This report summarizes analyses performed by David Coate Consulting (DCC) to assess and abate noise impacts caused by the cooling tower that services 9-17 Hartcourt Street in Boston, Massachusetts. I understand that the City of Boston measured cooling tower noise at this location, in response to a complaint about noise, and found noise levels to exceed City of Boston noise code (60 dBA during the day and 50 dBA during the night).
On November 13, 2006 I measured cooling tower noise near the balcony where the person that complained about the noise lives. Figure 1 shows the measured cooling tower noise (at maximum speed), compared with ambient noise levels, and noise levels caused by the nearby MBTA subway. Overall A-weighted cooling tower noise was 71.7 dBA which is substantially above the City of Boston noise code.
In order to attenuate noise levels to meet the Boston noise code, DCC recommended the installation of Industrial Acoustics Company (IAC) 5L silencers. Based on silencer insertion loss data provided by IAC, the expected resulting noise level at the balcony location would be 50 dBA. However, because this value would be just at the nighttime limit, DCC also recommended the installation of a Variable Frequency Drive (VFD). A VFD detects interior cooling demands, exterior temperature and humidity, and automatically adjusts fan speed accordingly to meet the demand. Consequently, at night when cooling demands are reduced, fan speed is automatically reduced which in turn reduces cooling tower noise. The fan noise level is exponentially related to fan speed, so small reductions in fan speed can provide large noise reductions.
Noise Measurements after Abatement
On May 25, 2007 DCC measured cooling tower noise to assess the effectiveness of the installed silencers and VFD. Figure 2 shows measured ambient noise and cooling tower noise at the balcony location with the tower at 100%. Since ambient noise levels are very close to cooling tower noise levels, ambient noise levels must be subtracted out to obtain true cooling tower noise levels. The ambient noise level was at 52.1 dBA, and 53 dBA with the tower at 100%. Since the addition (or subtraction) of noise levels is logarithmic, cooling tower noise would have been at 50 dBA or slightly lower to result in the measured 53 dBA value.
Noise levels vary with fan speed according to equation 1:
(1) SPL= 30log(S)
Where S is fan speed
SPL is sound pressure level
During the noise measurements on May 25, the VFD was governing fan speed at 16%. Applying that value to equation 1 and the 50 dBA cooling tower noise level results in an overall noise level of 26 dBA. This noise level was far below the ambient noise level, so it was difficult to ascertain whether or not the cooling tower was on. Logarithmically adding 26 dBA and 52.1 dBA (ambient) results in 52.1 dBA which is in good agreement with the measured value of 53.6 dBA with the tower at 16%.
In conclusion, based on the testing and noise measurements, cooling tower noise levels are now at 50 dBA at the balcony location when the tower is at 100%. This value meets the nighttime Boston noise code. Further, because of the VFD, cooling tower noise levels drop considerably lower than 50 dBA when cooling demands are reduced, particularly at night.