SYSTEM AND METHOD FOR ACTIVELY REDUCING NOISE PASSING THROUGH AN OPENING IN A SOUND BARRIER

Abstract

 Described and claimed is a system for actively reducing noise passing through an opening in a sound barrier, the system comprising sound blocking arrangements. Each sound blocking arrangement comprises a sound source, a sensor and a control circuit. The sensor of each sound blocking arrangement is arranged to sense a sound characteristic in a location in the opening. The control circuit of each sound blocking arrangement is adapted to generate from the signal representative of a sensed sound characteristic a control signal for the sound source of the respective sound blocking arrangement and the sound source of each sound blocking arrangement is adapted to emit sound in accordance with the received control signal. The control signal for each sound blocking arrangement is determined such that the sound characteristic sensed by the sensor of the respective sound blocking arrangement is minimized and the distance between the location in which the sensor of the respective sound blocking arrangement senses a sound characteristic and the sound source of the respective sound blocking arrangement is kept as small as possible.

Description

[0001] The present invention relates to a system for actively reducing noise passing through an opening in a sound barrier. The invention further relates to a method for actively reducing noise passing through an opening in a sound barrier.

[0002] Different kinds of active noise reducing or noise canceling systems which use sound from a secondary sound source to reduce or cancel unwanted sound, i.e., noise, from another sound source have been known for some time. One particular kind of noise reducing system is primarily provided for reducing noise passing through a well-defined opening or interface in a sound barrier. Such systems may, for example, be used for reducing noise entering a room through a window, wherein a room is an example of a confined space delimited by walls that form a sound barrier and a window is an example of a defined opening or interface.

[0003] An exemplary system of the above kind is described in DE 10 2005 016 021 A1. The system comprises a plurality of secondary sound sources, a control circuit, a reference sensor and an error sensor which are used to reduce noise passing through an opening in a sound barrier into a confined space. The noise is created outside of the confined space by a generic primary sound source and enters the inside of the confined space through an opening in the sound barrier in form of sound waves. For reducing the noise inside the confined space, the secondary sound sources are arranged in the opening facing towards the confined space such that they can emit sound waves towards the confined space. The reference sensor is arranged between the primary sound source emitting the sound waves that are perceived as noise inside the confined space and the secondary sound sources. The error sensor is arranged inside the confined space, i.e., on that side of the opening facing away from the primary sound source.

[0004] Both the error sensor and the reference sensor are connected to the control circuit for transmitting an error signal and a reference signal, respectively. The control circuit calculates control signals for driving the secondary sound sources such that the overall noise or sound level at the error sensors is minimized. To this end, the system requires both the reference signal captured outside of the confined space and the error signal captured inside the confined space. The reference signal is used to provide an open-loop or non-feedback control of the sound emitted by the secondary sound sources as the incoming noise is picked up by the error sensor before it reaches the confined space. The error signal is used to improve the open-loop control by picking up the noise not canceled by the system.

[0005] This system has been found disadvantageous as the reference sensor has to be arranged on that side of the sound barrier facing away from the confined space, i.e., between the secondary sound sources and the primary sound source. In many applications an arrangement of a reference sensor outside of the confined space will not be possible.

[0006] A system without a reference sensor is described in DE 10 2007 012 611 A1. The active noise control system described here also uses a plurality of secondary sound sources, a control circuit and a plurality of error sensors and is also designed for reducing noising passing through a sound barrier by shielding the sound that enters the confined space through a defined opening. The secondary sound sources are arranged in the opening. They face towards the inside of the confined space and, thus, away from any primary sound source arranged outside the confined space and emitting the sounds that are conceived as noise inside the confined space, i.e., on that side of the sound barrier facing away from the primary sound sources. One focus of this system is the arrangement of the secondary sound sources and their directional characteristic. For optimizing the attenuation of noise inside the confined space the secondary sound sources are arranged such that their directional characteristics meet the directional characteristics of the noise at the defined opening. Control signals for the secondary sound sources are generated by a control circuit from error signals picked up by the error sensors. These error sensors are preferably arranged in an equidistant matrix arrangement inside the confined space.

[0007] The above active noise control system may improve the quality of the noise reduction inside the confined space. However, as the system does not use a reference sensor and the error sensors are arranged inside the confined space, i.e., in the propagation direction of the sound waves which are conceived as noise on the far side of the secondary sound sources, a timely adaption to changing noise is not possible. The system is rather only capable of reducing stationary noise.

[0008] In view of the above it is an object of the present invention to provide a system for actively reducing noise passing through an opening in a sound barrier that overcomes at least some of the disadvantages of the systems disclosed in the prior art. In particular, the system should be able to adapt itself to changing noise and not require a reference sensor arranged between the secondary sound sources and a primary sound source generating the sound waves perceived as noise.

[0009] In a first aspect the problem is solved by a system for actively reducing noise passing through an opening in a sound barrier. The opening is delimited by a boundary. The system comprises one or more sound blocking arrangements. Each sound blocking arrangement comprises a sound source, a sensor and a control circuit. Each sound blocking arrangement is adapted to be arranged at the opening such that the sound source of the sound blocking arrangement is arranged to emit sound and the sensor of the sound blocking arrangement is adapted to sense a sound characteristic in form of a sound pressure and/or a sound velocity and/or a sound intensity in a location in the opening. The sensor of each sound blocking arrangement is adapted for transmitting a signal representative of the sensed sound characteristic to the control circuit of the respective sound blocking arrangement. The control circuit of each sound blocking arrangement is adapted to receive the signal from the sensor of the respective sound blocking arrangement, generate from the received signal a control signal for the sound source of the respective sound blocking arrangement and transmit the determined control signal to the sound source. The sound source of each sound blocking arrangement is adapted to receive the control signal transmitted by the control circuit of the respective sound blocking arrangement and emit sound in accordance with the received control signal. The control signal for each sound blocking arrangement is determined such that the sound characteristic sensed by the sensor of the respective sound blocking arrangement is minimized. For each sound blocking arrangement the distance between the location in which the sensor of the respective sound blocking arrangement senses a sound characteristic and the sound source of the respective sound blocking arrangement is not greater than the distance between the location in which the sensor of the respective sound blocking arrangement senses a sound characteristic and the sound source of any other sound blocking arrangement of the system.

[0010] In other words, the system according to the present invention comprises at least one and preferably a plurality of sound blocking arrangements. Each of the sound blocking arrangements comprises a sound source, a sensor and a control circuit. The sound source may, for example, be a loudspeaker. The sound source which can also be referred to as a secondary sound source should be adapted to emit sound in the frequency range of the noise that shall be reduced. The sensor is adapted to sense sound characteristic and could, for example, be formed by a microphone adapted to sense a sound pressure. Finally, the control circuit can, for example, be formed by a single microcontroller or a plurality of electronic elements. Both the sensor and the sound source of each sound blocking arrangement are functionally connected to the control circuit such that signals can be transmitted from the sensor to the control circuit and from the control circuit to the sound source. The sensor and the sound source can be connected directly to the control circuit. However, it is also possible that intermediate elements are arranged between the control circuit and the sensor or the sound source, respectively.

[0011] The sensors and sound sources of all sound blocking arrangements are arranged at the opening in the sound barrier. The opening may, for example, be a window opening to a room inside a house where the walls of the room form a sound barrier. The opening could also be a door to a department store where the walls of the department store form a sound barrier. In another example, the sound barrier is formed by a machine housing or machine frame surrounding a machine and the opening is provided in the machine housing. In this example, the primary sound source generating the noise may be the machine inside the machine housing and the exterior is largely shielded from the noise generated by the machine by the machine housing that forms the sound barrier. To further reduce the noise, the system according to the present invention can be placed around the opening in the machine housing.

[0012] In any case, the opening forms a path for sound from primary sound sources arranged on one side of the sound barrier to the other side of the sound barrier. The sound of these primary sound sources is unwanted on the other side of the sound barrier and, therefore, conceived as noise. The sound blocking arrangements are provided for reducing the noise as it passes through the opening. The opening itself is delimited by a boundary or border, for example, a window frame, and may extend in plane.

[0013] Each of the sensors is arranged such that it can sense a sound characteristic in form of a sound pressure and/or a sound velocity and/or a sound intensity in a different location in the opening than any of the other sensors, i.e., the sound characteristic is measured in as many different positions as there are sound blocking arrangements in the system. A signal representative of the sound characteristic sensed by each of the sensors is transmitted from each sensor to the control circuit of the respective sound blocking arrangement.

[0014] The signals representative of a sound characteristic are received at the respective control circuits and used to determine a control signal for the sound sources of the respective sound blocking arrangements. The control signal is determined with the object to minimize the sensed sound characteristic at the location where the sensor of the respective sound blocking arrangement senses the sound characteristic.

[0015] It should be noted that the term "to minimize" is not understood as reaching an absolute minimum. It is understood in that a relative, technically feasible minimum shall be reached. In this regard, minimizing the sound characteristic sensed by a sensor refers to the aim of reaching a technically feasible minimum sound characteristic, e.g., a minimum sound pressure. Methods of determining a control signal for a sound source to minimize a sound characteristic at a distant location are well known in the prior art. It is, for example, possible to use a linear predictor algorithm to determine a control signal.

[0016] Finally, the control signal determined by a control circuit is transmitted to the sound source of the respective sound blocking arrangement. Here, the control signal is received and used to drive the sound source to generate sound waves in accordance with the control signal. The sound waves emitted by the sound source than interfere with the sound waves received from the primary sources and reduce the sound characteristic at the corresponding location where the sound characteristic is sensed by the sensor of the respective sound blocking arrangement, i.e., the corresponding sensing location. As the sound characteristic reduced or minimized at the sensing location is again sensed or picked up by the respective sensor, a closed-loop or feedback-loop control of each sound blocking arrangement is provided. Prior art systems of a similar kind relied largely on open loop control for reducing changing noise passing through an opening in a sound barrier.

[0017] To make sure that the sound characteristic, e.g., the sound pressure, can be successfully reduced as much as technically feasible, the distance the sound wave travels between each sound source and the corresponding sensing location has to be as small as possible to reduce the time a single loop of the closed-loop control requires. To this end, it is required that the distance between the sensing location of each sound blocking arrangement and the corresponding sound source is not greater than the distance to any sound source of any other sound blocking arrangement.

[0018] The system according to the present invention advantageously provides a means for reducing noise passing through an opening in a sound barrier using a closed-loop control which does not require a sensor arranged between the opening and the primary sound source. Further, as a closed-loop control with a sensor arranged in close vicinity to the sound source of the respective sound blocking arrangement is provided, the control response time of each sound blocking arrangement is small enough to respond to changing noise.

[0019] In a preferred embodiment the sound source and the sensor of each sound blocking arrangement are arranged in a common housing. Further, the control circuit of each sound blocking arrangement is preferably arranged in the common housing with the sensor and the sound source of the respective sound blocking arrangement. Thus, the sound blocking arrangements can be provided in a compact form which has advantages both in regard to installing the sound blocking arrangement in an opening and in regard to the control response time of the sound blocking arrangements. Both the travel or propagation time of sound emitted by the sound source of each sound blocking arrangement to the respective sensor and also the signal propagation time in the circuits of the sound blocking arrangement can be reduced further. This improves in particular the quality of the reduction of time-variant noise.

[0020] It is further preferred that for each sound blocking arrangement the sensor and the sound source of the sound blocking arrangement are arranged such that a travel time of sound emitted by the sound source to the sensor is minimized. Once again, the term "to minimize" does not refer to an absolute minimum in the travel time but to a relative, technically feasible minimum.

[0021] In a preferred embodiment each sound blocking arrangement comprises an analog digital converter for converting an analog signal of the sensor of the same sound blocking arrangement to a digital signal with a sampling rate. The sensor of each sound blocking arrangement has a sensor cutoff frequency. The sampling rate of the analog digital converter of each sound blocking arrangement exceeds the sensor cutoff frequency at least by a factor of two.

[0022] In other words, each of the sound blocking arrangements comprises an analog digital converter which converts the analog signal sensed by the sensor of the sound blocking arrangement into a digital signal. The analog digital converter can be part of the sensor, the control circuit or a separate element arranged between the sensor and the control circuit of a sound blocking arrangement. Thus, the signal transmitted from the sensor to the control circuit can be transmitted in form of a digital signal, an analog signal or can be converted from an analog signal to a digital signal while being transmitted from the sensor to the corresponding control circuit. The sound characteristic can only be sensed by the sensor in a limited frequency range due to inherent structural limitations of the sensor. The upper end of the frequency range is commonly referred to as the cutoff frequency or corner frequency and may, for example, be defined as the frequency at which the output power of the circuit drops approximately to half. The cutoff frequency of the sensor can, for example, be 10 kHz. The sound characteristic of sound waves impinging on the opening at frequencies above the cutoff frequency are not picked up or only picked up with a lower sensitivity. Thus, the sensor serves as a low pass filter.

[0023] As the analog digital converter operates at a sampling rate exceeding the cutoff frequency of the sensor at least by a factor of two, the Nyquist-Shannon sampling theorem is satisfied. Thus, the system does not require any additional analog low-pass filter or anti-aliasing filters which delay signal processing and increase the control response time of the sound blocking arrangements. In other words, by using a high sampling rate, additional filters can be avoided which reduces the time each loop in the closed-loop control of each of the sound blocking arrangements requires. Therefore, the system has further improved capabilities of attenuating or reducing time-variant noise.

[0024] It is further preferred that each sound blocking arrangement comprises the analog digital converter with the sampling rate. The sound source of each sound blocking arrangement has a sound source cutoff frequency and the sampling rate of the analog digital converter of each sound blocking arrangement exceeds the sound source cutoff frequency of the respective sound blocking arrangement at least by a factor of two. Here, in the same manner as previously described, the sound source itself is used as a low pass filter which avoids the need of reconstruction filters for isolation of one or more desired part of the analog signal and also increases the control response time of the sound blocking arrangements.

[0025] In a preferred embodiment each sound blocking arrangement comprises a switching amplifier for amplifying the control signal generated by the control circuit of the respective sound blocking arrangement. The switching amplifier of each sound blocking arrangement generates a pulsed analog control signal, wherein the pulsed analog control signal is used for driving the sound source of the respective sound blocking arrangement. A switching amplifier is a digital amplifier that can advantageously be provided on the same chip as the control circuits determining the control signal for the sound sources. Thereby, the overall dimension and power consumption of the system can be reduced as compared to the use of an analog amplifier. Switching amplifiers generate a pulsed analog output signal with a sampling rate that corresponds to the sampling rate of the analog digital converter. Commonly, a reconstruction filter is required between a switching amplifier and a sound source for demodulation of the analog pulsed output signal for driving the output source. However, in the present embodiment the frequency of the pulsed output signal of the switching amplifier exceeds the cutoff frequency of the sound source at least by a factor of two. Hence, the limited frequency response of the sound source itself smoothens the pulsed output signal and no reconstruction filter is required. This further reduces the control response time of the sound blocking arrangements.

[0026] In addition, the system preferably comprises a plurality of sound blocking arrangements. The sensor of at least one sound blocking arrangement is adapted for transmitting the signal representative of the sensed sound characteristic to the control circuit of at least one other sound blocking arrangement. The control circuit of the at least one other sound blocking arrangement is adapted to receive the signal representative of the sensed sound characteristic from the sensor of the at least one sound blocking arrangement and to generate the control signal from all signals representative of sensed sound characteristics received by the control circuit. For example, the signals captured by the sensors of two adjacent sound blocking arrangements of the system can be exchanged to improve the noise reduction.

[0027] It is furthermore preferred if the system comprises a plurality of sound blocking arrangements arranged in a common elongated housing, wherein the elongated housing extends along a longitudinal direction and the sound sources of the plurality of sound blocking arrangements are aligned along the longitudinal direction. In other words, the sound blocking arrangements or at least some of the sound blocking arrangements of the system are arranged in a common housing which is formed as an elongated bar or beam. Such bars can, for example, be arranged along the boundary delimiting the opening to the confined space.

[0028] In a second aspect the problem is solved by a method for actively reducing noise passing through an opening in a sound barrier, wherein the opening is delimited by a boundary. The method comprising the following steps:

• sensing a sound characteristic in form of a sound pressure and/or a sound velocity and/or sound intensity of sound waves in one or more locations in the opening,
• determining from each of the sensed sound characteristics a control signal for a separate sound source arranged in the opening and
• emitting sound with each of the sound sources in accordance with the control signal determined for the respective sound source.

[0029] The control signal for each sound source is determined such that the sensed sound characteristic from which the control signal for the respective sound source has been determined is minimized. The distance between the location at which the sound characteristic is sensed from which the control signal for a specific sound source is determined and the respective sound source is not greater than the distance between the location at which the sound characteristic is sensed from which the control signal for the specific sound source is determined and any other sound source used in the method.

[0030] In a preferred embodiment the sound sources are arranged such that a travel time of sound emitted by each of the sound sources to the location where the sound characteristic is sensed from which the control signal of the respective sound source is determined is minimized.

[0031] It is further preferred that in each location the sound characteristic is sensed up to a sensing cutoff frequency and an analog signal representative of the sensed sound characteristic is converted to a digital signal with a sampling rate exceeding the sensing cutoff frequency up to which the respective sound characteristic is sensed at least by a factor of two.

[0032] Further, each sound source preferably has a sound source cutoff frequency and each signal representative of a sensed sound characteristic is converted to a digital signal with a sampling rate exceeding the sound source cutoff frequency of the sound source for which the control signal from the respective sensed sound characteristic is determined at least by a factor of two.

[0033] In another preferred embodiment each control signal determined for one of the sound sources is converted from a digital control signal to a pulsed analog control signal, wherein the pulsed analog control signal is used for driving the respective sound source.

[0034] Furthermore, the control signal for at least one of the sound sources used in the method is preferably additionally determined from a sound characteristic sensed at a location which is at a distance from the respective sound source that is not smaller than the distance between the location at which the sound characteristic is sensed from which the control signal is additionally determined and at least one other sound source used in the method.

[0035] Finally, a plurality of sound sources used in the method are preferably arranged in a common elongated housing, wherein the elongated housing extends along a longitudinal direction and the plurality sound sources are aligned along the longitudinal direction.

[0036] The definitions and additional aspects of the various embodiment of the system according to the present invention also apply to the method according to the present invention. In addition, the different embodiments of the method according to the present invention share the advantages of the embodiments of the system according to the present invention comprising corresponding features.

[0037] In the following, exemplary embodiments of the system and the method according to the present invention will be described with reference to the drawings, wherein

Fig. 1

shows a first exemplary arrangement of a plurality of exemplary embodiments of systems according to the present invention arranged in an opening in a sound barrier,

Fig. 2

shows a second exemplary arrangement of a plurality of exemplary embodiments of systems according to the present invention arranged in an opening in a sound barrier,

Fig. 3a

shows a perspective view of an exemplary embodiment of a system according to the present invention,

Fig. 3b

shows a sectional view of the exemplary embodiment of Fig. 3a along the line A-A,

Fig. 4a

shows a perspective view of another exemplary embodiment of a system according to the present invention,

Fig. 4b

shows a sectional view of the exemplary embodiment of Fig. 4a along the line B-B,

Fig. 5

shows a block diagram of an exemplary embodiment of a system according to the present invention,

Fig. 6

shows a block diagram of another exemplary embodiment of a system according to the present invention,

Fig. 7

shows a schematic representation of a control path of an exemplary embodiment of a system according to the present invention and

Fig. 8

shows a flow chart of an exemplary embodiment of a method according to the present invention.

[0038] In the Figures corresponding elements of different exemplary embodiments may be designated with like reference numerals.

[0039] Fig. 1 shows a confined space 1 in form of an indoor room 1 of a building. The walls 2 of the indoor room 1 and, in particular, the wall 2 comprising an opening 3 form a sound barrier 2. Two of the six walls delimiting the confined space 1 are not shown in Fig. 1. The opening 3 could, for example, be a window 3. The opening 3 could also be described as an extended interface to an environment. In the example shown in Fig. 1 the opening 3 extends along a plane and is delimited by a boundary in form of a window frame (not shown). Outside of the confined space 1 one or more undefined primary sound sources (not shown) are present. The primary sound sources emit sound waves 5 that can enter the confined space 1 through the opening 3 in the sound barrier 2. Inside the confined space 1 the sound waves 5 of the primary sources are conceived as noise.

[0040] To reduce the noise inside the confined space 1, six systems 7 for actively reducing noise according to the present invention are arranged along the window frame delimiting the opening 3. The opening 3 has a rectangular shape with four sides. On each of the four sides at least one system 7 for actively reducing noise is arranged. Each of the systems 7 comprises a plurality of sound blocking arrangements arranged in a bar-shaped common housing 9. In Fig. 1 most parts of the sound blocking arrangements are hidden arranged inside the respective housings 9. The only parts that are visible are sound sources 11 in form of loudspeakers 11 and sensors 12 for sensing a sound characteristic in form of microphones 12. To keep Fig. 1 neat and tidy only some of the sound sources 11 and sensors 12 have been designated with reference numerals. As can be seen in Fig. 1, the sound sources of each system 7 are arranged along a common longitudinal direction into which the elongated housings of the systems 7 extend. The sound sources 11 of opposing systems 7 are arranged such that they face towards each and emit sound in parallel to the plane in which the opening extends. The details of exemplary embodiments of systems 7 for reducing noise will be explained in more detail with reference to Figs. 3a to 7. In the example shown in Fig. 1 all sensors 12 are arranged in the same plane facing towards the confined space 1. However, this is not a necessary requirement.

[0041] Fig. 2 also shows a confined space 1 in form of an indoor room 1 with a similar arrangement of an opening 3 in a sound barrier 2 and primary sound sources arranged outside the confined space 1 and emitting sound waves 5 that are perceived as noise inside the confined space 1 as in Fig. 1, i.e., on that side of the sound barrier 2 and the opening 3 facing away from the primary sources. Four systems 13, 15 for reducing noise or noise reduction systems 13, 15 are arranged in the plane of the opening 3. Each of the systems 13, 15 comprises three sound blocking arrangements of which the sound sources 17, 19 in form of loudspeakers 17, 19 and the sensors 12 for sensing a sound characteristic in form of microphones 12 are shown in Fig. 2. Two of the noise reduction systems 13 are arranged along the upper and lower boundary of the opening 3, i.e., the window frame. The sound sources 17 of these system 13 are arranged in the same manner as in Fig. 1, i.e., they face towards each other. The remaining two systems 15 are arranged in the opening 3 such that they extend in parallel to the upper and lower systems 13. The sound sources 19 of these systems face away from the opening and towards the confined space 1. All sensors 12 are aligned in the same plane and face towards the confined space 1. For the sake of brevity, further details of Fig. 2 will not be described in more detailed as they correspond to Fig. 1. In particular, potential embodiments of the noise reducing system 13, 15 will be described in more detail with regard to Figs. 3a to 7. Please note that in Fig. 2 only some of the sound sources 17, 19 and only some of the sensors 12 have been designated with reference numerals.

[0042] Figs. 3a and 3b show a schematic representation of an exemplary embodiment of a system 21 for reducing noise according to the present invention. The system 21 comprises three sound blocking arrangements 23. Each of these sound blocking arrangements 23 comprises a sound source 25 in form of a loudspeaker 25, a sensor 27 for sensing a sound characteristic in form of a sound pressure and a control circuit 29. The sensors 27 are provided as microphones 27. The sound blocking arrangements 23 are arranged in a common elongated housing 31. The elongated housing 31 extends along a longitudinal direction 33. The sensors 27 of the sound blocking arrangement 23 are aligned along the longitudinal direction 33. Likewise, the sound sources 25 of the sound blocking arrangements are arranged along the longitudinal direction 33. In the embodiment shown in Fig. 1, all sound sources 25 and sensors 27 of the different sound blocking arrangements 23 are arranged on the same outer surface 35 of the common housing 31 and all arranged along the longitudinal axis 33. The exemplary embodiment of a noise reduction system 21 shown in Figs. 3a and 3b could, for example, be used in the arrangements shown in Figs. 1 and 2.

[0043] From Fig. 3a it can be taken that the sensors 27 are arranged in close proximity to the sound sources 25 of the respective sound blocking arrangements 23 to minimize the travel or propagation time of sound from the sound source 25 to the corresponding sensors 27 but without being placed in the primary sound field of the sound sources 25. In particular, the sensors 27 are arranged such that each sensor 27 is not arranged in a smaller distance to any other sound source 25 than to the sound source 25 of the respective sound blocking arrangement 23. In the exemplary embodiment shown in Figs. 3a and 3b, the distance between a sensor 27 and the two adjacent sound sources 25 is the same such that the sound pressure sensed by the sensor 27 can not only be used by the respective sound blocking arrangement 23 but also transmitted to an adjacent sound blocking arrangement 23 for improving the quality of the noise reduction. To this end an additional sensor 37 is provided which generates an additional sound pressure signal for one of the sound blocking arrangements 23. The operation of the system 21 will be explained in more detail with reference to Figs. 5 to 6.

[0044] An alternative exemplary embodiment of a noise reduction system 21 is shown in Figs. 4a and 4b. The system 21 shown in Figs. 4a and 4b is very similar to the system 21 shown in Figs. 3a and 3b. For the sake of brevity only the differences between the embodiments will be described in more detail. As can be seen in Figs. 4a and 4b, the sensors 27 are not arranged on the same outer surface 35 as the sound sources 25 but on another outer surface 39 of the housing 31 extending perpendicular to the outer surface 35 on which the sound sources 25 are arranged. Nevertheless, the sensors 27 are all aligned along the longitudinal direction 33. The system 23 shown in Figs. 3a and 3b could, for example, be used as noise reducing systems 15 in the arrangement shown in Figs. 1 and 2.

[0045] Fig. 5 shows a block diagram of a system 41 according to the present invention. The block diagram could be realized in any of the noise reduction system 7, 13, 15, 21 shown in Figs. 1 to 4b. The system 41 comprises a plurality of sound blocking arrangements 43 each comprising a sensor 45, a control circuit 47 and a sound source 49. The sensors 45 are formed as microphones and provided for sensing a sound characteristic in form of a sound pressure. A signal representative of sound pressure is transmitted from each sensor 45 to the control circuit 47 of the respective sound blocking arrangement 43. Here, the signal representative of the sound pressure is received and a control signal for the sound source 49 of the respective sound blocking arrangement 43 is determined. The control signal is determined such that the sound characteristic, i.e., the sound pressure, sensed at the sensor 45 of the respective sound blocking arrangement is minimized. Once the control circuit 47 has determined a control signal, the control signal is transmitted to the corresponding sound source 49, which is in turn driven in accordance with the control signal determined by the control circuit 47. In Fig. 5 details of the different sound blocking arrangements 43 including any filters, amplifiers or analog digital converters have been omitted. A detailed representation of an embodiment of a sound blocking arrangement including these elements will be described with reference to Fig. 7.

[0046] An alternative block diagram of a system 51 for reducing noise according to the present invention is shown in Fig. 6. The system 51 comprises four sound blocking arrangements 53 each comprising a sensor 55 and a sound source 57 which can be formed as the sensors and sound sources of the previously described embodiments. Contrary to the preceding embodiment, the control circuits 59 of two sound blocking arrangements 53 have been combined into a single unit. The signals representative of sound pressure are transmitted from the sensors 55 of both sound blocking arrangements 53 that are connected to the same control circuit 59. Here, both signals are used for determining control signals for the respective sound sources 57 that result in a minimized sound pressure at the location of the sensor 53 if the sound sources 57 are driven according to the control signal. By combining signals from a plurality of sensors 55, the quality of the noise reduction can be increased.

[0047] To further improve the quality of the noise reduction by the system 51, each of the control circuits 59 has an input line 61 where output signals of other control circuits 59 of the same system 51 or even output signals of control circuits from other noise reduction systems received through a system-wide input 63 can be received. These additional input signals can further be used to improve the quality of the noise reduction provided by the system 51. However, it has to be taken into consideration that the more input signals are used, the more the control response time of the system 51 increases which in turn reduces the capability of the system 51 to adapt to changing noise. Both control circuits 59 also comprise output lines 65 for transmitting output signals to other control circuits 59 or other sound reducing systems through a system-wide output 67.

[0048] Fig. 7 shows an embodiment of a control loop 69 schematically describing the operation of a sound blocking arrangement 71 as realized, for example, in the sound blocking arrangements 43 which are shown in Fig. 5. The sound blocking arrangement 71 comprises a sensor 73, an analog amplifier 75, an analog digital (A/D) converter 77, a control circuit 79, a switching amplifier or the class-D amplifier 81 and a sound source 83. The symbol shown between the sound source 83 and the sensor 73 schematically represents the travel or transit time 85 that sound waves require to travel from the sound source 83 to the sensor 73.

[0049] In the control loop 69 shown in Fig. 7 the sensor 73 picks up a sound characteristic in form of sound pressure and transmits a signal representative of this sound characteristic to the analog amplifier 75. The sensor 73 only picks up a sound characteristic of sound waves up to a sensing cutoff frequency above which the sensitivity of the sensor 73 drops off. Thus, the sensor 73 essentially functions as a low-pass filter for frequencies above the sensing cutoff frequency.

[0050] The signal representative of a sound characteristic is amplified in the analog amplifier 75 before it is converted into a digital signal in the A/D converter 77. The A/D converter 77 operates with a sampling rate exceeding the sensing cutoff frequency at least by a factor of two. Hence, due to the low-pass filter function of the sensor 73 and the high sampling rate of, for example, 20 kHz no anti-aliasing filters or additional low-pass filters are required which delay the signal between the sensor 73 and the control circuit 79. Thus, by using a sufficiently high sampling rate at the A/D converter 77, the control response time of the control loop 69 can be decreased which in turn improves the ability of the sound blocking arrangement 71 to respond to changing noise.

[0051] The control circuit 79 operates as previously described and determines a digital control signal for the sound source 83 which could be formed, for example, as a loudspeaker. The digital control signal is transmitted to the switching amplifier 81 which combines the function of an amplifier 87 and a digital to analog converter 89. The switching amplifier 81 generates as output a pulsed analog control signal having a pulse rate that corresponds to the sampling rate of the A/D converter 77. In the exemplary embodiment shown in Fig. 7 the sampling rate has not only been chosen such that it exceeds the cutoff frequency of the sensor 73 but also exceeds the cutoff frequency of the loudspeaker or sound source 83 at least by a factor of two. Thus, no reconstruction filter is required for smoothing the output of the switching amplifier 81 before it can be used for driving the sound source. Hence, the control response time of the control loop 69 is further reduced as the control signal is not slowed down by a reconstruction filter. Further, the switching amplifier 81 can be placed on the same chip as the control circuit 79 which reduces the size of the entire circuit. Furthermore, the power consumption as compared to a conventional digital-to-analog converter and an analog amplifier are reduced.

[0052] Finally, Fig. 8 shows an exemplary embodiment of a method for actively reducing noise passing through an opening in a sound barrier according to the present invention, wherein the opening extends is delimited by a boundary. In a first step 91 a sound characteristic of sound waves is sensed in a plurality of locations in the opening. The sound characteristic is sensed up to a sensing cutoff frequency, i.e., an upper limit of the frequency spectrum that can be sensed by the sensors. In a second step 93 analog signals representative of the sensed sound characteristic are converted to digital signals using a sampling rate that exceeds the sensing cutoff frequency at least by a factor of two. Thereby, it is ensured that the Nyquist-Shannon sampling theorem is satisfied and no additional low-pass filters or anti-aliasing filters are required which increases the control response time of the method.

[0053] In a third step 95 control signals for sound sources are generated or determined from the now digital signals representing the sensed sound characteristic. The digital control signals are then converted back in a fourth step 97 to pulsed analog control signals with a pulse rate exceeding a cutoff frequency of the sound sources at least by a factor of two. Thus, no further reconstruction filters are required for smoothing the control signal. This also reduces the control response time of the method according to the present invention. Finally, in fifth or last step 99 the sound sources are driven with the pulsed analog control signals generated in the fourth step 97. The sound pressure generated by the combination of the output of the sound sources and the noise that shall be reduced is then picked up again in the first step 91. Here, the distance between the sound sources and the locations in which the sound pressure is sensed has been minimized to make sure that the control response time and, therefore, the ability of the method to respond to changing noise is optimized.

Claims

1. A system (7, 13, 15, 41, 51) for actively reducing noise passing through an opening (3) in a sound barrier (2), the system (7, 13, 15, 41, 51) comprising one or more sound blocking arrangements (23, 43, 53, 71), wherein the opening (3) is delimited by a boundary,
wherein each sound blocking arrangement (23, 43, 53, 71) comprises a sound source (11, 17, 19, 25, 49, 57, 83), a sensor (12, 27, 45, 55, 73) and a control circuit (29, 47, 59, 79),
wherein each sound blocking arrangement (23, 43, 53, 71) is adapted to be arranged at the opening (3) such that the sound source (11, 17, 19, 25, 49, 57, 83) of the sound blocking arrangement (23, 43, 53, 71) is arranged to emit sound and the sensor (12, 27, 45, 55, 73) of the sound blocking arrangement (23, 43, 53, 71) is arranged to sense a sound characteristic in form of a sound pressure and/or a sound velocity and/or a sound intensity in a location in the opening (3),
wherein the sensor (12, 27, 45, 55, 73) of each sound blocking arrangement (23, 43, 53, 71) is adapted for transmitting a signal representative of the sensed sound characteristic to the control circuit (29, 47, 59, 79) of the respective sound blocking arrangement (23, 43, 53, 71),
wherein the control circuit (29, 47, 59, 79) of each sound blocking arrangement (23, 43, 53, 71) is adapted to receive the signal from the sensor (12, 27, 45, 55, 73) of the respective sound blocking arrangement (23, 43, 53, 71), to generate from the received signal a control signal for the sound source (11, 17, 19, 25, 49, 57, 83) of the respective sound blocking arrangement (23, 43, 53, 71) and to transmit the determined control signal to the sound source (11, 17, 19, 25, 49, 57, 83),
wherein the sound source (11, 17, 19, 25, 49, 57, 83) of each sound blocking arrangement (23, 43, 53, 71) is adapted to receive the control signal transmitted by the control circuit (29, 47, 59, 79) of the respective sound blocking arrangement (23, 43, 53, 71) and to emit sound in accordance with the received control signal,
wherein the control signal for each sound blocking arrangement (23, 43, 53, 71) is determined such that the sound characteristic sensed by the sensor (12, 27, 45, 55, 73) of the respective sound blocking arrangement (23, 43, 53, 71) is minimized and
wherein for each sound blocking arrangement (23, 43, 53, 71) the distance between the location in which the sensor (12, 27, 45, 55, 73) of the respective sound blocking arrangement (23, 43, 53, 71) senses a sound characteristic and the sound source (11, 17, 19, 25, 49, 57, 83) of the respective sound blocking arrangement (23, 43, 53, 71) is not greater than the distance between the location in which the sensor (12, 27, 45, 55, 73) of the respective sound blocking arrangement (23, 43, 53, 71) senses a sound characteristic and the sound source (11, 17, 19, 25, 49, 57, 83) of any other sound blocking arrangement (23, 43, 53, 71) of the system (7, 13, 15, 41, 51).

2. The system (7, 13, 15, 41, 51) according to claim 1, wherein the sound source (11, 17, 19, 25, 49, 57, 83) and the sensor (12, 27, 45, 55, 73) of each sound blocking arrangement (23, 43, 53, 71) are arranged in a common housing (9, 31),
wherein further the control circuit (29, 47, 59, 79) of each sound blocking arrangement (23, 43, 53, 71) is preferably arranged in the common housing (9, 31) with the sensor (12, 27, 45, 55, 73) and the sound source (11, 17, 19, 25, 49, 57, 83) of the respective sound blocking arrangement (23, 43, 53, 71).

3. The system (7, 13, 15, 41, 51) according to any of the preceding claims, wherein for each sound blocking arrangement (23, 43, 53, 71) the sensor (12, 27, 45, 55, 73) and the sound source (11, 17, 19, 25, 49, 57, 83) of the sound blocking arrangement (23, 43, 53, 71) are arranged such that a travel time (85) of sound emitted by the sound source (11, 17, 19, 25, 49, 57, 83) to the sensor (12, 27, 45, 55, 73) is minimized.

4. The system (7, 13, 15, 41, 51) according to any of the preceding claims, wherein each sound blocking arrangement (23, 43, 53, 71) comprises an analog digital converter (77) for converting an analog signal of the sensor (12, 27, 45, 55, 73) of the same sound blocking arrangement (23, 43, 53, 71) representative of the sensed sound characteristic to a digital signal with a sampling rate,
wherein the sensor (12, 27, 45, 55, 73) of each sound blocking arrangement (23, 43, 53, 71) has a sensing cutoff frequency and
wherein the sampling rate of the analog digital converter (77) of each sound blocking arrangement (23, 43, 53, 71) exceeds the sensing cutoff frequency at least by a factor of two.

5. The system (7, 13, 15, 41, 51) according to any of the preceding claims, wherein each sound blocking arrangement (23, 43, 53, 71) comprises the analog digital converter (77) with the sampling rate,
wherein the sound source (11, 17, 19, 25, 49, 57, 83) of each sound blocking arrangement (23, 43, 53, 71) has a sound source cutoff frequency and
wherein the sampling rate of the analog digital converter (77) of each sound blocking arrangement (23, 43, 53, 71) exceeds the sound source cutoff frequency of the respective sound blocking arrangement (23, 43, 53, 71) at least by a factor of two.

6. The system (7, 13, 15, 41, 51) according to claim 5, wherein each sound blocking arrangement (23, 43, 53, 71) comprises a switching amplifier (81) for amplifying the control signal generated by the control circuit (29, 47, 59, 79) of the respective sound blocking arrangement (23, 43, 53, 71),
wherein the switching amplifier (81) of each sound blocking arrangement (23, 43, 53, 71) generates a pulsed analog control signal from the control signal transmitted by the control circuit (29, 47, 59, 79), wherein the pulsed analog control signal is used for driving the sound source (11, 17, 19, 25, 49, 57, 83) of the respective sound blocking arrangement (23, 43, 53, 71).

7. The system (7, 13, 15, 41, 51) according to any of the preceding claims, wherein the system (7, 13, 15, 41, 51) comprises a plurality of sound blocking arrangements (23, 43, 53, 71),
wherein the sensor (12, 27, 45, 55, 73) of at least one sound blocking arrangement (23, 43, 53, 71) is adapted for transmitting the signal representative of a sensed sound characteristic to the control circuit (29, 47, 59, 79) of at least on other sound blocking arrangement (23, 43, 53, 71) and
wherein the control circuit (29, 47, 59, 79) of the at least one other sound blocking arrangement (23, 43, 53, 71) is adapted to receive the signal representative of a sensed sound characteristic from the sensor (12, 27, 45, 55, 73) of the at least one sound blocking arrangement (23, 43, 53, 71) and to generate the control signal from all signals representative of sensed sound characteristics received by the control circuit (29, 47, 59, 79).

8. The system (7, 13, 15, 41, 51) according to any of the preceding claims, wherein the system (7, 13, 15, 41, 51) comprises a plurality of sound blocking arrangements (23, 43, 53, 71) arranged in a common elongated housing (9, 31), wherein the elongated housing (9, 31) extends along a longitudinal direction (33) and the sound sources (11, 17, 19, 25, 49, 57, 83) of the plurality of sound blocking arrangements (23, 43, 53, 71) are aligned along the longitudinal direction (33).

9. A method for actively reducing noise passing through an opening (3) in a sound barrier (2), wherein the opening (3) is delimited by a boundary, the method comprising the following steps:

• sensing a sound characteristic in form of a sound pressure and/or a sound velocity and/or a sound intensity of sound waves in one or more locations in the opening (3),

• determining from each of the sensed sound characteristics a control signal for a separate sound source (11, 17, 19, 25, 49, 57, 83) arranged in the opening (3) and

• emitting sound with each of the sound sources (11, 17, 19, 25, 49, 57, 83) in accordance with the control signal determined for the respective sound source (11, 17, 19, 25, 49, 57, 83),

wherein the control signal for each sound source (11, 17, 19, 25, 49, 57, 83) is determined such that the sensed sound characteristic from which the control signal for the respective sound source (11, 17, 19, 25, 49, 57, 83) has been determined is minimized and
wherein the distance between the location at which the sound characteristic is sensed from which the control signal for a specific sound source (11, 17, 19, 25, 49, 57, 83) is determined and the respective sound source (11, 17, 19, 25, 49, 57, 83) is not greater than the distance between the location at which the sound characteristic is sensed from which the control signal for the specific sound source (11, 17, 19, 25, 49, 57, 83) is determined and any other sound source (11, 17, 19, 25, 49, 57, 83) used in the method.

10. The method according to claim 9, wherein the sound sources (11, 17, 19, 25, 49, 57, 83) are arranged such that a travel time (85) of sound emitted by each of the sound sources (11, 17, 19, 25, 49, 57, 83) to the location where the sound characteristic is sensed from which the control signal of the respective sound source (11, 17, 19, 25, 49, 57, 83) is determined is minimized.

11. Method according to claim 9 or 10, wherein in each location the sound characteristic is sensed up to a sensing cutoff frequency and
wherein signals representative of the sensed sound characteristic are converted to digital signals with a sampling rate exceeding the sensing cutoff frequency up to which the respective sound characteristic is sensed at least by a factor of two.

12. Method according to one of claims 9 to 11, wherein each sound source (11, 17, 19, 25, 49, 57, 83) has a sound source cutoff frequency and
wherein each signal representative of a sensed sound characteristic is converted to a digital signal with a sampling rate exceeding the sound source cutoff frequency of the sound source (11, 17, 19, 25, 49, 57, 83) for which the control signal from the respective sensed sound characteristic is determined at least by a factor of two.

13. Method according to claim 12, wherein each control signal determined for one of the sound sources (11, 17, 19, 25, 49, 57, 83) is converted from a digital control signal to a pulsed analog control signal, wherein the pulsed analog control signal is used for driving the respective sound source (11, 17, 19, 25, 49, 57, 83).

14. Method according to one of claims 9 to 13, wherein the control signal for at least one of the sound sources (11, 17, 19, 25, 49, 57, 83) used in the method is additionally determined from a sound characteristic sensed at a location which is at a distance from the respective sound source (11, 17, 19, 25, 49, 57, 83) that is not smaller than the distance between the location at which the sound characteristic is sensed from which the control signal is additionally determined and at least one other sound source (11, 17, 19, 25, 49, 57, 83) used in the method.

15. Method according to one of claims 9 to 14, wherein a plurality of sound sources (11, 17, 19, 25, 49, 57, 83) used in the method are arranged in a common elongated housing (9, 31), wherein the elongated housing (9, 31) extends along a longitudinal direction and the plurality sound sources (11, 17, 19, 25, 49, 57, 83) are aligned along the longitudinal direction.

Drawing