SPECIFICATIONLow noise fluid moving systemThe invention relates to a low noise fluid moving system comprising a casing having a rotating impeller housed therein, and a sound chamber connected to the inner chamber of the casing by at least one sound passage opening.
The substantial source region for the rotary sound in case of a radial fluid moving system without a guide wheel resides in the casing tongue at which the peripheral wall of the casing changes over into a duct via a bending area. At this point, the helically shaped casing has the smallest diameter. In case of axial fluid moving systems, the radial guide blades carrying the bearing for the axle of the impeller and being arranged transversely in the flow channel form the substantial zones of sound sources.
It has been known to provide in case of fluid moving systems a perforated wall in the bending zone of the casing tongue, or of the guide blades respectively, to arrange therebehind a resonator (EP Patent Application 0 039 459). The resonator is a A/4-type resonator for the substantial interfering sound frequency. A stationary wave is formed in it which is phased oppositely to the oscillation of the rotary sound thus extinguishing part of said rotary sound. The interfering rotary sound of the fluid moving system is substantially reduced accordingly. It is a disadvantage of the known sound compensation that the ;1/4-resonator must be tuned to the respective parasitic frequency, which, however, is substantially dictated by the speed of the fluid moving system.If its speed is changeable, a constant tuning of the length of the ;1/4-resonator is necessary subject to the respective speed.
It is the object of the invention to provide a lownoise fluid moving system of the above mentioned type in which the sound compensation is automatically adapted to different speeds of the device.
To solve this problem, the invention provides in the sound chamber a counter-sound source which is excited by a frequency changing subject to the speed of the impeller.
The counter-sound source is an active sound source, e.g. a loudspeaker. Said sound source is excited by a frequency changing responsive to the speed of the impeller. Thus, the frequency of the counter-sound source is automatically adapted to the respective speed of the impeller. The countersound source produces a sound which, if possible, is oppositely phased to the interfering amount of frequency of the rotary sound, to ensure that said amount of the rotary sound and the counter-sound are mutually extinguished at least partly. As a result thereof, the noise level will be iower.
In contradistinction to prior art, the fluid moving device of the invention comprises an "active" counter-sound source. In other words, it is notprovided with a resonator having a fixed or variable resonance frequency, but with a compensating counter-sound source effective in the substantial source region of the rotary sound.
Preferably, the counter-sound source is excited via an adjustable phase shifter circuit by which the phase position of the counter-sound can be so adjusted that it is phase-shifted by 1 800 relative to the phase position of the rotary sound. Such phase shifting can be easily performed and controlled manually, because the phase of the exciter signal of the counter-sound source must be shifted as long as to reach the lowest rotary sound level for the human sense of hearing. It is also possible to automatically adjust the phase shifter circuit so that it is setting the corresponding phase position which is most effective. To this effect, a sound transducer taking up the rotary sound and a corresponding controller are required.
The frequency by which the counter-sound source is excited, can be obtained by a sensor which is responsive to the elements rotating with the drive shaft of the impeller. It is possible, in this regard, to secure to the shaft of the impeller a coding disk whose marks pass by a contactless sensor which generates corresponding pulses. By a respective distribution of the marks at the coding disk, the produced frequency may correspond to a specific harmonic of the rotary sound dictated by the speed. As a rule, the principal interfering frequency in the noise of fluid moving systems can be considered the blade frequency(speed x number of blades of the impeller) andmultiple thereof.By deviating the exciter frequency for the counter-sound source from theimpeller speed via a speed pick-up, it is ensured that the frequencies of the counter-sound sourceare always exactly equal to those of the primaryaerodynamic pressure fluctuations.
According to another variant of the invention, the counter-sound source is excited by at least one pressure sensor converting the noises intoelectric oscillations. Said pressure sensor may be amicrophone that is provided in the source regionof the interfering noise. Via a control circuit, thecounter-sound source is so controlled that thepressure fluctuations formed at the point of the pressure sensor have oniy a minimum amplitude.
According to an advantageous embodiment of the invention, there are provided several countersound sources which are excited by different frequencies. Said counter-sound sources may becontrolled either in common by one sensor or theymay be controlled by different sensors. Said sensors which may be situated at various points of the casing, may be of different frequencysensitivities.
In the fluid moving machine of the invention, the shape and geometric dimensions of the innerchamber of the housing need not be changed tomount the counter-sound sources. Therefore, thecharacteristics and efficiency of the fluid movingmachine remain unchanged.
For a better understanding of the presentinvention and to show how the same may becarried into effect, reference will now be made, byway of example, to the accompanying drawings, in which: Figure 1 is a schematic perspective view of a radial fluid moving system;Figure 2 is a schematic view showing the excitation of the counter-sound source; andFigure 3 is a second embodiment showing the excitation of the counter-sound source.
Figure 1 shows a radial fluid moving device comprising a casing 10 having a helically shaped peripheral wall 11 continued by the external wall of a tangential duct 12.
In the casing 10, the impeller 13 with blades14 is pivotally attached. The driven shaft to which the impeller 13 is secured is not illustrated inFigure 1 so as to have a better survey. The point at which the peripheral wall 11 of the casing with respect to the axle of the impeller 13 - has the shortest radius is occupied by the casing tongue 1 5 which consists of a bending region in which the casing wall 11 changes over into the upper wall 1 6 of the duct 12. As a matter of fact, the casing tongue 15 is the substantial source of the inconvenient rotary sound.
The casing tongue 1 5 is a perforated wall containing a great number of holes extending into the closed sound chamber 1 7 arranged behind the casing tongue 1 5. All of the walls of the sound chamber 1 7 are closed, except for the openings in the casing tongue 1 5 and for one opening at which a loudspeaker 1 8 is mounted. The excitation of the latter will be still explained hereinafter, and it forms the counter-sound source which produces in the sound chamber 17 acoustic oscillations which are transmitted to the holes of the casing tongue 15, and which, in said holes, are oppositely phased to the rotary sound component to be suppressed.
According to Figure 2, a coding disk 20 mounted at the drive shaft 1 9 of the impeller 13 is provided with peripheral marks for instance, slots, colour marks, metal elements or the like. A sensor 21 mounted beside the coding disk 20 rotating with the drive shaft 1 9 produces an electric pulse with each passing mark. The pulses are supplied to a phase shifter circuit 22 in which the phase position of the pulses can be changed, for instancemanually. In a very simple design, the phase shifter circuit 22 consists of a delay circuit, e.g. anRC-member.
The output of the phase shifter circuit 22 is connected via an amplifier to a loudspeaker 1 8.
The frequency by which the latter is excited isequal to the r.p.s. of the drive shaft 10 multiplied by the number of marks on the coding disk 20. If the loudspeaker is to be excited at a lower frequency, a frequency divider circuit can be connected behind the sensor 21 producing thepulses to correspondingly reduce the frequency.
According to the principle of the invention, the counter-sound source consisting of the loudspeaker 1 8 produces a sound having a frequency equal to that of an interfering component of the rotary sound. By suitably setting the phase shifter circuit 22 and the amplifier, the corresponding rotary sound component and the counter-sound produced by the loudspeaker 1 8 at least partly extinguish themselves mutually within the range of the casing tongue 1 5.
In the embodiment of Figure 3, the casing tongue 10 houses a microphone 24 to receive the rotary sound produced at the casing tongue 1 5 and to supply corresponding electric signals to an amplifier 25. Via a phase shifter circuit 22, the output signal of the amplifier gets to the loudspeaker 18 mounted in the sound chamber 1 7 just as shown in the first embodiment. The phase shifter circuit 22 is so adjusted that the amplitude of the audible rotary sound is of a minimum value. In addition, the level of the counter-sound source can be changed at the amplifier 25.
According to another alternative, the microphone for the control of the counter-sound source is mounted at an optional point within or outside the fluid moving device, where the reduction of the noise level is particularly desirable, e.g. in a channel connected to the fluid moving device.
The shape of the holes at the casing tongue 1 5 is optional. it is also possible to provide in place of a perforated wall an opening closed by a diaphragm.
The counter-sound source may be also a diaphragm disposed in the casing tongue and excited directly by an oscillation exciter (structureborne noise exciter).
The invention can be used with all fluid moving devices, e.g. fans, blowers, ventilators, one- or multiple-stage blowers, compressors and pumps.