Rooftop plant relocation, Yorkshire

The challenge

The owner of a city-centre office building wanted to shift three groups of existing rooftop plant to new positions on the same roof: a bank of condensers, an air handling unit and its associated kit. Nothing new was being added and the capacity was unchanged, but the council still required a noise assessment at the neighbours.

The neighbours are close: another office building about twenty metres away, and a hotel about twenty-five, which is sensitive at night as well as during the day. The council's test was strict: the plant must be no louder than the background noise already there.

There was also a trap in the wording. Where the character of the noise cannot be evidenced at design stage, the council directs that a penalty be added to the result, and that penalty alone would have eaten most of the available margin.

What we did

We monitored for forty-eight hours from a position representing the neighbours, covering day, evening and night.

The important decision was to measure each item of plant individually, close up, while it was running normally, before it was moved. That serves two purposes. It gives real source levels instead of manufacturer claims, and it lets us establish the character of the noise: whether it whines, hums or thumps, or is simply steady. Every unit turned out to be steady and featureless, which meant the precautionary penalty did not apply. Evidence removed it; an assumption would have carried it.

We then calculated what reaches each neighbour from the new positions, including the effect of the parapet running round the roof, which screens every unit from both buildings.