West site

The West monitoring site, estimated to be 2.5 km away from the nearest wind turbine generator (turbine BH), was equipped with two long-term B&K monitoring stations to measure both inside and outside noise (Appendix B). The microphone used was a B&K type 4952 outdoor microphone and a weather station was established at 4 metres above ground level. For this site, wind direction between 53˚–143˚ (98˚±45˚) was considered downwind from the nearest turbine. There were tall trees and other vegetation in the areas adjacent to the house and the site was found to be exposed to frequent high wind speeds.

In the absence of pre-construction background data, a 40dB(A) default criterion was applied at this site for wind turbine generator wind speeds at cut-in and rated power conditions.

The house was occupied most of the time during the monitoring period. Equipment for the indoor measurements was withdrawn from the site before the end of the monitoring period at the owner’s request.

Data analysis

The inside microphone was placed within an empty bedroom next to a window facing the wind farm. The room was still used by the residents, but only rarely; so it provided a good site for monitoring indoor noise with reduced ambient noise and noise from other sources. Throughout the study, there were 5,347 valid data points, of which 1,451 pairs of internal records were available under downwind conditions.

The outside microphone was about 10 m to the south of the house, and was surrounded by a few trees and other vegetation in the garden of the residence. After elimination of noise from other sources, 2,859 valid data points remained, of which 464 were recorded during downwind conditions.

A-weighted and C-weighted levels

Where not affected by other noise sources, indoor A-weighted sound pressure levels were typically low (refer to Figure 26a and b). C-weighted levels were also low and but exceeded the adopted 60dB(C) limit occasionally at high local or wind turbine generator wind speeds (refer to Figure 26c and d).  

Figure 26        A-weighted and C-weighted levels versus local and wind turbine generator wind speed, inside the house at the West site

Statistical trends for A-weighted levels were typically higher for measurements outside the house than inside (Figure 27a and b). The simple low frequency criterion of 60dB(C) was exceeded for a large fraction of 10-minute intervals.

A-weighted and C-weighted descriptors were well correlated with local wind speeds (Table 14), possibly reflecting contributions from the surrounding trees and vegetation.

Total measured noise (without correction for background) met the baseline criterion of 40dB(A) in Figure 28b for the range of environmental conditions experienced during the study.

Figure 27        A-weighted and C-weighted levels versus local and wind turbine generator wind speed, outside at the West site

Table 14         Coefficient of determination (R²) for polynomial best fits of the acousticdescriptors versus wind turbine generator and local wind speeds, at the West site

Descriptor	vs wind turbine generator wind speed		vs local wind speed
	General	Downwind	General	Downwind
LAF90	0.248	0.444	0.711	0.683
LCeq (Inside)	0.215	0.650	0.586	0.678
LCeq (Outside)	0.193	0.482	0.794	0.705

 

Figure 28        Statistical descriptor L_AF90 versus (a) local and (b) wind turbine generator wind speeds, outside at the West site

 

Low frequency noise

Graphs of recommended Danish EPA low frequency noise descriptors against wind speeds at the nearest turbine and local wind speeds demonstrate increasing trends (Figure 29a and b). A saturation zone is observed at high wind turbine generator wind speeds for the difference in C-weighted and A-weighted levels at high wind speeds (Figure 29d). Levels exceeding Danish EPA criterion of 20dB(A) and L_Ceq-L_Aeq differences above 15–20dB(A) are common for this location.

The increasing trend is also notable for the outdoor L_pA,LF measurements (Figure 30a and b). From a statistical perspective, magnitudes outside of the house were higher than those recorded inside. Trends of the approximating curves for the C-weighted and A-weighted level differences outside were not similar to those inside the house. This suggests the presence of other mechanisms generating noise at higher frequencies; and possible influence of the house noise attenuation which becomes more effective at higher frequencies.

Relatively high magnitudes of low frequency noise descriptors for low wind turbine generator wind speeds, combined with high data scatter suggests that the wind farm may not be controlling low frequency impact at the location.

Figure 29        Low frequency noise descriptors versus local and wind turbine generator wind speed, inside at the West site

Figure 30        Low frequency noise descriptors versus local and wind turbine generator wind speed, outside at the West site

 

Shutdown period analysis

The organised shutdowns covered a range of environmental conditions (refer to Table 15). Influences of obvious noises from other sources during the shutdown periods for outdoor measurements were negligible. Since the internal measurement system had been withdrawn earlier, only one shutdown period was available (Shutdown 1: Downwind). Rather than decreased A-weighted levels; these were marginally higher during the shutdown (Table 16).

L_{pA, LF} levels were below the conservative 20dB(A) criterion in both of the cases. C-weighted levels can be considered as marginally higher during the operation period; however they were still below 60dB(C) by a significant margin. DEFRA low frequency noise criteria were satisfied for a lesser fraction of the time during operation (80%) compared with the shutdown period (100%). It should be noted that local wind speeds for the comparative operating period were significantly higher. This may have increased background noise level significantly.

Periods before and after the shutdown were characterised by negligible changes in A-weighted levels and notable increase in C-weighted magnitudes, although they were still significantly less than the 60dB(C) limit (Table 17). There were significant variations in L_{pA, LF} levels during the shutdown, but the upper limit of the range was marginally higher for the operating periods. It should be noted that the shutdown was characterised by wind speeds at the nearest turbine that were about 1 m/s lower than those measured within the adjacent operating periods. Lower wind speeds may have influenced these differences; and some local sources may also have contributed. In part, analysis of outdoor data during similar periods confirmed this view

Outdoor measurements indicated that values of the acoustic descriptors, including the low frequency noise parameters, were generally higher during the shutdown periods (Table 18). This may indicate that overall noise levels were not controlled by the wind farm during the study; and differences between them were too small to be detected by comparison of similar periods.

Table 15         Wind speed and direction for shutdown and comparative operation periods, at the West site

General conditions	Local wind		Wind turbine generator
	Wind speed, m/s	Wind direction, deg	Wind speed, m/s	Wind direction, deg
Shutdown
Shutdown 1: Downwind	1.9–2.3	155–170	8.2–9.1	131–135
Shutdown 2: Crosswind 1	3.6–3.9	60–62	9.3–10.3	26–29
Shutdown 3: Upwind 1	2.5–3.7	321–329	8.0–9.3	302–307
Shutdown 4: Upwind 2	0.8–2.7	338–11	10. –11.0	301–303
Shutdown 5: Crosswind 2	1.3–2.5	203–209	6.7–9.0	209– 217
Shutdown 6: Crosswind 3	2.0–2.0	184–190	5.8– 6.6	181–184
Operational (indoor)
Comparison 1: Downwind	4.4–6.2	122–135	7.9–9.1	123–134
Operational (outdoor)
Comparison 1: Downwind	1.3–2.7	184–204	7.9–9.1	148–151
Comparison 2: Crosswind 1	1.6–1.9	47–58	9.5–10.5	14–18
Comparison 3: Upwind 1	2.8–3.7	329–34	8.5–8.9	303–310
Comparison 4: Upwind 2	1.3–1.7	339–348	10.1–11.4	311–314
Comparison 5: Crosswind 2	0.9–2.4	177–211	8.1–8.6	222–223
Comparison 6: Crosswind 3	1.0–1.4	169–184	5.4–6.6	191–199

 

Periods adjacent to Shutdowns 5 and 6 (Crosswind 2 and Crosswind 3) were not affected by noises from other sources and a summary of wind and acoustic parameters for them can be found in Table 19. Generally the low frequency descriptors are greater during the adjacent operation periods. It should be noted that both the wind speeds at the nearest turbine and the local wind speeds were higher during the wind farm operation.

Only one shutdown period is available for measurements inside of the West site. Spectral content averaged over the shutdown periods shows decrease of low frequency content (up to 250Hz) compared to a similar operational period not affected by other noise sources (Figure 31a). There is a local maximum at 50Hz which is unlikely to add a tonal character to the noise. Increases in the low frequency content can be also seen by comparison of the averaged spectra for the periods adjacent to the shutdown (Figure 31b).

Table 16         Comparison of acoustical descriptors for the shutdown and similar period, inside the house at the West site

Shutdown period	Inside
	LAeq, dB(C))	LCeq, dB(C)	LpA, LF, dB(A)	DEFRA
Shutdown
Shutdown 1: Downwind	25–29	37–47	8–20	100%
Operational
Comparison 1: Downwind	24–26	46–48	16–19	80%

 

Table 17         Comparison of acoustic descriptors for shutdown and adjacent periods, inside the house at the West Site

Shutdown period 1	Descriptors (inside)
	LAeq, dB(A)	LCeq, dB(C)	LpA, LF, dB(A)	DEFRA
Before	25–27	46– 50	17–21	0%
Shutdown	25–29	37–47	8–20	100%
After	26–28	51–53	22–23	0%

 

The variability of levels is different for measurements outside the house. Spectral components for the shutdown periods (measured outside) are consistently higher for almost the entire frequency range and wind conditions (Figure 32). Adjacent periods for Shutdown 5 and 6 were not affected by noises from other sources and the relevant spectra are presented in Figure 33. Low frequency components are greater for the operating periods adjacent to Shutdown Crosswind 2, however this change is not obvious for Shutdown Crosswind 3. As mentioned above, this effect may be caused by differences in wind speeds before and after the shutdown or possible influences of other noise sources.

Comparisons of adjacent shutdown and operating periods (Figure 33) indicated that spectral components for the shutdowns sometimes exceeded the magnitudes of records acquired during the electricity generating modes; which means that contributions from the wind farm at the measurement site were insignificant and the noise levels were controlled by other sources.

Table 18         Comparison of acoustical descriptors for the shutdown and similar period, outside at the West site

Shutdown period	Outside
	LAeq, dB(A)	LCeq, dB(C)	LAF,90 dB(A)	LA, LF ,dB(A)		DEFRA
Shutdown
Shutdown 1: Downwind	43–49	74–78	37–44	38–45	0%
Shutdown 2: Crosswind 1	44–48	61–64	41–45	30–33	0%
Shutdown 3: Upwind 1	40–45	61–66	36–41	31–37	0%
Shutdown 4: Upwind 2	24–33	34–53	22–28	8–21	100%
Shutdown 5: Crosswind 2	30–36	43–53	27–33	18–22	100%
Shutdown 6: Crosswind 3	24–26	54–56	23	17–19	100%
Operational
Shutdown 1: Downwind	27–29	52–54	27–28	22–23	100%
Shutdown 2: Crosswind 1	34–35	46–49	32–33	20–21	100%
Shutdown 3: Upwind 1	34–42	56–65	30–37	27–36	0%
Shutdown 4: Upwind 2	23–25	46–47	22–24	14–17	100%
Shutdown 5: Crosswind 2	23	47–47	22	14–15	100%
Shutdown 6: Crosswind 3	22–24	46–48	22	14–17	100%

 

(a)

(b)

Figure 31       Unweighted spectra for shutdowns under downwind conditions and adjacent operating periods, inside the house at the West site

 

Table 19         Comparison of acoustic descriptors for shutdown and adjacent periods, outside at the West site

Shutdown	Period	Descriptors (outside)
		LAeq, dB(A)	LCeq, dB(C)	LpA, LF, dB(A)	DEFRA
Shutdown 5: Crosswind 2	Before	29–32	57–62	24–27	0%
	Shutdown	30–35	39–53	16–22	100%
	After	28–33	53–61	21–26	0%
Shutdown 6: Crosswind 3	Before	32–33	62	28–29	0%
	Shutdown	24–26	54–56	17–19	100%
	After	26–27	50–53	22–24	0%

 

Figure 32        Unweighted spectra for shutdowns and similar operating conditions, outside at the West site

(a)

(b)