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Title:
INTEGRAL DEVICE FOR ACTIVE CONTROL OF NOISE IN DUCTS
Document Type and Number:
WIPO Patent Application WO/1995/024171
Kind Code:
A1
Abstract:
An active pressure fluctuation control system to control noise in ducts including a vibratable element (2) within a duct (1) which can be adapted to vibrate so as to form a cancelling waveform to attenuate sound wave within the duct system (1).

Inventors:
Cook, Douglas R.
Donovan, Joseph B.
Application Number:
PCT/US1995/002435
Publication Date:
September 14, 1995
Filing Date:
March 01, 1995
Export Citation:
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Assignee:
NOISE CANCELLATION TECHNOLOGIES, INC.
International Classes:
G10K11/178; H04B11/00; (IPC1-7): A61F11/06; H03B29/00; H04B15/00
Foreign References:
US4947434A1990-08-07
US4709234A1987-11-24
US5103482A1992-04-07
US4565940A1986-01-21
US4787071A1988-11-22
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Claims:
CLAIMS
1. An active pressure fluctuation control system for use in ducts, said system comprising an elongated duct means of a specific geometrical crosssection adapted to carry pressure fluctuations, a section means within said duct means adapted to be vibrated to produce counter fluctuations to attenuate said pressure fluctuations, and a source of alternating voltage connected to said section means to cause it to vibrate to produce said counter fluctuations.
2. A system as in claim 1 wherein said section means comprises a crosssectional portion of said duct means with a layer of material that changes shape as a result of an electric field being imposed on it.
3. A system as in claim 1 wherein said section means includes a material layer that changes shape as a result of an electric field being imposed thereon.
4. A system as in claim 3 wherein said material is PVDF.
5. A system as in claim 3 wherein said material is a piezoelectric ceramic.
6. A system as in claim 3 wherein said material is an electrostrictive ceramic.
7. A system as in claim 1 wherein said section means includes a material layer that changes shape as a result of a magnetic field being imposed on it.
8. A system as in claim 7 wherein said material is Terfenol.
9. A system as in claim 1 wherein said section means comprises a patch in said duct means including a layer of material that changes shape as a result of an electric field being imposed thereon.
10. A system as in claim 1 wherein said section means comprises a flow modulated portion with a layer thereon of a material which changes shape as a result of an electric field being imposed upon it.
11. A system as in claim 1 wherein said section means includes a Bourdon tube means extending out from said duct means, said Bourdon tube means having a layer thereon of a material which changes shape as a result of an electric field being imposed upon it.
12. A system as in claim 1 wherein said section means includes a vibratable diaphragm which is actuated by a material which changes position as a result of an electric field being imposed thereon.
13. A system as in claim 12 wherein said section means also includes a lever arm actuated by changing positions of said material.
14. A system as in claim 1 wherein said section means includes a diaphragm means suspended within said duct means and adapted to fluctuate transversely to the direction of the pressure fluctuations.
Description:
INTEGRAL DEVICE FOR ACTIVE CONTROL OF NOISE IN DUCTS

Broad Description of the Subject Technology

This invention is related to the control of pressure pulsations in a fluid or gas carried by a duct or piping system. These systems are commonly used for the conduction of exhaust gases, cooling fluids, hydraulic fluids, or other fluids used in industrial processes. The pulsations can have several negative consequences. For one, they can cause offensive noise to radiate from the end of the piping system or the duct walls or they can cause excessive wear, and eventually fatigue, of the mechanical system components. They can also cause process inconsistencies which affect the quality of the final product. The source of the pressure pulsations may be an internal combustion engine, pump or similar device. Control of the fluctuations is proposed by using devices constructed of materials which change shape as a result of various electrical and magnetic fields being imposed on them. Some examples of these materials are: PVDF, Piezoelectric Ceramics, Electrostrictive Ceramics, and Magneto strictive materials like Terfenol.

Background Art

Several means have historically been used to quiet noise in ducts. The most common approach is to apply a passive silencer. Such silencers typically have dramatic flow area changes, resonant tubes, and/or dissipative media. These silencers are typically large, heavy, expensive, and offer a great deal of flow resistance.

An alternative to a passive only system is an active system in combination with a low-flow-restriction passive muffler which effectively eliminates high frequency noise. An active noise cancellation system functions by generating pressure pulsations that cancel the offending noise that remains. These systems typically have as a component one or several moving-coil-type loudspeakers. The loudspeakers are relatively inefficient producers of sound and thus require much electrical power to drive them. They are also lacking in durability and are limited in the type of environments in which they can function. This is especially important since the duct may be carrying hot exhaust gases, bulk solids, or other substances which are inhospitable to loudspeakers. Temperature extremes and moisture can damage or at least limit the performance of the loudspeakers. Finally, loudspeakers operate over a fairly broad bandwidth but are ineffective within the relatively small bandwidth at the low frequencies necessary for noise cancellation. Thus, they require enclosures to "tune" the active system to the frequencies of the offending noise. These enclosures are necessarily large and may be expensive.

Active-only noise cancellation systems are applicable when the offending noise has only low frequency content or as active control technology permits. Regardless, the

drawbacks of the moving-coil-type loudspeaker are the same In this case, however, the bandwidth requirements may be greater

Materials which can change shape as a result of various electrical and magnetic fields being imposed on them have been used for the generation of sound These materials, when integrated into an appropriately designed sound radiating mechanism, have the potential to replace moving-coil-type loudspeakers They have been shown to be smaller, lighter, less expensive, and more durable Relatively few applications have been for the generation of low frequency sound, the vast majority being tweeters or ultrasound generators Patents in which one of the objects of the invention was the generation of low frequency sound using such materials include U S Patent No 's

3,588,381, 4,386,241, 5,014,321, 5,132,942, and 5,185,549 These materials generally produce small displacements, and thus small pressure pulsations, when energized Attempts to amplify the sound generated by acoustic means are found in U S Patent No 's 4,885,781 (actually a sensor but conceivably operated in reverse) and 4,979,219 Attempts to amplify the sound generated by mechanical means are found in U S Patent No "s 4,547,631 and 4,706,230

Broad Disclosure of the Invention

This invention provides a means of reducing noise in a duct using different types of materials- magnetostrictive and piezoelectric Use of these flexible films allows for direct attachment to the structure for control This invention contains several embodiments The means of reducing noise radiated from a duct using various types of materials can be achieved in the following ways

(a) In the first embodiment of the invention, a section of the duct is made from the material This method is an integral system and has benefits in that it breaks up the structural path as well as reducing radiated noise

(b) In a second embodiment, a patch of the material replaces a section of the duct wall Ease of implementation is a major benefit of this use

(c) In a third embodiment, the material is placed on the exterior of the duct wall which is constructed to perform as a flow modulator by dynamically varying the duct cross-sectional area The benefits are two-fold, flow modulation and active tuning

(d) In a fourth embodiment, the material is used in conjunction with a device resembling a "Bourdon Tube" type direct displacement pressure measurement device connected to the duct at a single point The device would convert small displacements into the pressure pulsations necessary for the active noise cancellation

(e) In a fifth embodiment, the material is used to drive single point excitation on the duct using some form of connecting pin/rod allowing mechanical amplification

(f) In the final embodiment, the material drives a diaphragm placed in a duct It should be pointed out that the duct can be either circular, rectangular or any shape in cross-section In view of the shortcomings of moving coil-type loudspeaker-based noise cancellation systems, it is an object of this invention to provide a system which overcomes said shortcomings A further object of this invention is to provide a means of quieting noise in a duct more effectively

Another object of this invention is to provide a duct quieting system of low weight and low cost

A still further object of this invention is to provide a duct quieting system having less flow resistance than means heretofore existing

Yet another object of this invention is to provide a duct quieting system wherein the active component of the system is integral with the duct

These and other objects will become apparent when reference is had to the following drawings in which Figure 1 shows a longitudinal sectional view of a duct to which the device has been attached The original duct material is actually replaced by one of the materials discussed

Figure 2 shows a longitudinal sectional view of a duct to which the device has been attached in place of a section of the duct wall Figure 3 shows a longitudinal sectional view of a duct to which the device has been attached on the exterior of a duct The device is used in conjunction with a flow modulator system

Figure 4 shows a longitudinal sectional view of a duct with the device used in conjunction with a "Bourdon Tube" type system Figure 5 shows the device used in conjunction with a mechanical amplification system

Figure 6 shows a longitudinal sectional view of a duct and the device used with a diaphragm in the flow stream

Figure 7 shows a perspective view of the device illustrated in Figure 6 Figure 8 is a graph of the sound in a duct before control

Figure 9 is the sound plot in the duct after noise control Figure 10 and 11 show the two plots of broadband noise in a duct before and after control

Figure 12 is a cross sectional view taken on line 12-12 of Figure 4, and

Figure 13 is a cross sectional view taken on line 13-13 of Figure 4

In Figure 1 duct 1 is shown as a sectional view of a circular or rectangular duct with a piezo portion 2 forming an integral part thereof I e , the material can form a complete band or section or be only on the top and bottom of the duct The controller 3 produces an alternating voltage source which drives the material 2 and generates pressure pulsations that couple with the duct interior pressure pulsations Material 2 is driven at suitable amplitudes and frequencies to reduce the noise radiated from the duct Microphones 10 and 11 are used to measure the initial sound, IS, and the final sound, FS to allow the controller 3 to adapt so as to vary the vibration of 2 if sound IS varies

The configuration shown in Figure 2 shows a section 2 of duct 1 being made of the material The section dimensions are optimized to obtain appropriate amplitude and frequency response Controller 3 produces an alternating voltage source to vibrate 2 Again, microphones 10 and 11 allow controller 3 to vary the vibration In this case they are part of the duct

Another version of the invention is shown in Figure 3 wherein duct 1 has a flow modulated section 4 A film or layer of the material 2 is placed on the exterior of this section to enhance acoustic performance over a specified range set by the specific geometry of the modulated section The exact thickness and specific configuration of the film or layer 2 is adjusted to additionally control the duct radiated noise response The drive signal is supplied by controller 3

Microphone 10 is used to detect IS and microphone 11, which can be outside the end of the duct 1, is used to detect and measure the final sound FS to allow controller 3 to adapt to changes It should be noted that if the sound IS never changes a complex adaptive controller is not needed, only an alternating voltage source

Figure 4 shows the material 2 used in conjunction with a device resembling a "Bourdon tube" type direct displacement pressure measurement device. Such a device consists of a curved, perhaps spiral, flattened tube 4 sealed on one end and attached on the other end to the pipe or duct 1 The device converts small displacements into pressure pulsations in the duct The material, suitably attached to the tube either along its flattened surfaces or otherwise linked, then actuates the device Energy is supplied to the material using some reasonable source of alternating voltage 3 The device's material and dimensions may be chosen as to "tune" its response to the offending noise In the extreme case of a flattened duct, the device is constructed out of two curved parallel plates and the result is something like a line source If the material is applied to both sides of the tube then each side may be actuated out of phase to maximize the pressure pulse

In the Figure, A is shown as the offending noise, B is the same as A but inverted 180° and C is the noise that goes back up the duct. The noise generally travels in a plane wave as indicated by the dotted lines.

Figure 12 is a cross-sectional view taken on line 12-12 through duct 1 and shows the general configuration.

Figure 13 is a cross-sectional view taken along line 13-13 of Figure 4 and shows the "Bourdon" tube 4 with the material layer or film 2 thereon. The tube 4 can be fashioned of copper and be tuned geometrically to offending frequencies.

Figure 5 shows the material 2 used to drive a pivot arm 7 attached to a structural diaphragm 6. The diaphragm can be attached to the duct 1 using some compliant element. Two pivots 5, one attached to the duct and the other attached to the diaphragm are spaced to achieve the desired mechanical amplification. Using the appropriate coupling between the pivot arm, the material and the diaphragm, the amplitude can be controlled for optimum generation of pressure pulsations in the duct. The material is actuated by an alternating voltage source 3. There is a flexible surround 8 which holds the diaphragm 6 in place.

A shows the plane wave of the noise to be canceled and B shows the pulsating counter waves. C shows the resulting noise wave after attenuation. A very small deflection of diaphragm 6 results in effective attenuation. The mechanism can be adjusted by increasing or decreasing the D and E dimensions of the pivot arm or "teeter- totter". Material 2 is driven by alternating current source 3.

Figure 6 shows the material 2 located in the duct 1. The material, mounted to a substructure 9 is driven by alternating voltage source 3. The diaphragm made of material 2 can be sized appropriately to couple with the frequency range of interest.

Best Mode Requirement

An example of the latter embodiment shown in Figure 6 is a best mode of the invention. The device, as shown in Figure 7, consists of a pipe 1 constructed of Polyvinyl Chloride (PVC) six inches in diameter and filled with air. Pressure pulses are transmitted along the pipe. A three inch diameter steel circular diaphragm 8 is suspended within the pipe by cables or rods 9. A piezoelectric ceramic material 2 is attached to the diaphragm and is wired to a source of alternating voltage 3 which is controlled by a computer (not shown). The computer is programmed to cancel the pressure pulses coming down the pipe with pressure pulses generated by the diaphragm vibrating transversely to the flow. The cancellation can be verified by a microphone (also not shown).

Two types of pressure fluctuations can be canceled by this invention; a tone or broadband noise. Figures 8 and 9 show the effectiveness of canceling a tone as measured

at the microphone Figure 8 shows the sound pressure vs frequency before application of the invention Figure 9 shows the sound pressure vs frequency after application of the invention Figures 10 and 1 1 show the effectiveness of canceling broadband noise Figure 10 shows the sound pressure vs frequency before application of the invention Figure 11 shows the sound pressure vs frequency after application of the invention

Having described the invention attention is directed to the claims defining the same and it will be obvious to those of ordinary skill in the art that many changes and modifications can be made to the claims without departing from the scope thereof