January 30, 2003
by Paul Gipe
Both versions of the AirX monitored at the Wulf Test were qualitatively and quantitatively quieter than the Air 403.
Disclosure: All AirXs tested and a Whisper H40 were provided by Southwest Windpower in lieu of payment for noise measurements by Paul Gipe.
Southwest Windpower’s Air 403 was notoriously noisy. While not the noisiest wind turbine ever made, its widespread distribution caused an unusual number of noise complaints.
In late 2001 Southwest Windpower asked me to measure the noise from a preproduction version of the AirX. Beginning in 2002 and through the first half of 2002, I conducted a series of noise measurements on two versions of the AirX. For lack of a better descriptor I called them AirX.1 and AirX.2. I also measured the power curve on a third version, the AirX.3 but I have not made any noise measurements on this unit.
I’ve withheld publishing the data collected until now for several reasons. First, I had other obligations. Second, I considered the versions I tested defective products and returned them to the manufacture for correction. They were defective because they failed to meet the manufacturer’s power curve. While the noise measurements on these units were complete, the fact that the product would likely be modified by the manufacturer to correct the power curve defect suggested that the final production version would not be represented by my tests.
The final version I received, the AirX.3, operated similarly to the AirX.2 and the noise emissions are likely comparable. Further, Southwest Windpower has been shipping various versions of the AirX for more than one year. I have no idea how my versions compare with current production.
For a complete description of the rationale for testing, the methods employed, the terminology used, and my test equipment, see Noise from Small Wind Turbines: An Unaddressed Issue.
Critical to understanding the following data is the difference between sound pressure levels of ambient noise, and noise with the turbine operating. It is the difference between ambient and the turbine noise plus ambient that determines the noisiness of the wind turbine. The difference determines the noise from the turbine at a specified distance from the turbine. This calculation in sound pressure level is then used to arrive at the source emission strength in sound power level (LWA). The latter is the noise at the source, the wind turbine. This value can be used to project noise at varying distances from the turbine and can be used to compare one wind turbine with another.
To recapitulate earlier results on the Air 403, in winds from 8-10 m/s (18-22 mph), the Air 403 would emit an annoying “buzz” most of the time. The Air 403 sound power level or emission source strength (LWA) was 88 dBA at 8 m/s and 91 dBA at 10 m/s.
The AirX is so significantly quieter than the Air 403 that it was difficult to measure a sufficient difference between ambient and turbine plus ambient in wind speeds greater than 18 mph (8 m/s) to calculate valid values for sound power levels above this speed. In the accompanying chart for the SPL Summary for the AirX.1, the ambient noise can be seen rising steeply with increasing wind speed (wind noise in nearby willows) while the noise from the AirX rises less rapidly.
As seen in the second chart, Calculated Emission Source Strength, both the AirX.1 and the AirX.2 emitted about the same amount of noise. The sound power level for the AirX.1 and AirX.2 at 8 m/s (18 mph) is 80 dBA or 8 dBA quieter than the Air 403.
The AirX accomplishes this by stalling the rotor or turning the turbine off so the blades don’t begin to flutter. This is a very effective method for reducing the noise emitted by the AirX, but it does so at a cost. The AirX fails to meet its power curve by a wide margin.