Low frequency transducers having resonances below about 1 kHz have numerous applications, one of which is as a low frequency sonar projector. This acoustic wavelength corresponding to these frequencies is on the order of the size of naval mines, and thus can hunt for and/or classify them, as well as objects of similar size. Also, wavelengths of this size permit sonar location of buried objects, a task of interest to a wide range of commercial and governmental concerns. Unfortunately, current underwater projectors at these frequencies are large and heavy, and are cumbersome to use on many underwater vehicles.
Another application for low frequency transducers is that of active noise control. Essentially, there are two schemes by which to control unwanted sound and vibration. One, passive, adds additional mass, stiffness, or damping; the other, active, uses destructive interference of the sound or vibration field. Passive control techniques are best suited for applications where the frequency band of the disturbance is above 1 kHz. On the other hand, active control has found use in applications where the frequency range of interest is between 50 Hz and 1 kHz. The use of adaptive and smart structures for the active control of vibrations is based on the successful combination of sensor, actuator, and electronic control systems. Active control structures can eliminate structural vibrations from, e.g., a piece of manufacturing equipment or a helicopter rotor, remarkably improving the lifetime of each by reducing wear. Likewise, minimizing cabin noise in an aircraft or duct noise in a building leads to a much higher comfort level for the people in side. In active control, a sensor/actuator combination which is located on the surface of the vibrating structure is used to detect and suppress the disturbance. The vibration signal picked up by the sensor is sent to the appropriate electronic circuitry and is subsequently used to drive the actuator such that it has the same magnitude but opposite phase (or opposite time delay) as the disturbance.
Current state of the art in active vibration control is that the sensor and electronic control systems are more technologically advanced than the actuator components. Control systems have benefitted from faster and cheaper microelectronics available from the computer industry. Likewise, a wide variety of sensors have been developed including fiber optic, piezocomposite accelerometers, and acoustic pressure sensors. Sensor selections can now be based on application specific needs. This means that the weakest link in active control systems is in actuator technology.
In systems aiming to cancel structure-borne sound, a pressing need is for an actuator panel whose bandwidth contains about 50 Hz to 1 kHz, and has a linear near-field velocity (displacement) profile coupled with high force capability. An additional consideration is for the actuators to be rugged enough for placement in applications where they may be leaned on or pushed against without damage. Because many active control systems are in environments where they are placed in large sheets (panels), such as in large vibrating machinery mounts in power plants, an actuator must be physically rugged enough to withstand normal forces and hazardous exposures.
Many active control systems utilize either hydraulics or large, heavy electro-magnetic force transducers as the actuator component. These technologies may often be constrained by packaging limitations as well as high cost. In recent years, piezoelectric materials either in the form of piezoceramic-polymer composites, multilayer stacks, or flexor-type configurations have been studied for active vibration control applications. Multilayer stacks and piezoceramic-polymer composites are characterized as generating high force/low displacement, whereas the flexors exhibit low force/high displacement capabilities.
Another type of actuator, called the xe2x80x9ccymbal,xe2x80x9d effectively bridges the gap between the high force/low displacement multilayer stacks and the low force/high displacement flexors. Cymbal actuators show excellent potential for many active vibration control applications because they are simple to manufacture (resulting in low cost), exhibit thin profile, ruggedness, adaptability to panel form, and tailorable device characteristics.
Accordingly, an object of the invention is to reduce the cost of active electro-acoustic transducers by use of inherently inexpensive cymbal-type actuators.
Another object is to enable such a transducer to operate at lower frequencies, particularly between 50 Hz and 1 kHz.
Another object is to do the foregoing with a transducer that is inherently rugged.
Another object is to do the foregoing with a transducer whose near field velocity is substantially linear.
Another object is to do the foregoing in a manner which permits a designer to tailor the trade off inherent in acoustic transducers between transducer force and displacement.
Another object is to provide an acoustic projector operating at 1 kHz or less that is small, lightweight, and has low vehicle volume occupation.
In accordance with these and other objects made apparent hereinafter, the invention concerns an electro-acoustic transducer having a plurality of cymbal-type acoustic elements; and a pair of plates containing the cymbals such that the cymbals are disposed in mechanical and electrical parallel arrangements between the plates. It has found that, when driven in piston mode, i.e., at or below the transducer""s fundamental, or piston, mode, the cymbals are mechanically reactive (i.e., they are moving in-phase with each other) such that they move together as a unit. This results in an overall system of greater inertia, and hence lower resonant frequency and corresponding lower frequency band of operation. Further, because the magnitude of acoustic output depends on the number of actuators, this structure inherently produces more force than individual actuators, making total force output a matter of design choice. This, coupled with apriori knowledge of the design of individual cymbal actuators, permits tailoring of the panel""s force-displacement tradeoff to specific design needs. Because operation is in the piston-mode, the transducer is void of higher order plate modes, and motion of the panel is up and down in a linear fashion, resulting in a linear near field velocity. The cymbal-panel design is inherently sturdy, small, and lightweight.
These and other objects are further understood from the following detailed description of particular embodiments of the invention. It is understood, however, that the invention is capable of extended application beyond the precise details of these embodiments. Changes and modifications can be made to the embodiments that do not affect the spirit of the invention, nor exceed its scope, as expressed in the appended claims. The embodiments are described with particular reference to the accompanying drawings, wherein: