Sound Advice
Volume 12, Issue 2, 2009

Can You Hear It Now?

Recently, Wieland Acoustics had the pleasure of working with one of America's largest environmental engineering firms on a project here in the Southern California area. A new pumping station had been completed and was now in operation, but the vertical exhaust fan used to ventilate the basement was generating too much noise. The situation was compounded by the fact that the fan was located between two large parallel buildings (see below). This increased the fan noise level due to reverberation and focused the noise towards the property line. The project was required to meet a stringent noise standard at the property line, but was exceeding it by a substantial amount.

We were able to take extensive noise measurements at the pumping station and in the immediate vicinity to identify not only the level of noise being generated but the frequency content of the noise as well. The latter information is especially important because it determines which potential mitigation measures will be most effective and appropriate.

The noise measurement data was used to calibrate a 3-D noise model of the pumping station that was constructed using SoundPLAN software. The figure (right) provides a view of the calibrated noise contours for the exhaust fan. You can clearly see the focusing effect resulting from the parallel buildings (indicated by the shaded white areas). The noise levels are shown through the color spectrum from red (loudest) and orange to green and blue (quietest).

With the calibrated model in place, it was relatively straightforward to analyze various methods of reducing the fan noise to a level that complies with the noise standard at the property line. These methods included various combinations of installing a silencer on the fan exhaust, wrapping a dense lagging material around the exhaust stack, and enclosing the exhaust stack on three sides with a barrier material (the fourth side of the enclosure would be the adjacent building--see picture at left). 

Each combination of potential mitigation methods was presented to our client along with the resulting noise level expected at the property line. This allowed the client to assess the cost and benefits of each combination, and to make their selection of mitigation method accordingly.

Noting the predominance of blue and green, the figure at right indicates the huge drop in noise levels that can be achieved in this situation with the addition of exhaust fan silencers, lagging and an enclosure. Clearly, this is an vast improvement and benefit to the project owner and the local community.

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