Thus, the present invention is occupied with power generating plants in general, but it is particularly directed to adress prob- lems associated with wind power plants, and that particular ap- plication of the invention will hereinafter be discussed for illumi- nating but not in any way restricting the invention.

It is pointed out that said device will also control the power gen- eration of the wind turbine as a unit of such a wind power plant, but it is here more appropriate to speak about control of sound emission, since the control may theoretically involve e. g. a change of the pitch of the turbine blades resulting in a change of the sound power level while keeping the electric power gener- ated constant. Control of the operation of the wind turbine may also be a correct description of the object of said device.

It is well known that wind power plants, which may have a large' number, such as 50, of wind turbines or only one single wind

turbine, generate noise. This noise is spread over long dis- tances and may cause noise annoyance. This noise may be ei- ther aerodynamic noise, i. e. sound emanating from the blades of the wind turbine moving in the air, or mechanical noise emanat- ing primarily from the gear box of the wind turbine.

However, the aerodynamic sound from the movement of the blades through the air may cause problems and different acous- tic limits are set, by national authorities and future owners of a wind power plant planned, for sound emanating from the opera- tion of the wind turbine at different places where this is impor- tant, especially existing and possibly future residential areas.

Normally, a sound pressure level above 40 dB on such a place will not be accepted. However, this sound pressure level is not only dependent upon the speed of the blade tip of the wind tur- bine, the power generated by the wind turbine and the distance between the wind turbine and said place, but a number of other parameters may influence this sound pressure level.

The sound pressure level on said place is for instance depend- ent upon the temperature gradient of the air in the vertical di- rection. When the temperature increases with the height above the ground, i. e. the temperature gradient is positive, such as e. g. during the night, especially when there is no noticeable wind, the sound waves from the wind turbine will be bent down- wardly, so that the sound pressure level on said place may be higher than expected at a given power generated by the turbine.

In such a so-called temperature inversion, the sound may propagate over long distances with a comparatively small damping.

Furthermore, the magnitude of the wind has a major influence upon the sound pressure level on said place. The wind is nor- mally stronger at a distance above the ground than close to the ground, which causes a refraction of the upwind (as seen from

said place) sound waves having a result being similar to that of a positive temperature gradient. Moreover, the blowing direction of the wind is of importance, and down wind as seen from the wind turbine towards said place results in a higher sound pres- sure level on said place than headwind.

This means in practise that changes of the meteorological con- ditions from one period of the day to another may cause major influences upon the sound pressure level on said place for a given power generated by the wind turbine.

Furthermore, the nature of the surface over which the sound travels from said wind turbine is of great importance. Absorption of sound is normally much larger for land at such a surface than for water. There is also a difference between a smooth ground and a more irregular ground resulting in a larger sound absorp- tion.

High frequency components of the sound are more easily ab- sorbed than low frequency components, and if the distance from the wind turbine is considerable, such as at least a few kilome- tres, substantially only low frequency components, such as e. g. below 300 Hz, are still there. When said surface is land also the low frequency components will be affected by absorption if the distance is large enough, but when the surface is water such low frequency components may travel over long distances.

Accordingly, these low frequency components of the sound gen- erated by the operation of a wind turbine normally determine how the wind power plant may be operated while observing said acoustic limits on a certain place at far distance from the plant.

The sound pressure level of ambient sound, such as from the wind, traffic and the like, is also of importance for the way the sound generated by the wind turbine is experienced. Accord- ingly, when the sound pressure level of ambient sound is higher

than a high sound pressure level of the sound emanating from the operation of the wind turbine on said place the latter sound may not cause any disturbances at all, but a lower sound pres- sure level of that sound may be quite annoying when the sound pressure level of ambient sound is low, such as when there is practically no wind near the ground.

When wind power plants are planned and sited today the worst possible case with respect to the generation of sound on a place distant from the location of a wind turbine is considered and can determine the location of a wind power plant and the dimen- sioning of the wind turbines thereof with respect to the maximum blade tip speed and power that may be generated.

This means that the wind power plants in operation today can be located at a greater distance from said critical places and/or di- mensioned for a maximum power generated thereby being lower than the circumstances mostly prevailing allow. Such exagger- ated distances increase the costs of transmission lines and other equipment associated with the wind power plant especially when this is located at sea. Furthermore, the wind power plant will under many conditions, such as when said vertical tem- perature gradient is negative and/or the wind is blowing from said place towards the wind turbine, be restricted to generate less power than would be allowed for ensuring that the acoustic limits set are always observed. It is pointed out that the prob- lems and considerations mentioned above are also applicable to other types of power generating plants, such as plants utilizing water power.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an apparatus of the type defined in the introduction enabling an improvement of the operation of a power generating plant.

This object is according to the invention obtained by providing an apparatus according to the independent claim 1.

Such an apparatus allows an operation of the plant being opti- mum with respect to the conditions prevailing for the moment.

Accordingly, the invention is based on establishing a measure of how sound emanating from the operation of said unit will propa- gate between the location of the unit and a place where prede- termined acoustic limits are set for this sound, and making the control of the sound emission of the unit, such as a wind turbine, dependent upon this propagation. This means in practise that in the case of a wind turbine this could have higher power rating or be driven to generate more power than normally is accepted considering the risks of not observing said acoustic limits, since said appliance has through said measurement and calculation shown that this is possible. Furthermore, it is possible to design the plant to have a larger production capacity, so that it may generate more power under normal operation conditions, while still by the sound control method performed through the appa- ratus of the present invention, fulfil sound emission criteria dur- ing extreme conditions. Control of a wind turbine may include changing the pitch of the blades, the number of revolutions per minute of the generator or the yaw of the turbine when the gen- eration of such a high power is not allowed.

According to a preferred embodiment of the invention said ap- pliance comprises a sound receiving means, as a microphone, adapted to measure sound pressure levels of sound at said place, and the appliance is adapted to indirectly calculate said propagation of sound by presenting a value of the sound pres- sure level measured to said device. This is a very simple and cost efficient way to realize an apparatus according to the pres-4 ent invention, and the independent claim 1 is to be interpreted to also include this possibility to have in the extreme case only one microphone constituting said appliance by measuring the sound pressure level of sound at said place and indirectly cal-

culating said propagation of sound by sending the measurement results further for use by said device when controlling the unit.

An apparatus according to a preferred embodiment of the pres- ent invention is defined in appended claim 4. By using such a reference sound source in said appliance and generating a sound signal at said plant and determining how the sound pres- sure level thereof is changed in the direction towards said place it will be possible to obtain an accurate estimation of the nature of the sound generated by the unit on said place, so that the operation of the wind turbine may be adapted to observe the acoustic limits set for the sound emanating from the unit on said place.

According to another preferred embodiment of the invention said appliance comprises means adapted to measure meteorological parameters in the region of the plant and an arrangement adapted to calculate the sound pressure level of sound emanat- ing from the operation of the wind turbine on said place on the basis of these measurements and information about the sound power level of the sound emitted by the unit. This embodiment utilizes the understanding of the present inventors that the propagation of sound from the plant to a distant place is associ- ated with the meteorological conditions prevailing. By measuring these meteorological conditions this propagation of sound may be calculated and an expected value of the sound pressure level on said place of sound emanating from the operation of the unit, such as a wind turbine, at a given sound power level of the sound from the wind turbine may be established. It is pointed out that"information about the sound power level"in this context is to be interpreted to also cover the case of indirectly knowing this by for instance calculating the sound power level from cer- tain parameters of the plant and the conditions."Region"is also to be interpreted broadly, and said measurement is taken place at one or several locations between the plant and said place.

Said meteorological parameters may for instance be the height

gradient of the wind velocity and the height gradient of the air temperature.

According to another preferred embodiment of the invention, said device is adapted to carry out said control so that a maxi- mum of power is generated by the unit while observing said acoustic limits set for the sound emanating from the unit on said place.

According to another preferred embodiment of the invention, which is mostly coinciding with the embodiment last mentioned, said device is adapted to carry out said control of the unit so that a maximum revenue of the operation of the plant is possible to obtain while observing said acoustic limits set for the sound emanating from the wind turbine on said place.

According to another preferred embodiment of the invention the apparatus comprises means adapted to measure the sound pressure level of ambient sound on or near said place and send information thereabout to said device adapted to control the op- eration of the unit so that the sound pressure level of the sound emanating from the unit on said place will not be perceived. It may in many cases be convenient to use the sound pressure level of the ambient sound as a, said acoustic limit on said place, since sound emanating from the unit, such as a wind turbine, having a sound pressure level being below the sound pressure level of ambient sound will normally not constitute any problems, and it will not be a disturbance to people staying on said place depending on the masking at that place.

According to another preferred embodiment of the invention said- device is adapted to control the operation of the unit while ob- serving the threshold of hearing as a said acoustic limit of the sound pressure level for the sound emanating from the unit on said place. The threshold of hearing may be a suitable acoustic limit when the sound pressure level of ambient sound is below

that threshold and the latter will as a said acoustic limit put too strong restrictions on the operation of the unit. The sound pres- sure level of ambient sound may especially indoors be lower than the threshold of hearing.

Other acoustic limits or immission descriptors on said place may be authority regulations or requirements decided by the owner of the plant, i. e. a customer of the company producing and install- ing the plant.

According to other preferred embodiments of the invention a threshold of vibrations causing rattle-sound indoors can be ob- served as a said acoustic limit, and there may also be an acoustic limit set for the infrasound frequency range (<20 Hz), i. e. sound that may not be audible by the human ear.

According to another preferred embodiment of the invention said sound pressure level measuring means comprises at least one array of sound pressure level measuring members, which results in a reliable result of said measurement and a possibility to control the operation of the plant close to said acoustic limits.

According to another preferred embodiment of the invention the distance between adjacent members of said array is a fraction of X, in which k is the wave length of the sound of most importance with regard to said acoustic limits on said place."A fraction" means that said distance is less than S. The sound having a fre- quency corresponding to a wave length being approximately twice the distance between adjacent such members will appear especially clear and strong and be measured very accurately by said members. The sound of most importance is normally sound with lower frequencies, primarily below 300 Hz, and especially in the frequency range of 0 Hz-150 Hz.

According to another preferred embodiment of the invention said, means adapted to measure the sound pressure level of a refer-

ence sound signal is robust in the sense that it is designed to be substantially insensitive to the environmental conditions. This is important for delivering reliable measurement results, and said means should for instance be designed or protected to keep self induced noise on a lower level than the sound pressure level of the sound signal from the sound source to be measured.

Said reference sound source may have different locations, but it is preferred to arrange it on the unit, especially when this'is a wind turbine, when the plant has only one unit. When the plant has a plurality of units it is only necessary to arrange one refer- ence sound source but advantageous to arrange at least two said sound sources at opposite ends of a region covered by the units. The number of reference sound sources in a plant is de- pending upon the specific conditions for each plant.

Said reference sound source could be adapted to generate a sound signal having a lower sound power level than that of the sound typically generated by the unit, so that it will not cause any problem on said place by itself.

According to another preferred embodiment of the invention the apparatus comprises means adapted to store historical data of the operation of the plant, such as sound pressure levels, values related to the unit performance such as electric power generated by the unit, feed-in prices to an electric grid to which the plant is connected, meteorological conditions and other environmental conditions relevant for the sound propagation for possibly future evaluations. This means that the operation of the plant may be continuously improved by learning from the past.

The invention also relates to a method for operation of a plant having at least one unit according to the appended independent method claim. The advantages of that method and the preferred embodiments thereof defined in the claims dependent upon said independent method claim appear without any doubt from the

discussion above of preferred embodiments of the apparatus according to the invention.

The invention also relates to methods for siting of a power gen- erating plant having at least one unit according to the corre- sponding appended claims. Such methods may have impact on the result of said siting, since they allow an improved determina- tion of characteristics, such as the electric power rating of the unit and the plant, siting in a more appropriate location and im- proved siting-flexibility and cost optimization than the methods for siting of a plant already known and discussed above being only based upon a calculation of said worst circumstances.

The invention also relates to a method for operation of a power generating plant having an apparatus according to any of the appended apparatus claims comprising the steps of: collecting information enabling prediction of future values of pa- rameters important for the operation of the plant, carrying out a prediction of said future values of said parame- ters, comparing said predicted parameter values with economic in- formation, such as electricity energy feed-in prices and present and/or future demands of the consumers of electric power, determining the operation of the plant in dependence of the re- sult of said comparison so as to optimise the revenue obtained by the operation.

The invention also relates to a computer program and a com- puter readable medium according to the corresponding ap- pended claims. It is easy to understand that the methods ac- cording to the invention defined in the appended sets of method claims are well suited to be carried out through program instruc-

tions from a processor that may be influenced by a computer program provided with the program algoritm-steps in question.

Other advantages and advantageous features of the invention appear from the following description and the other dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS With reference to the appended drawings, below follows a spe- cific description of preferred embodiments of the invention cited as examples.

In the drawings: Fig 1 is a very schematical view illustrating a wind power plant with surroundings, Fig 2 is a very schematical view illustrating the phenomena of a positive temperature gradient and/or a positive wind gradient upon the propagation of the sound generated by a wind turbine, Fig 3 is a schematical view illustrating how the sound power- level of sound emanating from the operation of a wind turbine may be obtained, Fig 4 is a very schematical view illustrating the construction and function of an apparatus for operation of a wind power plant according to a preferred embodiment of the invention, Fig 5 is a view corresponding to that of fig 4 of a wind power plant having a plurality of wind turbines,

Fig 6 is a view illustrating a part of an apparatus according to a further preferred embodiment of the invention, Fig 7 is a flow chart illustrating a method for siting a wind power plant according to a preferred embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Fig 1 schematically illustrates a power generating plant in the form of a wind power plant 1 located at sea and having a plural- ity of wind turbines 2. As an example it may be mentioned that a turbine with a radius of 45 m may at a wind speed of 12 m/s and 17 revolutions per minute generate 3,5 MW. The blades 3 of such a wind turbine may accordingly extend far from the hub 4 of the turbine, so that the velocity of the blade tips may be high even if the number of revolutions of the turbine will be low. The movement of these blades through the air causes the aerody- namic sound discussed above. It is pointed out that some power plant parts, such as the generator associated with each turbine, gearbox, converters, transmission lines and so on have been omitted in the figures for making the illustrations clearer.

The passage of the turbine blades through the air generates a noise having a comparatively wide bandwidth being most im- portant for the frequency range of 20-4 000 Hz. The noise of the higher of these frequencies will be substantially totally ab- sorbed by the air already at short distances from the wind tur- bines. However, the low frequency components will travel longer, experiencing less absorption and these require a sub- stantial distance between the location 5 of the wind turbines and a place 6 where predetermined acoustic limits are set for sound emanating from the operation of the wind turbine. Such a place can be residential areas.

Fig 2 illustrates schematically the propagation of sound waves emanating from a wind turbine in case of a positive temperature gradient or when the wind speed is considerably higher at a distance from the ground than close to the ground, so that a so- called refraction phenomena occurs. It is shown how the sound waves 8 are refracted and by that bent back towards the ground, which means that the sound pressure level on the place 6 will be higher than would there be for instance no temperature gradient or a negative temperature gradient bending sound waves up- wardly instead.

Fig 3 schematically illustrates how a sound receiving means in the form of a microphone 20 may be calibrated for later on being used for delivering values of the sound power level of sound generated by the operation of a wind turbine. The microphone is arranged on the hub 4 of the wind turbine for measuring the sound generated by the operation of the wind turbine there.

However, this microphone 20 is arranged too close to the sound source (the wind turbine) for being able to directly measure the sound power level. For such measurement it is necessary that the distance between the sound source and the sound receiving means is that great that the sound receiving means may con- sider the sound source as a point sound source. This means that the microphone may only be used for indirectly measuring the sound power level once it has been calibrated. Such a cali- bration takes place by arranging a further sound receiving means 21 on the ground at a distance from the base of the wind turbine fundament being at least as large as the height of the wind turbine from the base to the maximum height of a blade tip.

The wind turbine is then operated and the sound power level is measured by the sound receiving means 21 while comparing the results with measurement results simultaneously obtained by the sound receiving means 20. After this calibration has taken place the sound power level may later on during the operation of the plant be indirectly measured by the microphone 20.

Fig 4 illustrates an apparatus for operation of a wind power plant according to a preferred embodiment of the invention. This ap- paratus has one reference sound source 9 arranged on the wind turbine 2 and adapted to generate a reference sound signal that may be discriminated by sound receiving means from ambient sound and directed towards a said place 6. The reference sound signal has preferably a sound pressure level arriving to said place 6 being below the sound pressure level of the ambient sound there or even below the threshold of hearing. There are sound analysis methods based on adaptive signal analysis or m- sequence correlation techniques allowing discrimination of sound down to 15 dB lower than ambient sound.

The apparatus further comprises means 10 adapted to receive said sound signal and measure the sound pressure level thereof. This means 10 is located on the path of the sound sig- nal towards said place 6, but it may also be located on said place, which also includes close therebehind. The means 10 comprises an array of sound pressure level measuring members 11 in the form of microphones, which preferably are arranged so as to define a surface being directed substantially perpendicu- larly to the direction of said sound signal towards the place 6.

The members 11 may here form a cross and may be 12 to the numbers, and they are normally 5-20 and preferably 10-15.

These members are robust in the sense that they are designed to be substantially insensitive to the environmental conditions, which means that for instance wind or snow will not disturb the operation thereof by causing self induced noise being higher than the sound pressure level of the sound signal from the sound source to be measured. The distance between adjacent members 11 of the array is preferably a fraction of X, in which k is the wave length of the sound of most importance with regard to said acoustic limits on said place, which in practise mostly, means a distance of 1-3 meters.

The reference sound source 9 is preferably adapted to primarily emit sound having a frequency below 300 Hz, since only such sound may still be audible when the sound emanating from the. operation of the wind turbine will reach the place 6, so that it is only interesting to investigate how sound of such frequencies is influenced when propagating through the air. The previous part of this paragraph is essentially valid for propagating through air over water, but when the ground is constituted by land, this has such an impedance that the low frequency content of the sound deriving from the operation of the wind turbine will experience higher absorption, in fact it has the largest absorption ability at frequencies of 100-300 Hz, so that the preferable frequency- range for a reference sound source will in that case be some- what higher than for the case of a wind power plant located far out at sea.

It is illustrated how the array of microphones 11 is sending in- formation about the sound pressure level of the sound emanat- ing from the sound source 9 to an arrangement 12 also receiving information about the sound power level of the sound emitted by the reference sound source for calculating the sound pressure level on said place 6 of the sound generated by a given opera- tion of the wind turbine. The sound emanating from the reference sound source may here and in the other embodiments be discriminated from ambient sound by carrying out coherence measurements. This may here be done by means of the micro- phones 11 and the microphone 14 arranged on different loca- tions. When coherence is obtained it may be excluded that other sound than that emanating from the turbine is included. Accord- ingly, by establishing a relation between the sound pressure level of the sound signal at a determined frequency-range and the sound power level of the reference sound signal emitted a corresponding relation for that frequency-range of the sound' generated by the wind turbine and the sound emanating from the operation of the wind turbine on said place 6 may be estab- lished. This calculation will accordingly automatically consider

the different phenomena, such as temperature gradients, wind gradients, moisture content of the air and the like, mentioned above, through said real measurement.

It is here important to discriminate a sound from the reference sound source from true ambient sound and sound emanating from the wind turbine. This may be done through different tech- niques, for instance by using an omni-directional microphone in combination with a microphone for which the receiving direction is changed for determining from what direction most sound ar- rives. Another possibility is to use a reference source with a deterministic signal and the coherence technique. A further pos- sibility is to use the technique of forming an average of the sig- nals received by the microphone over a longer period of time, which means that stationary sound tends to increase in impor- tance, whereas varying ambient sound tends to disappear.

The result of the calculations of the arrangement 12 is sent to a device 13 for controlling the wind turbine 2, which is adapted to control the wind turbine depending upon these results for ob- serving the acoustic limits set for the sound emanating from the wind turbine on said place 6. The transmission of information between different parts of the apparatus is preferably enabled through a wire-less communication system but not restricted thereto.

It is illustrated in fig 4 how the apparatus may also comprise means 14 adapted to measure the sound pressure level of am- bient sound on or near said place 6 and send information there- about to said arrangement 12 for considering the sound pres- sure level of ambient sound as an acoustic limit when determin- ing the possible operation of the wind turbine. Furthermore, means 17 may be there for calibrating the reference sound source 9 for instance by measuring the current and voltage of the reference sound source or the sound power level of the sound signal emitted by the reference source. The arrangement 12 may also be associated with means 18 adapted to store his-

torical data of the operation of the plant, such as sound pres- sure levels, feed-in prices to an electric grid to which the plant is connected, meteorological conditions and other environmental conditions relevant for the sound propagation for possibly future evaluations.

Fig 5 illustrates an embodiment of the present invention in which the wind power plant has a plurality of wind turbines, and in such a case reference sound sources 9 are instead arranged at opposite ends of a region covered by the wind turbines.

Fig 6 illustrates how the apparatus may also comprise means 15 adapted to measure meteorological parameters, such as tem- perature gradients and wind gradients. These may be arranged on a mast 16 or be performed by a sodar system. In this case the device 12 is adapted to consider the values of these mete- orological parameters so established when controlling the op- eration of the wind turbine. These meteorological measurements may further improve the accuracy of the calculation of the sound pressure level of the sound emanating from the wind turbine on the place 6.

It is obvious that the invention offers the possibility to operate a wind power plant while optimising the revenue obtained by the operation. In the case that the electricity network connected to the wind power plant allows feeding in more electric power and the wind is strong enough for enabling this the apparatus ac- cording to the invention may determine that the power genera- tion may be increased without any sound problems or it may not be increased because any acoustic limits set will then not be observed any longer. This way of proceed may increase the revenue for the owner of the plant. Conversely, there may be no demand from the electricity network of more electric power, but a wind enabling an increase of the power generation is there and the apparatus according to the invention reports that the sound will not constitute any problem. The power generation

may then be increased and the energy may be stored in any magazine, such as in potential energy of any medium or the like, which will also contribute to an increase of said revenue. It is also possible to collect information enabling prediction of future values of parameters important for the operation of the plant, such as winds around the plant and near said place, and then carry out a prediction of the future values of these parameters, so that they may be compared with economic information, such as electricity energy feed-in prices and present and/or future demands of the consumers of electric power for determining the operation of the plant in dependence of the result of said com- parison so as to optimise the revenue obtained by the operation.

The invention also comprises a method for siting of a wind power plant having at least one wind turbine, and the steps of such a method is schematically illustrated in fig 7. A reference sound source 9 according to above is firstly arranged (A) on the planned location of the wind turbine, such as a mast for pros- pecting purposes, the mast being equipped with meteorological measurement devices. The reference sound source is then brought (B) to generate a sound signal that may be discrimi- nated by sound receiving means from ambient sound in the di- rection towards a place where predetermined acoustic limits are set for sound emanating from the operation of the wind turbine.

The sound signal is then received and the sound pressure level thereof is measured (C) on said place or on the path of the sound signal towards said place. Meteorological parameters in the region of said planned location of the wind turbine are also measured in this step, or they are simulated. The sound pres- sure level on said place of the sound that would be generated by a given operation of the wind turbine is calculated (D) on the basis of information about the sound pressure level of the sound signal on the location of said measurement and the sound power level thereof at the sound source. Finally, the result of said cal- culation and information obtained in step (C) are used for de- termining characteristics, as dimensions, of the wind turbine and

the wind power plant sited and/or the result of the calculation is used (E) for determining whether the location of the wind power plant is to be changed. Such a method for siting of a wind power plant may result in savings of financial means while reducing the environmental impact of the wind power plant during the later operation thereof. In alternative embodiments the meteorological measurements may be replaced by simulations of the meteoro- logical conditions, in another alternative embodiment the sound measurements may be replaced by calculation methods for sound propagation.

The invention is of course not in any way restricted to the pre- ferred embodiments described above, but many possibilities to modifications thereof would be apparent to a person with ordi- nary skill in the art.

"On the path of the sound signal towards said place"as used in the claims is to be interpreted broadly and comprises also a o- cation on a continuation of said path behind or beside said place.

It is pointed out that a said acoustic limit may be freely chosen by the operator of the wind power plant and also be changed when desired.

It is pointed out that said place can be freely chosen.

The reference sound source may be there in a different number than shown and it may also be otherwise located, such as be- tween two wind turbines.

It will also be possible to add equipments for measuring sound generated in water by wind turbines located at sea for studying the influence of such sound upon the life in the sea. In this case said place will be located under water. This embodiment is

schematically indicated in Fig 1 by a sound receiving means 22 arranged in the sea.

It would be possible to present the data of the measurements and/or the calculations carried out to e. g. the public through all types of especially wire-less communication systems, the Inter- net, telephone, radio, television and so on.

It is pointed out that after some period of time of operation of the wind power plant, such as a couple of years, that many data may have been collected, that the equipment of the apparatus may be simplified. For instance, it may then be possible to re- move the reference sound source and only rely upon coherence' measurements. Furthermore, the meteorological measurements may be reduced to the extent and also to the frequency and historical data thereof be used for the calculations instead.