Acoustic Pressure Sensor Research Articles

Tilted $c$-Axis Thin-Film Bulk Wave Resonant Pressure Sensors With Improved Sensitivity

Aluminum nitride thin-film bulk wave resonant pressure sensors employing c- and tilted c-axis texture, have been fabricated and tested for their pressure sensitivities. The c-axis tilted film bulk acoustic resonators pressure sensors demonstrate substantially higher-pressure sensitivity compared to its c-axis oriented counterpart. More specifically, the thickness plate quasi-shear resonance has demonstrated the highest pressure sensitivity while further being able to preserve its performance in liquid environment.

Pressure Dependence of Laser-Induced Dielectric Breakdown in Water

The effects of pressure on laser-induced breakdown events at superthreshold irradiances in water were investigated over a range of pressures from 0 to 1380 bar. Breakdown events were generated using 5-ns Nd:YAG laser pulses of wavelength 532-nm. Observations of breakdown events were made using imaging and single detector techniques. Applications for use as a static and/or acoustic pressure sensor were explored. Using imaging techniques over a range of static pressures from 0 to 275 bar, it was observed that the laser-induced breakdown threshold increased with increasing pressure. An ability to infer acoustic pressure via optical breakdown techniques using imaging was demonstrated. Single detector measurements over the same pressure range were observed to follow a similar trend.

Spectral analysis of internally leaking shut-down valves

We herein describe the results obtained from a set of experiments conducted on a shut-down valve, the objective of which was to identify the degree of internal leakage through the valve in its closed state. Leakage in a valve is not only a concern in terms of safety, but can also contribute to high maintenance costs and the loss of supply. Any improvements to the methods available for monitoring the condition to the valve may yield substantial cost savings and safer working conditions. All the experiments were conducted after causing damage to a valve and by subsequent monitoring of the vibrations caused by the resulting leak, using different types of instrumentation, including acoustic emission and dynamic pressure sensors. We carried out an analysis of the data obtained, in which our primary focus was the spectrum of frequencies obtained from each of the signals emitted. The methods we used deemed to be effective for the detection of leaks. The quantification of the leaks is a rather more complex matter that warrants further investigation, however. Further research is also suggested on the effect of external disturbances, the influence of the experimental parameters, and the applicability of analytical models.

Sensing with hollow-core photonic bandgap fibers

This paper examines the unique properties of hollow-core photonic bandgap fibers and discusses potential sensing applications of such fibers. Guidance of light in air instead of silica reduces the effect of material on light propagation and is advantageous to applications such as fiber gyroscopes. An air–silica microstructure has novel mechanical and thermal properties and can be beneficial to applications such as acoustic pressure sensors. Holey cladding provides extra flexibility for geometry modification through post thermal treatment or selective filling of the air holes and can be exploited for developing novel devices such as in-line polarizers/polarimeters, modal interferometer, wavelength filters and sensors. The confinement of gas and liquid phase materials and optical guided modes simultaneously within the hollow core allows strong light/sample interaction over an extended length and may be useful for the development of highly sensitive gas and liquid sensors.

Eruption source processes derived from seismic and acoustic observations of the 25 September 2007 Ruapehu eruption—North Island, New Zealand

Mt. Ruapehu erupted on 25 September 2007 at 20:26 PM NZDT (8:26 UT) generating a steam column to about 15,000 ft (4600 m), a directed ballistic and surge deposit of coarse blocks and ash to the north of the Crater Lake, and initiated lahars in the Whangaehu catchment and Whakapapa ski field. The eruption was recorded on three broadband seismometers and two acoustic pressure sensors which indicated broadly coherent waveform characteristics across the network. The eruption had a duration of less than 1 min as indicated by strong seismic and acoustic pulses. The acoustic wave coincided with observed very long-period (VLP) seismic signal (2–25 s period). The acoustic pulse had a positive pressure (indicating an explosion) and travelled at a velocity of ∼ 320 m/s. The syn-eruptive VLP was composed of surface waves having strong radial and transverse components. The immediate aftermath of the eruption produced about 4 min of continued high amplitude spasmodic tremor signals that probably resulted from post-explosion phase associated vent backfill/stabilisation. The eruption was preceded by minor volcano-tectonic earthquakes and tremor bursts which began 10 min before the main eruption. Two smaller VLP signals were associated with these tremor pulses at 20:17 and 20:25. These pre-eruptive VLP signals had particle motions indicating body and surface waves from a sub-surface source region. Modelling of the VLP sources using a 3D finite-difference method shows that that the pre-eruption VLP signals are consistent with a volume fluctuation at 3–7 km depth from a radially symmetric source. The main eruption VLP was consistent with a south and downward directed single force in response to a northward directed jet and northward directed ballistic deposits.

Spherical harmonic beamforming for room acoustic analysis.

We describe the potential of using a spherical beamforming microphone array to investigate the spatial correlation of sound fields in rooms. We are building spherical microphone arrays consisting of many acoustic pressure sensors mounted appropriately on the surface of a rigid sphere. Our associated spherical eigenbeamformer decomposes the sound-field into spatially orthonormal spherical harmonics up to third-order. We refer to the signals from the eigenbeamformer as eigenbeams. All eigenbeams have phase centers at the physical center of the array. Due to the orthonormal property of the eigenbeamformer, a diffuse field ideally results in zero correlation between the eigenbeams. Therefore, by measuring the cross-correlation between the eigenbeam signals, one can investigate the proximity to the isotropy (or “diffuseness”) of the sound-field. Simultaneously, the underlying eigen-beam patterns can be steered without effecting the orthonormality property. How the cross-correlation function changes with general orientation of the eigenbeams is another potential measure for sound field diffuseness in rooms. We will show some real room measurements demonstrating the potential usefulness of this approach.

Acoustic pressure sensor

An acoustic pressure sensor includes a stack of one or more acoustically responsive elements which are contained within a housing. An external acoustic pressure to be sensed is transmitted to the stack via a diaphragm which exists in an end cap closing a top portion of the housing. The outer edge of the diaphragm is in direct contact with the outer edge of the housing. This area enhancement increases the basic sensitivity of the sensing stack without the need of a gap. The diaphragm allows flexure to occur in a controlled fashion. The housing is sealed at the top and bottom by end caps, the top end cap defining a diaphragm area, which diaphragm deflects to thereby apply pressure to the stack to convert acoustic signals into electrical signals.

Fast direction of arrival algorithm based on vector-sensor arrays using wideband sources

An acoustic vector sensor (AVS) can capture more information than a conventional acoustic pressure sensor (APS). As a result, more output channels are required when multiple AVS are formed into arrays, making processing the data stream computationally intense. This paper proposes a new algorithm based on the propagator method for wideband coherent sources that eliminates eigen-decomposition in order to reduce the computational burden. Data from simulations and lake trials showed that the new algorithm is valid: it resolves coherent sources, breaks left/right ambiguity, and allows inter element spacing to exceed a half-wavelength.

SAW based passive sensor with passive signal conditioning using MEMS A/D converter

A monolithically packaged surface acoustic wave (SAW) radio transponder and pressure sensor are developed for the application to a tire pressure monitoring system (TPMS). The device contains the wireless transponder, which converts analog signal into digital one without any auxiliary electronic circuits and transmits the converted data wirelessly. No power sources are needed for wireless transponder and pressure sensor. The touch-mode pressure sensor converts externally applied pressure into capacitance, and the SAW radio transponder radiates sensor values with pulse train to the interrogation (measurement) unit. The realization of the mechanical A/D conversion is possible since the SAW radio transponder is connected to the touch-mode capacitive pressure sensor. The SAW radio transponder and touch-mode sensor are fabricated using a surface micromachining and a bulk micromachining technologies, respectively. The performance of the integrated, passive and wireless pressure sensor meets the design specifications such as linearity, sensitivity and noise figure. Finally, experimental results for the radio transponder and the sensor without power source are presented.

Further developments in the active control of free‐field sound using near‐field energy‐based error signals

Practical efforts to actively control sound often require error sensors located in the acoustic or geometric near field of sound sources. Unfortunately, when using conventional acoustic pressure sensors, control performance often becomes very sensitive to sensor location. Optimal sensor positions are difficult to ascertain or maintain with changing system conditions. Alternate types of error sensors often lead to fewer position‐dependent complications, producing global sound field control with greater ease and consistency. The distinct spatial and spectral uniformities of potential, kinetic, and total energy density for near‐field error signal measurements will be discussed for the sound control of sources located in a free field. Numerical results will be compared to experimentally measured results.

Vacuum sealed surface acoustic wave pressure sensor

A vacuum sealed SAW pressure sensor is disclosed herein, which includes a sensing element configured as a SAW device (e.g. SAW resonator or SAW delay line) supported by a thin diaphragm. The substrate material can be implemented as a quartz wafer (i.e., “base” wafer). The SAW device can be configured on one side of the wafer and the diaphragm etched on the opposite side. A quartz micromachined pressure sensor can thus be realized, which operates based on a variation of the surface wave velocity of a SAW device situated on the thin diaphragm. The SAW sensor is generally sealed in a vacuum and diaphragm sustains the sensor, thereby implementing a sensor on a wafer scale while allowing for a cost reduction per chip.

Apparatus and method for measuring multi-phase flows in pulp and paper industry applications

The consistency of a pulp slurry in a pipe 12 is determined using a pair of effective sound speeds a 1eff and a 2eff of the fluid/pipe system. The pair of effective system sound speed measurements are taken at two sensing regions X 1 , X 2 along the pipe wherein each of the sensing regions comprises a different system cross sectional area compliance. The pair of effective system sound speeds a 1eff and a 2eff are provided to signal processing logic 160 , which determines at least the composition of the fluid 12 flowing in the pipe 112 . The effective system sound speeds a 1eff and a 2eff may be provided by a pair of sound speed meters positioned at sensing regions X 1 , X 2 wherein the sound speed meters utilize a spatial array of acoustic pressure sensors placed at predetermined axial locations along the pipe 112 . One technique uses acoustic spatial array signal processing techniques with the direction of propagation of the acoustic signals along the longitudinal axis of the pipe 112.

Eigenbeam beamforming for microphone arrays

In this talk, we describe a beamforming spherical microphone array consisting of many acoustic pressure sensors mounted appropriately on the surface of a rigid sphere. The associated beamformer is implemented in a two-stage process: a first stage that decomposes of the sound-field into spatially orthonormal spherical harmonics, which are called eigenbeams, and a second stage that combines the eigenbeams to form the overall output beampattern. It is shown that this approach allows for a computationally efficient, flexible, and scalable beamformer structure. As an example of beamforming flexibility, an acoustic zoom feature will be described. This application enables continuous variability of the beamwidth in a simple and elegant manner. Simultaneously, the array beampattern can be steered without affecting the beampattern shape. There are many applications for such a feature, one being the coordination of the acoustic and image scenes of microphone and a video camera.

Active control of free-field sound using near-field energy-based error signals

Practical efforts to actively control sound often require error sensors located in the acoustic or geometric near field of primary and secondary sources. Unfortunately, when using conventional acoustic pressure sensors, control performance often becomes very sensitive to sensor position, and optimal sensor positions are difficult to ascertain or maintain with changing system conditions. Other types of error sensors or combinations of error sensors may produce fewer position-dependent complications, yielding excellent global sound-field control with greater ease and consistency. This paper compares the use of potential, kinetic, and total energy density error signals in the near field of a primary and secondary source in free space. It also discusses the distinct spatial and spectral uniformities of these quantities and their impact on the active control of sound.

Bending mode effect on sensitivity of plate surface acoustic wave pressure sensors

The bending mode effect on sensitivity of pressure sensors operating with surface acoustic waves (SAW) propagating over the surface of a plate is analyzed using Tiersten's perturbation integral for frequency shifts in a piezoelectric resonator due to the presence of initial fields. For a plate subjected to nonpure bending, the SAW speed, whose shift in response to pressure determines the sensitivity, can be defined locally through the local variations of the effective material constants. Three common bending modes of pure bending, bending under a concentrate load, and bending under a uniformly load are analyzed and compared.

Analysis of surface acoustic wave pressure sensors

Analysis of surface acoustic wave pressure sensors

Coherent signal-subspace processing of acoustic vector sensor array for DOA estimation of wideband sources

This paper deals with the estimation of the direction of arrival (DOA) for wideband coherent sources using acoustic vector sensor array. By extending the idea of wideband focusing into acoustic vector sensor array processing, a wideband DOA estimation approach based on coherent signal-subspace processing is developed for acoustic vector sensor array. Finally, simulation results are presented, to compare the proposed method with its counterpart based on acoustic pressure sensor array, the advantages of the proposed algorithm are identified.

Analysis of surface acoustic wave pressure sensors

The authors perform sensitivity analysis theoretically on three surface acoustic wave (SAW) pressure sensor structures and two material selections. The analyses take into consideration the effects of mounting structures, ways of transferring pressure to the sensing element, and various physical and geometrical parameters. It is shown that pressure-induced bending produces a larger change of wave speed than pressure-induced extension. A proposed shallow shell structure is demonstrated to increase the sensitivity of SAW pressure sensors, and for this structure, Si SAW exhibits a slightly higher sensitivity than Ge SAW.

Acoustic pressure-vector sensor array

Pressure-vector sensors measure both scalar and vector components of the acoustic field. December 2003 measurements at the NUWC Seneca Lake test facility verify previous observations that acoustic ambient noise spectrum levels measured by acoustic intensity sensors are reduced relative to either acoustic pressure or acoustic vector sensor spectrum levels. The Seneca measurements indicate a reduction by as much as 15 dB at the upper measurement frequency of 2500 Hz. A nonlinear array synthesis theory for pressure-vector sensors will be introduced that allows smaller apertures to achieve narrow beams. The significantly reduced ambient noise of individual pressure-vector elements observed in the ocean by others, and now at Seneca Lake, should allow a nonlinearly combined array to detect significantly lower levels than has been observed in previous multiplicative processing of pressure sensors alone. Nonlinear array synthesis of pressure-vector sensors differs from conventional super-directive algorithms that linearly combine pressure elements with positive and negative weights, thereby reducing the sensitivity of conventional super-directive arrays. The much smaller aperture of acoustic pressure-vector sensor arrays will be attractive for acoustic systems on underwater vehicles, as well as for other applications that require narrow beam acoustic receivers. [The authors gratefully acknowledge the support of ONR and NUWC.]

An improved water-filled impedance tube.

A water-filled impedance tube capable of improved measurement accuracy and precision is reported. The measurement instrument employs a variation of the standardized two-sensor transfer function technique. Performance improvements were achieved through minimization of elastic waveguide effects and through the use of sound-hard wall-mounted acoustic pressure sensors. Acoustic propagation inside the water-filled impedance tube was found to be well described by a plane wave model, which is a necessary condition for the technique. Measurements of the impedance of a pressure-release terminated transmission line, and the reflection coefficient from a water/air interface, were used to verify the system.