The Fitzwilliam Substation in Fitzwilliam New Hampshire was producing tonal noise that was objectionable to neighbors. The town of Fitzwilliam contracted David Coate Consulting (DCC) to assess the severity of the noise issue and recommend appropriate mitigation measures. The photograph above shows the cantilevered sound wall (during construction) designed by David Coate Consulting and Burns & McDonnell Engineering.
Figure 1 shows tonal substation noise measured (pre-sound wall) at a neighbor's house in the vicinity of the substation. The tonal peaks are as much as 15 dB higher than sound pressure levels at adjacent frequency bands. The Environmental Protection Agency (EPA) defines “Pure Tone” as the following: “Any sound which can be distinctly heard as a single pitch or a set of single pitches. For the purposes of this ordinance, a pure tone shall exist if the one-third octave band sound pressure level in the band with the tone exceeds the arithmetic average of the sound pressure levels of the two contiguous one-third octave bands by 5 dB for center frequencies of 500 Hz and above and by 8 dB for center frequencies between 160 Hz and 400 Hz and by 15 dB for center frequencies less than or equal to 125 Hz.” (US EPA Model Noise Ordinance, 1979). Using the 2:20 am data as an example and performing the previously cited calculation, the 250 Hz tone exceeds the arithmetic average of its adjacent bands by 12 dB, well in excess of the 8 dB EPA pure tone definition.
Figure 2 shows CADNA modeling of a three-sided sound wall and the shielding effects that would benefit residents adversely impacted by transformer noise.
Pre- and post sound wall noise measurements by the acoustical consultants showed that the actual noise reduction of the wall was approximately 10 dBA at affected residences- in agreement with the CADNA modeling. The large reduction in transformer noise afforded by the sound wall has resulted in acceptable noise levels for residents in the area.