The tonality is a notable factor during the development of wind farms. Tonality is a component of acoustic noise emission from the wind turbine which annoys the surrounding area of the wind farm. This paper focuses to evaluate the tonality of the wind turbine acoustic measurement data based on the standard IEC 61400-11 AMD 2018: ed. 3.1. The narrowband analysis of the wind turbine tonal noise component is fully evaluated as given in the standard. The identification of tone and classification of spectral lines is performed for all spectra of the specified wind speed bin. The results are obtained based on the IEC approach: masking level, tonality, and tonal audibility for the sample data of a specific wind speed bin. The audible tones are found for all spectra and reported in the results. The IEC method gives a good prospect of tonality assessment for wind turbine noise measurement from this result.
Wind energy is one of the harmless renewable sources of energy that attract worldwide energy industries. But the noise generates by the wind turbine during the energy generation is a more risk factor in the environment . Noise emission in wind turbines is a serious environmental issue and is highly considered and assessed in many countries. The limitations and guidelines are prepared and provided to the wind farm for noise assessment. The noise in wind turbines is produced from two sources: aerodynamic noise and mechanical noise. Due to the interaction between air and turbine, the aerodynamic noise is created and this is classified into turbulence noise, tonal noise, and airfoil noise . The tonal noise is one of the components that makes risk factors even at low levels. The residential surrounding environment of wind farms may affect due to the increasing annoyance of tonal components. In some countries, there is a penalty for high annoyance due to tonal components .
The international standard of ISO 1996-2:2017 explains the components for evaluation of tonal audibility using psychoacoustic principles  and the procedure for tonal analysis implies the energy ratio of a tonal component to masking noise with the masking index obtained from the test conducted repeatedly in the machine ISO/PAS 20065: 2016. The assessment of tones based on the same manner of the listener is provided in the NZ6808. It used the concept of Psychoacoustics in the critical band for assessing tonal components. This standard includes adjustments in tonality in the sound levels of the wind farm . Most of the wind farm complaints are the high annoyance caused by the tonal component. The complaints are found from the survey and the potential of tonal noise that annoyed is obtained and penalized. The extent of the tone appropriate to the penalty is imposed in BS 4142 and ESTU-97. The international electrotechnical commission for wind turbine generator systems gives the standard of acoustic noise measurement technique in part 11 (IEC61400-11). In this standard, the narrowband analysis is used to find the tones in the noise for different wind speeds . Evans et al  reviewed and compared the tonal noise assessment regulations used in Australia as well as internationally. In these methodologies, the penalty has been added to the measured spectrum when the peak frequency exceeded the criterion level. And also they studied the differences in regulations for tonal noise from different sources. Arana et al  studied the tonality assessment based on IEC 61400-11 and used this procedure to develop the algorithm to evaluate the tonal noise from wind turbines. Cooper et al  identified the frequency level of audible tones in the residence nearer to the wind farm using IEC 61400-11 assessment method. They resulted in the tonal frequency of 124HZ being audible in the larger and smaller wind speed conditions at night. Also, the wind speed and direction in which the audible tones evolved were assessed based on the standard. Kobayashi and Yokoyama  used the narrowband analysis in wind turbine noise for roughly estimating tonal audibility. The maximum tonal component level and average masking noise levels present in the critical band centered by the tonal component were compared in the roughly estimated tonal audibility applied in the FFT spectrum. Liu et al  evaluate the tonality by using the new method of Gabor filtering used for the separation and processing of noise in the sound from the wind turbine. They evaluate the noise that the procedure illustrated in the IEC standard.
The major objective of this paper is to evaluate the tonal components of IEC 61400-11 sample spectra of wind turbine noise. The narrowband analysis of wind turbine noise is based on the procedure of standard IEC61400 part 11 editions 3.1. By using this principle, the masking level, tonality, and tonal audibility of the 30 sample spectra are evaluated. This paper consists of a work methodology explaining the principle and method of evaluation in section 2. The noise measurement and tonality analysis method are explained in section 3. The results obtained from the analysis are discussed in section 4 and the summary of the future scope of this work is discussed in the conclusion of section 5.
II. WORK METHODOLOGY
The tonality analysis of the sample spectra for wind turbine noise measurement is evaluated in this paper. The evaluation of tonal components is based on the principle and procedures of the IEC 61400-11 standard. The A-weighted sample spectra of tonal noise are measured at the energy average period of 10s is considered in this paper. The narrowband analysis sample spectra at the specified bin center wind speeds are evaluated hence the frequency range for the analysis is limited from 20HZ to 11200Hz. And its frequency resolution for the spectra is 2Hz. In this sample analysis, 30 spectra are considered for tonality evaluation. The wind speed bin should contain a narrowband spectrum of not less than 6 identified tones of similar origin thus it determining the tonal audibility. If the tone has a similar origin, it is taken as a single tone. The detailed methodology of tonality analysis based on the IEC standards is given in the flow chart in fig.1.
The authors acknowledge the National Institute of Wind Energy (NIWE) under the Ministry of New and Renewable Energy, Government of India, Chennai, Tamilnadu, for extending technical support for this paper.
The evaluation of tonal components in the wind turbine noise measurement data was performed based on the principles and procedure of standard IEC61400-11 ed.3.1. the narrowband analysis procedure was applied for the entire tonality analysis. The possible tones and classification of spectral lines are explained for the first spectrum S01 of the specified wind speed bin. The tonal audibility of all the spectra of the wind speed bin was calculated and explained in the graphs. The obtained results were possible tone, tone frequency, critical bandwidth, energy average, masking level, tonality, and audibility. The tonality evaluation of the IEC approach was done in a 10s energy average period. The uncertainties in the IEC method may affect the wind turbine noise quality. The accuracy and effectiveness of the method were further studied as a future assessment.
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 Product data: pulse wind turbine sound power determination according to IEC 61400-11 Type-7914 https://www.bksv.com/media/doc/bp2322