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Nov 09, 2022 353 likes •464 views

23rd International Congress on Acoustics A simplified A simplified method method for for determination determination of of amplitude amplitude modulation of modulation of audible audible and and inaudible inaudible wind

SLIDE 1

A A simplified simplified method method for for determination determination of

f “amplitude

“amplitude modulation” modulation” of

f audible

audible and and inaudible inaudible wind wind turbine turbine noise noise

Steven Cooper

23rd International Congress on Acoustics

SLIDE 2

Cape Bridgewater Windfarm Study – Modulation

f A-weighted level for different power settings

SLIDE 3

Simultaneous Inside and Outside Measurements – Leonards Hill Wind Farm (Victoria, Australia)

Courtesy of L Huson & Associates

SLIDE 4

FFT (0-50 Hz, 400 lines)

5 10 15 20 25 30 35 40 45 50 [Hz] 20 30 40 50 60 70 80 90 100 [dB/20u Pa] centre upwind (Real) centre downwind (Real) 5 10 15 20 25 30 35 40 45 50 [Hz] 20 30 40 50 60 70 80 90 100 [dB/20u Pa] centre upwind (Real) downwind centre (Real)

50% power 14% power

SLIDE 5

CBW Turbine 13 at 50% power

FFT vs Time (0-50 Hz, 400 lines) FFT (0-50 Hz, 400 lines) 1/3 Octave Bands (A-weighted)

SLIDE 6

Manual Detection of AM

Some methods have used A-weighted levels or peaks in the A-weighted 1/3
ctave band spectra to then be subject to assessment in 10 second blocks
f time for which the median of the maximum and minimum over a 10

minute sample is used to determine the AM

Other proposals have nominated the use of an FFT analysis of the blade

pass frequency but without due consideration of the BT = 1 issue

Based upon a modified Den Brook Methodology the use of 1/3 octave as

described above have been used for comparison purposes

On the basis of an A-weighted spectrum, then the criticism of non-

compliance with BT = 1 is reduced because of the attenuation of the A- weighting filter

SLIDE 7

25 Hz 1/3 octave band time trace and movie plot of turbine

25 Hz 1/3 Octave Band Time Splice CPB vs. Time (1/3 octave bands)

SLIDE 8

Modulation Indices derived from alternative method (A-weighted spectra)

SLIDE 9

Modulation Indices derived from alternative method (linear spectra)

SLIDE 10