaged over the working day or week, so a compressor that runs intermittently is measured on the exposure that produces. A screw compressor cycling on and off through a shift can sit well under the limit even if its peak noise, taken in isolation, looks alarming on paper.

Where a workshop sits against these numbers is a question a noise assessment answers for the real layout, the real duty cycle and the other machinery running alongside the compressor.

Sound pressure level is what a microphone actually picks up at a given point, usually one metre from the compressor casing, and it is the figure closest to what you would hear standing next to the machine. Sound power level is different: it describes the total acoustic energy the compressor radiates in every direction. Neither is wrong. They answer different questions, and a manufacturer's datasheet will usually only tell you which one it has given if you check the label closely.

The distance the figure was taken at matters because sound pressure falls away as you move from the source. Roughly speaking, sound pressure level drops by about 6 dB each time the distance from the machine doubles, so a reading taken at one metre and a reading taken at four metres will differ even for the identical compressor. That is why a listing needs to state both the level and the distance it was measured at: a bare number with no distance attached tells you almost nothing about what the workshop floor will actually sound like.

Comparing two compressors on noise alone

Before setting one figure against another, check that both are the same kind of measurement, sound pressure against sound pressure or sound power against sound power, and that any sound pressure figures were taken at the same distance. A compressor rated at 70 dB(A) at one metre is not quieter than one rated at 68 dB(A) at four metres; the second number was always going to read lower simply because the microphone was further away. Where a datasheet gives a sound power figure instead, it can be a useful way to compare machines regardless of where they will be sited, but it will not tell you directly what you will hear standing beside one in a small plant room.

An enclosure round the compressor, a plant room with a door between the machine and the workshop, and distance from the operator all cut the noise a compressor puts into a room, and they do it before anyone has to put in ear defenders. Hearing protection has its place, but it protects the person wearing it and nobody else on the floor, so it belongs at the end of the list.

Enclosures

An acoustic enclosure surrounds the compressor itself, usually with foam or mineral wool lining and a ventilated air path so the motor and element do not overheat inside it. It reduces the sound reaching anyone standing near the machine, but it only does that job if the ventilation is sized for the heat the compressor actually produces. An enclosure that traps heat will shorten the life of the compressor faster than the noise it was fitted to stop.

Plant rooms

Putting the compressor in its own room, with the door shut and a duct run to the workshop, moves the noise source away from the people working near the tools. This is usually the more effective option where the workshop already has a spare room or a lean-to that can take the pipework, because a wall and a door reduce sound more reliably than a light enclosure panel. It also keeps the heat, oil mist and vibration of the compressor out of the working area, which matters for reasons beyond noise.

Distance

Sound pressure falls off with distance from the source, so moving a compressor a few metres further from the operator's usual position reduces what reaches their ears without any equipment at all. This is the cheapest control available and the one most often overlooked when a compressor gets sited wherever there happens to be floor space. It works alongside an enclosure or a plant room.

Hearing protection

Ear defenders or plugs protect the person wearing them, and only that person, for as long as they are worn correctly. They are the right answer when the other controls have been applied and a residual noise level remains, not a substitute for reducing the noise at its source. Relying on hearing protection as the first response leaves everyone else in the room, and anyone who takes their protection off for a moment, unprotected.