WO1996002910A1

WO1996002910A1 - Active duct silencer kit

Info

Publication number: WO1996002910A1
Application number: PCT/US1995/008144
Authority: WO
Inventors: Dexter G. Smith; Deborah P. Mendat; Steven Graham
Original assignee: Noise Cancellation Technologies, Inc.
Priority date: 1994-07-15
Filing date: 1995-06-29
Publication date: 1996-02-01

Abstract

An active duct silencer (120) that can be furnished complete or in a kit which utilizes feedback microphones (56, 57) and microprocessor (55) with a fast adapting algorithm to quiet noise in the duct (11).

Description

ACTIVE DUCT SILENCER KIT

This invention relates to an active duct silencer which is adapted to be furnished in a kit form as either separate electronic components or assembled in a duct section that will be integrated into a planned duct system or retrofitted into existing duct work.

Such a kit contains active electronics which comprise a microprocessor control board with inputs and outputs for digital signal processing, an AC to DC power supply, two microphones and a speaker with connecting cables. Sensors in a duct fitted with the kit or a duct silencer supplied therewith sample the sound field in a duct at a position some distance from the silencer inlet. A digital signal processor analyzes that data, filters it and outputs an anti-noise wave which is an inverted version of the offending noise. Both the counter and original noise are summed and a sample of the mixed noise is taken near the outlet which is used to continually adapt the signal processing for maximum attenuation. For optimal performance of the kit or unit, the duct noise must have good coherence properties, i.e., the noise upstream and at the outlet is the same with the active unit turned off. Such an arrangement can provide up to 12 dB of attenuation within low- frequency one-third octave bands.

These units solve the existing problem of currently available duct silencers which have poor low-frequency performance and in the case of higher performance units, they have substantial air pressure drop. The instant invention provides low frequency performance with reduced pressure drop and can be easily installed by technicians of normal skill. The lengths of duct units can vary from three feet to ten feet and the cross sectional dimensions can include 6x12, 9x12, 12x12, 12x18, 12x24, 15x15, 18x18, 18x24, and 24x24. Accordingly, it is an object of this invention to provide an improved duct silencer to deal with low frequency noise while minimizing pressure drop throughout the duct.

Another object of this invention is to provide a retrofit kit for active noise cancellation of existing or planned ductwork.

A further object of this invention is to provide a duct section with both active and passive quieting which can be inserted into an existing or future duct system.

These and other objects will become apparent when reference is had to the accompanying drawings in which

Figure 1 is a typical linear flow noise cancellation application, Figure 2 is a block diagram representation for the system seen in Figure 1 , Figure 3 is a cross-sectional view of the physical arrangement of a first embodiment of this invention showing a passive lining,

Figure 4 is a frequency versus pressure level plot of the embodiment of Figure 3 with random noise, Figure 5 is a frequency versus pressure level chart of the operation of the embodiment of Figure 3 as per Figure 4 with the active noise on and off,

Figure 6 is a frequency versus pressure level plot of the embodiment of Figure 3 with recorded fan noise, Figure 7 is a frequency versus pressure level plot of the operation of the embodiment of Figure 3 as shown in Figure 6 with the active noise on and off,

Figure 8 is a cross-sectional view of the physical arrangement of a second embodiment of this invention,

Figure 9 is a frequency versus pressure level plot of the operation of the second embodiment of Figure 8 for random noise,

Figure 10 is a frequency versus pressure level plot of the operation of the second embodiment with the active noise on and off,

Figure 11 is a cross-sectional view of a third embodiment of the physical arrangement of this invention, Figure 12 is a plot of frequency versus pressure of random noise being tested on the third embodiment,

Figure 13 is a plot of pressure versus frequency of the third embodiment handling random noise with the active noise on and off,

Figure 14 is a plot of pressure versus frequency of the third embodiment handling 160 Hz random noise,

Figure 15 is a pressure versus frequency plot of the third embodiment with active noise on and off,

Figure 16 is a block diagram of the control system.

Detailed Description

A typical linear flow noise cancellation application is seen in Figure 1. Noise 100 enters sound conductor 101 which could be a pipe or duct and propagates at the speed of sound At some point in the duct 101, noise 100 is measured by reference sensor 102. Digital signal processor system (DSP) 105 calculates a signal to attenuate noise 100 and injects this signal into duct 101 through cancellation transducer 103 The residual noise after mixing noise and anti-noise is measured by sensor 104. The residual error sensor 104 signal and the reference sensor 102 signal are digitally processed by DSP system 105 to continually generate a signal that minimizes the residual error signal power seen at sensor 104. Figure 2 shows a block diagram representation for the system seen in Figure 1 and the associated DSP system to continuously attenuate the noise in sound conductor 101 in Figure 1. Figure 2 assumes that the system depicted in Figure 1 can be broken down into components and modeled by linear, time invariant filters For example, the acoustic path the noise travels can be broken down into a component from the reference sensor to the point in space where the noise and the anti-noise mix and a component from there to the residual error sensor.

The components of the physical system are seen in block 110. The transfer function Pi l l represents the transmission path of the noise 100 from the reference sensor 112 to the cancellation transducer 113. Noise 100 is sensed by sensor 114. Block Fl 15 represents the acoustic feedback path from cancellation transducer 113 to the reference sensor 112. Block SI 13 represents the cancellation transducer 113. Block El 16 represents the transmission path from the cancellation transducer 117 to the residual error sensor 118. Reference sensor 112 is depicted as a summer because it senses both the noise 100 and the cancellation signal after passing through 113 and 1 15. Summer 117 is a depiction of noise 100 after transmission path Pi l l and cancellation signal 119 through cancellation transducer 113 mixing.

The adaptive noise canceller used in this invention is seen in block 120. Signal 121 is the reference signal, signal 122 is the residual error signal and signal 119 is the cancelling signal. Blocks A 123, B124 and Cl 32 are adaptive Finite Impulse Response (FIR) filters. The purpose of filter B 124 is to model the acoustic feedback of cancellation signal 119 through SI 13 and Fl 15. Signal h(n) 126 is then the best estimate of noise in the duct after subtracting the acoustic feedback signal at summer 127. Filter A 123 then shapes the measured reference signal 121 to account for its propagation through P in the duct and for cancellation signal 119 distortion through S. Filter C 125 is an estimate of canceling signal 119 through path S and E.

When the system is canceling, filter A 123 is adjusted by adapter 2 128 to rrύnimize residual error signal 122. To calibrate the system, filter A weights are set to zero and noise generator 129 is turned on. Adapterl 130 then adjusts B 124 filter weights to model the path SF. Adapter 3 131 adjusts C125 filter weights to model the path SE. Weights from filter C125 are then used in filter C132 during system cancellation to ensure convergence of the filter A123 weights.

Figure 3 shows a first embodiment of this invention designated as 10. It consists of a duct section 11 having outer walls 12 and inner walls 13. An acoustic dampening material 14 is located between said walls such as Fiberglas or a similar material. An upstream microphone (mic 1) is placed adjacent the inlet end of the duct 11 and a residual microphone (mic 2) is placed near the outlet end. A housing 15 houses a speaker 16 which is adapted to broadcast counter noise into the duct. Holes 17 may be provided in the walls of the duct and the dampening material. Additional noise dampening material is packed around the speaker 16 to keep housing resonance to a minimum. Figure 4 shows a 13.2 dB reduction in the plots of the unit with random noise from 44 Hz to 355 Hz at an air velocity of 2000 feet per minute (fpm). The duct tested was sheet metal of 300 x 300 xlOOO mm with a 1 inch Fiberglas lining Figure 5 shows the octave band center plotted versus dB for the Figure 3 embodiment. When a recorded fan noise was used on the same embodiment varying degrees of attenuation were obtained depending on the tone as shown in Figure 6 Figure 7 shows the frequency versus dB plot with the results plotted as a bar graph

Figure 8 shows a second embodiment of the silencer unit 20. A duct 21 having inner walls 22 and outer walls 23 has an inlet end adjacent the noise source and an outlet end opposite. A housing 24 encloses a speaker 25. An acoustic deadening material 26 is packed between the inner and outer walls. As shown, the spacing between the inner and outer walls where the speaker is housed is much greater than where microphones 26 and 27 are located. Also the inner walls are formed as curves 28 or angles 29 to prevent disturbances in the air flow. Again holes may be provided in the sheet metal and dampening material in front of the speaker and in front of the microphones as well Also, acoustic foam can be packed around the rear of the speaker Figures 9 and 10 show a graph and bar chart of the attenuation results when 100 Hz to 300 Hz of random noise was run through the duct of the embodiment of Figure 8 at 2000 fpm A 104 dB attenuation was achieved.

Figure 11 shows a third embodiment of the invention generally designated as 30. It consists of a duct 31 having inner walls 32 and outer walls 33 with curved transition portions 34. Microphones 35 and 36 are embedded in acoustic foam 37 between the inner and outer walls A housing 38 encloses speaker 39 which is driven to produce counter noise into the duct As shown in Figures 3, 8 and 11 the residual microphone 2 is placed slightly downstream of the center of the axis of the diaphragm of speakers 16, 25 and 39 A foam or Fiberglas is used to pack around speaker 39 which, as shown, projects directly into the inner duct space. Holes 40 are provided for microphone 1 and microphone 2 Figures 12 and 13 depict a plot and bar graph of the attenuation of random noise from 44 Hz to 355 Hz running through the duct at a flow rate of 1000 fpm Figures 14 and 15 show a graph and bar graph of a random noise centered at 160 Hz passing through the duct at 1000 fpm The space between the inner and outer wall in Figure 11 is at least 25% on each side of the total duct width

Figure 16 shows a block diagram of the system 50 which includes an AC to DC power supply 51, a microphone preamp circuit 52 and a power amplifier 53 connected to speaker 54 An active microprocessor unit 55 is connected to preamp 52 as are microphones 56 and 57 The ACU or active microprocessor 55 is a fast adapting digital unit Having described the invention it will be obvious to those of ordinary skill in the art that many changes and modifications can be made without departing from the scope of the appended claims.

Claims

1. An active noise attenuation kit for retrofitting existing duc$s for reducing undesirable noise associated with the operation of the ducts, said kit comprising a first microphone means adapted to be placed upstream in a duct so as to sense the initial disturbing noise, a second microphone means adapted to be placed further downstream in the flow carrying the offending noise, speaker means adapted to be placed downstream from said first microphone means and approximately opposite said second microphone means, active control means adapted to receive signals from said first microphone means representing the noise to be attenuated, driving said speaker means so as to produce equal amplitude, and opposite phase noise to cancel said offending noise and adapted to receive signals from said second microphone means to measure its effectiveness in cancellation. •

2. A kit as in claim 1 wherein said active control means includes a digital signal processor and an AC to DC power supply means.

3 A kit as in claim 2 wherein said kit further includes a field adjustment unit means.

4 A kit as in claim 3 wherein said kit further includes a separate power amplifier means for said speaker means.

5 A duct silencer unit for installing into existing or planned air duct systems, said unit comprising a section of duct having outer walls of a predetermined size for installation into a duct system, said duct being open at each end, one end being defined as an inlet end, an upstream microphone means located in said duct near said inlet end and adapted to generate sensing signals in response to noise waves entering said duct, speaker means located in said duct along a wall and downstream from said upstream microphone means and adapted to generate counter noise waves to attenuate said noise waves entering from said inlet end, residual microphone means located in said duct downstream from said speaker means and adapted to sense the combined noise and counter waves and to generate a residual signal in response thereto, electronic control means including digital signal processing means adapted to receive said sensing signal, produce a signal in relation thereto to drive said speaker means to produce counter noise waves and to adjust said driving signal in response to said residual signal, and passive quieting means covering at least two lengths of said duct walls to attenuate the higher frequency components of the entering noise.

6. A unit as in claim 5 wherein said duct has inner walls generally parallel to said outer walls and said passive quieting means is between said inner and outer walls.

7. A unit as in claim 6 and including noise dampening means surrounding said speaker means.

8. A unit as in claim 6 wherein the speaker means is between said inner and outer walls on one side of said duct and both microphone means are located on the opposite side of said duct between the inner and outer walls of said duct, the space between the inner and outer walls around said speaker being greater than the space between said walls where said microphone are located.

9. A unit as in claim 6 wherein the space between said inner and outer walls is greater than 25% of the width of said duct.

10 A unit as in claim 9 wherein said speaker means is located between said inner and outer walls.

11 A unit as in claim 10 wherein both said microphone means are located between said inner and outer walls.

12. A unit as in claim 5 wherein said electronic control means includes a microphone preamplifier means and a power amplifier means.

13 A unit as in claim 12 and including an AC to DC power supply means.

14 A unit as in claim 5 including a field adjustment unit means.