This letter, submitted to the Massachusetts Department of Environmental Protection (MA DEP) details the appropriate methodology for surveying turbine noise post-construction. The procedure was developed under the guidance of acoustician, Richard James of E-coustics Solutions. The criteria for compliance is specific to the Massachusetts state law regarding noise and will differ from other jurisdictions. However, the procedure should be consistent for all noise surveys. The full report can be accessed by clicking on the link(s) at the bottom of the page.
This letter, submitted to the Massachusetts Department of Environmental Protection (MA DEP) details the appropriate methodology for surveying turbine noise post-construction. The procedure was developed under the guidance of acoustician, Richard James of E-coustics Solutions. The criteria for compliance is specific to the Massachusetts state law regarding noise and will differ from other jurisdictions. However, the procedure should be consistent for all noise surveys. The full report can be accessed by clicking on the link(s) at the bottom of the page.
Noise Test Procedure for Industrial Wind Turbines
1) Intent: To accurately identify the actual acoustical noise levels emanating from the industrial wind turbine, including infrasound, low frequency sound and Aerodynamic Amplitude Modulation ("AAM") levels. This acoustical analysis will be performed by an independent acoustical professional utilizing acceptable industry procedure especially ANSI S12.18 Procedures for Outdoor Measurement of SPL and ANSI S12.9 Part 3- Short Term Measurements with Observer Present.
2) Purpose: To determine if the industrial wind turbine complies with the noise limits stipulated in the MaDEP, Section 310 CMR 7.10 Noise (which were adopted by Scituate). Noise compliance will be determined as follows:
a) Continuously measure the noise level time history of the wind turbine(s) operating at full-load (or greatest noise emissions if different from full load) for the wind speed and other meteorological condition(s) identified in any complaints (or during calm surface winds (<5mph) in the absence of a complaint) through the turbine shut down sequence to full stop. Continue the measurement during the full stop condition for at least 10 minutes to measure the background sound level. From this measurement determine the background sound level (LA90) for the wind turbine "off" condition and the average and maximum sound levels (LAeq and LAmax) for the wind turbine "on" condition. If necessary, the background sound level data shall be selected to be free of short duration sounds, wind induced noise, insect noise and other sounds that are not considered to be part of the long term background sound level as defined in ANSI S12.9 Part 3 5.4 long term background sound.
b) Inspect measurements for Pure tones by comparing the average of the adjacent octave band sound pressure levels, if the tone is steady over time, and the background sound pressure levels (assuming the sound in the adjacent octave bands is not fluctuating). If the tone or background sounds are fluctuating, more advanced tests for tones covered in ANSI standards should be applied.
3) Criteria: A noise source will be considered to be violating the Department's noise regulation (310 CMR 7.10) if the source:
a) Increases the broadband sound level by more than 10 dB(A) above ambient (e.g. the background sound level as LA90), or
b) Produce a "pure tone" condition - when any octave-band center frequency sound pressure level exceeds the two adjacent center frequency sound pressure levels by 3 decibels or more.
These criteria are measured both at the property line and at the nearest inhabited residence.
"Ambient" is defined as the background A-weighted sound level that is exceeded 90% of the time, measured during the quietest time that occurs during normal equipment operating hours. Ambient" may also be established by other means with consent of the Department.
4) Instruments: Acoustic instruments shall comply with the American National Standards Institute (ANSI) or equivalent international standard. The sound level meter shall be capable of simultaneously measuring and storing un-weighted octave-bands (8 to 8kHz) and over-all sound levels in dBA, dBC and dBLin, (or dBZ) in intervals no longer than 0.100 to 0.125 seconds.
Sound level meters shall comply with the latest version of ANSI S1.4-1983 requirements for Class 1 (or Type 1) precision. Acoustic calibrators shall comply with ANSI S1.4-1984 for Class 1 (or Type 1) precision. Each microphone used for outdoor measurements shall be fitted with a windscreen appropriate for wind speeds at the microphone. Sound level meters used for pure tone analysis shall comply with ANSI (or equivalent) requirements for octave band center frequencies.
5) Calibration: Acoustic instruments used for measurements shall have current calibration certificates from an independent laboratory, which are traceable to a measurement standard established by the National Institute of Standards and Technology (NIST). The sound level meter's measurement accuracy shall be confirmed with an acoustic calibrator mounted over the microphone prior to commencement of the acoustical test.
6) Weather Conditions: This test is required to have moderate to strong wind speeds at the hub (12 to 18 meters per second, 27 to 40 mph) and calm to light (0 to 2.2 m/s, 0 to 5 mph) at ground level with no precipitation.
7) Measurements: The sound meter or microphone shall be mounted on a tripod approximately 5 feet above the ground. The tripod shall be positioned to avoid reflections; at least 25 feet away from all large vertical surfaces, (buildings, walls, and solid fences), 25 feet or more away from vegetation including trees that may produce leaf rustle, and at least 5 feet away from smaller surfaces (poles, posts and tree trunks) to comply with ANSI S12.9-1993/Part3).
8) Observations: During measurement periods, acoustic observations are required to identify all audible noise sources with and without the noise source under test operating. Other required comments are; date & time, weather conditions; ground level wind speed & direction).
9) Wind Turbine Owner: The wind turbine operator shall provide critical information about wind turbine operation during the testing period. The purpose of this requirement is to verify that the wind turbine controls are set for normal operating mode, and to document measurements of wind and operating conditions important to interpreting the sound measurements. This information may be obtained from the historical SCADA data in a resolution no greater than one minute increments showing wind speed at hub & direction, nacelle orientation, blade pitch setting(s), hub rpm, and generator output or load.
10) Instrument Setup: Prior to the first measurement, the sound level meter shall be verified with the acoustic calibrator and at the completion of all sound measurements. The sound level meters shall be configured to measure and store un-weighted octave bands sould pressure levels (8Hz to 8kHz minimum) plus over-all dBA, dBC and dBLin, (or dBZ if dBLin not available) sound levels at intervals no longer than 0.125 seconds (e.g "Fast" setting). Due to the potential for strong wind turbine generated aerodynamic amplitude modulation; the sound meter response shall be set to fast. The information collected during the measurements shall be reported in total along with the final values for the LAMax and LA90 compliance metrics. This will provide for peer review and archival documentation should the need arise.
11) Test Locations: Nearest three to five residences, with a clear line-of-sight view of the wind turbine, ranging from about 600 feet to 4,000 feet.
12) Test Procedure: With the wind turbine operating start the sound meter measurements and continue for 10 minutes, then shut the turbine off and continue for 10 minutes. Repeat this procedure at each measurement location to determine the maximum sound level emanating from the industrial wind turbine.
