Marty Johnson

Reduction of sound transmission into a circular cylindrical shell using distributed vibration absorbers and Helmholtz resonators

A modal expansion method is used to model a cylindrical enclosure excited by an external plane wave. A set of distributed vibration absorbers (DVAs) and Helmholtz resonators (HRs) are applied to the structure to control the interior acoustic levels. Using an impedance matching method, the structure, the acoustic cavity, and the noise reduction devices are fully coupled to yield an analytical formulation of the structural kinetic energy and acoustic potential energy of a treated cylindrical cavity. Lightweight DVAs and small HRs tuned to the natural frequencies of the targeted structural and acoustic modes, respectively, result in significant acoustic and structural attenuation when the devices are optimally damped. Simulations show that significant interior noise reduction can only be achieved by adding damping to both structural and acoustic modes, which are resonant in the frequency bandwidth of interest. In order to be independent of the azimuth angle of the excitation and to avoid unwanted modal interactions, the devices are distributed evenly around the cylinder in rings. This treatment can only achieve good performance if the structure and the acoustic cavity are lightly damped.

An equivalent source technique for calculating the sound field inside an enclosure containing scattering objects

The equivalent source method has previously been used to calculate the exterior sound field radiated or scattered from bodies in the free-field. In this paper the method is used to calculate the internal pressure field for an enclosure which can have arbitrary boundary conditions and may include internal objects which scatter the sound. Some of the equivalent source positions are chosen to be the same as the first order images of the source inside the enclosure, some are positioned within the scattering objects, and the remainder are positioned on a spherical surface some distance outside the enclosure. The normal velocity on the surfaces of the scattering objects and the enclosure walls is evaluated at a larger number of positions than there are equivalent sources. The sum of the squared difference between this velocity and that expected because of the admittance of the boundary, is minimized by adjusting the strengths of the equivalent sources. The convergence of the method is checked by evaluating the velocity at a larger number of monitoring positions. Example results are presented for the sound field and frequency response inside a damped rectangular enclosure, which compare very well with the conventional modal model. The effect of having rigid spheres inside the enclosure are then investigated, and it is found that the effect is significant even some distance from the spheres and at frequencies for which the size of the sphere is small compared to a wavelength. Finally the effect of a nonlocally reacting boundary condition is illustrated by assuming that one of the walls of the enclosure is an elastic plate.

The effect of actuator and sensor placement on the active control of rotor unbalance

Vibration and Acoustic Labs Mechanical Engineering Virginia Tech, Blacksburg, VA 24061-0238

Development of an Adaptive Helmholtz Resonator for Broadband Noise Control

This paper presents the development of adaptive Helmholtz resonators aimed at controlling broadband disturbance for the reduction of noise transmission into rocket payload fairing. Helmholtz resonators are commonly used for narrow band control application and so are designed to present the lowest amount of damping yielding maximum impedance. For this particular application however, optimal damping ratios usually superior to 4% are required. This relatively high level of damping permits more lightweight and compact design options to be considered that are not possible for low damping applications. Two design solutions are presented. The first tunes the resonator by varying the length of an accordion neck. The second varies the HR opening using an iris diaphragm. The characteristics of these two devices are measured, and a solution to maintain the damping level relatively constant is also proposed. Finally, experimental result obtained in a large cylinder representative of a payload fairing using 8 adaptive resonators is presented.© 2004 ASME

Multiple-array passive acoustic source localization in urban environments

In many situations of interest, obstacles to acoustic wave propagation such as terrain or buildings exist that provide unique challenges to localization. These obstacles introduce multiple propagation paths, reflections, and diffraction into the propagation. In this paper, matched field processing is proposed as an effective method of acoustic localization in a two dimensional scattering environment. Numerical techniques can be used to model complex propagation in a space where analytical solutions are not feasible. Realistically, there is always some uncertainty in model parameters that in turn can adversely affect localization ability. In particular, uncertainty in array location, sound speed, and various parameters affecting inter-array coherence only are investigated. A spatially distributed, multiarray network is shown to mitigate the effects of uncertainty. Multiarray inverse filter processing techniques are evaluated through perturbation of uncertain model parameters. These techniques are more accurate and flexible to implement than other matched field processing methods such as time reversal.

OPTIMIZATION OF DISTRIBUTED VIBRATION ABSORBERS FOR SOUND TRANSMISSION INTO A COMPOSITE CYLINDER

This paper presents an analytical model of an acoustic wave exciting a baffled, simply supported, cylinder. The sound transmitted into the interior acoustic cavity is then calculated using a fully coupled structural-acoustic model. It is shown that at low frequencies only the lower order circumferential modes are efficiently excited by the external acoustic wave. A set of lightweight distributed vibration absorbers (DVAs) is applied to the structure in an attempt to control both the structural vibration and interior acoustic levels at low frequencies. It is shown that by tuning a set of optimally damped DVAs to a structural resonance can result in the efficient addition of damping to that structural mode. Therefore, sound transmitted by resonant structural modes can be effectively controlled. It is also shown that to be effective the absorbers must be positioned on the maxima of the modes and also distributed evenly around the cylinder in rings containing sufficiently large numbers of DVAs to avoid unwanted modal interaction. Experimental results validating this approach are also presented.

Payload noise suppression using distributed active vibration absorbers

The authors are developing a cost- and weight-effective means for achieving an improved low- and mid-frequency acoustic environment in payload fairings for rockets at lift-off. The solution will be an active noise control system with an optimum selection of distributed active vibration absorbers (DAVAs) and acoustic actuators. High sound pressure inside a launch vehicle fairing during lift-off can damage delicate equipment in the payload. Space launch vehicle payload noise is a very important problem in the successful launch and deployment of space instruments and equipment. Measurements taken during the first few seconds of launch show very high sound pressure level (SPL) in the low frequency range of 60 to 250 Hz. High SPL is a severe problem because interior noise impinges on the instruments and equipment in the payload and can lead to their vibrational failure. Engineers have made moderate progress in addressing this problem by strengthening the instruments and by applying passive noise control treatment to the fairing. Both strategies incur significant penalties of added weight and financial cost and reduced allowable payload size. For further progress in suppressing low-mid frequency noise, another way is needed. The authors are developing a hybrid passive/active noise control system based on emerging technology of distributed active vibration absorbers (DAVAs). DAVAs are constructed from acoustic foam and area-distributed actuators. Passively it behaves as a tuned mass damper at low frequencies and a viscous damper at high frequencies. Actively a DAVA produces mechanical forces that are directed to reduce fairing vibrations.

Pilot study: Exposure and materiality of the secondary room and its impact in the impulse response of coupled‐volume concert halls

What does one room sound like when it is partially exposed to another (acoustically coupled)? More specifically, this research aims to quantify how operational and design decisions impact aural impressions in the design of concert halls with acoustical coupling. By adding a second room to a concert hall, and designing doors to control the sonic transparency between the two rooms, designers can create a new, coupled acoustic. Concert halls use coupling to achieve a variable, longer, and distinct reverberant quality for their musicians and listeners. For this study, a coupled‐volume shoebox concert hall was conceived with a fixed geometric volume, form, and primary‐room sound absorption. Aperture size and secondary‐room sound‐absorption levels were established as variables. Statistical analysis of sound decay in this simulated hall suggests a highly sensitive relationship between the double‐sloped condition and (1) Architectural composition, as defined by the aperture size exposing the chamber and (2) Mater...

Development of an Analog Controller for Tuning an Adaptive-Passive Control Device

Adaptive-passive devices such as adaptive Helmholtz Resonators (HR) and tunable vibration absorbers have been shown to be suitable for controlling both narrowband disturbances and lightly damped structural/acoustic modes driven by broadband disturbances. In order to track changes in the disturbance or changes in the modes, the natural frequency of the absorber, ωn , is tuned to match the observed signals. This is achieved by altering some physical parameter of the control device such as the stiffness of a vibration absorber or the neck cross-sectional area of a Helmholtz resonator. In order to automatically adjust these devices, control systems and tuning algorithms have been developed, most of which involve a digital controller. However, this paper looks specifically at the development of a simple analog controller used to drive a DC motor in order to tune a mechanical device. A two sensor dot product method is employed where one sensor is placed inside of the control device, such as a Helmholtz Resonator, and the other on/in the system under control, such as in a room. The outputs from the two sensors are multiplied together and subsequently low passed in order to extract a low frequency “DC” voltage which acts as an error signal. The error signal is related to the relative phase of the two sensor signals and determines the direction in which the device should be tuned. When the two signals are 90° apart, the system is tuned (i.e. the inner product produces zero DC level). If the drive frequency ω is different than the tuned frequency, then the system is mis-tuned. The relationship between the mis-tuning, ωn -ω, and the error is not linear, but for small perturbations a linear approximation can be used to investigate the stability and performance of the system. The gradient of the function is shown to be largest when the mis-tuning error is zero and is inversely proportional to the damping level in the control device. Once stability of the system has been ensured the ability of the system to track changes in drive frequency is investigated experimentally. The control system is demonstrated using an adaptive Helmholtz resonator which has a variable cross-sectional neck via an iris diaphragm. The iris is controlled using a small DC motor; two microphones (one mounted internally and one externally) are used to supply the driving signal to the circuit.Copyright

Active control of transmission of turbulent boundary layer noise using smart foam elements

Results concerning the active control of transmission of turbulent boundary layer noise using smart foam elements will be presented. Smart foam consists of a cylindrical shaped PVDF film embedded within partially reticulated polyurethane acoustic foam. A smart foam element is an active–passive control device utilizing the passive absorption characteristics of the foam (effective at high frequencies) and the electrically driven PVDF film as the active element (effective at lower frequencies). This lightweight, compact arrangement lends itself well to the control of aircraft cabin noise resulting from an exterior turbulent boundary layer. Six smart foam elements (each 2 in. thick×4 in.×5 in.) are used to control the sound transmission through a 0.05 in. thick×10 in.×20 in. aluminum plate mounted in a wind tunnel. The sound transmission is monitored by measuring the pressure inside an externally mounted anechoic box that encloses the plate. Various control strategies are tested and results compared.