Jerome P. Smith

A broadband passive–active sound absorption system

In recent years, hybrid absorption systems have been implemented which achieve high sound absorption over a broad frequency range. This work is an experimental study of a broadband hybrid absorption system which is comprised of a layer of sound-absorbing material (the passive component) positioned at a distance from a movable wall (the active component) inside an impedance tube. The movable wall is used to impose desired boundary conditions in the cavity behind the passive layer, thereby increasing the absorption of the system at frequencies where the passive material is not independently effective. Both pressure-release (i.e., minimizing the pressure at the back surface of the layer) and impedance-matching (i.e., minimizing the reflected wave from the layer) boundary conditions are studied. The performance of the hybrid system for these two boundary conditions is compared for broadband disturbances over a frequency range of 100–1000 Hz. The unmodified passive system showed absorption coefficients greater...

Control of Broadband Acoustic Radiation with Adaptive Structures

Active Structural Acoustic Control has been demonstrated for harmonic disturbances on various structures. This paper presents an extension of this work to the attenuation of acoustic radiation from planar structures subject to broadband disturbances. An adaptive, multiinput multi-output (MIMO), feedforward broadband acoustic control system has been developed. The control approach is the least mean squares (LMS) algorithm. The compensators are adaptive finite impulse response (FIR) filters. The control inputs are implemented with piezoceramic actuators. Both far-field microphones and polyvinylidene fluoride (PVDF) distributed structural sensors are used as error sensors. Both the actuator locations and the PVDF sensor configurations were optimally chosen to minimize radiated sound power assuming multiple frequency excitation. The disturbance is band-limited zero mean white noise and is implemented with a point force shaker. In the control of harmonically excited systems, satisfactory attenuation is possible with a single-input single-output (SISO) controller. In contrast, for systems excited with broadband disturbances, a MIMO controller is necessary for significant acoustic attenuation. Experimental results for the control of a simply supported plate are presented.

Active structural acoustic control of radiation due to broadband disturbances

Active structural acoustic control (ASAC) using the adaptive feedforward LMS algorithm has been demonstrated for narrow‐band (harmonic) disturbances. This paper presents an extension of this work to the attenuation of acoustic radiation from structures subject to broadband (random) disturbances. The control compensators are adaptive finite impulse response (FIR) filters. The LMS control algorithm is implemented on a floating point digital signal processor. The control inputs are implemented with piezoelectric ceramic actuators (PZT), and both polyvinylidene flouride (PVDF) structural sensors designed to observe the efficient acoustic radiating modes and far‐field microphones are used as error sensors. In the control of harmonically excited systems, satisfactory attenuation is achieved with a single‐input, single‐output (SISO) controller. In contrast, for systems excited with broadband disturbances, a multi‐input, multi‐output (MIMO) controller is necessary for significant acoustic attenuation. A MIMO broa...

Active noise control on a large‐scale ducted fan with inlet wave‐number sensing

Active noise‐control (ANC) methods for reduction of turbofan inlet noise have been previously demonstrated on a business‐sized turbofan engine. The potential of ANC is investigated here on a large 4‐ft‐diameter ducted fan which has a geometry and tone frequencies which are more characteristic of current and future large production engines. The control inputs are implemented with a single circumferential array of acoustic drivers constructed with rare‐earth magnets. Both far‐field pressure transducers and an inlet wave‐number sensing technique are investigated as error‐sensing strategies to reduce the 2BPF tone of the ducted fan (approximately 1000 Hz.) The wave‐number sensing strategy involves filtering and combination of inlet‐mounted microphone signals to result in the observation of a particular wave number propagating axially out of the inlet. The ducted fan is equipped with a rotating microphone array in the inlet, which allows for modal analysis of the inlet acoustic field before and after control. ...

Active control of jet engine inlet noise

This paper summarizes the recent advances in the active control of tonal inlet noise from an operational Pratt and Whitney JT15D turbofan engine. The most recent results show that multiple circumferential control source arrays reduce the radiation spillover to the sideline that is often observed with a single control array. An optimal control configuration results in near global attenuation in all directions of the directivity field, and a sound power reduction of 5.1 dB. A power reduction of 4.1 dB was obtained over a desired sector spanning the angles from 40 to 60 degrees in the far field using a properly configured control system. Results using a hybrid passive‐active inlet have also been obtained showing that a passive liner in combination with an active control system will eliminate much of the sideline control spillover. An inlet‐mounted sensing strategy using a wave‐number observation technique was implemented and the preliminary results show the potential of the method for eliminating far‐field m...

Passive–active broadband sound absorption

Researchers have proposed hybrid systems to achieve high sound absorption over a wide frequency range. The passive component of the system is comprised of a layer of sound absorbing material positioned at a distance from a movable wall, leaving an air space. The wall is the active component and is used to increase the absorption of the system at frequencies where the passive system is not effective. The control input that drives the active wall is determined in order to achieve a desired boundary condition at the back of the absorbing layer. Both pressure‐release (i.e., minimizing the pressure at the back surface of the layer) and impedance‐matching (i.e., minimizing the reflected wave in the cavity) conditions have been proposed to increase the absorption of the system. The performance of the hybrid system for these two boundary conditions is compared for broadband disturbances over a frequency range of 0–1000 Hz. The passive system showed absorption coefficients greater than 0.7 only above 500 Hz. The i...

Active noise control for attenuation of low‐frequency broadband jet engine exhaust noise

Low‐frequency broadband jet engine exhaust noise radiated into the surrounding area has been a problem for facilities that perform ground jet engine run up operations such as military hush‐houses. This low‐frequency acoustic radiation is capable of propagating over long distances, creating a noise and vibration problem in nearby communities. In this paper, active noise control (ANC) is experimentally implemented to achieve attenuation of low‐frequency turbo‐fan jet engine exhaust noise in a nearby area. The control method is the feedforward filtered‐x LMS algorithm and is implemented for both single‐input, single‐output and multiple‐input, multiple‐output systems. The control inputs are generated by filtering a reference signal through adaptive FIR filters before being input to the control loudspeakers, and microphones are used to generate the reference and error signals. Attenuations of up to 15 dB are achieved in the 1/3‐octave bands at the error sensor locations. The results also demonstrate a large ar...