KR100344595B1 - Wall cavity microphone turbulence shield - Google Patents

Abstract

The cavity is provided outside of the duct or fan outlet in the ANC system so that the duct liner or porous material divides the interior of the duct from the cavity. The microphone is positioned in the cavity. The baffle of porous material is preferably positioned within the cavity to divide the cavity into a plurality of chambers to reduce internal flow circulation in the cavity.

Description

Wall cavity microphone disturbance shield {WALL CAVITY MICROPHONE TURBULENCE SHIELD}

It is well known in the art of acoustic sensing in duct active noise control (ANC) systems that the rejection of pressure pulsations due to system input or error microphone disturbances is important for the ability to cancel noise. have. This so-called "flow noise" reduces the coherence between the sensing and error microphones and the level of interference is directly related to the level of performance that can be achieved in noise cancellation. For example, in a feedforward ANC system, a sense-error microphone interference level of 0.99 is needed to achieve a 20 dB cancellation level and an interference of 0.9 reduces the cancellation level to 10 dB. In the case of the collocated feedback approach (so-called "TCM" in the case of tightly coupled monopoles), the flow noise level on the input microphone is the lowest sound that can be achieved by active cancellation if the system is fully operational. Limit the performance of the system. In addition, the high-amplitude low-frequency flow pulsations sensed by the microphone cause disruptive high amplitude speaker operation and system instability. If a sufficient wind screen or shield does not allow adequate rejection of disturbances, the only solution is to move the ANC system to a quieter area away from disturbed areas, especially near fans. . This solution actually increases the cross sectional area of the duct size of the ANC system and limits product integration since the microphone cannot be located near the fan discharge. A typical flow shielding method used for standard ANC, HVAC is the use of a flush-mounted microphone located under standard duct liner material. This structure provides an inexpensive and efficient means of preventing unnecessary flow noise. However, under extreme disturbance conditions encountered near the fan outlet, this method is insufficient. Thus, essentially this solution requires longer system lengths since the microphone must be spaced from the fan outlet.

The present invention dramatically improves performance by placing the microphone outside the cavity of the duct liner so that the microphone can be positioned at the fan outlet while avoiding problems associated with disturbances. Additionally, the microphone may be divided into a plurality of chambers by one or more porous partitions, and at least one of the chambers may be disposed in a chamber disposed in a cavity containing the microphone.

1 illustrates an ANC system for a ducted system using an adaptive feedforward approach.

2 illustrates an ANC system for a ducted system using a collocated feedback approach.

3 is a sectional view of a first embodiment of a disturbing shield of the present invention;

4 is a sectional view taken along line 4-4 of FIG.

FIG. 5 is a cross sectional view of the FIG. 3 embodiment rotated 90 ° relative to the duct; FIG.

Fig. 6 is a sectional view of a second embodiment of the present invention.

Fig. 7 is a sectional view of a third embodiment of the present invention.

Fig. 8 is a sectional view of a fourth embodiment of the present invention.

9 is a sectional view of a fifth embodiment of the present invention.

10 is a graph showing interference between two microphones in the same cross plane with different flow shields.

FIG. 11 is a sectional view corresponding to FIG. 5 but with two microphones; FIG.

12 is a sectional view corresponding to FIG. 3 but with two microphones;

13 is a graph showing interference between faced and non-faced duct liners.

Fig. 14 is a sectional view of a sixth embodiment of the present invention.

Figure 15 is a cross sectional view taken along line 15-15 of Figure 14;

<Description of Symbols for Main Parts of Drawings>

10: duct

11: duct liner

12: fan

14: control speaker

16: microphone

30: cover member

32: joint

It is an object of the present invention to allow the positioning of the sensor microphone in a position adjacent to extreme disturbance areas while maintaining a high level of interference between the input and error microphones.

Another object of the present invention is to provide a disturbance shield for a microphone.

It is a further object of the present invention to provide a microphone disturbance shield suitable for positioning in the region of the outlet of the fan. These objects and other objects which will become apparent below are achieved by the present invention.

Basically, the sensor microphone is located outside the cavity of the duct liner of the duct of the ANC system. A baffle of porous material is provided in the cavity to reduce internal flow recirculation.

1 and 2, reference numeral 10 denotes the duct used for the controlled distribution of air and is lined with standard duct liner material such as 2.54 cm (1 inch) thick glass fiber insulation. Preferably, the duct liner material is porous but resistant to flow through due to the presence of a facing, such as covering or coating. One such suitable material is a plastic coated matt faced fiberglass mat available from the Manville Fiber Glass Group under the trade name Lincoustic. The upstream fan 12 generates aerodynamically induced noise, such as trailing-edge noise, which is mainly attributable to the noise mechanism that goes down the duct. An effective means of controlling the low frequency of this recently developed noise is active noise control, whereby the control loudspeaker 14 is used to generate an opposite sign pressure distribution in order to "clear" unnecessary noise. This cancellation is performed by echoing the sound back towards the source (ie, pure reaction system), absorbing acoustic energy by the control speaker 14, or by a combination of these instruments. 1 and 2 show two main means for achieving duct ANC (active noise control), in which FIG. 1 is an adaptive feedforward scheme and FIG. 2 is a juxtaposition feedback scheme. For the adaptive feedforward approach of FIG. 1, the sensor microphone 16 is an adaptive DSP (Digital Signal Processor) controller (DSP) that detects ongoing noise and compensates for signals suitable for time delay, attenuation of acoustic amplitude, duct mode, speaker dynamics, and the like. 18) feeds this signal forward and sends the signal to the control speaker 14, which cancels the noise. The downstream error microphone 20 detects residual noise. The signal from the downstream microphone 20 is used to employ the coefficients of the DSP 18 in a manner that minimizes the residual noise signal at the error microphone 20. The juxtaposition feedback method shown in Fig. 2 uses a microphone 22 that measures and adds fan noise and speaker noise, and an analog controller 24 and a control speaker 14. The signal from microphone 22 is input to analog controller 24 which continuously adjusts the output of control speaker 14 to minimize the signal detected by microphone 22. In general, the adaptive feedforward scheme of FIG. 1 performs better than the feedback scheme of FIG. 2, but at the expense of system length and cost.

In the case of the two duct ANC systems, in order to minimize the space required for the system, the microphone 16 or as close as possible to the outlet of the fan (i.e., the minimum length D), i. 22) It is advantageous to position the system as closely as possible the distance between them. However, the disturbance T in the region near the fan discharge is maximal and hinders the perfect use of this approach. Such disturbing pulsations may exceed 50% of the average flow rate in the duct 10. Pressure oscillations caused by disturbing patterns that collide on the microphones 16, 22 generate a "flow noise" signal that is combined with the acoustic pressure vibrations. Flow noise limits the amount of noise cancellation that can be performed by the ANC system. For example, if the flow noise is lower than the acoustic noise, the attenuation will be limited to the portion of the acoustic signal that can be detected above the flow noise floor. Also, if the flow noise is greater than the acoustic noise, the ANC system disperses the flow noise through the control speaker, thereby acting as a noise generator rather than a noise attenuator. To increase the attenuation capability of the ANC system, there are four options: (1) to move the ANC system downstream of the quieter flow zone, or (2) the signal from the microphone 16 before being input into the controller 18 or Sensors with appropriate signal conditions to electronically filter flow noise from signals from microphone 22 before being input to controller 24, or (3) to electronically separate flow induced noise from noise traveling in duct 10. Use a microphone, or (4) use a disturbing forced shield around the microphone 16, 20, 22 to selectively reduce the strength of the disturbing energy at the microphone face relative to the strength of the acoustic noise signal propagating in the duct 10. Can be considered. In option (4), the use of disturbing forced shields around the sensor microphone is a preferred option, and the unique high performance shield concept is the subject of the present invention.

The present invention not only provides additional shielding when used with the microphones 20, 22, but also allows positioning of the microphone 16 in the disturbance (T) region associated with the discharge of the fan 12. Referring now to FIGS. 3 and 4, it is clear that the cover member 30 defines a three-dimensional cavity 32 having an opening side. The cavity extends in the flow direction indicated by the arrows in FIG. 3 and in one embodiment has dimensions of 50.8 cm (20 inches) long, 25.4 cm (10 inches) wide and 7.62 cm (3 inches) deep. Another embodiment is 25.4 cm (10 inches) by 12.7 cm (5 inches) by 7.62 cm (3 inches). The cover member 30 has a peripheral flange 30-1 to allow fastening of the cover 30 to the duct 10. Porous member 34 that is the same material as the duct liner 11 and is faced or covered divides the cavity 32 into chambers 32-1 and 32-2. Accordingly, the porous material 34 defining the liner 11 and the partition wall enhances flow attenuation by facing or covering. The microphone 16 is illustrated as being positioned in the upstream chamber 32-1, but may be positioned in the chamber 32-2. The opening 10-1 is present in the duct 10 or near the outlet of the fan 12 and is sized to correspond to the open side of the cover 30 and only the duct liner 11 is ducted from the cavity 32. Overlaid by the duct liner 11 to separate the interior of 10. The cover 30 is secured to the outside of the duct 10 by screws 38 or other suitable attachment means. Since air leakage can generate flow noise and the cover needs to be airtight and actuated to contain sound, a gasket or seal may be installed between the flanges of the cover 30 or duct 10 if desired or desired. Can be.

In operation, fan generated noise easily passes through the duct liner 11 to the chambers 32-1 and 32-2 of the cavity 32, which are sensed by the microphone 16. The duct liner 11 provides a restriction on the disturbance T flowing into the cavity 32. In addition, porous material 34 that separates cavity 32 into chambers 32-1 and 32-2 attenuates internal flow recycling between chambers 32-1 and 32-2. Attenuation of the internal flow recirculation may be significant in order to average the noise generated in the liner by disturbances that facilitate the passage of disturbing flow through the duct liner 11 overlapping the openings 10-1. It is the result of associating size. Porous material 34 acting as a partition wall effectively counteracts the significant magnitude of the effect of the opening 10-1 on the flow circulating in the cavity 32.

5 is the same as the apparatus of FIGS. 3 and 4 except that the cover 30 is rotated 90 degrees. As a result of changing the positioning, the length of the duct for installing the cover 30 becomes shorter, the flow direction extension distance of the opening 10-1 becomes shorter, and the two chambers 32-1 and 32-2 It is present at the same position with respect to the length of the duct 10. The operation of the apparatus in FIG. 5 is the same as that of the apparatus in FIGS. 3 and 4.

In the embodiment of FIG. 6, the partitions defined by the porous material 34 are smaller than the chambers 32-1, 32-2 in which the cavities 32 correspond to chambers 132-1, 132-2, 132-3. And replaced by porous material partitions 134-1 and 134-2. The microphone 16 can be positioned satisfactorily in one of the chambers 132-1, 132-2, and 132-3, but by positioning in the chamber 132-2, the chamber 132-2 has three out of six sides. While the chambers 132-1 and 132-3 are made of porous material, only two of the six sides are made of porous material. The extra side of the porous material in the chamber 132-2 provides extra means against the recirculating flow effect as described above as in the smaller chambers compared to the chambers 32-1, 32-2. Otherwise, the apparatus of FIG. 6 is the same as the operation of the apparatus of FIGS. 3 and 4.

In the embodiment of FIG. 7, the duct 10 is unlined such that the opening 10-1 provides free communication between the interior of the duct 10 and the cavity 32. Thus, the porous material 111 is positioned in the cover 30, extends into the cavity 32 to approach the other open side of the cover 30, and flush surface embedded in the inner surface of the duct wall 10. To provide. The cavity 32 is then divided into chambers 232-1 and 232-2 by the porous material 234 forming the partition. Although it extends through one side, especially the bottom, of the cover 30 to be embedded in the floor rather than positioning the microphone 16 as a whole within the chamber 232-1, in another embodiment of the present invention the microphone is further extended into the cavity. You can also In addition, the microphone only needs to respond to the acoustic pressure in the cavity 32 and can be in fluid communication via a tube or the like while being positioned on the outer surface of the cavity 32. Since the porous material 111 is positioned in the cover 30, the chambers 232-1 and 232-2 are smaller than the chambers 32-1 and 32-2 if the dimensions of the cover 30 are the same. The operation of the device of Fig. 7 is the same as that of Figs. If desired, the porous material 111 alone or in combination with the porous material 234 may fill most or all of the cavity 32. This option is enhanced by placing the microphone 16 in a buried manner.

In the embodiment of FIG. 8, the porous material 334 is parallel to and spaced apart from the duct liner 11 to form a partition in the cavity 32 that is divided into chambers 332-1 and 332-2. 32). Chamber 332-1 and porous material partition 334 are positioned between cavity chamber 332-2 and inside duct 10 to provide additional means for reducing internal flow circulation to chamber 332-2. to provide. Thus, the microphone 16 is preferably positioned within the chamber 332-2. The operation of the embodiment of Figure 8 is basically the same as that of Figures 3 and 4.

In the Figure 9 embodiment, the cover 430 is three-dimensional parabolic in the cross section shown. Because of the parabolic shape, short wavelength (ie, wall derived disturbance) noise reaching the cavity 432 is reflected by the inner surface of the cover 430 at the parabolic focal point. This focusing of short wavelength disturbing pressure pulsations allows greater averaging to minimize the effect. Long wavelength acoustic energy (ie fan noise) is maintained substantially unaffected by the presence of the reflection surface. The microphone 16 is positioned in an insulating material 434 that defines a partition of the parabola, preferably partitioning the cavity 432 into the chambers 432-1, 432-2. Although a parabolic shape is shown specifically, there are other shapes, such as some of the three-dimensional ellipses, with a focus where noise is reflected. Except for focusing the disturbing noise and positioning the microphone 16 at its focus, the embodiment of FIG. 9 works the same as the apparatus of FIGS. 3 and 4 with regard to reducing internal flow circulation.

10 shows two identical in the same cross section obtained from a 25 ton air conditioner (VPAC) unit vertically stacked rotated 90 ° immediately downstream of the fan discharge, with disturbance levels equal to 25 to 30% based on bulk average speed. The performance of the wall cavity disturbance shield of the present invention mounted flush against interference between well shielded microphones is shown. The addition of liner was less effective below 20 Hz and the interference increased dramatically above 20 Hz. The addition of a cover [50.8 cm × 25.4 cm × 7.62 cm (20 inches × 10 inches × 3 inches)] has been found to significantly improve interference across the entire band, and the vertical orientation of the porous material to dampen the internal flow circulation. By installing three internal baffles or bulkheads, the interference is further improved, especially at low frequencies.

One or more additional microphones may be added to the cavity 32 and summed to improve the interfering acoustic energy measured from the fan 12. For example, interfering acoustic energy is increased at a rate of 6 dB per fold of the number of microphones / sensors, while non-interfering sound, i.e. disturbing noise, is added as 3 dB per fold of the number of mics / sensors. Microphones / sensors are spaced longer than the disturbing interference length. Referring now to Figure 11, it is similar to Figure 5 except that microphone 16-1 is added and positioned within cavity 32-2. Since the microphones 16, 16-1 are at the same distance from the fan 12, they are directly connected to the adder 50. Referring now to FIG. 12, the cover 30 is similar to FIG. 11 except that the cover 30 is rotated 90 degrees clockwise. This causes microphone 16-1 and chamber 32-2 to be downstream of microphone 16 and chamber 32-1 with respect to fan 12. Thus, there is a time delay Δτ between the microphones 16 and 16-1 and the microphone 16 is connected to the adder 50 through the time delay 60 while the microphone 16-1 is connected to the adder ( 50) directly.

As described above, the duct liner 11 and the partition walls 34, 134-1, 134-2, 234, 334, and 434 are porous glass fibers with facings or coverings that exhibit greater resistance to flow through. Are made of the same material. Figure 13 shows an increase in interference due to the presence of facings on the fiberglass.

Referring now to FIGS. 14 and 15, the housing or protrusion 110-1 defining the cavity 532 can be integrated with the duct or fan outlet 110. The main advantage of this embodiment is that airtightness is ensured since only the opening for electrical connection to the microphones 16, 16-1 in the chambers 532-1, 532-2 is required in the housing 110-1. Additionally, housing 110-1 may be made of the same material as duct or fan outlet 110 and have the same sound / noise absorption capacity. The embodiments of FIGS. 14 and 15 correspond directly to the embodiment of FIG. 12 and correspond to the FIG. 11 embodiment if rotated by 90 °, but the embodiments of FIGS. 3 to 9 also change the duct without changing their basic functions. It may be provided with a cover integral with. Thus, an example of a modified embodiment of the device of Figures 3-9, which shows a cover integral with the duct or fan outlet, would not need additional information or explanation.

Although preferred embodiments of the present invention have been described and illustrated, other changes will occur to those skilled in the art. For example, more chambers may be formed and the chambers may be maintained in unequal sizes and / or different orientations. In addition, the microphone only needs to respond to acoustic pressure in the cavity, eliminating the need for physical positioning of the microphone within the cavity. This will be helpful if the cavity dimensions are small compared to the microphone and baffle. Therefore, it is intended that the subject matter of the invention be limited by the claims appended hereto.

Claims (14)

A duct with a noise generating fan 12 and a fan outlet with flow disturbances therein and an air distribution means defined by the duct 10 including the fan outlet and at least partially having a wall, an inner surface and a disturbing shield In an active noise cancellation system, The disturbance shield, Means for defining a cavity (32, 432, 532) having an open end and defining a cavity positioned adjacent the duct such that the open end corresponds to an opening in the wall; Flow damping means (11, 111) positioned between the inner surface and the cavity; At least one acoustic sensing means (16, 16-1) positioned to respond to acoustic pressure in the cavity and separated from the inner surface by the flow damping means, And the cavity is divided into a plurality of chambers. The disturbing shield of claim 1, wherein said flow damping means is a simple physical barrier between said inner surface and said at least one acoustic sensing means. The disturbing shield of claim 1, wherein said flow damping means is at least partially filled in said cavity. The method of claim 1, wherein the at least one sound sensing means is at least partially positioned in the first chamber (332-2) of the plurality of chambers, the second chamber (332-1) of the plurality of chambers is A disturbing shield, positioned between the first chamber and the inner surface of a plurality of chambers. The disturbing shield of claim 1, wherein said at least one acoustic sensing means (16) is positioned in one of said plurality of chambers. 6. A disturbing shield as claimed in claim 5, further comprising second sound sensing means (16-2) at least partially positioned in a second of said plurality of chambers. The disturbance shield of claim 6, wherein the first chamber of the plurality of chambers is positioned upstream of the second chamber of the plurality of chambers. The disturbing shield of claim 1, wherein the cavity has a three-dimensional parabolic shape. The disturbing shield of claim 1, wherein flow damping means is used to divide the cavity into a plurality of chambers. 10. The disturbing shield of claim 9, wherein said at least one acoustic sensing means is positioned within said flow damping means that divides said cavity into a plurality of chambers. The disturbing shield of claim 1, wherein the cover has a parabolic shape. 12. A disturbing shield as set forth in claim 11, wherein said at least one acoustic sensing means is located in flow damping means that divides the cavity into a plurality of chambers. The disturbing shield of claim 1, wherein the cover and the cavity formed thereby are formed to echo sound into focus. The disturbing shield of claim 13, wherein said at least one sound sensing means is positioned at said focal point.