J. James Esplin

A high amplitude, time reversal acoustic non-contact excitation (trance)

This paper describes the principle behind a high amplitude non-contact acoustic source based on the principle of time reversal (TR), a process to focus energy at a point in space. By doing the TR in an air filled, hollow cavity and using a laser vibrometer in the calibration of the system, a non-contact source may be created. This source is proven to be more energetic than an off the shelf focused ultrasound transducer. A scaled up version of the proposed source has the potential to allow nondestructive evaluation processes that require high amplitude, such as nonlinear elastic wave spectroscopy (NEWS) techniques.

Toward a high power non-contact acoustic source using time reversal

Over the last decades, nonlinear acoustic techniques have been developed to detect mechanical damage in solids. They have been proven to be far more sensitive to early damage than standard linear acoustic techniques. Unfortunately, they often require high amplitude waves to propagate within the sample. To be practical in industrial applications, signals have to be generated without contact. Currently available non-contact transducers are generally not powerful enough. A first step toward the creation of a high amplitude non-contact acoustic source will be described. This source is based on the principle of focusing energy on the surface of a sample using Time Reversal in a hollow cavity. By using a laser vibrometer for the necessary calibration of the system we are able to use a full non-contact process. New development in signal processing allows a much cleaner signal generation than usually achieve with time reversal. This source is proven to be much more energetic than current off the shelf non-contact...

The effects of non-cardioid directivity on incidence angle estimation using the polar energy time curve

Assessment of desirable reflections and control of undesirable reflections in rooms are best accomplished if the reflecting surfaces are properly localized. Several measurement techniques exist to identify the incident direction of reflected sound, including the useful polar energy time curve (Polar ETC), which requires six cardioid impulse response measurements along the Cartesian axes. The purpose of this investigation is to quantify the incidence angle estimation error introduced into the Polar ETC by non-cardioid microphone directivities. The results demonstrate that errors may be minimized with a cardioid-family microphone possessing a certain range of directivities and by maximizing the measurement signal-to-noise ratio.

A cavitation threshold for transient signals applied to laboratory-scale sparker-induced pulses

The phenomenon known as cavitation can occur when a volume of liquid is subjected to a pressure that falls below a “cavitation threshold”. Following this cavitation inception, a rupturing of the fluid or rapid growth of microbubbles occurs. The cavitation threshold is typically thought to be equal to the vapor pressure of the fluid; however, laboratory experiments involving underwater high-amplitude sparker-induced pulses have demonstrated that this is not necessarily the case. This presentation introduces a generalized threshold for transient acoustic pulses based on previous work of a threshold for constant-frequency transient signals. The output of this transient cavitation threshold will be compared against simulation and experiment.

A two-dimensional model for control of centrifugal fan inlet noise in a notebook computer

Previous work on active control of exhaust noise from small centrifugal fans demonstrated significant reductions of the blade passage frequency (BPF) tone. A fan and heat-sink were placed within a mock-up notebook computer case, and control of the fan exhaust noise was measured. It was found that control of the BPF in the exhaust did not significantly affect noise radiated from the fan inlets into the notebook casing, suggesting that exhaust noise and inlet noise may be controlled separately without one adversely affecting the other. In the current work, a two dimensional half-space, source coupling model has been developed to calculate the field within the notebook casing caused by the inlet noise. As a first approximation, free-space boundary conditions were used. A two-dimensional space was constructed to test the model, and error sensor placement was predicted. Measurements of radiated sound power show significant reduction of the blade passage frequency tone. Factors influencing experimental agreemen...

Active control of centrifugal fan noise: Experimental results.

Previous work by these authors in active control of axial fans suggests an approach that can be successful in applying active control to small centrifugal fans used in fan trays and laptop computers. The modeling and analysis strategies developed for axial fans were modified for use with centrifugal fans mounted in a rectangular exhaust duct. Experimental verification allowed for proper inclusion of damping in the model. By minimizing the sound power radiated from the duct, optimal error sensor placement was predicted. Experimental results verified the effectiveness of placing the error sensor at these locations. Using predicted control source and error sensor locations, the rectangular duct was replaced by a centrifugal fan and duct attached to a heat sink, with the total dimensions being the same as the previous rectangular duct. The experimental results indicate that significant global reduction of the radiated tonal fan noise can be achieved.

Active control of centrifugal fan noise: Modeling design guidelines

Information technology (IT) noise is very prevalent in todays society. Active noise control (ANC) has shown promise in minimizing the effect of fan-induced IT noise on users. Much of the previous research has concentrated on axial cooling fans, such as those found in desktops and servers. This approach was based on the concept of minimizing radiated acoustic power in a model of the fan radiation, and using those results to determine appropriate nearfield locations for the error sensor(s). This paper describes modifications to this previous method to develop a modeling approach to implement active noise control with a centrifugal blower, such as those found in fan trays and laptop computers. This model has been used to predict tonal noise inside and outside the duct, as well as how to best develop an ANC system for such an idealized setup. Differences between the axial fan model and the centrifugal blower model are discussed.

Active control of a centrifugal fan in a mock laptop enclosure

Active noise control (ANC) has shown promise in minimizing the effect of fan noise on users. Recent research by the authors has developed a model which is used to implement ANC on the inlets of centrifugal cooling fans. This model is based on minimizing radiated acoustic power in a model of the fan radiation and using those results to determine appropriate nearfield locations for the error sensor(s). Though this approach has been experimentally verified in an idealized setting, it was not verified in a more realistic situation. This paper describes how this model was expanded from its idealized setting to a mock laptop enclosure. When necessary modifications to the model were made, tonal noise can be predicted in the nearfield of the fan inlets, which allows one to develop an effective compact, realistic ANC setup for use in the mock laptop enclosure. With this ANC setup, significant global reduction of the inlet tonal noise can be achieved.

The effects of nonideal microphone directivity patterns on directional impulse response measurements.

An acoustician can identify and treat problematic surfaces to reduce or eliminate unwanted reflections only if he knows the origins of those reflections. Several measurement techniques exist for the purpose of identifying these origins, including the Polar ETC method, which requires six cardioid impulse response measurements along Cartesian axes. This presentation will explore two implementations of the method using either a microphone positioner and six sequential cardioid measurements (as originally intended) or four simultaneous measurements from a tetrahedral subcardioid microphone array (originally intended for Ambisonic recordings, but also usable to synthesize the six cardioid measurements). It will compare the two approaches and investigate typical errors introduced by nonideal cardioid directivity patterns. The presentation will also discuss the capabilities of a new method, based on a Cartesian array of seven omnidirectional microphones, and explore the effects of nonideal omnidirectional patterns.