Index of content:
Volume 106, Issue 1, July 1999
- NOISE: ITS EFFECTS AND CONTROL 
106(1999); http://dx.doi.org/10.1121/1.427050View Description Hide Description
In this paper, analytical and experimental investigations into active control of multiple tones in a three-dimensional enclosure are presented. The frequency range of the disturbance is chosen to encompass one or more structural and/or enclosure resonances. Distributed piezoceramic actuators and distributed polyvinylidene fluoride (PVDF) sensors are mounted on one of the enclosure boundaries. Acoustic sensors are used inside and outside the enclosure. An analytical formulation based on a state-space model is developed for local vibration control on an enclosure boundary and/or local noise control inside the enclosure for bandlimited disturbances. The error sensor signals are minimized by using digital feedforward control schemes based on filtered-U gradient descent algorithms. Analytical predictions from the current work are found to compare well with the results obtained in our previous experimental investigations. The current efforts are relevant to sound fields such as those enclosed in rotorcraft cabins.
106(1999); http://dx.doi.org/10.1121/1.427051View Description Hide Description
The concept of a weak sound radiating cell is proposed to reduce the low-frequency radiated noise from structures. The cell consists of two mechanically coupled surfaces such that, when placed on a vibrating structure, the response of the two surfaces are nearly out-of-phase and nearly of the same strength over a wide frequency range. This structure response leads the cell to behave as an acoustic dipole and thus a poor sound-radiating source. The control of low-frequency structurally radiated noise is then achieved by covering the structure with an array of these weak radiating cells, i.e., surface treatment. Thus the surface treatment essentially transforms the response of the structure to that of a distributed array of dipoles yielding a low sound radiating structure. A theoretical model of a single weak radiating cell applied to a simple piston structure was developed and experimental verification was performed. Overall sound power level reductions of over 6 dB were experimentally achieved between 400 and 1600 Hz with maximum reductions of over 30 dB at discrete frequencies.