WO2021089095A1

WO2021089095A1 - Masking tonal noise from a wind turbine

Info

Publication number: WO2021089095A1
Application number: PCT/DK2020/050299
Authority: WO
Inventors: Mranal GUPTA; Luis Daniel COSTA GONCALVES SILVA CRUZ; Kaj Dam MADSEN
Original assignee: Vestas Wind Systems A/S
Priority date: 2019-11-04
Filing date: 2020-11-04
Publication date: 2021-05-14

Abstract

A method of masking tonal noise from a wind turbine. A background masking noise level is obtained, which is indicative of a level of background masking noise which partially masks the tonal noise. An additional masking noise level is calculated on the basis of the background masking noise level and a target total masking noise level. Additional masking noise is generated which further masks the tonal noise. A level of the additional masking noise is based on the calculated additional masking noise level. The level of the additional masking noise target is controlled in accordance with a predefined total masking noise level, and as a result the total masking noise level may be prevented from becoming undesirably high.

Description

MASKING TONAL NOISE FROM A WIND TURBINE

FIELD OF THE INVENTION

The present invention relates to a method, and associated apparatus, for masking tonal noise from a wind turbine.

BACKGROUND OF THE INVENTION

A known method of masking pure tones within sound from a noise generating source is described in US 2010/0272285. A sound exhibits a pure tone when the sound pressure in a given one-third octave band is at least five decibels above the sound level in each of two adjacent one-third octave bands. One or more masking sounds are generated which are capable of masking only the pure tones.

A problem with US 2010/0272285 is that the masking sounds may have a high energy which results in an undesirably high total sound level. This may cause a violation of regulations. It may also cause psycho-acoustic annoyance since the sound may not be natural after the additional of the masking sound, especially if the method is used to mask tones with high amplitude.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a method of masking tonal noise from a wind turbine, the method comprising: obtaining a background masking noise level which is indicative of a level of background masking noise which partially masks the tonal noise; calculating an additional masking noise level on the basis of the background masking noise level and a target total masking noise level; and generating additional masking noise which further masks the tonal noise, wherein a level of the additional masking noise is based on the calculated additional masking noise level.

A second aspect of the invention provides apparatus for masking tonal noise from a wind turbine, the apparatus comprising: a masking noise source; and a masking noise control system configured to obtain a background masking noise level which is indicative of a level of background masking noise which partially masks the tonal noise, calculate an additional masking noise level on the basis of the background masking noise level and a target total masking noise level; and instruct the masking noise source to generate additional masking noise which further masks the tonal noise, wherein a level of the additional masking noise is based on the calculated additional masking noise level.

The present invention controls the level of the additional masking noise target in accordance with a predefined total masking noise level. As a result, the total masking noise level may be prevented from becoming undesirably high.

The use of a target total masking noise level may also enable the total masking noise level to be accurately controlled, reducing uncertainty during the operation of the wind turbine. This reduced uncertainty enables components of the wind turbine to be designed on the basis of the target total masking noise level.

The additional masking noise level may be calculated on the basis of a difference between the target total masking noise level and the background masking noise level.

The method may further comprise determining a parameter (such as an operating condition of the wind turbine, a wind condition or a tone line level) and setting the target total masking noise level on the basis of the parameter.

The method may further comprise determining an operating condition of the wind turbine (for instance power, rotor speed, turbine speed, blade pitch, or an operating condition of an auxiliary such as a cooling fan) and setting the target total masking noise level on the basis of the operating condition of the wind turbine.

Alternatively, the target total masking noise level may be fixed, or it may vary on the basis of some other parameter such as wind speed or tone line level.

Optionally the target total masking noise level plateaus at high turbine speeds.

The method may further comprise identifying a frequency of the tonal noise.

The obtained background masking noise level may be indicative of an energy level of background masking noise summed over a pair of critical bands on either side of the frequency of the tonal noise. Alternatively, the obtained background masking noise level may be indicative of an energy level of background masking noise summed over a single critical band including the frequency of the tonal noise.

The background masking noise level may be a masking noise level according to the International Standard IEC 61400-11 , Edition 3, section 9.5.6, pg 37. Alternatively, the background masking noise level may be determined according to another standard, or in a non-standard way.

An energy level of the additional masking noise summed over the pair of critical bands may be based on the calculated additional masking noise level. For instance, the energy level may equal the additional masking noise level.

The background masking noise level may be obtained by estimation on the basis of a time of year, for instance the season. This enables seasonal variations to be taken into account.

The background masking noise level may be obtained by estimation on the basis of an operating condition of the wind turbine (for instance rotor speed and/or blade pitch and/or an operating condition of an auxiliary such as a cooling fan).

The background masking noise level may be obtained by estimation on the basis of a wind condition, such as wind speed.

The estimate of the background masking noise level may be obtained by a combination of two or more of the parameters mentioned above. For example, it may be obtained by estimation on the basis of a time of year and on the basis of an operating condition of the wind turbine.

The background masking noise level may also be obtained by measuring sound in a vicinity of the wind turbine with a sensor - either as an alternative to estimation, or as part of such an estimation.

Optionally the background masking noise level is obtained by simulation.

The additional masking noise may be generated by a loudspeaker and/or by one or more further sources. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 shows a wind turbine system with a loudspeaker for generating additional masking noise;

Figure 2 shows a sound pressure time series recorded by the microphone;

Figure 3 shows a sound pressure level spectrum for a 10s time period, including tonal noise;

Figure 4 shows tonal noise in the spectrum of Figure 3, and background masking noise in a pair of critical bands;

Figure 5 shows the spectrum of Figure 4 with additional masking noise added;

Figure 6 shows background masking noise energy levels and a pair of target masking level curves;

Figure 7 shows the spread of the background masking energy;

Figure 8 is a pair of graphs contrasting high masking and low masking;

Figure 9 shows the graphs of Figure 8 with tonal noise added;

Figure 10 shows a low background masking noise with a high additional masking noise; Figure 11 shows total masking based on Figure 10;

Figure 12 shows a high background masking noise with a low additional masking noise; and

Figure 13 shows total masking based on Figure 12.

DETAILED DESCRIPTION OF EMBODIMENT(S)

Figure 1 shows a wind turbine system including a horizontal axis wind turbine 1. The wind turbine 1 comprises a tower 2 supporting a nacelle 4 to which a rotor is mounted. The rotor comprises a plurality of wind turbine blades 3 that extend radially from a central hub. In this example, the rotor comprises three blades 3.

The system includes an International Electrotechnical Commission (IEC) microphone 5 which records a time history of sound pressure as shown in Figure 2. A ten second period 10 of this time history is shown in the lower part of Figure 2. Figure 3 is a spectrum of sound pressure versus frequency, presented as a bar graph with each bar representing sound pressure level (in dB(A)) in a respective frequency range. The range of each bar may be the same for each bar, or each bar may represent a one-third octave band. Typically, each bar covers a range of 1-2 Hz, although for ease of illustration Figure 3 gives a lower resolution example in which each bar represents a range of the order of 10 Hz.

The spectrum of Figure 3 is based on the ten second time period 10 from Figure 2 and includes a tonal noise 20 - i.e. a pure tone which occupies a narrow frequency range. If each bar represents a 10 Hz range, then the tonal noise 20 in this example may be in the 300-31 OHz frequency range. Figure 4 shows the tonal noise 20, and background masking noise 21 , 22 in a pair of critical bands 21a, 22a on either side of the frequency of the tonal noise 20. In this example the critical bands 21a, 21b each have a range of about 30 Hz, but in other embodiments the widths of the critical bands 21a, 21b may be higher or lower.

The background masking noise 21 , 22 only partially masks the tonal noise 20, so the tonal noise 20 may be annoying.

The background masking noise 21, 22 may originate from a number of sources, including aero-acoustic noise from the rotor, ambient noise (for example seasonal noise from flora and fauna), or noise from wind turbine auxiliaries such as cooling fans.

The level of the background masking noise 21, 22 in the critical bands 21a, 21b can thus vary depending on a number of factors, including wind speed, turbine operating conditions, time of year and so on. Figure 6 is a graph illustrating how the level of the background masking noise 21, 22 in the critical bands 21a, 21b can vary with respect to turbine speed (in rpm).

Each data point on Figure 6 represents an energy level (in dB(A)) obtained by summing the six bars in Figure 4 associated with the background masking noise 21, 22. This gives an indication of the energy or power level of the background masking noise in the critical bands 21a, 21b. Note that the energy level in the 10Hz range containing the tonal noise 20 may be excluded from the sum. The masking energy levels shown in Figure 6 may be determined according to the International Standard IEC 61400-11 Ed.3, section 9.5.6, pg 37. Alternatively, the masking energy levels may be determined in any other suitable way.

Two exemplary data points 30, 31 are labelled in Figures 6 and 7, each associated with the same turbine speed S. The data point 30 has a higher energy level than the data point 31. As explained above, the difference in these energy levels may be due to a number of factors including operating conditions of the wind turbine (for instance blade pitch angle, time of year and so on) and wind conditions such as wind speed.

As shown in Figure 7, there is a large spread in the energy level of the background masking noise, generally varying between an upper boundary 34 and a lower boundary 35. This spread has a significant effect on the masking effect as shown in Figures 8 and 9.

Figure 8 contrasts a spectrum of a low background masking noise 40 (which peaks at a relatively low sound pressure level 42) with a spectrum of a high background masking noise 41 (which peaks at a relatively high sound pressure level 43). Figure 9 adds a tonal noise 50 which peaks at a level 51. As can be seen in Figure 9, the tonal noise 50 is masked more by the high background masking noise 41 than by the low background masking noise 40.

A method of masking tonal noise will now be described with reference to Figures 4 and 5.

First, a frequency of the tonal noise 20 is identified. The tone may be a stationary tone, or its frequency may vary depending on turbine speed.

Next a background masking noise level is obtained, which is indicative of an energy level of the background masking noise 21 , 22 summed over the critical bands 21a, 22a adjacent to the tonal noise 20 on either side.

The background masking noise level may be obtained by estimation on the basis of a time of year and on the basis of various operating conditions of the wind turbine. For example, the data point 30 may be obtained on the basis of a known rotor speed, pitch angle, and time of year. This may be achieved by inputting the time of year and operating conditions into a look-up-table, or by inputting them into a theoretical model which obtains the background masking noise level by simulation.

Alternatively, the background masking noise level may be obtained by measuring sound in a vicinity of the wind turbine with a sensor, for instance the microphone 5.

Next, an additional masking noise level is obtained on the basis of the background masking noise level and a target total masking noise level. Figures 6 and 7 show a target curve 32 indicating how the target total masking level may increase with turbine speed. Optionally the target total masking noise level may plateau at high turbine speeds, as indicated by an alternative target curve 33 in Figure 6. The target curve 32, 33 is predefined, and data recording the target curve (i.e. the relationship between turbine speed and target total masking level) is stored in a memory - for instance in the form of a lookup table.

The target curve 32, 33 could be generated based on simulations and measured experience.

In this example, the target total masking level is defined by only a single parameter: the turbine speed. Other parameters, such as tone line level, could govern the masking energy target curve (i.e. the other parameters could be part of the lookup table).

The target total masking level is set by determining the turbine speed, and setting the target total masking noise level accordingly. So for the turbine speed S associated with data points 30 and 31, a target total masking noise level Lpn (target) is read from the lookup table.

The additional masking noise level is then calculated on the basis of a difference between the target total masking noise level and the background masking noise level, by the formula:

Lpn (additional) = Lpn (target) - Lpn (ambient + WTG) where Lpn (additional) is the additional masking noise level, Lpn (target) is the target total masking noise level, and Lpn (ambient + WTG) is the background masking noise level. The background masking noise includes ambient noise, together with masking noise coming from the wind turbine itself.

Once Lpn (additional) has been calculated, then additional masking noise is generated accordingly which further masks the tonal noise 20. An example is shown in Figure 5. Figure 4 shows a spectrum without additional masking noise, and Figure 5 shows a spectrum with the additional masking noise added. It can be seen that the masking noise 21b, 22b in Figure 5 further masks the tonal noise 20 so that the tonal noise 20 no longer stands out.

An energy level of the additional masking noise, summed over the pair of critical bands, is based on the calculated additional masking noise level Lpn (additional), so that the total energy level of the masking noise 21b, 22b in the critical bands 21a, 22a is equal to Lpn (target).

As shown in Figure 7 for the two data points 30, 31, the level 30a of the additional masking noise associated with the data point 30 is less than the level 31a of the additional masking noise associated with the data point 31.

The additional masking noise may be generated by a loudspeaker 101 shown in Figure 1, and/or by one or more further sources. Such further sources may include the wind turbine rotor (the noise of which can be controlled by controlling the rotor speed and blade pitch); static aero-dynamic devices on the blades 3 such as vortex generators, or dynamic aerodynamic devices on the blades 3 such as flaps.

Figure 10 shows an example of low background noise 40, tonal noise 50 and high additional masking noise 61. Figure 11 shows the total masking noise 62 generated by the sum of the low background noise 40 and the high additional masking noise 61.

Figure 12 shows an example of high background noise 41 , tonal noise 50 and low additional masking noise 63. Figure 13 shows the total masking noise 64 generated by the sum of the high background noise 41 and the low additional masking noise 63.

Since the background noise 41 of Figure 12 has a higher energy level than the background noise 40 of Figure 10, the additional masking noise 63 has a lower energy level than the additional masking noise 61. A Gaussian masking spectrum profile is scaled as required, so that the energy level of the additional masking noise in the critical bands is approximately Lpn (additional). The additional masking noise 61, 63 is then generated in accordance with the scaled masking spectrum profile.

The system of Figure 1 includes various apparatus for masking tonal noise as described above. The additional masking noise source in this example is a loudspeaker 101. A masking noise control system 100 is configured to obtain a background masking noise level which is indicative of a level of background masking noise which partially masks the tonal noise. This may be obtained from the microphone 5, another microphone on the tower or nacelle, or by estimation or simulation as described above. The system 100 is configured to calculate an additional masking noise level on the basis of the background masking noise level and a target total masking noise level; and instruct the masking noise source 101 to generate additional masking noise which further masks the tonal noise. As described above, a level of the additional masking noise may be based on the calculated additional masking noise level.

The examples given above show only a single tonal noise 20, 50 but there may be multiple tones which are simultaneously masked by respective multiple additional masking noises. Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

Claims

1. A method of masking tonal noise from a wind turbine, the method comprising: obtaining a background masking noise level which is indicative of a level of background masking noise which partially masks the tonal noise; calculating an additional masking noise level on the basis of the background masking noise level and a target total masking noise level; and generating additional masking noise which further masks the tonal noise, wherein a level of the additional masking noise is based on the calculated additional masking noise level.

2. The method of claim 1 , wherein the additional masking noise level is calculated on the basis of a difference between the target total masking noise level and the background masking noise level.

3. The method of claim 1 or 2, further comprising determining a parameter, and setting the target total masking noise level on the basis of the parameter.

4. The method of any preceding claim, further comprising determining an operating condition of the wind turbine, and setting the target total masking noise level on the basis of the operating condition of the wind turbine.

5. The method of claim 4, wherein the operating condition of the wind turbine is power, rotor speed, turbine speed, blade pitch, or an operating condition of an auxiliary.

6. The method of any of claim 3, 4 or 5, wherein the target total masking noise level plateaus at high turbine speeds.

7. The method of any preceding claim, further comprising identifying a frequency of the tonal noise, wherein the obtained background masking noise level is indicative of an energy level of background masking noise summed over a pair of critical bands on either side of the frequency of the tonal noise.

8. The method of claim 7, wherein an energy level of the additional masking noise summed over the pair of critical bands is based on the calculated additional masking noise level.

9. The method of any preceding claim, wherein the background masking noise level is obtained by estimation on the basis of a time of year.

10. The method of any preceding claim, wherein the background masking noise level is obtained by estimation on the basis of an operating condition of the wind turbine.

11. The method of any preceding claim, wherein the background masking noise level is obtained by measuring sound in a vicinity of the wind turbine with a sensor.

12. The method of any preceding claim, wherein the background masking noise level is obtained by simulation.

13. The method of any preceding claim, wherein the additional masking noise is generated by a loudspeaker.

14. The method of any preceding claim, wherein the additional masking noise is generated by a loudspeaker and one or more further sources.

15. Apparatus for masking tonal noise from a wind turbine, the apparatus comprising: a masking noise source; and a masking noise control system configured to obtain a background masking noise level which is indicative of a level of background masking noise which partially masks the tonal noise, calculate an additional masking noise level on the basis of the background masking noise level and a target total masking noise level, and instruct the masking noise source to generate additional masking noise which further masks the tonal noise, wherein a level of the additional masking noise is based on the calculated additional masking noise level.

16. Apparatus according to claim 15, wherein the apparatus is configured to mask tonal noise from the wind turbine by a method according to any of claims 1 to 14.