City-centre retail condensers, South West

The challenge

A shop in a conservation area needed to replace two ageing air conditioning condensers. To satisfy the heritage constraints, the new units were to come off the exposed roof and tuck into a rear escape area, mounted high up behind the existing parapet where nobody would see them.

That solved the visual problem and created an acoustic one. The new position is about six metres from the nearest home, where the old one had been well away from it, and the replacement units are louder at source than the ones being taken out. On paper, the scheme moved noisier equipment to within six metres of somebody's window.

What we did

We left monitoring equipment running for a full week at a position representing the neighbour's exposure. A week matters here: it captures how quiet the street gets on a Sunday night, not just a convenient weekday. The shop's own units were switched off throughout, so they could not flatter the results by inflating the background.

The council publishes no fixed number for plant noise, so we applied the standard test, that the plant should not be louder than the background already there, and set the limits from the measured data.

Then we worked the new units through to the neighbour: two of them running together, the distance, and crucially the parapet they now sit behind, which blocks the direct line to the neighbour's windows.