Acoustic Diffraction Effects Research Articles

A Method for Correcting Signal Aberrations in Ultrasonic Indoor Positioning.

The increasing focus on the development of positioning techniques reflects the growing interest in applications and services based on indoor positioning. Many applications necessitate precise indoor positioning or tracking of individuals and assets, leading to rapid growth in products based on these technologies in certain market sectors. Ultrasonic systems have already proven effective in achieving the desired positioning accuracy and refresh rates. The typical signal used in ultrasonic positioning systems for estimating the range between the target and reference points is the linear chirp. Unfortunately, it can undergo shape aberration due to the effects of acoustic diffraction when the aperture exceeds a certain limit. The extent of the aberration is influenced by the shape and size of the transducer, as well as the angle at which the transducer is observed by the receiver. This aberration also affects the shape of the cross-correlation, causing it to lose its easily detectable characteristic of a single global peak, which typically corresponds to the correct lag associated with the signal's time of arrival. In such instances, cross-correlation techniques yield results with a significantly higher error than anticipated. In fact, the correct lag no longer corresponds to the peak of the cross-correlation. In this study, an alternative technique to global peak detection is proposed, leveraging the inherent symmetry observed in the shape of the aberrated cross-correlation. The numerical simulations, performed using the academic acoustic simulation software Field II, conducted using a typical ultrasonic chirp and ultrasonic emitter, compare the classical and the proposed range techniques in a standard office room. The analysis includes the effects of acoustical reflection in the room and of the acoustic noise at different levels of power. The results demonstrate that the proposed technique enables accurate range estimation even in the presence of severe cross-correlation shape aberrations and for signal-to-noise ratio levels common in office and room environments, even in presence of typical reflections. This allows the use of emitting transducers with a much larger aperture than that allowed by the classical cross-correlation technique. Consequently, it becomes possible to have greater acoustic power available, leading to improved signal-to-noise ratio (SNR).

Simulating room transfer functions between transducers mounted on audio devices using a modified image source method.

The image source method (ISM) is often used to simulate room acoustics due to its ease of use and computational efficiency. The standard ISM is limited to simulations of room impulse responses between point sources and omnidirectional receivers. In this work, the ISM is extended using spherical harmonic directivity coefficients to include acoustic diffraction effects. These effects occur in practice when transducers are mounted on audio devices of finite spatial extent, e.g., modern smart speakers with loudspeakers and microphones. The proposed method is verified using finite element simulations of various loudspeaker and microphone configurations in a shoebox-shaped room. It is shown that the accuracy of the proposed method is related to the sizes, shapes, number, and positions of the devices inside a room. A simplified version of the proposed method, which can significantly reduce computational effort, is also presented. The proposed method and its simplified version can simulate room transfer functions more accurately than currently available image source methods and can aid the development and evaluation of speech and acoustic signal processing algorithms, including speech enhancement, acoustic scene analysis, and acoustic parameter estimation.

Acoustic Simulation for Performance Evaluation of Ultrasonic Ranging Systems

The recent growing interest in indoor positioning applications has paved the way for the development of new and more accurate positioning techniques. The envisioned applications, include people and asset tracking, indoor navigation, as well as other emerging market applications, require fast and precise positioning. To this end, the effectiveness and high accuracy and refresh rate of positioning systems based on ultrasonic signals have been already demonstrated. Typically, positioning is obtained by combining multiple ranging. In this work, it is shown that the performance of a given ultrasonic airborne ranging technique can be thoroughly analyzed using renowned academic acoustic simulation software, originally conceived for the simulation of echographic transducers and systems. Here, in order to show that the acoustic simulation software can be profitably applied to ranging systems in air, an example is provided. Simulations are performed for a typical ultrasonic chirp, from an ultrasound emitter, in a typical office room. The ranging performances are evaluated, including the effects of acoustic diffraction and air frequency dependent absorption, when the signal-to-noise ratio (SNR) decreases from 30 to −20 dB. The ranging error, computed over a point grid in the space, and the ranging cumulative error distribution is shown for different SNR levels. The proposed approach allowed us to estimate a ranging error of about 0.34 mm when the SNR is greater than 0 dB. For SNR levels down to −10 dB, the cumulative error distribution shows an error below 5 mm, while for lower SNR, the error can be unlimited.

Simulating Signal Aberration and Ranging Error for Ultrasonic Indoor Positioning.

Increasing efforts toward the development of positioning techniques testify the growing interest for indoor position-based applications and services. Many applications require accurate indoor positioning or tracking of people and assets, and some market sectors are starting a rapid growth of products based on these technologies. Ultrasonic systems have already been demonstrating their effectiveness and to possess the desired positioning accuracy and refresh rates. In this work, it is shown that a typical signal used in ultrasonic positioning systems to estimate the range between the target and reference points—namely, the linear chirp—due to the effects of acoustic diffraction, in some cases, undergoes a shape aberration, depending on the shape and size of the transducer and on the angle under which the transducer is seen by the receiver. In the presence of such signal shape aberrations, even one of the most robust ranging techniques, which is based on cross-correlation, provides results affected by a much greater error than expected. Numerical simulations are carried out for a typical ultrasonic chirp, ultrasonic emitter, and range technique based on cross-correlation and for a typical office room, obtained using the academic acoustic simulation software Field II. Spatial distributions of the ranging error are provided, clearly showing the favorable low error regions. The work demonstrates that particular attention must be paid to the design of the acoustic section of the ultrasonic positioning systems, considering both the shape and size of the ultrasonic emitters and the shape of the acoustic signal used.

Characterization of the effects of ground boards on acoustic signals

International and Federal regulations stipulate the acquisition of aircraft noise shall be conducted using inverted pressure microphones over a round ground board. These ground boards are used to provide an acoustically hard reflecting surface, limiting the effects of the potentially absorptive ground local to the test location. The microphone location is also specified to be offset from the center of the board to limit the effects of acoustic diffraction off the board edge. In order to determine the effects of the ground board on the measured acoustic signal, a comprehensive measurement campaign was undertaken at NASA Langley Research Center. The experimental setup included multiple ground board configurations placed on top of a sand pit, in an otherwise anechoic chamber. Ground board configurations included a microphone inverted and offset over the round ground board, a microphone offset and flush mounted in the ground board, and a microphone flush mounted in the sand. A detailed frequency sweep from a known source was then used to investigate the ground board effects on the recorded signal. Normal impedance measurements were also acquired to determine the reflection coefficients of the sand and ground boards. Experimental results are discussed, along with implications for future research and development.International and Federal regulations stipulate the acquisition of aircraft noise shall be conducted using inverted pressure microphones over a round ground board. These ground boards are used to provide an acoustically hard reflecting surface, limiting the effects of the potentially absorptive ground local to the test location. The microphone location is also specified to be offset from the center of the board to limit the effects of acoustic diffraction off the board edge. In order to determine the effects of the ground board on the measured acoustic signal, a comprehensive measurement campaign was undertaken at NASA Langley Research Center. The experimental setup included multiple ground board configurations placed on top of a sand pit, in an otherwise anechoic chamber. Ground board configurations included a microphone inverted and offset over the round ground board, a microphone offset and flush mounted in the ground board, and a microphone flush mounted in the sand. A detailed frequency sweep from a ...

The effects of diffraction on the frequency difference and frequency sum autoproducts

Previously, a remote sensing technique termed frequency difference matched field processing was developed for source localization in the shallow ocean (Worthmann et al., 2015, 138, 3549-3562). In this technique, field measurements at the in-band frequency are shifted down (or up) in frequency through the use of the bandwidth-averaged frequency-difference (or frequency-sum) autoproduct, a nonlinear construction made from field amplitudes and averaged over the available signal bandwidth. These bandwidth-averaged autoproducts may have phase structure similar to genuine acoustic fields at out-of-band frequencies when the original acoustic field is well-described by a sum of ray-path contributions. While ray theory may be a useful field description in many situations, it does not include diffraction. In this presentation, the effects of acoustic diffraction on the autoproduct are analyzed in an environment where diffraction varies in importance depending on the spatial location. Specifically, the behavior of t...

Photoacoustic and laser-induced evaporation effects in liquids

It is shown experimentally that the photoacoustic pressure response in absorbing condensed matter to laser pulses with high frequency modulation of intensity can be used to monitor displacement of the irradiated surface. Some aspects of the processes under investigation are also discussed, including acoustic diffraction effects, features of acoustic reflection from liquid–air and liquid–solid boundaries, explosive boiling, and interference between thermoacoustic and vaporization pressure signals.

Numerical and experimental studies on the effects of stage risers

The acoustic effects of stage risers, especially on the sound of lower string instruments, are numerically and experimentally analyzed. To discuss the effects of the vibration of riser’s boards due to the mechanical force from an instrument, a structural–acoustical coupling approach is applied based on the mode expansion and the boundary element technique. Measurement results of the mechanical force from real instruments are used in the numerical study. The vibration of the top board of a riser affects the sound field only around the natural frequencies of the board and the cavity of the riser. In contrast, the acoustic diffraction due to the riser affects the sound field in a wide frequency range. The riser’s sideboard that faces to receiving points increases the sound pressure levels because it reflects waves diffracted at the riser’s edge to the front. To verify the numerical results, the effects of acoustic diffraction due to risers are especially investigated in detail through a scale model experiment.

Acoustic diffraction effects at the Hellenistic amphitheater of Epidaurus: Seat rows responsible for the marvelous acoustics

The Hellenistic theater of Epidaurus, on the Peloponnese in Greece, attracts thousands of visitors every year who are all amazed by the fact that sound coming from the middle of the theater reaches the outer seats, apparently without too much loss of intensity. The theater, renowned for its extraordinary acoustics, is one of the best conserved of its kind in the world. It was used for musical and poetical contests and theatrical performances. The presented numerical study reveals that the seat rows of the theater, unexpectedly play an essential role in the acoustics--at least when the theater is not fully filled with spectators. The seats, which constitute a corrugated surface, serve as an acoustic filter that passes sound coming from the stage at the expense of surrounding acoustic noise. Whether a coincidence or not, the theater of Epidaurus was built with optimized shape and dimensions. Understanding and application of corrugated surfaces as filters rather than merely as diffuse scatterers of sound, may become imperative in the future design of modern theaters.

Observation of acoustically induced space charge gratings in CdS

Static space charge gratings, produced via a nonlinear acoustoelectric interaction, have been directly observed in CdS by means of laser diffraction. Spatial and temporal filtering techniques were used to separate the required optical information from extraneous signals due to background and acoustic diffraction effects.

Experiments on thermoacoustic arrays with laser excitation

A recent theoretical formulation by Westervelt and Larson predicts the development of highly directive sound beams in water through the thermalization of modulated laser light. The present paper reports the experimental validation of this theory with measurements carried out in a fresh water lake. Additional theory pertinent to practical implementation is also presented and verified. The experimental apparatus includes an optically pumped laser system operating in the conventional mode at wavelengths in either the red or infrared regions of the optical spectrum. Modulation of the stochastic light burst is provided by a half-wave Pockels cell. The acoustic measurements began with an existence test which confirmed that an acoustic signal is produced by thermo-optic demodulation of laser light in the water. The sound pulse was generated in a beam with a width and source level that are in reasonable agreement with theory. Acoustic diffraction effects associated with laser beams of finite size were also examined, as were the nearfield effects of this type of array. The relation of laser technology to further work in the blue–green spectrum is analyzed. Subject Classification: [43]20.55; [43]85.40; [43]35.65.