VAN VEEN, Theo, Antoon (Helsingborg 47, TD Schiedam, NL-3124, NL)
CLAIMS
1. Method for the reduction of sound travelling in a wave-guide in the air above the ground, comprising the provision below the ground of at least a top layer which, at the interface of the top layer and air, has an acoustic im- pedance which substantially matches the acoustic impedance of the air.
2. Method according to claim 1, wherein water is excluded as much as possible from said at least one top layer. 3. Method according to claim 1 or 2, wherein the top layer is part of an assembly comprising at least two superimposed layers with different density and/or different speed of sound and/or different thickness.
4. Method according to claim 3, wherein the assem- bly of at least two superimposed layers is designed to create a mass-spring system.
5. Method according to claim 3 or 4, wherein the assembly of at least two superimposed layers provides a mode coupling to secondary waves (i.e. particles moving perpen- dicularly to the direction of sound propagation) and/or primary waves (i.e. seismic waves, longitudinal sound propagation) and/or Rayleigh waves (i.e. rotation of particles) and/or Love waves below the ground surface.
6. Method according to any of the claims 3-5, wherein the assembly of superimposed layers comprises at least three superimposed layers with an alternation of two different layer types.
7. Method according to claim 6, wherein the assembly comprises alternating layers of sand and polystyrene foam.
8. Method according to any of the previous claims, wherein the effective acoustic impedance of the layer or layers in the ground is matched for the incident sound wave properties, such as for example frequencies, direction of sound propagation, depending on the type of layer (s), the dimensions of the layers and orientation in the ground.
9. Method according to any of the previous claims, wherein each layer below the ground is oriented in a direction perpendicular to the maximum intensity of the sound.
10. Method according to any of the previous claims, wherein below the ground further a grid structure is provided for scattering and/or absorbing the sound. 11. Method according to claim 10, wherein the grid structure comprises bars.
12. Method according to claim 11, wherein the grid structure comprises cylindrical structures, e.g. pipes, filled with air. 13. Method according to claim 12, wherein the cylindrical structures have holes facing the air.
14. Method according to any of the previous claims, for the reduction of aircraft sounds, especially in the range of 15 Hz to 40 Hz, during the first part of take- off, wherein said layer (s) below the ground are provided in a range starting at the lateral side of the runway and extending until at least 100 meters from the runway.
15. Method according to claim 14, wherein said range extends until a distance of at least 300 meters from the runway.
16. Method according to claim 15, wherein said range extends until a distance of at least 500 meters from the runway. |
Method for the reduction of sound
The invention relates to a method for the reduction of sound travelling in a wave-guide located in the air above the ground.
Under specific circumstances noise, for example aircraft noise during the first part of take-off when the aircraft is gaining speed at a runway, can be very disturbing at a great distance from the source of the noise. This especially applies for sound frequencies in the range from 15 Hz to 40 Hz. In the above example of aircraft noise an increase of low frequency noise above all occurs when the aircraft still has contact with the runway, and runs its engines at high power, whereas further the wind conditions are such that the wind is blowing substantially towards an area of interest (the area suffering from the disturbing noise) with the right (vertical) wind gradient. Other factors which may be of influence are, among others, the (vertical) temperature gradient and/or a combination of wind and temperature gradients.
Under such conditions a situation may be created in which noise emanating from the source of noise in a specific range of angles will propagate above the ground in a so-called wave-guide between the ground surface and a virtual upper limit. Sound waves will successively reflect downwards at the upper limit due to the (vertical) gradient (the effective speed of sound increases with altitude) in the sound speed caused by the summation of the effects of wind gradient and temperature gradient, and upon reaching the ground surface will reflect upwards again, and so on. Basically the reflection at the ground surface is a result of an acoustic impedance boundary present at the ground.
The above wave-guide effect especially occurs in a range of sound frequencies from about 15 Hz to about 40 Hz.
The sound propagating through this wave-guide is less attenuated during propagation in comparison to the attenuation that occurs with spherical propagation of sound.
State of the art attempts to reduce disturbing noise substantially comprise structures such as shields, hills, trees and vegetation located between the source of noise and the area of interest. When applied for reducing noise from aircraft at an airfield, such known structures are not compatible with safety requirements calling for free, unobstructed areas alongside the runway. Moreover such structures only are effective in reducing high frequency noise, whereas noise in the above range of 15 Hz to 40 Hz hardly is reduced.
In view of the above it is an object of the pre- sent invention to provide an improved method for the reduction of sound travelling in a wave-guide in the air above the ground.
In accordance with the present invention the method comprises the provision below the ground of at least a top layer which, at the interface of the top layer and air, has an effective acoustic impedance which substantially matches the acoustic impedance of the air.
As a result the ground surface no longer acts as an acoustically λ hard' surface which reflects the sound up- wards. Because of the improved impedance match between the air and the adjoining top layer of the ground the sound now can enter the ground where it is propagated and thus absorbed by means of different mechanisms. For example an excitation of the ground structure may be caused which absorbs the sound energy; or a coupling to pores in the ground filled with air may be caused. For the lower frequencies mentioned above the former mechanism is expected to be the most practical.
In a preferred embodiment of the method according to the present invention water is excluded as much as possi-
ble from said at least one top layer. Such a measure is effective in improving the air to ground coupling (impedance matching) . When there is much water in the ground, the ground reflection and thus the wave-guide for ground waves is more pronounced (as can be seen in the difference between summer and winter: in summertime less sound energy is reflected by the acoustically softer ground) .
A positive side-effect of providing a significant reduction of water at the surface is a substantial reduction of the possibility for the creation of local fog which is an important gain for the safety and capacity of an airport.
Preferably, further, the top layer is part of an assembly comprising at least two superimposed layers with different density and/or different speed of sound and/or different thickness. As a result the transmission of low frequency sound through the layers in the ground below the ground surface may be optimized in the manner of a low frequency sound filter.
The absorption of the sound propagating in the ground may be enhanced further when, in accordance with yet another embodiment of the present method, the assembly of at least two superimposed layers is designed to create a mass- spring system.
Other measures resulting in such an improved ab- sorption are provided by methods wherein the assembly of at least two superimposed layers provides a mode coupling to secondary waves (i.e. particles moving perpendicularly to the direction of sound propagation) and/or primary waves (i.e. seismic waves, longitudinal sound propagation) and/or Rayleigh waves (i.e. rotation of particles) and/or Love waves below the ground surface.
In a practical embodiment of the method according to the present invention the assembly of superimposed layers comprises at least three superimposed layers with an alter- nation of two different layer types. That means that succes-
sive horizontal layers are created in which all odd layers are of the same type, as are the even layers.
It is noted that λ of the same type' does not necessarily mean that such layers have the same dimensions (es- pecially thickness) , but it merely tries to express that such layers are constituted of the same (or equivalent) materials .
For example it is possible that the assembly comprises alternating layers of sand and polystyrene foam. Such layers may be designed for an optimal angle of incidence of the sound and frequency concerned, and even for multiple sounds and multiple frequencies. Depending on the density and the speed of sound in the layers the assembly can be optimized for the absorption of the fore mentioned low fre- quency noise.
As a further possibility to improve the method according to the present invention, the effective acoustic impedance of the layer or layers in the ground is matched for the incident sound wave properties, such as for example fre- quencies, direction of sound propagation, depending on the type of layer (s) in the ground.
When, in accordance with another embodiment of the method according to the present invention each layer below the ground is oriented in a direction perpendicular to the maximum intensity of the sound, the sound wave will contact the ground most effectively regarding the air to ground coupling.
According to another embodiment of the present method, below the ground further a grid structure is pro- vided for scattering the sound. Such a grid, for example, may comprise bars. The bars may be optimized by dimension and distance from each other. Also the density and speed of sound in the bars and in the surrounding ground may be optimized with respect to the scattering of sound.
As an example, in addition to the layered structures, cylindrical structures with air inside can be located just below the ground surface. When excited by pressure fluctuations of the low frequency sound at the right acous- tical modes, these structures are able to absorb acoustical energy. This absorption can be achieved by transfer of acoustic energy to vibration energy and heat in the pipes which are located below the surface. The vibration energy will be absorbed by the surrounding ground. Care should be taken to keep water out of these pipes. The sound absorption of the pipes can be increased by making holes in the pipe which face the air. These holes will actuate the modes of the pipe more efficiently by the pressure changes of the sound impinging onto the pipe in the ground. In a practical embodiment of the method according to the present invention, meant for the reduction of aircraft sounds, especially in the range of 15 Hz to 40 Hz, during the first part of take-off, said layer (s) below the ground are provided in a range starting at the lateral side of the runway and extending until at least 100 meters from the runway. The range can vary depending on the required noise reduction in a specific situation.
The effectiveness of such a method is further improved when said range extends until a distance of at least 300 meters from the runway, and more preferably when said range extends until a distance of at least 500 meters from the runway.
It is noted that the present method for reducing sound travelling in a so-called wave-guide may be combined with known methods of absorbing sound by means of, for example, sound absorbing structures (e.g. shields) or vegetation (which structures, when used in the context of reducing aircraft noise can be positioned at a larger distance from the runway such as not to create any safety risk) . The sound re-
duction achievable with the method in accordance with the invention may be considerable (up to βdB and more) .
Further it is noted that the method in accordance with the present invention also may be applied in other situations, such as on aircraft carriers or at locations where reduction of high intensity and/or low-frequency noise sources is required.
The invention is not limited to the embodiments described above which may be varied widely within the scope of the invention as defined by the appending claims.
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