Frederic G. Pla

Active noise control using noise source having adaptive resonant frequency tuning through variable ring loading

A noise source for an aircraft engine active noise cancellation system in which the resonant frequency of noise radiating structure is tuned to permit noise cancellation over a wide range of frequencies. The resonant frequency of the noise radiating structure is tuned by a plurality of drivers arranged to contact the noise radiating structure. Excitation of the drivers causes expansion or contraction of the drivers, thereby varying the edge loading applied to the noise radiating structure. The drivers are actuated by a controller which receives input of a feedback signal proportional to displacement of the noise radiating element and a signal corresponding to the blade passage frequency of the engines fan. In response, the controller determines a control signal which is sent to the drivers, causing them to expand or contract. The noise radiating structure may be either the outer shroud of the engine or a ring mounted flush with an inner wall of the shroud or disposed in the interior of the shroud.

Active vibration control of structures undergoing bending vibrations

An active vibration control subassembly for a structure (such as a jet engine duct or a washing machine panel) undergoing bending vibrations caused by a source (such as the clothes agitator of the washing machine) independent of the subassembly. A piezoceramic actuator plate is vibratable by an applied electric AC signal. The plate is connected to the structure such that vibrations in the plate induced by the AC signal cause canceling bending vibrations in the structure and such that the plate is compressively pre-stressed along the structure when the structure is free of any bending vibrations. The compressive prestressing increases the amplitude of the canceling bending vibrations before the critical tensile stress level of the plate is reached. Preferably, a positive electric DC bias is also applied to the plate in its poling direction.

Method and apparatus for synchronizing rotating machinery to reduce noise

A system for reducing noise created by multiple rotating machines by synchronizing the machines so as to establish a phase relationship between the machines which minimizes the noise. One or more feedback sensors, such as microphones, are placed so as to sense the noise to be reduced. The noise signals from the microphones are sent to a controller. Tachometer signals of the rotational speed of each machine are also sent to the controller. The controller generates an output signal in response to the inputs from the microphones and the tachometers that is fed to one or more of the rotating machines in order to establish the desired phase relationships.

Active control of aircraft engine noise using vibrational inputs

An active noise control system which minimizes noise output by creating a secondary, cancelling noise field using vibrational inputs. The system includes one or more piezoceramic actuators mounted to the inner surface of the shroud of an aircraft engine. The actuators can be either mounted directly to the shroud or to one or more noise cancelling members which are resiliently mounted the shroud. Transducers are also provided for sensing the noise generated by the engine and producing an error signal corresponding to the level of noise sensed. A controller sends a control signal to the actuators in response to the error signal, thereby causing the actuators to vibrate and generate a noise field which minimizes the total noise emanating from the engine. The piezoceramic actuators can be thin sheets of piezoceramic material or can be in the form of a piezoelectric-driven mechanical lever arrangement.

Active noise control of aircraft engine discrete tonal noise

An active noise control subassembly for an aircraft engine. An aircraft engine noise radiating panel is bendably vibratable to generate a canceling noise generally opposite in phase to at least a portion of the discrete tonal noise produced by the engine. A piezoceramic actuator plate is vibratable by an applied electric AC signal. The plate is connected to the panel such that vibrations in the plate cause bending vibrations in the panel and such that the plate is compressively prestressed along the panel when the panel is free of bending vibrations. The compressive prestressing increases the amplitude of the canceling noise before the critical tensile stress level of the plate is reached. Preferably, a positive electric DC bias is also applied to the plate in its poling direction to increase the amplitude of the canceling noise before the sum of the AC signal and DC bias exceeds the depolarization voltage in a direction opposite to the poling direction.

Active noise control using a tunable plate radiator

An active noise control subassembly for reducing noise caused by a source (such as an aircraft engine) independent of the subassembly. A noise radiating panel is bendably vibratable to generate a panel noise canceling at least a portion of the source noise. A piezoceramic actuator plate is connected to the panel. A back plate is spaced apart from the first plate and the panel with the panel positioned between the source noise and the back plate. A pair of spaced-apart side walls each generally abut the panel and the back plate so as to generally enclose a back cavity. A mechanism is provided for varying the panel resonating frequency by varying the state of the back cavity (such as by varying its fluid pressure and/or volume) while the panel is undergoing bending vibrations.

Active noise control using noise source having adaptive resonant frequency tuning through variable panel loading

A noise source for an aircraft engine active noise cancellation system in which the resonant frequency of a noise radiating element is tuned to permit noise cancellation over a wide range of frequencies. The resonant frequency is tuned by adjusting the size of a frame which encloses the noise radiating element. One or more expandable elements are disposed in the frame to produce expansion and contraction of the frame. The elements are actuated by a controller which receives input of a feedback signal proportional to displacement of the noise radiating element and a signal corresponding to the blade passage frequency of the engines fan. In response, the controller determines a control signal which is sent to the elements and causes the frame to expand or contract.

Effects of attenuation, dispersion, and high sound‐pressure levels on acoustic wave distortion in horns

High‐power sound sources have received a lot of attention in the past few years due to renewed interest in industrial applications of high‐intensity sounds such as the acoustic agglomeration of aerosols or combustion enhancement. Most high‐power sound sources require a horn to match the source impedance to the medium where the sound is radiated. Such horns introduce distortion in the initial waveform, which can be detrimental to the agglomeration or combustion enhancement process. Boundary‐layer attenuation smooths the wave shape while dispersion breaks up the symmetry of the waveform. Horn‐induced dispersion is usually the dominant dispersion mechanism, resulting in strong peaks in the waveform. Finally, due to the very high acoustic levels at the horn throat, finite‐amplitude effects are responsible for a significant amount of distortion at high frequencies. Simple examples of waveform distortion due to these various mechanisms are shown. The effects of sound‐pressure level, horn design, and frequency o...

The influence of high‐intensity sounds on the performance of air warning sirens

The use of sirens as air warning devices has been widespread for several hundred years. Due to the very high sound‐pressure levels generated, finite‐amplitude effects can result in large losses leading to poor sound generation efficiency. Although much work has been done on siren design, the importance of finite‐amplitude effects has been neglected in the recent past, even though a review of earlier works shows that researchers such as Reyleigh, in 1903, King, in 1919, and later, Hart, in 1926, noticed the extra acoustic losses associated with high‐intensity sound sources. Comparisons between linear and nonlinear models of wave propagation in a siren horn illustrate the importance of these types of losses, and show how they can be minimized. Acoustic saturation, which limits the sound‐pressure level that can be perceived by a receiver at a given distance from a high‐intensity sound source, and the implications of finite‐amplitude effects on the rating of high‐intensity sound sources in a laboratory enviro...

Use of special waveforms for optimum efficiency of high‐intensity sound sources

High‐intensity sound sources, such as sirens, have received much attention in the recent past due to renewed interest in industrial applications of high‐intensity acoustics. As a result of the very high sound pressure levels required (155–165 dB), finite amplitude effects must be taken into account in the design of sound generators. A time domain solution of the second‐order nonlinear wave equation is used to predict the behavior of initially nonsinusoidal plane waves, and is compared with a frequency‐domain approach. Results for initially sinusoidal, rectangular, and inverse‐shock waves are presented. It is shown that the shock formation distance for an initially inverse shock wave is π times the shock formation distance for an initially sinusoidal wave, and that the wave distortion actually results in an amplification of the fundamental, thus increasing the efficiency of the sound generation process. The consequences of wave distortion on several practical high‐intensity sound sources is discussed.