Time-domain Acoustic Simulations Research Articles

The Potential of Focusing Acoustic Retroreflectors for Architectural Surface Treatment

How much sound can a building surface reflect to a source, the location of which is not exactly known? This paper considers this question particularly for a planar surface acting as an array of retroreflectors, or of focusing retroreflectors. The question is investigated using finite-difference time-domain acoustic simulation, using ideal retroreflective patches achieved by space-reversal, and focusing achieved by delays. Extensive (7.2 × 7.2 m) and local (2.4 × 2.4 m) ideal planar reflector arrays were investigated at distances of 1.5 to 4 m from sources that were within a 2.4 × 2.4 m square plane. Patch sizes ranged from 0.3 m squares to the full reflector size. Physically realizable non-ideal focusing retroreflectors based on parabolic trihedra were also investigated. With sufficiently large patches, ideal focusing retroreflector arrays consistently outperform non-focusing retroreflector arrays. A large focusing retroreflector array has the potential to provide retroreflected energy levels (speech and A-weighted) from the first reflection to a source at 2 m distance comparable to the diffuse field energy level of acoustically supportive reverberant rooms. A small focusing retroreflector array returns less sound, but still much more than a single reflection from an equivalent specularly reflecting surface. Results from parabolic trihedra demonstrate that retroreflected energy levels similar to those from ideal surfaces can be achieved by architectural form. Challenges in translating these concepts to practical design solutions are discussed.

Development and validation of low-intensity pulsed ultrasound systems for highly controlled in vitro cell stimulation

This work aims to describe the development and validation of two low-intensity pulsed ultrasound stimulation systems able to control the dose delivered to the biological target. Transducer characterization was performed in terms of pressure field shape and intensity, for a high-frequency range (500 kHz to 5 MHz) and for a low-frequency value (38 kHz). This allowed defining the distance, on the beam axis, at which biological samples should be placed during stimulation and to exactly know the intensity at the target. Carefully designed retaining systems were developed, for hosting biological samples. Sealing tests proved their impermeability to external contaminants. The assembly/de-assembly time of the systems resulted ~3 min. Time-domain acoustic simulations allowed to precisely estimate the ultrasound beam within the biological sample chamber, thus enabling the possibility to precisely control the pressure to be transmitted to the biological target, by modulating the transducer's input voltage. Biological in vitro tests were also carried out, demonstrating the sterility of the system and the absence of toxic and inflammatory effects on growing cells after multiple immersions in water, over seven days.

Control-Oriented Methods for Turbomachinery Noise Simulation

This paper presents an innovative stability analysis and design approach for time-domain impedance boundary conditions to simulate noise propagation and radiation from a lined turbomachinery duct in the presence of a mean flow. A control-oriented model is developed for the stability analysis of the impedance boundary condition by using generalized function at the lining surface. The mean flow effect and sound propagation are considered in the model as well. Then, the numerical stability issue is analyzed by using the Bode plots before stabilized accordingly by employing the phase lead compensator method, which results in a rational transfer function. Finally, the corresponding time-domain implementation is achieved by using the so-called controllable canonical form rather than an inconvenient convolution operation. The performance of the current proposed approach is first validated in an in-duct propagation case by comparing to analytical solutions obtained by employing the Wiener–Hopf method and then demonstrated in a couple of duct acoustic problems with representative turbomachinery setups. The innovative cross-disciplinary nature of the current proposed approach can shed light on impedance problems and is very useful to time-domain acoustic simulations for turbomachinery applications.

Analysis of bats’ gaze and flight control based on the estimation of their echolocated points with time-domain acoustic simulation

Bats detect objects by echolocation. With those echoes, they can describe the shape of objects as well as position, and then approach or avoid them. To reveal which part of objects bats gaze at, we simulated the echoes that bats hear, and estimate the bats’ echolocated points by time-domain 2D-acoustic-simulation based on the behavioral data during obstacle-avoiding flight. First, we constructed a microphone array system in an experimental chamber and attached telemetry-microphones to Japanese horseshoe bats. With that measurement system, we obtained the timing, positions and directions of emitted pulses during obstacle-avoiding flight. Using these data, we simulated the echoes returning from the obstacles (acrylic boards) and investigated how the bats show spatial and temporal changes in the echolocated points of objects as they became familiar with the space. In a comparison between before and after the habituation in same obstacle layout, there are differences in the wideness of echolocated points on objects. By flying same layout repeatedly, their echolocating field become narrower. This study suggests the help of acoustic simulation to understand the way bats see the world. [This research was supported by Scientific Research on Innovative Areas of JSPS, and the JST PRESTO program.]

GPU-Based Acceleration for FDTD Method of Acoustic Field Analysis

The finite difference time domain (FDTD) method is a numerical technique used in time-domain acoustic simulation. However, the huge computing capacity has been an important limiting factor for its applications. In this paper, we will present a graphics processing units (GPU) - based parallel FDTD framework and its successful application to the estimation of time-series sound pressure data. Compared with the computer, the runtime of acoustic field FDTD simulation on GPU is dramatically decreased. The acceleration in runtime has made such study possible, and will pave the way for other studies of large-scale computational acoustic problems which were previously impractical.

Numerical absorbing boundary conditions based on a damped wave equation for pseudospectral time-domain acoustic simulations.

In the context of wave-like phenomena, Fourier pseudospectral time-domain (PSTD) algorithms are some of the most efficient time-domain numerical methods for engineering applications. One important drawback of these methods is the so-called Gibbs phenomenon. This error can be avoided by using absorbing boundary conditions (ABC) at the end of the simulations. However, there is an important lack of ABC using a PSTD methods on a wave equation. In this paper, we present an ABC model based on a PSTD damped wave equation with an absorption parameter that depends on the position. Some examples of optimum variation profiles are studied analytically and numerically. Finally, the results of this model are also compared to another ABC model based on an hybrid formulation of the scalar perfectly matched layer.

An Improvement of Absorbing Boundary Condition Based on Algorithm of Constrained Interpolation Profile Method for Finite-Difference Time-Domain Acoustic Simulation

In this study we examined an improved reasonable absorbing boundary condition (ABC). We examined ABC on the basis of the algorithm of the constrained interpolation profile (CIP) method for finite-difference time-domain (FDTD) acoustic simulation. Analysis of one-dimensional sound propagation with CIP-ABC was implemented. The results obtained in this study clarified that improved CIP-ABCs have better absorption performance than first-order Mur's ABC. That is, CIP-ABCs maintain a reflection coefficient that is lower than that of Mur's ABC in all frequency ranges.

Optimum Loss Factor for a Perfectly Matched Layer in Finite-Difference Time-Domain Acoustic Simulation

A perfectly matched layer (PML) is commonly used in finite-difference time-domain (FDTD) simulation to absorb outgoing waves and thereby reduce artifactual reflections from the computational domain boundaries. However, previous two-dimensional studies have noted that increasing the PML loss factor does not monotonically improve the PML's performance. This paper evaluates the PML in three-dimensional FDTD acoustic simulations. It confirms the existence of an optimum loss factor, with higher values degrading PML performance. An empirical formula is offered for estimating the optimum loss factor for a linear or a quadratic profile, that depends on the PML depth, sound speed, and grid resolution.