Patent Description:
A method for controlling the amplitude modulation of noise generated by a wind turbine is disclosed in <CIT>. The method comprises: determining a sound characteristic of a turbine sound wave generated by a wind turbine; and generating an additive sound wave based on the sound characteristic such that a resultant sound wave is produced having both a peak-to-peak amplitude that is smaller than a peak-to-peak amplitude of the turbine sound wave and an average sound pressure level that is greater than an average sound pressure level of the turbine sound wave. The additive sound wave has an average sound pressure level that is less than the average sound pressure level of the turbine sound wave.

A problem with the method of <CIT> is that if the phase difference between the turbine sound wave and the additive sound wave is not close to <NUM> degrees, then constructive interference may occur which causes the peak-to-peak amplitude of the resultant sound wave to be too high. This may happen due to an error in determining the phase of the turbine sound wave, for instance due to interference from another sound source. Also the phase difference between the turbine sound wave and the additive sound wave may vary from place to place, being <NUM>° in some places but not in others.

A first aspect of the invention provides a method of masking wind turbine noise from a wind turbine according to claim <NUM>.

A second aspect of the invention provides a wind turbine system according to claim <NUM>.

The invention recognizes that generating masking noise with a modulation depth which is either zero or relatively low (less than the modulation depth of the wind turbine noise) makes it less likely to cause constructive interference.

The modulation depths and levels of the various noise signals described herein may be in dB(A) or an equivalent unit of sound pressure level. That is, the modulation depth of the resultant noise may be less than the modulation depth of the wind turbine noise in dB(A) or an equivalent unit of sound pressure level. Similarly the average level of the resultant noise may be greater than an average level of the wind turbine noise in dB(A) or an equivalent unit of sound pressure level. The masking noise may either be un-modulated masking noise with a substantially constant level in dB(A) or an equivalent unit of sound pressure level. The amplitude-modulated masking noise, in an example not forming part of the claimed invention, may have a modulation depth which is less than the modulation depth of the wind turbine noise in dB(A) or an equivalent unit of sound pressure level.

The masking noise is un-modulated masking noise with a substantially constant level. This is advantageous because it does not require a measurement of the phase of the wind turbine noise, and the risk of constructive interference is minimised or avoided entirely.

Alternatively, in an example not forming part of the claimed invention, the masking noise may be amplitude-modulated masking noise which is phase-shifted relative to the wind turbine noise and has a modulation depth which is less than the modulation depth of the wind turbine noise. The advantage of this arrangement is that a given reduction in modulation depth may be achieved with a lower increase in average level, compared with un-modulated masking noise.

The un-modulated masking noise may not have a substantially constant level at all times, but rather may have a level which varies to adapt to changes in wind speed. For instance the un-modulated masking noise may have a substantially constant level during a time of constant wind speed, but its level may vary up or down at other times to adapt to changes in wind speed.

Optionally the wind turbine noise is amplitude-modulated over a series of two or more modulation cycles, and the masking noise is un-modulated masking noise with a substantially constant level over the series of two or more modulation cycles. Optionally the wind turbine noise is amplitude-modulated over a series of peaks and troughs, and an average level of the masking noise is greater than the troughs of the wind turbine noise.

Optionally an average level of the masking noise is greater than or equal to the average level of the wind turbine noise.

The method further comprises measuring a parameter which is directly or indirectly indicative of the modulation depth of the wind turbine noise; and controlling an average level of the masking noise in accordance with the parameter.

The average level of the masking noise is controlled in accordance with the parameter so that it is greater than or equal to the average level of the wind turbine noise.

The parameter may be directly indicative of the modulation depth of the wind turbine noise - for instance it may be obtained from a noise meter which directly measures the wind turbine noise. Alternatively the parameter may be indirectly indicative of the modulation depth of the wind turbine noise - for instance it may be a wind speed, or a wind turbine operating parameter such as power or rotor speed. In this case the modulation depth of the wind turbine noise may be estimated based on the wind speed or wind turbine operating parameter - for instance using a look-up-table.

Optionally the resultant noise has a modulation depth less than or equal to 3dB(A).

Optionally the resultant noise has a modulation depth greater than 2dB(A).

Optionally the average level of the resultant noise is greater than the average level of the wind turbine noise by at least 2dB(A) or 3dB(A).

Optionally the masking noise is generated by a loudspeaker.

Optionally the resultant noise has a modulation depth at a receptor which is less than a modulation depth at the receptor of the wind turbine noise and an average level at the receptor which is greater than an average level at the receptor of the wind turbine noise.

Optionally the masking noise is either un-modulated masking noise with a substantially constant level, or, in an example not forming part of the claimed invention, amplitude-modulated masking noise which is phase-shifted at the receptor relative to the wind turbine noise and has a modulation depth at the receptor which is less than the modulation depth at the receptor of the wind turbine noise.

Optionally the masking noise is generated by a sound-generating device such as a loudspeaker, and the masking noise is un-modulated masking noise with a substantially constant level at the sound generating device. In this case the level of the un-modulated masking noise at the receptor may vary slightly due to meteorological transmission variations and reflections from objects around the receptor, but any such variation will tend to also affect the wind turbine noise so the masking effect is not influenced greatly.

A further aspect of the invention provides a method of masking wind turbine noise from a wind turbine, the method comprising: generating masking noise to produce resultant noise with a modulation depth which is lower than a modulation depth of the wind turbine noise and an average level which is greater than an average level of the wind turbine noise, wherein an average level of the masking noise is greater than the average level of the wind turbine noise.

<FIG> shows a wind turbine system including a horizontal axis wind turbine <NUM>. The wind turbine <NUM> comprises a tower <NUM> supporting a nacelle <NUM> to which a rotor <NUM> is mounted. The rotor <NUM> comprises a plurality of wind turbine blades <NUM> that extend radially from a central hub <NUM>. In this example, the rotor comprises three blades <NUM>.

<FIG> shows apparatus for masking wind turbine noise from the wind turbine <NUM>. Some of the elements of <FIG> are mounted on or near the wind turbine <NUM>, and shown in <FIG>. A turbine controller <NUM> receives one or more measurement signals from sensor(s) <NUM>, and calculates one or more associated parameters which it sends to a calculating unit <NUM>. The sensor(s) <NUM> may include a wind sensor mounted on the nacelle <NUM>; a power sensor measuring a power generated by the wind turbine, and/or a rotor speed sensor measuring a speed of the rotor <NUM>. The associated parameters sent to the calculating unit <NUM> may be a wind speed (for instance in m/s); or a wind turbine operating parameter such as power (for instance in W) or rotor speed (for instance in revolutions per minute or radians per second).

The turbine controller <NUM> passes the parameter(s) on to a calculating unit <NUM> that determines a suitable masking noise signal that is sent to a masking noise generator <NUM>. The masking noise generator <NUM> generates an amplifier drive signal based on the masking noise signal which it inputs into an amplifier <NUM>. The amplifier <NUM> drives a loudspeaker <NUM> with a loudspeaker drive signal based on the masking noise signal.

The turbine controller <NUM>, calculating unit <NUM> and masking noise generator <NUM> may be a computer or any other electronic device. The turbine controller <NUM>, calculating unit <NUM> and masking noise generator <NUM> may be embodied in different electronic devices or the same electronic device.

The elements <NUM>-<NUM> may be mounted to the wind turbine <NUM> as shown in <FIG>, or uniquely associated with it. So for example the turbine controller <NUM>, the calculating unit <NUM>, the masking noise generator <NUM> and the amplifier <NUM> may be housed in the nacelle <NUM>, in the tower <NUM> or adjacent to a base of the tower <NUM>. Exemplary positions for the elements <NUM>-<NUM> are shown in <FIG> but others are possible.

The wind turbine <NUM> may be one of a collection of wind turbines in a wind farm. In this case the wind farm may have a wind farm controller <NUM> which receives measurement signals from the sensor(s) <NUM> associated with the wind turbine <NUM> as well as sensors associated with other wind turbines in the wind farm. In this case the parameter(s) may be input to the calculating unit <NUM> by the wind farm controller <NUM> rather than the turbine controller <NUM>.

The loudspeaker <NUM> may be located in a variety of locations indicated in <FIG>, including on the ground - in an optimized height and distance from the base of the tower - as shown in <FIG>. Other placement options for the loudspeaker are possible including above or on top of the nacelle as shown at 24a; below or hanging from the nacelle as shown at 24b; inside the nacelle - radiating through an opening or potentially as an actuator using the nacelle wall as a noise radiating surface - as shown at 24c; on the tower at any height as shown at 24d; or inside the tower - potentially as an actuator using the tower wall as a noise radiating surface - as shown at 24e.

Preferably the loudspeaker <NUM> is omnidirectional so that it radiates sound approximately equally over all angles of azimuth.

<FIG> is a graph showing the sound pressure level in dB(A) measured at a receptor <NUM> (such as a sound meter) shown in <FIG> which is positioned at a suitable location such as the boundary of the wind farm, or the boundary of a neighbouring property.

The wind turbine <NUM> generates amplitude-modulated wind turbine noise <NUM> at the receptor <NUM> which is caused by the rotation of the blades <NUM> and can be heard as a "swooshing" sound. The average level of the wind turbine noise <NUM>, as well as the frequency and modulation depth of the amplitude modulation of the wind turbine noise <NUM>, will depend on the rate of rotation of the blades <NUM> as well as aerodynamic factors such as the pitch angle of the blades, the direction of the wind etc. In this example the frequency of the amplitude modulation of the wind turbine noise is about <NUM>.

Regulations consider not only the average level of the wind turbine noise <NUM> at the receptor <NUM>, but also its modulation depth in terms of peak-to-peak height. For instance if the modulation depth is greater than <NUM> dB(A) then a penalty may be incurred. This penalty is expressed as a <NUM> dB(A) penalty. So the regulation adds the <NUM> dB(A) penalty to the average level, and if the sum exceeds a threshold then the wind turbine noise is deemed to be above the legal limit.

In order to reduce the modulation depth of the wind turbine noise <NUM>, the loudspeaker <NUM> may be driven to produce un-modulated masking noise <NUM> at the receptor <NUM> as shown in <FIG>. The wind turbine noise <NUM> and the masking noise <NUM> combine additively to produce resultant noise <NUM> at the receptor <NUM>.

Table <NUM> below sets out the average sound pressure level and modulation depth at the receptor <NUM> of each of the noise signals of <FIG>.

The resultant noise <NUM> may have a modulation depth (<NUM> dB(A)) at the receptor <NUM> as indicated by a peak-to-peak height <NUM> in <FIG> which is less than the modulation depth at the receptor of the wind turbine noise (<NUM> dB(A)) as indicated by peak-to-peak height <NUM>. The resultant noise <NUM> may also have an average level (<NUM> dB(A)) at the receptor <NUM> which is greater than that of the wind turbine noise (<NUM> dB(A)).

The average level of the un-modulated masking noise <NUM> is controlled by the calculating unit <NUM> and the masking noise generator <NUM> to ensure that the modulation depth <NUM> at the receptor <NUM> of the resultant noise <NUM> is reduced to a desired level - for example <NUM> dB(A) as shown in <FIG>. This control may be achieved by the following method.

The sensor(s) <NUM> provide outputs which are indirectly indicative of the average level and modulation depth of the wind turbine noise <NUM> at the receptor <NUM>, and can be analysed to infer these properties. So for example the sensor(s) <NUM> may include a wind sensor mounted on the nacelle <NUM>. Higher wind speed will lead to a wind turbine noise with a higher average level and modulation depth. The relationship between these parameters can be measured by the sensor(s) <NUM> and the receptor <NUM> and then stored in a look-up-table, so for a given wind speed the lookup-table outputs an estimated average level and modulation depth of the wind turbine noise <NUM> at the receptor <NUM>. A similar principal can be used to estimate the average level and modulation depth <NUM> of the wind turbine noise <NUM> at the receptor <NUM> based on the output of a power sensor measuring a power generated by the wind turbine, or a rotor speed sensor measuring the speed of the rotor <NUM>.

The masking noise generator <NUM> analyses the parameter(s) received from the turbine controller <NUM>, estimates the average level and modulation depth <NUM> of the wind turbine noise <NUM> at the receptor <NUM> based on these parameters, and sets the average level of the masking noise <NUM> accordingly so that the modulation depth <NUM> of the resultant noise <NUM> is reduced to an acceptable level.

The average level (<NUM> dB(A)) of the un-modulated masking noise <NUM> in the example of <FIG> is set to be <NUM> dB(A) greater than the average level (<NUM> dB(A)) of the wind turbine noise <NUM>. This ensures that the modulation depth <NUM> of the resultant noise <NUM> is reduced to <NUM> dB(A) to avoid a penalty under the regulations.

The resultant noise <NUM> has an average level (<NUM> dB(A)) which is <NUM> dB(A) greater than that of the wind turbine noise <NUM> (<NUM> dB(A)), but this is less than the <NUM> dB(A) penalty that would have been incurred if the modulation depth had not been reduced from <NUM> dB(A) to <NUM> dB(A).

Alternatively the average level of the un-modulated masking noise <NUM> may be set to be equal to or less than the average level of the wind turbine noise <NUM> if a higher modulation depth <NUM> of the resultant noise <NUM> is acceptable.

If the average level of the un-modulated masking noise <NUM> is reduced compared with <FIG>, then typically it remains greater than the level at the troughs of the wind turbine noise <NUM>, in other words greater than <NUM> dB(A) in this example.

The average level of the un-modulated masking noise <NUM> may also be increased compared with <FIG>. This will increase the average level of the resultant noise but will also decrease its modulation depth below <NUM> dB(A).

The masking noise signal output by the calculating unit <NUM> has a substantially constant level in dB(A), as does the loudspeaker drive signal input to the loudspeaker <NUM> and the sound pressure level in dB(A) of the masking noise at the loudspeaker <NUM>. Accordingly the level of the un-modulated masking noise <NUM> at the receptor <NUM> may be substantially constant as shown in <FIG>, although the level of the un-modulated masking noise <NUM> measured at the receptor <NUM> may vary slightly (for instance by <NUM> to <NUM> dB(A)) due to measurement errors, reflections or other artefacts. Variations of the order of <NUM> dB(A) may be caused by meteorological transmission variations, and variations of the order of <NUM> dB(A) may be caused by reflections from objects around the receptor <NUM>. Such reflections will most likely also affect the amplitude-modulated wind turbine noise <NUM> to the same degree, so the relative difference between the wind turbine noise <NUM> at the receptor and the masking noise <NUM> at the receptor will not change and the masking effect is not influenced greatly. In any event, any small variations in the level of the un-modulated masking noise <NUM> measured at the receptor <NUM> will be less than the modulation depth <NUM> of the wind turbine noise <NUM> at the receptor.

The wind turbine noise <NUM> is amplitude-modulated over a series of modulation cycles, five full modulation cycles being shown in <FIG>. The un-modulated masking noise <NUM> has a substantially constant level of <NUM> dB(A) over the series of modulation cycles shown in <FIG>, but its level may increase or decrease slowly at a later time to adapt to changes in wind speed.

Variations in the wind will cause associated variations in the level of the un-modulated masking noise <NUM> at the receptor <NUM>. Such variations will occur over a time scale of minutes, whereas on the shorter time scale of the amplitude-modulation the level of the un-modulated masking noise <NUM> at the receptor <NUM> can be considered quasi-static. In other words, there may only be minimal variations (no greater than <NUM> dB(A)) in the level of the un-modulated masking noise <NUM> at the receptor <NUM> over the time scale of, say, two modulation cycles of the amplitude-modulated wind turbine noise (about <NUM> seconds in the case of <FIG>).

<FIG> is a graph showing the sound pressure level in dB(A) measured at the receptor <NUM> in an alternative embodiment. In this case, the masking noise is amplitude-modulated masking noise 32a which is phase-shifted relative to the wind turbine noise <NUM> and has a modulation depth as indicated by peak-to-peak height <NUM>.

The resultant noise 33a has a modulation depth (<NUM> dB(A)) as indicated by a peak-to-peak height 34a in <FIG> which is less than the modulation depth of the wind turbine noise <NUM> (<NUM> dB(A)) as indicated by peak-to-peak height <NUM>. The resultant noise 33a also has an average level (<NUM> dB(A)) which is greater than that of the wind turbine noise (<NUM> dB(A)).

The modulated masking noise 32a has a modulation depth (<NUM> dB(A)) as indicated by peak-to-peak height <NUM> in <FIG> which is relatively low, less than that of the wind turbine noise <NUM> (<NUM> dB(A)). The relatively low modulation depth of the modulated masking noise 32a ensures that any constructive interference is kept relatively low. Also, the relatively low modulation depth <NUM> of the modulated masking noise 32a makes it insensitive to inaccuracy in the phase-shift. Ideally the modulated masking noise 32a is phase-shifted by <NUM>° relative to the wind turbine noise <NUM>, in other words the noise signals are in precise anti-phase. <FIG> shows a modulated masking noise 32a which is not precisely in anti-phase phase with the wind turbine noise <NUM> (rather it is phase-shifted by <NUM>°) but nevertheless it reduces the modulation depth 34a of the resultant noise 33a to an acceptable level.

The advantage of using the amplitude-modulated masking noise 32a of <FIG> is that a given reduction in modulation depth can be achieved with a lower increase in average level, compared with the un-modulated masking noise shown in <FIG>.

The masking noise <NUM> or 32a may be broadband white noise, but more preferably it only spans a limited frequency range which matches the frequency range of the wind turbine noise <NUM>. For instance the masking noise <NUM> or 32a may be band limited to a frequency range of <NUM> to <NUM>, or <NUM> to <NUM>. This minimises the amount of sound energy that must be produced.

Claim 1:
A method of masking wind turbine noise (<NUM>) from a wind turbine (<NUM>), the method comprising: generating masking noise (<NUM>) to produce resultant noise (<NUM>) with a modulation depth which is less than a modulation depth of the wind turbine noise and an average level which is greater than an average level of the wind turbine noise, wherein the masking noise is un-modulated masking noise with a substantially constant level, the method further comprising measuring a parameter which is directly or indirectly indicative of the modulation depth of the wind turbine noise and controlling an average level of the masking noise in accordance with the parameter so that the average level of the masking noise is greater than or equal to the average level of the wind turbine noise.