Mark Holdhusen

Anechoic chamber qualification: traverse method, inverse square law analysis method, and nature of test signal.

Qualification of anechoic chambers is intended to demonstrate that the chamber supports the intended free-field environment within some permissible tolerance bounds. Key qualification issues include the method used to obtain traverse data, the analysis method for the data, and the use of pure tone or broadband noise as the chamber excitation signal. This paper evaluates the relative merits of continuous versus discrete traverses, of fixed versus optimal reference analysis of the traverse data, and of the use of pure tone versus broadband signals. The current practice of using widely space discrete sampling along a traverse is shown to inadequately sample the complexity of the sound field extant with pure tone traverses, but is suitable for broadband traverses. Continuous traverses, with spatial resolution on the order of 15% of the wavelength at the frequency of interest, are shown to be necessary to fully resolve the spatial complexity of pure tone qualifications. The use of an optimal reference method for computing the deviations from inverse square law is shown to significantly improve the apparent performance of the chamber for pure tone qualifications. Finally, the use of broadband noise as the test signal, as compared to pure tone traverses over the same span, is demonstrated to be a marginal indicator of chamber performance.

Damping effects on the state-switched absorber used for vibration suppression

A state-switched device is conceptually capable of instantaneously changing its mass, stiffness, or damping. Such a device will exhibit different dynamical response properties (modes and resonance frequencies) depending on its current state. A state-switched vibration absorber exploits the state-switching concept for the purposes of enhanced vibration suppression. Between each state switch, it is fundamentally a passive vibration absorber, but one which exhibits a different tuning frequency for each possible state. A state-switched vibration absorber therefore has a greater effective bandwidth than a classical passive absorber. This paper considers the role of damping in the state-switching concept for a simple one-degree of freedom system and for a two-degree of freedom system. Certain values of damping in the system improve performance, while other values hinder the performance of the state-switched absorber, as compared to classical absorbers. The predicted performance of the system also depends upon the particular damping model used, such as viscous absorber or system modal damping. Damping values also affect the frequency of switch events that occur during the response of the system. In general, the highest relative performance of the state-switched absorber as compared to a classical vibration absorber occurs at low values of damping.

Experimental Vibration Control of a Single-Degree of Freedom System Using a State-Switched Absorber

A state-switched absorber (SSA) is a device capable of instantaneously switching between resonance frequencies, increasing its effective bandwidth as compared to classical tuned vibration absorbers (TVAs) for vibration control. Previous simulations show that for a system subjected to a multi-harmonic disturbance, the SSA reduces vibration more effectively than classical TVAs. This study considers the experimental performance of the SSA for vibration suppression of an elastically mounted lumped mass base. State switching is achieved using a clutch to connect or disconnect a coil spring in parallel with other coil springs. An SSA system outperforms a classical TVA system for several combinations of forcing frequencies.

Experimental Vibration Control of a Two-Degree of Freedom, State-Switched Absorber System

A State-Switched Absorber (SSA) is a device capable of instantaneously changing its stiffness, thus it can switch between resonance frequencies, increasing its effective bandwidth as compared to classical tuned vibration absorbers for vibration control. Previous theoretical simulations show that for a system subjected to a multi-harmonic disturbance, using an appropriate logic for switching states, the SSA reduces vibration more effectively than classical tuned vibration absorbers (TVA). This paper considers the experimental performance of the SSA for vibration suppression of an elastically mounted lumped mass base. State switching is achieved using magneto-rheological fluid to connect or disconnect a coil spring in parallel with other coil springs. The stiffness state is controlled by applying or removing a magnetic field across of the MR fluid. Experiments were performed over a range of forcing and tuning frequencies. The SSA system, optimally tuned, outperformed the optimal classical TVA system for all combinations of forcing frequencies.Copyright

Design features for free‐field qualification of a new semi‐anechoic room, and qualification performance

Precision qualification of a semi‐anechoic room requires careful attention to the sound source and traversing method. Prior work, with test sources mounted above the reflecting floor of such a room, has indicated the potential for image source problems in the resulting field. To address such shortcomings, the new Georgia Tech semi‐anechoic room was constructed with a recessed enclosure in the center of the floor. This enclosure permits the implementation of test sources coincident with the reflecting plane of the floor. In addition, prior work in an anechoic room has indicated the inadequacy of qualification traverses implemented at large spacings. To address this issue, hard‐points were designed and implemented within the room to permit installation of traverse cables extending radially from the in‐floor source enclosure out to the walls and corners. These traverse cables are an integral component of a custom continuous traverse system. The design features of the chamber which facilitate chamber qualific...

Role of damping in state-switched absorber for vibration control

A state-switched device is conceptually capable of instantaneously changing its mass, stiffness, or damping. Such a device will exhibit different dynamical response properties (modes and resonance frequencies) depending on its current state. A state-switched vibration absorber exploits the state-switching concept for the purposes of enhanced vibration suppression. Between each state switch, it is fundamentally a passive vibration absorber, but one which exhibits a different tuning frequency for each possible state. A state-switched vibration absorber therefore has a greater effective bandwidth than a classical passive absorber. This paper considers the role of damping in the state-switching concept for a simple one-degree of freedom system and for a two-degree of freedom system. Certain values of damping in the system improve performance, while other values hinder the performance of the state-switched absorber, as compared to classical absorbers. The predicted performance of the system also depends upon the particular damping model used, such as proportional, viscous, or modal damping. Damping values also affect the frequency of switch events that occur during the response of the system. In general, a state-switched absorber with optimized damping is more effective at vibration suppression as compared to a classical vibration absorber with optimized damping.

Yeast flocculation using acoustic agglomeration

A major cause of haziness in beer is due to yeast suspended in the liquid. The most common methods used to flocculate the yeast and have it settle out of the liquid solution are to use various chemicals, cold temperatures, and/or extended time. This research considers using acoustic agglomeration to cause the yeast to flocculate as a means to clarify beer. Acoustic agglomeration uses high intensity acoustic standing waves to clump fine particles together in order for them to become large enough to settle out of the fluid. In this work, an ultrasonic acoustic transducer will be implemented to achieve standing waves in the beer. In theory, these standing waves will cause the yeast particles to clump together and settle. The results from this approach will be compared to the traditional methods of yeast flocculation. This approach may lead to an increase in efficiency in clarifying beer without the use of undesirable chemicals. The preliminary work considered here is a proof of concept and will use visual inspection as a means of clarity comparison. Future work will use more in-depth laboratory analysis as a means of comparison of clarity.

An inexpensive approach to measure one‐third octave reverberation times.

Reverberation time is a common measurement taken in room acoustics. However, the software used in portable sound‐level meters can be rather expensive, making it challenging for those on a smaller budget to take these necessary measurements on site. The work here outlines an inexpensive method to instantaneously measure one‐third octave reverberation times using a portable experimental setup. The setup consists of a PCB random‐incidence microphone along with a National Instruments USB data acquisition system and a notebook PC. Using LABVIEW along with MATLAB software, code was written to process acoustic measurement data to achieve one‐third octave reverberation times with almost no additional processing time. The entire setup costs approximately 2000 dollars with educational discounts. This setup and procedure give similar results to the aforementioned sound‐level meters, but at a fraction of the cost.

Space‐dependent damping for reducing vibration of continuous systems

Structural vibration in continuous systems is a common source of problems for many mechanical systems. Much research has considered ways to reduce the vibration in continuous systems. Some research uses passive techniques that are simple to implement, whereas other methods implement active control that can be quite complex. The research presented here investigates the effect space‐dependent damping has on the vibration of continuous systems. Specifically, the work at hand will focus on the flexural vibration of beams. In a distributed damping beam system, the damping is variable as a function of position along the length of the beam. Such a system could be physically realized through the use of magneto‐rheological or electro‐rheological fluids. Along its length, a beam could be divided into several isolated sections containing such a fluid. By controlling the magnetic or electric field across each section of the beam, the damping would vary along the length of the beam. The work presented here optimizes t...

Experimental Validation of a State-Switched Absorber Used to Control Vibrating Continuous Systems

A state-switched absorber (SSA) is a device capable of instantaneously changing its stiffness, thus it can switch between resonance frequencies, increasing its effective bandwidth as compared to classical tuned vibration absorbers for vibration control. Previous numerical work has shown that an optimized SSA outperforms an optimized TVA at controlling vibrations of both a beam and a plate. This paper details the experimental validation of these simulation results. An SSA was realized by employing magneto-rheological elastomers to achieve a stiffness change. The stiffness of these elastomers is a function of the magnetic field put across them. Experiments were conducted on both a cantilever beam and a square plate clamped on all sides. Each system was excited by several two-frequency component excitations. For each forcing combination, several tuning configurations of the SSA were applied and the kinetic energy of the system was found. This observed performance was compared to the performance found through numerical simulations of a system with a similar tuning and excitation configuration. It was found that the observed performance follows closely with results found through numerical simulation.Copyright