PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

In situ measured facade sound insulation of wind turbine soundPontus THORSSON
Akustikverkstan AB, Lidköping, Sweden

ABSTRACTIn most countries there are regulations of wind turbine sound level outdoors at dwellings. Often there arealso regulations of the sound levels inside the dwelling, however not often directly aiming at wind turbine
sound. The sound level indoors from wind turbines has attracted more interest in the latest years, and thenespecially in the low frequency region (up to 200 Hz). Studies on the in situ sound level difference between
outside and inside of dwellings are however scarce. This paper presents the in situ measured sound level
difference for two Swedish houses in rural locations, both using a loudspeaker and using the wind turbine
sound as exciting signal. This is possible due to a 2 month long measurement series with simultaneous
sound recordings outside and inside. The sound pressure level differences from the two methods are shownto differ substantially.
Keywords: Wind turbine sound, sound insulation, low frequencies1. INTRODUCTIONWind turbines produce sound that, as all other community noise sources, can be perceived as
annoying and self-reported sleep issues [1,2]. Most countries have national regulations that limit thenoise exposure from wind turbine sound on the outside of dwellings. Some countries also have
explicit regulations on the sound levels indoor of dwellings, e g Denmark where there is a limit ofmaximum 20 dB(A) sound pressure level from wind turbines in the low frequency region, 10 – 160
Hz. The Danish regulations states that the low-frequency contribution shall be calculated using
previously measured sound pressure level differences that are given in the regulation text [3]. In theregulations from 2019 there are two different sound pressure level differences; one for regulardwellings, and one significantly lower sound pressure level difference for summer houses. Thesesound pressure level differences were measured in a number of Danish houses using a loudspeaker.
In Sweden there is no explicit regulation for wind turbine sound indoors of dwellings, and ageneral regulation for maximum sound pressure levels is instead used irrespective of sound source[4]. Those regulations state limits of A-weighted equivalent levels and linear equivalent levels in the
1/3 octave bands 31.5 – 200 Hz.
In the following, the sound pressure level difference measured using both loudspeaker and in situ
wind turbine sound is presented and compared to the Danish guidelines.

2. MEASUREMENT SERIES
A very long measurement series (roughly 2 months) was performed on two houses in Sweden.
The sound was recorded in audio quality during the whole series in order to ascertain that all
evaluated sound levels were coming from wind turbines only. The microphones were mounted on thefacade of each house on a flat measurement board, and equipped with double windscreens. No
high-pass filter was applied to the signal, and the sampling frequency used was 25.6 kHz thus
making it possible to evaluate sound pressure levels in the 1/3 octave bands up to at least 5 kHz.
Meteorological data was simultaneously recorded using a weather station mounted on a 10 m windmast. Thirty-five 10 minute long periods were chosen for the evaluation of the sound pressure level
differences. The periods were chosen subjectively for strong presence of wind turbine sound, mainlyidentified with perceived amplitude modulation.

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2.1 EVALUATION OF OUTDOOR SOUND LEVELS
Figure 1 shows the measured sound pressure level spectra in the 1/3 octave bands between 20 Hz
and 5 kHz for all periods on house 1. All evaluations have been made in the same way for bothhouses, but detailed measurement data is only presented for house 1. The presented spectra are themeasured levels, i e not corrected for the facade reflection. For the measurement periods with lowest
sound levels at high frequencies the noise floor of the measurement system was reached.
Outdoor microphone4555405035

Sound pressure level (dBA)

Sound pressure level (dB)

Outdoor microphone
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Figure 1 – Evaluated 1/3 octave band spectra outdoors for all measurement periods at house 1.
Left: unweighted with respect to frequency; Right: A-weighted levels.
From the data in Figure 1 it can be seen that the A-weighted total levels are in most cases
dominated by 1/3 octave bands below 1 kHz. This is due to the lowered sound power level from thewind turbines towards higher frequencies, and to air absorption. The high levels in somemeasurement periods at 1-2 kHz is caused by background disturbances, such as bird song or windhiss in nearby vegetation. This has been corroborated through listening to the recorded sound.
2.2 EVALUATION OF INDOOR SOUND LEVELS
The sound levels indoors have been evaluated through inspection of the variations of themeasured sound pressure level with time, and by subjective listening. In house 1, the measurement
room was not used and the background level was thus sufficiently low at most times to give relevant
indoor sound levels. The measured sound pressure level spectra for all measurement periods arepresented in Figure 2.
indoor microphone
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Indoor microphone
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4kFigure 2 – Evaluated 1/3 octave band spectra indoors for all measurement periods at house 1.
Left: unweighted with respect to frequency; Right: A-weighted levels.
Again, the limiting noise floor of the measurement equipment can be seen at the higher
frequencies. The measured indoor sound levels can be trusted up to the 630 Hz 1/3 octave band
because of the nosie floor.

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3. EVALUATION OF SOUND PRESSURE LEVEL DIFFERENCE
It is tempting just to subtract the levels in Figure 2 from the levels in Figure 1, but before that canbe done it is necessary to test if such a evaluation would give credible results. Here this is done byanalysing the correlation between outdoor and indoor levels in each 1/3 octave band. The result forthis is shown in Figure 3 for selected 1/3 octave bands.
20 Hz 1/3 octave band

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Figure 3 – Correlation between outdoor and indoor sound pressure levels in selected 1/3 octave bandfor all measurement periods at house 1. The x- and y-scales are equal in scale.
In the figures, blue stars represent levels evaluated through standard equivalent levels, i eremoving the portions that contain subjectively identified background disturbances and thenevaluating for the equivalent level. Red circles represent equivalent levels identified through theexistence of amplitude modulation in the A-weighted signal. The amplitude modulation is calculatedthrough∆𝐿!" =𝑝! + 𝑝!
𝑝! − 𝑝!

(1)

where p 0 is the steady-state RMS value of the sound pressure (corresponding to the equivalentlevel), and p f is the RMS value of the sound pressure at the blade pass-by frequency. Thisformulation comes from Lee et al [5].
The identification is made for all 10 second clips in each measurement period, and thecorresponding equivalent level (red circles) is evaluated from the identified 10 second clips.
It is clear in Figure 3 that there are differences from the methods for individual measurement
periods, but the overall trend and corresponding global results in each 1/3 octave band are very
similar. Note that the x- and y-scale have the same scaling in the graphs, so that a slope of 1 dB/dB
corresponds to a good correlation between indoor and outdoor levels. In the lower frequency bands
there is a clear correlation between outdoor and indoor levels, which shows that the indoor soundlevels are indeed coming from the outside. In general, the correlation in each 1/3 octave band should3828

be composed of two straight lines; one horizontal line that corresponds to the background level, andone sloped line that is caused by sound coming from outdoors. It has been validated that themeasurement periods are dominated by wind turbine noise, so it can be concluded that the indoor
sound levels are wind turbine sound up to 630 Hz. The sound pressure level differences for themeasurement periods are shown in Figure 4.

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Sound level difference ∆L (dB)

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Figure 4 – Sound pressure level difference for all measurement periods at house 1.
The grey area represents frequency bands where background noise dominates.

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Sound pressure level difference (dB)

Sound pressure level difference (dB)It can be argued that the higher values of the sound pressure level difference are more relevant,since background noise disturbances are more likely in the indoor measurement. Here the 70 % levelhas been chosen as relevant for the houses to not exaggerate the sound level difference. A final soundpressure level difference using this choice is shown in Figure 5 for both house 1 and house 2 in the1/3 octave bands between 31.5 and 200 Hz. These frequency bands are used in Swedish regulationsof sound pressure levels indoors.

2015105031,563125
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250

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Figure 5 – Sound pressure level differences for house 1 (left) and house 2 (right).
Red dashed line show the 70 % level.

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30302525201510Wind turbine noise
Loudspeaker
DK regular
DK summer house

5031,563
125
Frequency band (Hz)250

Sound pressure level difference (dB)

Sound pressure level difference (dB)

The facade sound insulation of both houses has also been measured using the loudspeaker methodin ISO 16283-3:2016. In those measurements a 15 inch loudspeaker was used for the 1/3 octave
bands from 125 Hz and upwards, and a high-power subwoofer was used for the 1/3 octave bands
from 20 to 125 Hz. The results from those measurements are shown in Figure 6 together with theresults in Figure 5, and with the Danish calculation values from [3]. In the figure it can be seen thatthe in situ measured sound pressure level differences using wind turbine noise as source isconsiderably lower in the low frequency range than both the loudspeaker measurements and thevalues given in the Danish regulations. For house 2 the in situ wind noise measurements are in
relatively close to both the loudspeaker measurements and the Danish values for summerhouses. The
difference between the measurements in house 1 and house 2 is that the measurement room in house
1 was on the upper floor, thus having sound transmission through the roof as well. It is clear thatusing measurement data from a loudspeaker measurement does not necessarily give representativevalues. Further studies on this is clearly needed.

201510Wind turbine noise
Loudspeaker
DK regular
DK summer house

5031,563
125
Frequency band (Hz)250

Figure 6 – Sound pressure level differences for house 1 (left) and house 2 (right).

4. CONCLUSIONS
It is shown here that it is possible to measure the sound pressure level difference for wind turbine
sounds in situ using the wind turbine as sound source. This is however only possible if it can beascertained that the sound on the outside is dominated by wind turbine noise in all relevant frequency
bands. From long measurement series it was shown here that the sound pressure level difference isacoustically relevant, e g it can be the dominating noise source, and the values can differ
significantly from measurements using loudspeakers. In one case presented here, the in situ
measured sound pressure levels were significantly lower than values that are available in theplanning stage of new wind turbines. More studies on relevant values for the sound insulation fromwind turbine sound are thus needed.

REFERENCES
1. Pedersen E Health aspects associated with wind turbine noise - results from three field studies. NoiseControl Engineering Journal 59(1):47-53, 2011.
2. Kageyama T, Yano T, Kuwano S, Sueoka S, and Tachibana H. Exposure-response relationship ofwindturbine noise with self-reported symptoms of sleep and health problems: A nationwide
socioacoustic survey in japan. Noise & Health , 18(81):53–61, 2016.
3. Miljøstyrelsen BEK nr 135 af 07/02/2019. Bekendtgørelse om støj fra vindmøller. Miljøstyrelsen,Denmark, 2019.
4. FoHMFS 2014:13. Folkhälsomyndighetens allmänna råd om buller inomhus. Folkhälsomyndigheten,Sweden, 2014.
5. Lee S, Kim K, Choi W, and Lee S. Annoyance caused by amplitude modulation of wind turbine noise.
Noise Control Engineering Journal 59(1):38–46, 2011.

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