Electrical audio signal processing systems and devices – Electro-acoustic audio transducer – Electromagnetic
Reexamination Certificate
1999-08-27
2001-02-27
Le, Huyen (Department: 2743)
Electrical audio signal processing systems and devices
Electro-acoustic audio transducer
Electromagnetic
C381S096000, C381S353000
Reexamination Certificate
active
06195442
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The proposed invention is related to the field of structural vibration and acoustic (noise) control.
2. Description of the Prior Art
A number of engineering applications can be described as a flexible structure surrounding a cavity. In these systems, structural vibrations induced by either force inputs or external acoustic pressure loads produce sound (or noise) within the cavity. Some common examples of acoustic cavities enclosed by a flexible structure include airplanes, trains, cars and spacecraft launch vehicles. A subset of this group of applications is civil structures or buildings, where relatively rigid walls are combined with flexible windows and other panels.
For the case of spacecraft launch vehicle fairings, the structure is typically constructed from a stiff composite material. The launch vehicle fairing is subjected to extremely high levels of structural vibration and noise at launch. This vibration and noise can damage delicate payloads. The acoustic response of the volume enclosed by the flexible composite structure is dominated at low frequency by very lightly-damped structural acoustic modes (50 Hz-250 Hz). In spacecraft applications, mass and volume are critical parameters, therefore there is a budget on how much noise treatment can be added to the fairing in order to mitigate noise and vibration. Passive methods for attenuating low-frequency disturbances such as foam linings and acoustic blankets are not effective at low frequency. Furthermore, they provide negligible structural vibration attenuation.
Active control strategies have been applied to reduce noise inside acoustic cavities such as aircraft fuselages and automobiles, and have demonstrated significant success in coupling to and attenuating low-frequency acoustic modes. (Fuller, C. R., et. al., “Experiments on Reduction of Aircraft Interior Noise Using Active Control of Fuselage Vibrations,”
J. Acoust. Soc. Am.,
78(S1), S79, 1985; Fuller, C. R., et. al., “Active Control of Sound Transmission/Radiation from Elastic Plates by Vibrational Inputs,”
J. Sound and Vibration,
136(1), pp. 1-15, 1990). However, these strategies have the disadvantage of requiring complex control algorithms, digital signal processing hardware, power amplifiers, signal conditioning, and extensive cabling, which greatly increases the mass of the system. In addition, active acoustic control techniques do little to reduce the vibration of the structure or to prevent noise from being transmitted through the structure. Finally these techniques have never been demonstrated at acoustic levels commensurate with space launch.
There are several accepted methods of reducing noise transmission from external sources into a cavity interior. Most methods are designed to reduce structural vibration by increasing the mass, stiffness or damping of the structure. Traditional, localized, reactive vibration-suppression devices such as vibration absorbers and tuned mass-dampers are very effective at increasing localized structural impedance and structural damping, respectively. (Bies, D., and Hansen, C.,
Engineering Noise Control, Theory and Practice
, E&FN SPON, 2
nd
edition, NY, 1996). The disadvantage of such devices is the necessity of additional mass for their operation. Furthermore, these devices act only on the structure, and do little to attenuate the acoustic dynamics of the cavity.
Recently, work has been done to combine active structural control with active acoustic control. (Jolly et. al.,
Hybrid Active
-
Passive Noise and Vibration Control System for Aircraft
, U.S. Pat. No. 5,845,236, 1998; Fuller, C. R.,
Apparatus and Method for Global Noise Reduction
, U.S. Pat. No. 4,715,559, 1987; Hodgson, et. al.,
Broadband Noise and Vibration Reduction
, U.S. Pat. No. 5,526,292, 1996; Majeed et. al.,
Active Vibration Control System for Attenuation Engine Generated Vibrations in a Vehicle
, U.S. Pat. No. 5,332,061, 1994). These methods utilize arrays of structural sensors such as accelerometers, and acoustic sensors such as microphones to sense disturbances and actively cancel them. Typically in these control systems, loudspeakers are driven by a control signal 180° out-of-phase with the sensed disturbance to provide cancellation. Also, active vibration absorbers, passive vibration absorbers, or structural actuators such as proof-mass actuators, piezoceramic actuators, or shakers, are used by the control scheme to control structural vibration. Although these active methods have been successful at reducing structural vibration and interior noise, they require a considerable degree of sophistication and hardware to implement. The mass of the hardware (including actuators, sensors, controllers, signal conditioning hardware, power amplifiers, mounting apparatus, and cabling) can become prohibitively large and negate the value of using such systems. Furthermore, complex control systems such as these have a greater chance of component failure, which can be catastrophic in critical applications.
SUMMARY OF THE INVENTION
The passive vibroacoustic device of the present invention consists of an acoustic diaphragm, a voice-coil, a magnet, a shunting resistor, and a base suspension. Within a flexible structure surrounding a cavity, the device operates to both reduce the structural vibration by increasing the mechanical impedance of the flexible structure and to dissipate acoustic energy in the cavity. The vibroacoustic attenuating device has numerous advantages over active vibration absorbers. It is completely passive in nature, not requiring cabling, power amplifiers, signal conditioning hardware, or centralized control schemes. The device is capable of coupling to and dissipating low-frequency acoustic modes of a cavity. It also acts as a collocated structural vibration damper and couples to and dissipates structural vibration modes. Unlike tuned vibration absorbers and similar devices which have a narrow targeted bandwidth of attenuation, this device targets multiple structural modes and acts over a wider bandwidth.
Furthermore, there is no possibility of instability or catastrophic failure since the device is completely passive. It is much less expensive to produce and more easily implemented than fully-active and hybrid control systems of the prior art. This device can easily be added onto existing spacecraft fairing structures without requiring redesign of the fairing. The added mass contributed by the device itself serves to attenuate both structural vibration and the acoustic cavity modes, providing more efficient use of the added mass.
The advantages offered by this invention and further novel details and features of this device will become readily apparent from the subsequent description and drawings of the preferred embodiment.
REFERENCES:
patent: 4715559 (1987-12-01), Fuller
patent: 5332061 (1994-07-01), Majeed et al.
patent: 5526292 (1996-06-01), Hodgson et al.
patent: 5586195 (1996-12-01), Ishigaki et al.
patent: 5812686 (1998-09-01), Hobelsberger
patent: 5845236 (1998-12-01), Jolly et al.
patent: 3524280 (1987-01-01), None
Fuller, C. R., et. al., “Experiments on Reduction of Aircraft Interior Noise Using Active Control of Fuselage Vibrations,”J. Acoust. Soc. Am., 78(S1), S79, 1985.
Fuller, C. R., et. al., “Active Control of Sound Transmission/Radiation from Elastic Plates by Vibrational Inputs,”J. Sound and Vibration, 136(1), pp. 1-15, 1990.
Bies, D., and Hansen, C., Engineering Noise Control, Theory and Practice, E&FN SPON, 2ndedition, NY, 1996.
Cazzolato, B., Novel Transduction Methods for Active Control of Sound Transmission into Enclosures. Ph.D. Dissertation, University of Adelaide, 1998.
Griffin Steven F.
Lane Steven A.
Callahan Kenneth E.
Le Huyen
The United States of America as represented by the Secretary of
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