John W. Parkins

Narrowband and broadband active control in an enclosure using the acoustic energy density

An active control system based on the acoustic energy density is investigated. The system is targeted for use in three-dimensional enclosures, such as aircraft cabins and rooms. The acoustic energy density control method senses both the potential and kinetic energy densities, while the most popular control systems of the past have relied on the potential energy density alone. Energy density fields are more uniform than squared pressure fields, and therefore, energy density measurements are less sensitive to sensor location. Experimental results are compared to computer-generated results for control systems based on energy density and squared pressure for a rectangular enclosure measuring 1.5 x 2.4 x 1.9 m. Broadband and narrowband frequency pressure fields in the room are controlled experimentally. Pressure-field and mode-amplitude data are presented for the narrowband experiments, while spectra and pressure-field data are presented for the broadband experiment. It is found that the energy density control system has superior performance to the squared pressure control system since the energy density measurement is more capable of observing the modes of a pressure field. Up to 14.4 and 3.8 dB of cancellation are achieved for the energy density control method for the narrowband and broadband experiments presented, respectively.

Error analysis of a practical energy density sensor

The investigation of an active control system based on acoustic energy density has led to the analysis and development of an inexpensive three-axes energy density sensor. The energy density sensor comprises six electret microphones mounted on the surface of a 0.025-m (1 in.) radius sphere. The bias errors for the potential, kinetic, and total energy density as well as the magnitude of intensity of a spherical sensor are compared to a sensor comprising six microphones suspended in space. Analytical, computer-modeled, and experimental data are presented for both sensor configurations in the case of traveling and standing wave fields, for an arbitrary incidence angle. It is shown that the energy density measurement is the most nearly accurate measurement of the four for the conditions presented. Experimentally, it is found that the spherical energy density sensor is within +/- 1.75 dB compared to reference measurements in the 110-400 Hz frequency range in a reverberant enclosure. The diffraction effects from the hard sphere enable the sensor to be made more compact by a factor of 3 compared to the sensor with suspended microphones.

Multiple resonator attenuating earplug

A non-vented shell earplug for insertion into the ear canal has multiple chambers configured as coupled acoustic resonators. Each chamber is provided with an acoustic element having inertia and resistance coupling the chamber to its neighbor to form multiple resonant chambers. The dimensions of the individual acoustic elements set the frequency response of the earplug, and determine the attenuation characteristics of the resonators. Unlike the single chamber resonator of the prior art, the multiple-resonator earplug provides attenuation in both low and high frequency segments of the noise spectrum. Additionally, the present invention teaches placing loudspeaker and/or sound sensing transducer for sound communication purposes and active noise control within the earplug, with their sound field patterns directed into a chamber for coupling to the ear canal. The use of multiple resonators enhances the high frequency communications response of the earplug.

Active control of energy density in three‐dimensional enclosures

Attenuating the sound pressure at a microphone in an enclosure typically results in a relatively small region of control, often referred to as a zone of silence. In an effort to increase the region of control for practical applications, as many as 30–50 microphones have been used to achieve a broader region of control. An alternative control method for achieving global control of the field, based on sensing and minimizing the total energy density at discrete locations, has been developed. Previous work using this method in one‐dimensional enclosures has indicated that significant improvement in overall attenuation is possible. This improvement can be attributed to the fact that sensing the energy density monitors all of the modes of the enclosure, and thereby avoids the spillover problem which often plagues control systems that minimize only pressure. The work reported here extends the energy density approach to three‐dimensional, rectangular enclosures. Numerical results are presented to compare the glob...

Experimental results using active control of energy density

Previous research reported has presented numerical results obtained, indicating that one can often achieve improved global control of an enclosed acoustic field by minimizing the acoustic energy density, rather than the acoustic pressure. Inexpensive sensor probes have been developed that are capable of sensing the pressure and velocity components of the acoustic field, for incorporation into an adaptive control system that minimizes the energy density. The control system has been implemented within an enclosed sound field and has been used to compare the global attenuation achieved by minimizing either the squared pressure or the energy density. The control system is capable of implementing control with multiple sources and/or sensors. Results are shown for the low modal density case in the enclosure and indicate the improved global attenuation that can be achieved using energy density control. As well, the dependence of the controlled field on error sensor location is shown to be substantially weaker wh...

Narrow‐band and broadband active control in an enclosure using energy density sensors

An active control system has been constructed based on the minimization of the energy density at discrete points within an enclosure. Though most control systems for use in enclosures are based on squared pressure as a cost function, a system based on energy density is more capable of sensing modes contributing to the acoustic field. The enclosure used is lightly damped and measures 1.5×2.4×1.9 m. A measurement system within the enclosure is capable of spatially sampling the pressure field and decomposing the field into complex modal amplitudes, which yield insight into the control phenomena. The control system can use squared pressure as well as energy density as a cost function for comparisons of the two methods. The squared pressure and energy density control are consistent with predictions of active control performance in the case of single‐frequency multiple sensor/source configurations. Global control of up to 18 dB is achieved. Broadband control results using a single sensor and control source are ...

Multi‐channel active control of acoustic energy density

Previous numerical work has indicated the potential advantages of minimizing the acoustic energy density for active control of sound in enclosures. Preliminary experimental results have generally confirmed this numerical work. A computerized scanning system has recently been implemented in an enclosure at Penn State Univ. University that provides greatly improved scanning capabilities to investigate the uncontrolled and controlled fields for active control experiments. Using this system, the active control of enclosed fields using energy density has been further investigated for both single‐channel and multi‐channel implementations. With the scanning capabilities, it is also possible to represent the experimentally measured field in terms of its modal components. Recent results that have been obtained will be presented, to show the control achieved using energy density, both in terms of the spatial dependence of the field, as well as in terms of its modal components.

The effects of microphone mismatch on the performance of a spherical acoustic vector‐field sensor

Elko previously established the performance improvement for intensity measurements of an acoustic sensor using two microphones embedded on the surface of a hard sphere to that of a sensor comprising two infinitely small microphones separated in space [G. Elko, Noise‐Con 91, pp. 525–532]. Elko derived his results for perfectly matched microphones, with regard to phase and sensitivity, in a plane‐wave field with a variable angle of incidence. The performance of a spherical sensor and a two‐point sensor are compared here for the case of a one‐dimensional standing wave field with an arbitrary reflection coefficient and incidence angle, when the measurement microphones have a phase and sensitivity mismatch. Results are generated using sensitivity and phase variations typically found in inexpensive electret microphones. The effect of bias errors in potential energy density, kinetic energy density, total energy density, and intensity are reported.

Modal decomposition results for active control of energy density

In recent years, an alternative sensing approach has been developed for active control, based on minimizing the acoustic energy density at the error sensor location(s). This new approach has been tested both numerically and experimentally, with the results indicating that one can often achieve improved global attenuation of the field by minimizing the acoustic energy density, rather than the sum of the squared pressures. Previous results from minimizing the energy density at the error sensors have concentrated on investigating the control that can be achieved by looking at the global energy in the field before and after control, and also by looking at the attenuation that can be achieved as a function of frequency. However, it has also been found that additional insight can be gained by examining the acoustic field in terms of the acoustic modes contributing to the acoustic field. This paper will present some of the modal decomposition results obtained for different active control approaches. These result...

Active control in three‐dimensional enclosures using multiple secondary sources and error sensors

The use of multiple secondary sources and multiple error sensors can significantly improve global attenuation whether one employs a control method based on the squared pressure or energy density. A single source positioned close to a pressure node will be inefficient at exciting the corresponding mode, therefore the secondary modes will dominate the pressure field, and attenuation is unlikely at the related frequency. Increasing the number of secondary sources improves the probability that at least one source will not lie close to a pressure node, thereby mitigating this problem. Problems also arise when error sensors are close to nodes. Adding multiple error sensors increases the probability that the sensors will be able to observe the dominant modes, which will yield improved attenuation. Using a greater number of error sensors than secondary sources will yield a determined control system, with a unique optimal solution. If more sources are used than sensors, an underdetermined control system will resul...