A METHOD AND A DEVICE FOR GENERATING LOW FREQUENCY SOUND AND USE OF THE DEVICE

Abstract: In a method and a device for generating or extinguishing low frequency sound having a high sound pressure, the wave shape is divided into air pulses with variable amplitude and with considerably higher frequency than that of the wanted tone. The air pulses thereby represent instantaneous values (sampling values) of the wanted wave shape which, thus, is formed by an imaginary curve connecting the amplitude values of the discrete air pulses. The air pulses are positive during one half-cycle and negative during the other half-cycle of the wave shape. The device comprises means to pump in high frequency pulses from a chamber (1) to the surroundings during a first period of time and during a second period of time high frequency air pulses are sucked into a said chamber (2) from the surroundings. The invention also relates to use of such a device, e.g., for actively extinguishing low frequency noise.

Description: A method and a devicce for generating low frequency sound and use of the device. FIELD OF THE INVENTION AND PRIOR ART The present invention relates to a method and a device for generating dynamic air flows for generating or extinguishing low frequency sound. "Low frequency" is to be given a very broad sense and is to be put in relation to the context in which the sounds in question are to be created. However, it may typically be frequencies below 500 Hz. The dynamic air flows in question may be created either for generating (increasing) sound energy or for absorbing (reducing) sound energy in the surrounding thereof. The object of generated sound energy may be to extinguish undesired sound in an airfield, i.e. for local active noise control. However, it is also conceivable that generated sound energy is used for reproduction of music or in any type of alarm or warning system. The use of the technique described for reduction of sound energy comprises so called global active control of sound. The invention is particularly, but not exclusively, directed to generation or absorption of low frequency sound having a high sound pressure, i.e. a high source strength. It is in particular in active noise control desired to generate such a sound, since on one hand the acoustic absorbents used for silencing of noise, for example in walls of a gas turbine, function well at high sound frequencies, but badly at low frequencies, and on the other in some cases, such as in a gas turbine, high sound powers, well in the order of 120-130 dB, occur at low frequencies, such as below 250 Hz. However, extinguishing of such sounds encounters considerable problems. It is not particularly hard to generate or absorb sound energy having a high frequency. A usual measure of the source strength of an acoustic source is the volume velocity of the air moved when generating sound, and it is sufficient to have small amounts of air per oscillation for being able to obtain a high source strength. An oscillation of a membrane having a surface of 1 dm 2 and an amplitude somewhat higher than one millimetre would for example at a frequency of 10 kHz result in a "pumping" of 100 litres of air per second, while a corresponding surface and a frequency of 20 Hz would require an amplitude of about 0,5 metre for obtaining the same source strength of 100 litres per second . This is of course very difficult to obtain, for what reason the arrangements known for obtaining low fre- quency sound having a high source strength are very complicated and costly. SUMMARY OF TH E INVENTION The object of the present invention is to provide a method and a device of the type defined in the introduction, which make it possible to generate and absorb sound energy with a frequency being low in the respective context by simple means, and which also enable obtaining of a comparatively high source strength . This object is according to the invention obtained by providing such a method, in which alternatingly during first and second periods of time firstly during a first period of time air is, in pulses with a high frequency, pumped out from a chamber to the surroundings and air is introduced into the chamber from an air tank between these pulses and during a second period of time air is in pulses with a high frequency sucked into a chamber and air is introduced into a said air tank from the chamber between the pulses, so as to create an air flow with frequency components having a substantially lower frequency than the fre- quency of the pulses. The periodicity for the air flow generated will thereby be determined by the time for said introduction of air and suction of air, respectively, as well as a device according to the appended independent device claim. High frequencies with which air is pumped out from or sucked into the chamber may in this way be translated to contain considerably lower frequency components while maintaining the source pressure, and since a high source pressure at a high frequency without any major problems may be generated in this way a high source strength with a low frequency may be obtained. By the fact that air movements of a certain type take place during an entire said period of time and then air movements of a certain other type take place during the consecutive period of time, two such consecutive periods of time of the method according to the invention will correspond to an entire oscillation of a membrane of for example a loud speaker, which oscillates with said low frequency components. It is here pointed out that "air tank" is to be given a very broad sense, and it is not at all necessary that the chamber adjoins directly to the air tank, but this could very well be connected to the chamber through an air conduit and thereby be located at a more or less important distance from the chamber. Moreover, it is important that such an air tank really exists, through which the chamber receives air during the first period of time or delivers air to during the second period of time, so that no disturbing sounds occur, which would be the case if air was introduced into and emitted from, respectively, the chamber directly to the surroundings without any such air tank. According to a preferred embodiment of the invention the amount of air of the pump pulses and suction pulses , respectively, is varied so that the amounts of air as a function of time substantially follow a determined signal. It is hereby obtained that the resulting air source is given source properties being similar to those of a membrane oscillating with a low frequency content for generating or absorbing sound energy. According to another preferred embodiment of the invention air is pumped out to the surroundings and sucked in from the surroundings to a said chamber by bringing a membrane forming a part of the wall delimiting the chamber to vibrate with a high frequency so as to change the volume of the chamber with such a high frequency. By utilizing a membrane in this way for gener- ating the high frequency pump and suction pulses, respectively, this result may easily be obtained, since a membrane will be brought to vibrate with a high frequency, for example by mag- netostrictive or other influence. The comparatively small amount of air such a membrane usually may move per oscillation is neutralized by the high frequency, so that a high source strength may be obtained, and this for a low frequency, for which a membrane may not be utilized when generating sound directly by the membrane. According to another preferred embodiment of the invention said amount of air is varied by varying the amplitude of the membrane, which may easily be achieved by controlling the influence on the membrane. It would also be possible to vary the amount of air by varying the length of the open times between the chamber and the surroundings at the pump and suction pulses. It is here to be mentioned that the invention is not restricted to the use of a membrane for obtaining said air pulses, but it would be well conceivable to accomplish this in another way, for ex- ample by connecting a compressed air member and a negative air pressure generating member, respectively, to a said cham- ber, in which these members may then be considered also to provide said air tank. However, exactly the use of a membrane has particular advantages, for what reason this embodiment is especially advantageous. According to another preferred embodiment of the invention air is pumped out from at least two chambers to the surroundings during said first period of time and air is sucked into at least two chambers from the surroundings during said second period of time, and the pumping out of and suction into, respectively, the chambers in question are timely displaced with respect to each other, so that the time intervals between two consecutive pump and suction pulses from one of the chambers in a group of at least two chambers are filled out by pump or suction pulses from the other chamber or chambers in the same group. By synchronizing the pumping and suction from at least two chambers in this way gaps in the volume flow to or from the sound generating device may be avoided and a sound being more free from disturbances and having a substantially ideal wave shape may be generated. A device according to the invention has advantages appearing without any doubt from the discussion above of the method according to the invention. According to a preferred embodiment of the invention said high frequency is more than 10 times, preferably more than 30 times, higher than said low frequency. This makes it possible to generate sound with low frequency and considerably higher source pressure than would such a sound generation have taken place in a conventional way, especially while utilizing a said membrane in both cases. According to another preferred embodiment of the invention the device comprises at least two said chambers with at least one said first and second valve each for connection to the sur- roundings and a said air tank, respectively. This is advantageous, since it enables both obtaining of a high source pressure if desired and a greater flexibility of the device with respect to the possibility to generate sound with different frequencies and source strengths. According to another preferred embodiment of the invention a said chamber may the for example be adapted to act during said first periods of time to pump out air through the first valve thereof to the surroundings and another chamber be adapted to act during said second periods of time to suck air thereinto from the surroundings, and the two chambers may then be connectable through their second valves to a said common air tank. This then means that the function of the two chambers may be complimentary, so that a first of the chambers, which causes a pumping of air in pulses out to the surroundings reduces the air pressure somewhat in the air tank, but this is then built up again through the communication of the air tank with the other chamber during the suction period of time. This means that both the pumping out and the suction in may take place at favourable air pressure conditions in the air tank. According to another preferred embodiment of the invention said air tank forms a container being air tight with respect to the sur- roundings and adjoining to said chamber. This constitutes a simple way to realize the air tank. According to another preferred embodiment of the invention the size of said chamber and air tank are within the micrometer re- gion, i.e. it is of a typical size for a micro electromechanic structure (MEMS). It will hereby be possible to incorporate a device of the type according to the invention in places where a small space is available or it is judged to be important that the device is hidden. It also enables a distribution of a compara- tively high number of such devices over a certain surface or within a certain space for obtaining optimum conditions for sound generation or active noise control . The invention also relates to a use of a device as above according to the appended use claim . Further advantages as well as advantageous features of the invention appear from the following description and the other dependent claims. BRI EF DESCRI PTION OF TH E DRAWI NG With reference to the appended drawing , below follows a description of preferred embodiments of the invention cited as ex- amples. In the drawing: Fig 1 is a very schematical cross section view through a de- vice according to a first preferred embodiment of the invention, Fig 2 is a graph illustrating the air flow to and from the surroundings versus time for a device according to Fig 1 , Fig 3 is a view corresponding to Fig 1 of a device according to a second preferred, simplified embodiment of the invention, and Fig 4 illustrates schematically from above a device according to a third preferred embodiment of the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE I NVENTION The construction and the function of a device according to a first preferred embodiment of the invention are schematically illustrated in Fig 1 , and this device has two chambers 1 , 2, which through a first valve 3, 4 each are connectable to the surroundings, and through two second valves 5, 6 and 7, 8, respectively, each are connectable to an air tank 9 included in the device, which is common for the two chambers. The air tank 9 is a container containing air with substantially the same air pressure as the surrounding air pressure and being hermetically closed with respect to the surroundings. The relationships between the volumes of the chambers and the air tank may in the practice be completely different than those shown in Fig 1 , which is only there for explaining the function of the device according to the invention. A part of the wall of the respective chamber is formed by a membrane 1 0, 1 1 , which may through a control unit 12 indicated only for one of the chambers be influenced to vibrate with a high frequency, for example within the range of 1 kHz - 10 kHz. The control unit 12 also controls the opening and closing of the first and second valves, but it is pointed out that these valves could also be of a passive type, i.e. be influenced by pressure differences between the surroundings and the chamber and the chamber and the air tank, respectively. The control unit is adapted to control the valves and the membranes in the following way for reducing a sound wave with low frequency and high source pressure: During a first period of time A (see Fig 2) the control unit brings the membrane 1 0 to oscillate with a high frequency and the control unit controls at the same time the valve 3 to open when the membrane swings upwardly as seen in Fig 1 and reduces the volume of the chamber at the same time as the second valves 5, 6 are kept closed , so that air is pumped out of the chamber 1 . When then the membrane 10 swings downwardly and the volume of the chamber 1 is increased the first valve 3 is closed and the second valves 5, 6 are opened, so that air from the air tank 9 is sucked into the chamber 1 . This sequency continues during a first period of time A, during which the second chamber 2 and the membrane 1 1 thereof are at rest. The amount of air contained in each pump pulse is varied during the period of time A so that these amounts of air as a function of time will substantially follow a sinus curve, such as illustrated in Fig 2. This is preferably achieved by changing the amplitude of the oscillations of the membrane 10 over the time, but it is also possible to control the open times of the first valve 3 or combine these two measures. When the first period A has elapsed the period B is started , during which air is sucked in in pulses from the surroundings into the chamber by bringing the membrane 1 1 to oscillate with a high frequency. When the membrane 1 1 swings downwardly (see Fig 1 ) the valve 4 is opened, at the same time as the second valves 7, 8 are kept closed and an air pulse is sucked into the chamber 2. When then the membrane 1 1 swings upwardly again as seen in Fig 1 and thereby the volume of the chamber is reduced the first valve 4 is kept closed and the second valves 7, 8 are opened, so that air from the chamber may be pumped out into the air tank, and that it is possible later on to obtain a negative air pressure in the chamber 2 with respect to the surroundings again when swinging the membrane downwardly again for sucking air into the chamber again. This procedure is carried out in a corresponding way as the procedure during the period A. A sound wave having a frequency being half the frequency of the frequency of the alterations between said periods of time is obtained in this way. It is important for the optimum function of the device that the air pressure in the respective chamber is not changed substantially during a said period of time, and it is therefore advantageous that the volume of the air tank is dimensioned to ensure that the maximum pressure difference does not exceed 50%. It is then advantageous to let one chamber act at a time and let these two chambers have a common air tank in the way shown in Fig 1 , since it means a requirement of an air tank being considerably smaller with respect to the volume than if two separate chambers with one air tank each had been used. However, it is in the practice well possible that the device has only one chamber 1 with one air tank 9, such as shown in Fig 3, and the membrane 1 0 and the valves 3, 5, 6 will then during a first period of time be controlled in the way shown at A in Fig 2 and during a second period of time be controlled in the way shown at B in Fig 2. A safety valve 13 located on the rear side of the air tank at a considerable distance from the place for pumping out to and suction in air from the surroundings is also shown in this Figure, and this valve is used for connecting the air tank to the surroundings should the air pressure therein be too high or too low. It is furthermore illustrated in Fig 4 that it is possible to arrange more than two chambers in a device of this type, and four chambers are there connected to the same air tank 9, and the two chambers 1 , 1 ' are adapted to act during the first period of time and two chambers 2, 2' to act during the second period of time. The action of the chambers acting during the same period of time are then preferably displaced in the way described further above, so that the gaps between the columns of amounts of air shown in Fig 2 are substantially filled out and there is substantially no distance between consecutive air amounts columns any longer, and the sound wave receives an appearance of a determined time signal being almost ideal. The invention is of course not in any way restricted to the preferred embodiment described above, but many possibilities to modifications thereof will be apparent to a person with ordinary skill in the art without departing from the basic idea of the invention as defined in the claims. More than two chambers could for example act during the same period of time for obtaining an optimum curve shape of the sound wave generated. It would also be possible that each chamber has its own air tank. It is also pointed out that it would be possible to let a small part of air flow into the left chamber 1 as seen in Fig 1 before the first valve 3 closes at the downward swing of the membrane, and it is neither necessary to directly open the first valve 3 when the membrane starts generating an over-pressure in the chamber when the membrane swings upwardly, but the opening may take place as of a determined level of the over-pressure.

Claims: Claims 1 . A method for generating dynamic air flows for generating or extinguishing low frequency sound, characterized in that alter- natingly during first and second periods of time firstly during a first period of time air is, in pulses with a high frequency, pumped out from a chamber (1 ) to the surroundings and air is introduced into the chamber from an air tank (9) between these pulses and during a second period of time air is, in pulses with a high frequency, sucked into a chamber (1 , 2) and air is introduced into a said air tank from the chamber between the pulses, so as to create an air flow with frequency components having a substantially lower frequency than the frequency of the pulses. 2. A method according to claim 1 , characterized in that the amount of air in the pump pulses and the suction pulses, respectively, is varied so that the amounts of air as a function of the time substantially follows a determined signal. 3. A method according to claim 1 or 2, characterized in that the air is pumped out to the surroundings and sucked in from the surroundings to a said chamber (1 , 2) by bringing a membrane (10, 1 1 ) forming a part of the wall delimiting the chamber to vibrate with a high frequency so as to change the volume of the chamber with such a high frequency. 4. A method according to claims 2 and 3, characterized in that said amount of air is varied by varying the amplitude of the membrane (10, 1 1 ). 5. A method according to claim 2 or claims 2 and 3 or 4, characterized in that said amount of air is varied by varying the length of the open times between the chamber (1 , 2) and the surroundings during the pump and suction pulses. 6. A method according to any of the preceding claims, characterized in that air is pumped out from at least two chambers (1 , 1 ') to the surroundings during said first period of time and air is sucked into at least two chambers (2, 2') from the surroundings during said second period of time, and that the pumping out of and suction into, respectively, the chambers in question are timely displaced with respect to each other, so that the time intervals between two consecutive pump and suction pulses from one of the chambers in a group of at least two chambers are filled out by pump or suction pulses from the other chamber or chambers in the same group. 7. A device for generating dynamic air flows for generating or extinguishing low frequency sound, characterized in that it comprises at least one chamber (1 ) containing air and connectable to the surroundings through a first valve (3) and to an air tank (9) included in the device through a second valve (5, 6), a first means (10) adapted to pump out air from said chamber to the surroundings through a said first valve, a second means (1 1 ) adapted to suck air into a chamber (2) from the surroundings through a first valve (4) as well as a unit (12) adapted to control said means to act alternatingly during one of first and second periods of time each, respectively, so that the first means during the first period of time (A) pumps air in pulses with a high frequency out of a said chamber to the surroundings and air is introduced into that chamber from said air tank between the pulses through a second valve (5, 6) belonging thereto and the second means during a second period of time (B) sucks air in pulses with a high frequency from the surroundings into a said chamber and air is introduced into the air tank from this chamber between the pulses through a second valve (7, 8) belonging thereto, so as to create an air flow with frequency components having a substantially lower frequency than the frequency of the pulses. 8. A device according to claim 7, characterized in that said high frequency is more than 10 times, preferably more than 30 times, higher than said low frequency. 9. A device according to claim 7 or 8, characterized in that the control unit (12) is adapted to co-ordinate the control of said means ( 10, 1 1 ) and first valves (3, 4) for varying the amount of air of said pump pulses and suction pulses, respectively, so that said amounts of air as a function of time substantially follow a determined signal. 10. A device according to any of claims 7-9, characterized in that each of said first and second means is formed by a separate membrane ( 10, 1 1 ) forming a part of the wall delimiting the respective chamber ( 1 , 2), and that the control unit (12) is adapted to control the membrane to vibrate with a high frequency for changing the volume of the chamber with such a high frequency. 1 1 . A device according to claims 9 and 1 0, characterized in that the control unit (12) is adapted to vary said amount of air by controlling the amplitude of the membrane ( 10, 1 1 ) to vary. 12. A device according to claim 9 or claims 9 and 10 or 1 1 , characterized in that the control unit (12) is adapted to control the variation of said amount of air by controlling the length of the open time of the first valve (3, 4) in question to the surroundings to vary. 13. A device according to any of claims 7-12, characterized in that one or a plurality of said first and second valves is/are passive, i.e. they are adapted to be controlled by pressure differences between the chamber (1 , 2) and the surroundings and the chamber and the air tank (9), respectively. 14. A device according to any of claims 7-12, characterized in that one or a plurality of said first and second valves is/are active, i.e. a control unit (12) is adapted to control opening and closing thereof. 1 5. A device according to any of claims 7-12, characterized in that it comprises at least two said chambers ( 1 , 2) with at least one said first (3, 4) and second (5, 6, 7, 8) valve each for connection to the surroundings and a said air tank (9), respectively. 16. A device according to claim 15, characterized in that it comprises one said chamber (1 ) adapted to act during said first periods of time to pump out air through the first valve thereof (3) to the surroundings and another chamber (2) adapted to act during said second periods of time to suck air thereinto from the surroundings. 17. A device according to claim 15 or 16, characterized in that it has at least two chambers (1 , 2) connectable through their second valves to said common air tank (9). 18. A device according to claim 15 or 16, characterized in that it comprises at least two said air tanks (9) and at least two said chambers are connectable to separate air tanks. 19. A device according to any of claims 15-18, characterized in that the control unit (12) is adapted to control at least two said means (10; 1 1 ) of the same type, first or second , arranged in a chamber (1 , 1 ', 2, 2') each to act alternatingly during the first or second period of time in question so that the time intervals between two consecutive pump or suction pulses from one of the chambers in a group of at least two chambers with means of the same type is filled out by pump or suction pulses from the other chamber or chambers in the same group. 20. A device according to any of claims 7-19, characterized in that it comprises more than two said chambers ( 1 , 1 ', 2, 2') connectable through first and second valves to the surroundings and an air tank, respectively. 21 . A device according to any of claims 7-20, characterized in that said air tank (9) forms a container being air tight with respect to the surroundings and adjoining to said chamber (1 , 2). 22. A device according to claim 21 , characterized in that the dimensions of the air tank (9) are adapted to the amount of air said means are adapted to pump out of and suck into, respectively, said chamber (1 , 2) during a said period of time, so that the air pressure in the air tank is not changed substantially dur- ing a said period of time. 23. A device according to claim 22, characterized in that the volume of the air tank (9) is dimensioned so as to ensure that the maximum pressure difference therein does not exceed 50% of the surrounding pressure. 24. A device according to any of claims 7-23, characterized in that the size of said chambers (1 , 2) and the valves (3, 4) is within the micro meter region , i.e. is of a typical size for a micro electromechanic structure (MEMS). 25. A use of a device according to any of claims 7-24 for generating low frequency sound having a high air pressure. 26. A use according to claim 25, characterized in that it takes place for actively extinguish low frequency noise.

Inventors: GUSTAVSSON MATS, GUSTAVSSON, MATS
Applicants: GUSTAVSSON MATS, SAAB AB, GUSTAVSSON, MATS
Application year: 2001
Main classification: G10K11
IPC codes: G10K11, G10K15, H04R1