Adaptive Helmholtz Resonator Research Articles

Acoustic-Structure Interaction in an Adaptive Helmholtz Resonator by Compliance and Constraint

Abstract The acoustic energy attenuation capabilities of traditional Helmholtz resonators are enhanced by various methods, including by coupled resonators, absorbing materials, or replacement of rigid walls with flexible structures. Drawing from these concepts to envision a new platform of adaptive Helmholtz resonator, this research studies an adaptive acoustic resonator with an internal compliant structural member. The interaction between the structure and acoustic domain is controlled by compression constraint. By applying uniaxial compression to the resonator, the flexible member may be buckled, which drastically tailors the acoustic-structure interaction mechanisms in the overall system. A phenomenological analytical model is formulated and experimentally validated to scrutinize these characteristics. It is found that the compression constraint may enhance damping capabilities of the resonator by adapting the acoustic-structure interaction between the resonator and the enclosure. The area ratio of the flexible member to the resonator opening and the ratio of the fundamental natural frequency of the flexible member to that of the enclosure are discovered to have a significant influence on the system behavior. These results reveal new avenues for acoustic resonator concepts exploiting compliant internal structures to tailor acoustic energy attenuation properties.

Adaptive Helmholtz resonator based on electroactive polymers: modeling, characterization, and control

This paper presents a new concept and strategy allowing adaptive control of a membraned Helmholtz resonator (HR) embedded in a melamine foam. The designed system aims to adapt the acoustic absorption performances and transmission loss in low frequencies (<500 Hz). The proposed concept consists in replacing the resonator front wall by an electroactive polymer membrane. The stiffness of the membrane can be controlled by an electric field, resulting in a resonance frequency shift. A 2D axisymmetric numerical model based on the finite elements method is developed to characterize the complex structure-acoustic coupling between the membrane, the HR and the host foam to determine the potential of the concept. Experimental measurements are then performed in an impedance tube and compared to numerical results. A feedforward algorithm based on neural networks allows the adaptivity of the membraned HR to the acoustic excitation variation inside the impedance tube.

Adjustable pipes and adaptive passive damping

Pipe natural frequencies are dependent on the geometry of the pipe where the pipe length is the main contributor in regulating the natural frequencies. The boundary conditions are another major contributor because the acoustic waves in the pipe will produce standing waves because of the reflections at the boundary conditions. Making one of the boundary conditions location in the pipe adjustable will allow for the length of the pipe to be modified and therefore change the natural frequencies of the pipe. This adjustable pipe provides a means for introducing undergraduate students to an inexpensive setup to test pipe length in changing the natural frequencies of pipes and the associated sound pressure level within the pipe. Helmholtz resonators are passive devices that can absorb acoustic energy from an acoustic enclosure. These resonators have three major variables that are used to determine the resonant frequency of the resonator and therefore the frequency at which the resonator will absorb acoustic energy. Designing a resonator where one of the variables can be altered allows the resonator to be tuned to a specific frequency. The adaptive Helmholtz resonator provides undergraduate students an inexpensive setup to tune a resonator and test how acoustic pressure is dampened by the energy absorption of the resonator.

FDTD simulation of the acoustic interaction between a cavity and an adaptive Helmholtz resonator

Helmholtz resonators are used to manipulate the eigen behaviour of the acoustic fluid in a cavity. Using the analogy to a mass damper, it is possible to tune the resonators' eigen frequency by handling the resonator volume as spring and the resonator neck as mass of a vibration absorber. Simulating the transient interaction of the HR with the acoustic sound field in a cavity in terms of active sound control, a high simulation performance is demanded. This can be provided by the finite difference time domain method. The simulations are performed with respect to the LOEWE-Zentrum AdRIA acoustics demonstrator. It depicts the abstraction of a real room. Therefore, it consists of six sound reflecting walls that are implemented through acoustic impedances in the time domain. The semi-passive system is implemented using MATLAB/Simulink. In addition, the simulations are validated applying an adaptive Helmholtz resonator inside the acoustic fluid of the acoustics demonstrator.

A sliding Goertzel algorithm for adaptive passive neutralizers

A common method used in tuning adaptive–passive tuned vibration neutralizers is to adjust its resonance frequency to match the excitation frequency, which has the characteristic that the phase angle between its vibrating mass and its support is −90°. A sliding-Goertzel algorithm is presented and demonstrated for extracting the vibration signals at the frequency of interest. The benefit of using the sliding Goertzel algorithm compared to other methods when used in vibration environments with multiple tones is that additional band-pass notch filtering is not required. This algorithm could also be used for adaptive tuned mass dampers, adaptive Helmholtz resonators, and adaptive quarter-wave tubes.

Adaptive Helmholtz Resonator in a Hydraulic System

Adaptive Helmholtz Resonator in a Hydraulic System

Adaptive Helmholtz resonators and passive vibration absorbers for cylinder interior noise control

This paper presents an adaptive-passive solution to control the broadband sound transmission into rocket payload fairings. The treatment is composed of passive distributed vibration absorbers (DVAs) and adaptive Helmholtz resonators (HR). Both the frequency domain and time-domain model of a simply supported cylinder excited by an external plane wave are developed. To tune vibration absorbers to tonal excitation, a tuning strategy, based on the phase information between the velocity of the absorber mass and the velocity of the host structure is used here in a new fashion to tune resonators to peaks in the broadband acoustic spectrum of a cavity. This tuning law, called the dot-product method, only uses two microphone signals local to each HR, which allows the adaptive Helmholtz resonator (AHR) to be manufactured as an autonomous device with power supply, sensor, actuator and controller integrated. Numerical simulations corresponding to a 2.8 m long 2.5 m diameter composite cylinder prototype demonstrate that, as long as the structure modes, which strongly couple to the acoustic cavity, are damped with a DVA treatment, the dot-product method tune multiple HRs to a near-optimal solution over a broad frequency range (40–160 Hz). An adaptive HR prototype with variable opening is built and characterized. Experiments conducted on the cylinder prototype with eight AHRs demonstrate the ability of resonators adapted with the dot-product method to converge to near-optimal noise attenuation in a frequency band including multiple resonances.

Control of sound transmission into a large composite cylinder using passive, adaptive–passive and active control strategies

This work is directed at the goal of controlling the high noise levels experienced in launch vehicle payload fairings. The problem frequency range is typically below 300 Hz where passive structural damping and sound absorption treatments perform inadequately. In this paper the work conducted at Virginia Tech over the last 3 years on techniques for reducing the interior noise levels at launch using an eight ft. diameter composite cylinder as a test platform is summarized. First the theoretical development will be presented with an overview of the mechanisms of sound transmission. Then, passive control methods using optimally damped structural distributed vibration absorbers (DVAs) and acoustic Helmholtz resonators will be presented. Both theoretical results and experimental results using high level acoustic excitation (132 dB) will be described. A set of adaptive Helmholtz resonators were also built and tested and are capable of tracking changes in acoustic resonances automatically. Finally, the results from a fully active structural acoustic control experiment using distributed active vibration absorbers will be presented.