Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
  • H04R1/00—Details of transducers, loudspeakers or microphones
  • H04R1/005—Details of transducers, loudspeakers or microphones using digitally weighted transducing elements
• • G—PHYSICS
  • G10—MUSICAL INSTRUMENTS; ACOUSTICS
  • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
  • G10K15/00—Acoustics not otherwise provided for
  • G10K15/04—Sound-producing devices

Definitions

• a method and a devicce for generating low frequency sound and use of the device are described.
• 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.
• 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.
• 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.
• 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.
• 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.
• the amount of air of the pump pulses and suction pulses 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.
• 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.
• 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.
• 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.
• 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.
• a membrane has particular advantages, for what reason this embodiment is especially advantageous.
• 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.
• a device according to the invention has advantages appearing without any doubt from the discussion above of the method according to the invention.
• said high frequency is more than 10 times, preferably more than 30 times, higher than said low frequency.
• 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.
• 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.
• 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.
• 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.
• 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).
• MEMS micro electromechanic structure
• the invention also relates to a use of a device as above according to the appended use claim .
• 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.
• Fig 4 illustrates schematically from above a device according to a third preferred embodiment of the invention.
• 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:
• 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 .
• 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 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.
• 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.
• 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.
• 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.
• Fig 4 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.
• 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.
• each chamber has its own air tank.

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• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Acoustics & Sound (AREA)
• Signal Processing (AREA)
• Multimedia (AREA)
• Reciprocating Pumps (AREA)
• Compressor (AREA)
• Piezo-Electric Transducers For Audible Bands (AREA)
• Transmitters (AREA)

Applications Claiming Priority (3)

Publications (2)

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• 2000
  • 2000-03-16 SE SE0000863A patent/SE518168C2/sv not_active IP Right Cessation
• 2001
  • 2001-03-16 DE DE60114184T patent/DE60114184D1/de not_active Expired - Lifetime
  • 2001-03-16 WO PCT/SE2001/000540 patent/WO2001069588A1/en active IP Right Grant
  • 2001-03-16 AT AT01915988T patent/ATE307374T1/de not_active IP Right Cessation
  • 2001-03-16 EP EP01915988A patent/EP1285432B1/de not_active Expired - Lifetime
  • 2001-03-16 AU AU2001242932A patent/AU2001242932A1/en not_active Abandoned

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