Patent Description:
<CIT> discloses an acoustic emission device for a wind turbine with a rotor-nacelle-assembly (RNA). A loudspeaker is arranged at the rear of the RNA, and radiates sound to the rear, in line with the rotor axis.

<CIT> discloses systems and methods for managing power plant acoustics. In various embodiments, a system is disclosed including at least one computing device configured to perform the following: determine a difference between an A-weighted sound decibel (dBA) level and a C-weighted sound decibel (dBC) level (ΔdBC-dBA) from a power plant system within a sound spectrum; compare the Δ dBC-dBA with a predetermined threshold difference for the sound spectrum; and provide instructions to increase the dBA level of a balancing sound in the spectrum proximate the power plant system in response to determining the Δ dBC-dBA exceeds the predetermined threshold difference.

A first aspect of the invention provides a method of masking tonal noise from a wind turbine according to claim <NUM>, the wind turbine comprising a noise generator and a rotor-nacelle-assembly (RNA), the RNA comprising a nacelle and a rotor configured to rotate about a rotor axis with a horizontal projection; the method comprising: feeding a drive signal into the noise generator so that the noise generator emits directional masking noise, wherein the directional masking noise at least partially masks tonal noise from the wind turbine and points in a masking direction with a horizontal projection, and wherein the horizontal projection of the masking direction points away from the horizontal projection of the rotor axis.

The method may further comprise rotating the RNA about a yaw axis to change a yaw angle of the RNA, wherein the masking direction does not change as the yaw angle changes.

The method may further comprise rotating the RNA about a yaw axis to change a yaw angle of the RNA, wherein the masking direction changes as the yaw angle changes.

Optionally the rotor defines an upwind direction, a downwind direction, an upstroke direction and a downstroke direction; and at least a component of the horizontal projection of the masking direction points in the upwind direction or the upstroke direction.

Optionally the horizontal projection of the rotor axis is oriented at an angle relative to the horizontal projection of the masking direction, and the method comprises: changing the angle by rotating the RNA about a yaw axis; and changing the drive signal in accordance with the change of angle, thereby changing a characteristic of the directional masking noise.

Optionally the horizontal projection of the masking direction is at an angle relative to the horizontal projection of the rotor axis; and a characteristic of the directional masking noise is set on the basis of the angle.

Optionally the characteristic of the directional masking noise is a frequency profile or sound power of the directional masking noise.

Optionally changing the drive signal causes a sound power of the directional masking noise to increase; the tonal noise is a directional tonal noise which points in a tonal noise direction with a horizontal projection; and the change of the angle causes the horizontal projection of the masking direction and the horizontal projection of the tonal noise direction to become more closely aligned with each other.

The method may further comprise generating directional natural masking noise, wherein the directional natural masking noise at least partially masks the tonal noise and points in a natural masking noise direction with a horizontal projection, wherein changing the drive signal causes a power of the directional masking noise to decrease; and the change of the angle causes the horizontal projection of the masking direction and the horizontal projection of the natural masking noise direction to become more closely aligned with each other.

The method may further comprise setting the masking noise direction by rotating the noise generator, or by selecting the noise generator from a set of noise generators. Optionally the masking noise direction is set on the basis of a location of a receptor, such as a neighbour of the wind turbine.

Optionally the RNA is oriented at a yaw angle, and the drive signal is based on the yaw angle.

Optionally the drive signal is enabled or changed on the basis of the yaw angle. Optionally the drive signal is fed into the noise generator only for a specific operational envelope.

A further aspect of the invention provides a method of masking tonal noise from a wind turbine, the wind turbine comprising a masking noise generator and a rotor-nacelle-assembly (RNA), the RNA comprising a nacelle and a rotor, wherein the rotor defines an upwind direction, a downwind direction, an upstroke direction and a downstroke direction, the method comprising: feeding a drive signal into the noise generator so that the noise generator emits directional masking noise, wherein the directional masking noise at least partially masks tonal noise from the wind turbine and points in a masking direction with a horizontal projection, and at least a component of the horizontal projection of the masking direction points in the upwind direction or the upstroke direction.

A further aspect of the invention provides a computer program product comprising software code adapted to control noise masking apparatus when executed on a data processing system, the computer program product being adapted to perform the method of any aspect of the invention.

A further aspect of the invention provides a wind turbine comprising a noise generator according to claim <NUM>, a control system; and a rotor-nacelle-assembly (RNA), wherein the RNA comprises a nacelle and a rotor configured to rotate about a rotor axis with a horizontal projection, the control system is configured to feed a drive signal into the noise generator so that the noise generator emits directional masking noise which at least partially masks tonal noise from the wind turbine and points in a masking direction with a horizontal projection, and the horizontal projection of the masking direction points away from the horizontal projection of the rotor axis.

<FIG> shows a wind turbine system <NUM> comprising a noise generator <NUM>; a control system <NUM>; and a rotor-nacelle-assembly (RNA) <NUM>, <NUM> mounted on a tower <NUM>. The RNA comprises a nacelle <NUM> to which a rotor <NUM> is mounted. The rotor comprises a plurality of wind turbine blades that extend radially from a central hub. In this example, the rotor <NUM> comprises three blades. The RNA <NUM>, <NUM> can be rotated about a vertical yaw axis to change its yaw angle.

The system <NUM> includes an International Electrotechnical Commission (IEC) microphone <NUM> which records sound pressure. <FIG> is a spectrum of sound pressure measured by the microphone <NUM> versus frequency, presented as a graph with each spectral line representing sound pressure level (in dB(A)) in a respective frequency range. The range of each spectral line may be the same for each spectral line, or each spectral line may represent a one-third octave band. Typically, each spectral line covers a range of <NUM>-<NUM>, although for ease of illustration <FIG> gives a lower resolution example in which each spectral line represents a range of the order of <NUM>.

The spectrum of <FIG> is based on a ten second time period, and includes a tonal noise <NUM> - i.e. a pure tone which occupies a narrow frequency range. If each spectral line represents a <NUM> range, then the tonal noise <NUM> in this example may be in the <NUM>-<NUM> frequency range. <FIG> shows the tonal noise <NUM>, and natural masking noise <NUM> in a pair of critical bands <NUM> on either side of the frequency of the tonal noise <NUM>. In this example the critical bands <NUM> each have a range of about <NUM>, but in other embodiments the widths of the critical bands <NUM> may be higher or lower.

The natural masking noise <NUM> only partially masks the tonal noise <NUM>, so the tonal noise <NUM> may be annoying.

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

The level of the natural masking noise <NUM> in the critical bands <NUM> can thus vary depending on a number of factors, including wind speed, turbine operating conditions, time of year and so on.

The control system <NUM> is configured to feed a drive signal into the noise generator <NUM> so that the noise generator <NUM> emits additional masking noise which at least partially masks the tonal noise <NUM> from the wind turbine as shown in <FIG> is a spectrum showing the total masking noise in the critical bands <NUM>. The total masking noise is made up by a combination of the natural masking noise <NUM> from the rotor <NUM> and other sources, and additional masking noise <NUM> from the noise generator <NUM>.

The tonal noise <NUM> may be directional. That is, the tonal noise <NUM> may point in a particular direction (or a finite number of directions). This needs to be considered if there is a neighbour near the system <NUM>. For example, a neighbour may not be affected as much by a tonal noise which points away from the neighbour. In this case, the additional masking noise <NUM> may not need to be as loud to successfully mask the tonal noise from the perspective of the neighbour.

In addition, the natural masking noise <NUM> may be directional. If the natural masking noise is pointing at the neighbour, then less additional masking noise may be required.

<FIG> show a possible directional profile of the tonal noise <NUM>.

As shown in <FIG>, the rotor <NUM> is configured to rotate in a rotor plane <NUM> and about a rotor axis <NUM>. The rotor <NUM> points in a rotor axis direction 7a. By definition, the horizontal projection of the rotor axis direction 7a points in the positive Y-direction, which is indicated in <FIG>.

Thus the rotor <NUM> defines an upwind direction, a downwind direction, an upstroke direction and a downstroke direction which are indicated in <FIG>. A Cartesian coordinate frame is also shown - with a vertical Z-axis, and horizontal X and Y axes. The upstroke direction is defined herein as the positive X-direction, and the upwind direction is defined as the positive Y-direction.

The wind blows in the downwind direction and from the upwind direction. The rotor <NUM> rotates clockwise when viewed from the upwind direction as shown in <FIG>. The downstroke (negative X) direction is the crosswind direction where the blades are moving down, and the upstroke (positive X) direction is the crosswind direction where the blades are moving up.

The tonal noise <NUM> may originate from a source (such as a fan) within the nacelle <NUM>, and it may be radiated by a radiating section <NUM> of the tower <NUM> on the upstroke side of the tower <NUM> as shown in <FIG>.

Since the tonal noise <NUM> is radiated from only one side of the tower <NUM>, it is directional and points in a tonal noise direction 20a. The tonal noise direction 20a may be defined as the direction of maximum sound level of the tonal noise. In this case the tonal noise <NUM> has only a single lobe, with a single associated direction 20a. In other examples the tonal noise <NUM> may have a finite number of multiple lobes, each with a respective direction.

In this case the tonal noise direction 20a is inclined down, and points in the upstroke direction. This is exemplary only, and different types of tonal noise may point in different directions. Alternatively, the tonal noise <NUM> may be omnidirectional.

<FIG> show a possible directional profile of the natural masking noise <NUM>. As explained above, the natural masking noise <NUM> may originate from many sources, including the movement of the rotor blades though the air.

The natural masking noise <NUM> points in a natural masking noise direction 21a. The natural masking noise direction 21a may be defined as the direction of maximum sound level of the natural masking noise <NUM>. In this case the natural masking noise <NUM> has only a single lobe, with a single associated direction 21a. In other examples the natural masking noise <NUM> may have a finite number of multiple lobes, each with a respective direction.

The natural masking noise direction 21a is illustrated as a vector with X,Y,Z components labelled 21x, 21y, 21z. The natural masking noise direction 21a is inclined down, and has a component 21x, 21y in each horizontal direction (X, Y).

The magnitude of the downwind component 21y shown in <FIG> is greater than the magnitude of the downstroke component 21x shown in <FIG>. This is exemplary only, and different types of natural masking noise may point in different directions.

A large component 21y of the natural masking noise direction 21a is expected to be in the downwind direction since much of the natural masking noise <NUM> originates from the wind's interactions with the blades.

The natural masking noise direction 21a may point directly downwind (i.e. without any crosswind component) but more typically a component 21x of the natural masking noise direction 21a is expected to be in the downstroke direction as shown in <FIG>, due to the tiltback of the rotor plane <NUM>. This tiltback causes the rotor blades to experience a higher wind speed during the downstroke than during the upstroke (since they are effectively moving into the wind during the downstroke). This is expected to result in the blades emitting more natural masking noise <NUM> on the downstroke side than on the upstroke side.

The control system <NUM> feeds a drive signal (or drive signals) into the noise generator <NUM> so that the noise generator <NUM> emits directional masking noise <NUM> shown in <FIG>. The directional masking noise <NUM> at least partially masks the tonal noise <NUM> from the wind turbine and points down in a masking direction 23a.

To distinguish over the natural masking noise <NUM>, the directional masking noise <NUM> is referred to below as directional masking noise <NUM> and the masking direction 23a as the additional masking noise direction 23a.

The additional masking noise direction 23a may be defined as the direction of maximum sound level of the additional masking noise <NUM>. In this case the additional masking noise <NUM> has a directional sound profile with only a single lobe. If the sound profile has multiple lobes then the additional masking noise direction 23a is defined by the largest lobe.

The additional masking noise direction 23a is illustrated as a vector with X,Y,Z components labelled 23x, 23y, 23z. The additional masking noise direction 23a is inclined down, and has a component 23x, 23y in each horizontal direction X, Y. The magnitude of the upwind component 23y (<FIG>) is less than the magnitude of the upstroke component 21x (<FIG>). This is exemplary only, and the additional masking noise direction 23a may be adjusted as required.

The noise generator <NUM> may be a loudspeaker, a set of loudspeakers, or any other element (or set of elements) which can be driven by an electrical drive signal (or signals) to produce a masking noise at the appropriate range of frequencies. In this example, the noise generator <NUM> is illustrated as a pair of loudspeakers mounted to the top of the tower. In other embodiments, described further below, the noise generator <NUM> may be carried by the nacelle <NUM> - for instance suspended under the nacelle <NUM> or housed inside the nacelle <NUM>. In either case, the noise generator <NUM> may also be mounted via a pivot to the tower or the nacelle, so it can be rotated relative to the RNA about a vertical yaw axis to change its yaw angle. This enables the additional masking noise direction 23a to be set by yawing the noise generator <NUM> on its pivot to change its yaw angle.

The noise generator <NUM> could have a preferential yaw angle (based on neighbors or the most dominant wind direction), or it could yaw in anticipation based on communication about incoming wind conditions from neighboring turbines.

<FIG> show the tonal noise <NUM>, the natural masking noise <NUM>, and the additional masking noise <NUM> in a single view.

<FIG> is a plan view of the RNA with the rotor oriented to face a southerly wind. Since <FIG> is a plan view, it shows the horizontal projections of the rotor axis <NUM> and the additional masking noise direction 23a. Note that the orientation of the (X,Y,Z) frame of reference is defined by the yaw angle of the RNA, which sets the rotor axis direction 7a indicated in <FIG>. The horizontal projection of the rotor axis direction 7a is defined as pointing in the positive Y-direction.

The noise generator <NUM> is oriented so that the horizontal projection of the additional masking noise direction 23a points at a neighbour <NUM> located west of the RNA in the upstroke direction.

<FIG> is a plan view showing the horizontal projections of the rotor axis direction 7a, the tonal noise direction 20a, the natural masking direction 21a and the additional masking noise direction 23a, in the case of <FIG>. When the RNA is set at the yaw angle shown in <FIG>, the horizontal projection of the rotor axis direction 7a is oriented at an angle θ of -<NUM>° relative to the horizontal projection of the additional masking noise direction 23a.

As shown in <FIG>, the horizontal projection of the additional masking noise direction 23a is oriented parallel with the horizontal projection of the tonal noise direction 20a. In other words, the additional masking noise <NUM> and the tonal noise <NUM> both point west, generally in the same (upstroke) direction towards the neighbour <NUM>. The horizontal projection of the natural masking direction 21a, on the other hand, is not aligned with the horizontal projection of the tonal noise direction 20a and points north-east, generally away from the neighbour <NUM>. Thus the natural masking noise <NUM> and the tonal noise <NUM> point in generally opposing directions. So from the perspective of the neighbour <NUM>, the natural masking noise <NUM> is relatively quiet and not effective in masking the tonal noise <NUM>. So the additional masking noise <NUM> is set by the control system <NUM> at a high sound power level. The length of the arrow representing the horizontal projection of the natural masking noise direction 23a is relatively long in <FIG> to schematically illustrate this.

<FIG> is a plan view of the RNA with the rotor oriented to face a westerly wind. Since the noise generator <NUM> is mounted to the tower <NUM>, the additional masking noise direction 23a does not change as the RNA rotates about the yaw axis. Therefore the horizontal projection of the additional masking noise direction 23a continues to point at the neighbour <NUM>, west of the RNA, which is now in the upwind direction.

<FIG> is a plan view showing the horizontal projections of the rotor axis direction 7a, the tonal noise direction 20a, the natural masking direction 21a and the additional masking noise direction 23a, in the case of <FIG>. As shown in <FIG>, the horizontal projection of the rotor axis direction 7a is now oriented at an angle of <NUM>° relative to the horizontal projection of the additional masking noise direction 23a (i.e. they are parallel).

The horizontal projection of the additional masking noise direction 23a is at right angles to the horizontal projection of the tonal noise direction 20a (which is now pointing north). From the perspective of the neighbour <NUM>, the natural masking noise <NUM> remains relatively quiet, but the tonal noise <NUM> is no longer pointing directly at the neighbour <NUM>, so less additional masking noise <NUM> is needed. The control system <NUM> changes the drive signal so that the additional masking noise <NUM> is reduced to a lower sound power level, compared with <FIG>.

<FIG> is a plan view of the RNA, with the rotor oriented to face an easterly wind. The horizontal projection of the additional masking noise direction 23a continues to point at the neighbour <NUM>, west of the RNA, which is now in the downwind direction.

<FIG> is a plan view showing the horizontal projections of the rotor axis <NUM>, the tonal noise direction 20a, the natural masking direction 21a and the additional masking noise direction 23a, in the case of <FIG>. The horizontal projection of the rotor axis direction 7a is now oriented at an angle of <NUM>° relative to the horizontal projection of the additional masking noise direction 23a (i.e. they are anti-parallel).

As with <FIG>, the horizontal projection of the additional masking noise direction 23a is at right angles with the horizontal projection of the tonal noise direction 20a (which is now pointing south). From the perspective of the neighbour <NUM>, the natural masking noise <NUM> is louder than in the case of <FIG>, so less additional masking noise <NUM> is needed. The control system <NUM> changes the drive signal, so that the additional masking noise <NUM> is reduced to a lower sound power level, compared with <FIG>.

<FIG> is a plan view of the RNA with the rotor oriented to face a northerly wind. The horizontal projection of the additional masking noise direction 23a continues to point at the neighbour <NUM>, west of the RNA, which is now in the downstroke direction.

<FIG> is a plan view showing the horizontal projections of the rotor axis <NUM>, the tonal noise direction 20a, the natural masking direction 21a and the additional masking noise direction 23a, in the case of <FIG>. The horizontal projection of the rotor axis direction 7a is now oriented at an angle θ of +<NUM>° relative to the horizontal projection of the additional masking noise direction 23a.

The horizontal projection of the additional masking noise direction 23a is directly opposed to the horizontal projection of the tonal noise direction 20a (which is now pointing east). From the perspective of the neighbour <NUM>, the natural masking noise <NUM> is now relatively loud, and the tonal noise <NUM> is pointing directly away from the neighbour <NUM> so the additional masking noise <NUM> can be reduced further. The control system <NUM> changes the drive signal so that the additional masking noise <NUM> is reduced to a lower sound power level, compared with <FIG>.

When the wind is westerly (<FIG>) or easterly (<FIG>) then the horizontal projection of the additional masking direction 23a is parallel to the horizontal projection of the rotor axis <NUM>. That is, the horizontal projection of the additional masking direction 23a points directly upwind or downwind.

For all other wind directions (including <FIG> and <FIG>) the horizontal projection of the additional masking direction 23a points away from the horizontal projection of the rotor axis <NUM> - i.e. it points "off-axis". In other words, the horizontal projection of the additional masking direction 23a has at least a component in a crosswind direction (upstroke or downstroke). This can be contrasted with <CIT>, in which the additional masking noise direction is parallel with the rotor axis for all wind directions. For the reasons explained above, it has been found to be beneficial to point the additional masking noise <NUM> "off-axis" (i.e. with at least a component in a crosswind direction) in order to provide more effective masking performance.

As described above, the horizontal projection of the rotor axis direction 7a is oriented at an angle θ relative to the horizontal projection of the additional masking direction 23a. Since the noise generator <NUM> is mounted to the tower <NUM> rather than the RNA, this angle θ changes as the RNA rotates about the yaw axis. As described above, a characteristic of the directional masking noise <NUM> may be set on the basis of this angle θ. For instance, the control system <NUM> may change the drive signal(s) driving the noise generator <NUM> in accordance with the change of angle, thereby changing a characteristic of the directional masking noise (for instance changing its sound power or frequency profile).

If the horizontal projection of the additional masking noise direction 23a and the horizontal projection of the tonal noise direction 20a are parallel or at least closely aligned with each other (as in <FIG>) then a high sound power level may be required from the noise generator <NUM>. Thus when these directions 23a, 20a become more closely aligned with each other, the control system <NUM> may change the drive signal(s) to cause a sound power of the additional masking noise <NUM> to increase.

If the horizontal projection of the additional masking noise direction 23a and the horizontal projection of the natural masking noise direction 21a are parallel or closely aligned with each other (as in <FIG> and <FIG>) then a low sound power level may be required from the noise generator <NUM>. Thus when these directions 23a, 21a become more closely aligned with each other, the control system <NUM> may change the drive signal(s) to cause a sound power of the additional masking noise <NUM> to decrease.

In the examples given above, the noise generator <NUM> comprises a pair of loudspeakers pivotally mounted to the tower <NUM>. <FIG> shows an alternative noise generator - in this case a set of eight loudspeakers <NUM>-<NUM>. The control system <NUM> is also shown, along the with the upwind, downwind, upstroke and downstroke directions. Each of the four directions has an associated quadrant <NUM>-<NUM>.

Each loudspeaker <NUM>-<NUM> is directional and directs the majority of its generated sound power in the direction its speaker cone is facing. Although <FIG> shows eight loudspeakers, any number of loudspeakers may be used and in any 2D or 3D orientation, for example an array of speakers may take the form of a sphere, or half sphere.

<FIG> shows an example in which a first subset <NUM>-<NUM> of the speakers is driven by the control system <NUM> to generate additional masking noise <NUM>. The arrow heads on the control lines from the control system <NUM> indicate the drive signals which are enabled to generate additional masking noise <NUM>. The additional masking noise <NUM> comprises a lobe formed from the superposition of the sound from the three loudspeakers <NUM>-<NUM>. The lobe provides a global maximum <NUM> which defines the additional masking noise direction 23a. The horizontal projection of the additional masking direction 23a points in the upwind direction and the upstroke direction.

The drive signals for the other five speakers <NUM>-<NUM> are disabled. The number of driven speakers in the first subset <NUM>-<NUM> is an example only, and in general the first subset may comprise one or more loudspeakers. Loudspeakers in the upwind and upstroke directions are chosen because they are the directions in which the natural masking noise is the least powerful and the tonal noise is most prevalent (see above).

The horizontal projection of the masking direction 23a points along the boundary between the upwind quadrant <NUM> and the upstroke quadrant <NUM> (so it effectively points in both quadrants). In this example, no additional masking noise is emitted in the downwind quadrant <NUM> or the downstroke quadrant <NUM>.

<FIG> can be contrasted with <CIT>, in which the additional masking noise direction points in the downwind direction. For the reasons explained above, it has been found to be beneficial for the additional masking noise direction 23a to point in the upwind quadrant <NUM> (where there may be less natural masking noise) or in the upstroke quadrant <NUM> (where there may be less natural masking noise and/ or more tonal noise).

<FIG> shows the system of <FIG> after a <NUM>° change in wind direction. In the case of <FIG>, the loudspeakers <NUM>-<NUM> are mounted to the tower. So as the RNA yaw angle changes, they do not move.

The loudspeaker <NUM> that was previously pointing in the upwind/upstroke direction, now points in the upwind direction. The loudspeaker <NUM> that was previously pointing in the upstroke direction, now points in the upwind/upstroke direction.

As a result, the loudspeaker <NUM> that was previously emitting additional masking noise in the upwind direction is disabled by the control system <NUM>, and the previously inactive loudspeaker <NUM> which is now pointing in the upstroke direction is enabled by the control system <NUM>.

Thus <FIG> show a method of setting the masking noise direction by selecting a subset of one or more noise generators from the set of noise generators <NUM>-<NUM>. As the yaw angle changes to the yaw angle shown in <FIG>, a different subset <NUM>-<NUM> of the loudspeakers is selected and driven, so the additional masking noise continues to point in the upwind and upstroke quadrants regardless of the yaw angle of the RNA.

<FIG> shows an alternative arrangement, for a case in which the loudspeakers <NUM>-<NUM> are mounted to the RNA. As the RNA yaw angle changes, the set of loudspeakers <NUM>-<NUM> rotates with the RNA. In the case of <FIG>, the first subset <NUM>-<NUM> of the speakers continue to be driven by the control system <NUM> to generate the additional masking noise in the upwind and upstroke quadrants.

If the loudspeakers are mounted to the RNA (as in <FIG>) and additional masking noise is only required in the upwind and upstroke quadrants for all operating conditions, then only three loudspeakers <NUM>-<NUM> may be required rather than the full array of eight loudspeakers <NUM>-<NUM>.

<FIG> shows an alternative method of driving the loudspeakers <NUM>-<NUM>. In this case, only a single loudspeaker <NUM> is driven to generate additional masking noise <NUM> pointing at a neighbour <NUM>.

<FIG> shows the system after a <NUM>° change in wind direction from <FIG>. In the case of <FIG>, the loudspeakers <NUM>-<NUM> are mounted to the tower, so as the RNA yaw angle changes they do not move. In the case of <FIG>, the loudspeaker <NUM> continues to be driven by the control system <NUM> to generate the additional masking noise pointing at the neighbour <NUM>.

Optionally the amplitude of the additional masking noise <NUM> is updated after the <NUM>° change of wind direction based on the inputs by the drive signal, which rely on the tonal noise and natural masking as described above in relation to <FIG>. Alternatively the amplitude may not change.

<FIG> shows an alternative arrangement, for a case in which the loudspeakers <NUM>-<NUM> are mounted to the RNA, so as the RNA yaw angle changes the set of loudspeakers <NUM>-<NUM> rotates with the RNA. In the case of <FIG>, the drive signal for the loudspeaker <NUM> is disabled by the control system <NUM>, and the drive signal for the loudspeaker <NUM> is enabled by the control system <NUM> to generate the additional masking noise pointing at the neighbour.

If there are multiple neighbours, a subset of one or more loudspeakers may be assigned to each neighbour to achieve similar results.

The methods of <FIG> and <FIG> are not mutually exclusive. Thus the control system <NUM> may drive one or more of the loudspeakers <NUM>-<NUM> to generate additional masking noise in a chosen quadrant (for instance upstroke or downstroke), at the same time as driving one or more of the loudspeakers <NUM>-<NUM> to generate additional masking noise which points at a neighbour.

To summarise the embodiments described above, a drive signal is fed by the control system <NUM> into a noise generator so that the noise generator emits directional additional masking noise <NUM> which at least partially masks tonal noise <NUM> from the wind turbine. The noise generator may be a single element (such a loudspeaker <NUM>) fed by a single drive signal. The additional masking noise <NUM> generated by the element is directional and points in a masking direction 23a. Alternatively the noise generator may comprise multiple elements (such as loudspeakers <NUM>-<NUM>) fed by multiple drive signals. In this case the superposition of the additional masking noise <NUM> generated by the multiple elements is directional and points in a masking direction 23a.

The rotor <NUM> defines an upwind direction, a downwind direction, an upstroke direction and a downstroke direction. In some embodiments, at least a component of the horizontal projection of the additional masking noise direction 23a points in the upwind direction or the upstroke direction. In some embodiments, at least a component of the horizontal projection of the additional masking noise direction 23a points in the upstroke direction or the downstroke direction.

The rotor <NUM> also defines an upwind quadrant <NUM>, a downwind quadrant <NUM>, an upstroke quadrant <NUM> and a downstroke quadrant <NUM>. In some embodiments, the horizontal projection of the additional masking noise direction 23a points in the upwind quadrant (<FIG>) or the upstroke quadrant (<FIG>). In some embodiments, the horizontal projection of the additional masking noise direction 23a points in the upstroke quadrant (7A) or the downstroke quadrant (<FIG>). In some embodiments, the horizontal projection of the additional masking noise direction 23a points in the upstroke quadrant (7A) or the downstroke quadrant (<FIG>) or the upwind quadrant (<FIG>).

The drive signal(s) may be fed into the noise generator only for a specific operational envelope. For instance the noise generator may be actuated by the control system <NUM> on the basis of lookup tables, which contain parameters such as, rpm, wind speed, power production, yaw angle, pitch angle, time of year, etc..

The control system <NUM> may be implemented in software, as a computer program product comprising software code adapted to control noise masking apparatus when executed on a data processing system, the computer program product being adapted to perform the method as described in any of the examples above.

Claim 1:
A method of masking tonal noise (<NUM>) from a wind turbine (<NUM>), the wind turbine comprising a noise generator (<NUM>) and a rotor-nacelle-assembly (RNA), the RNA comprising a nacelle (<NUM>) and a rotor (<NUM>) configured to rotate about a rotor axis (<NUM>) with a horizontal projection; the method comprising: feeding a drive signal into the noise generator so that the noise generator emits directional masking noise (<NUM>), wherein the directional masking noise at least partially masks tonal noise from the wind turbine and points in a masking direction (23a) with a horizontal projection, characterized in that the horizontal projection of the masking direction points away from the horizontal projection of the rotor axis.