Nomura Laboratory

Introduction

It is our pleasure and privilege to be able to introduce Applied Acoustics & Electronics Laboratory, UEC to the visitors from all over the world. The Laboratory is mainly concerned with Ultrasound-Electronics and Electro-Acoustics. In particular, fundamental studies on Nonlinear Acoustics are our central themes of unceasing research for new scientific developments and technologies in the 21st century. Modern acoustics pays much attention to nonlinear phenomena such as parametric array, streaming, acoustic radiation, cavitation, acoustic soliton, sonoluminescence, sonochemistry, and so on. Our research activities are focused on the following topics...

fig.1
Plane wave distortion of a finite amplitude sound wave in a rigid-wall tube. We can observe shock waves in the final stage.

1. Propagation of finite-amplitude ultrasound in a fluid

When the assumption that sound amplitude is infinitesimally small breaks down, the waveform distorts during propagation due to the inherent nonlinearity of a medium. This means that plenty of harmonics are generated in the beams. Using the oblate spheroidal coordinate system, we have developed a new model equation which describes well the nonlinear propagation of finite-amplitude beams in a viscous fluid. The model equation developed is powerful for predicting theoretically the spatial and/or temporal evolution of sound beams emitted from a focusing source with a wide-angle aperture. It must give more insight into the nonlinear behaviors of strongly focusing beams than other model equations.

fig.2

Theoretical three-dimentional views of the pressure amplitudes of the first three harmonics (l-r) in 1MHz focused beams from a circular concave source with a half-aperture angle of 30°and a focal length of 10cm. Source-face pressure is 220kPa.

(1) "Nonlinear sound field by interdigital transducers in water,'' Jpn. J. Appl. Phys. 47, 4076-4080 (2008).

(2) "Simulation of sound field in a tissue medium generated by a concave spherically annular transducer, " Ultrasonics 44, e271-274 (2006).

(3) "Elliptically curved acoustic lens for emitting strongly focused finite-amplitude beams: Application of the spheroidal beam equation model to the theoretical prediction, " Acoust. Scie. & Tech. 26, 279-284 (2005).

(4) "Two model equations for describing nonlinear sound beams, " Jpn. J. Appl. Phys. 43, 2808-2812 (2004).

(5) "A higher-order parabolic model equation for describing nonlinear propagation of ultrasound beams, " Acoust. Scie. & Tech. 25, 163-165 (2004).

(6) "Nonlinear propagation of focused ultrasound beams from a wide-angle aperture," Acustica・acta acustica 86, 446-456 (2000).

(7) "Model equation for strongly focused finite-amplitude sound beams," J.Acoust.Soc.Am. 107, 3035-3046 (2000).

(8) "A new theoretical approach to the analysis of nonlinear sound beams using the oblate spheroidal coordinate system," J.Acoust.Soc.Am. 105, 3083-3986 (1999).

(9) "Model equation of directive sound beams using the oblate spheroidal coordinate system," J.Acoust.Soc.Jpn. 55, 832-837 (1999).

2. Acoustic streaming and temperature elevation in confined beams

When ultrasounds are propagating down in a fluid, bulk movement of the medium appears in the beams as a by-product of the sounds. Usually the flow velocity is not so fast as to be easily observed by simple measuring equipment. However, the velocity increases with the sound pressure and time-independent current of the fluid circulates globally in a closed tube. Such macroscopic fluid movement is called Eckart-type acoustic streaming. In addition to the streaming, temperature in the beam is elevated locally due to sound energy dissipation. Generally, the flow induced in the beam reduces the elevation of temperature by cooling. Ultrasound heating and flow cooling are being investigated experimentally and theoretically...

Axial velocity profiles of acoustic streaming generated in a 1.8MHz focused ultrasound beam. The velocities are measured at different times using a laser Doppler velocimeter after the sound is switched on. Dots are experimental results and solid lines are theoretical prediction.

(1) "Generation and visualization of acoustic streaming by bulk acoustic waves from interdigital transducers," Trans. on IEICE. J91-A, 1141-1148 (2008).

(2) "Local control of Eckart streaming nera focus of concave ultrasound source with two coaxially arranged transducers," Jpn. J. Appl. Phys. 45, 4448-4452 (2006).

(3) "Acoustic streaming and temperature elevation in focused Gaussian beams," J.Acoust.Soc.Jpn.(E) 18, 247-252 (1997).

(4) "Buildup of acoustic streaming in focused beams, "Ultrasonics 34, 763-765 (1996). 

(5) "Time evolution of acoustic streaming from a planar ultrasound source," J.Acoust.Soc.Am. 100, 132-138 (1996). 

(6) "Acoustic streaming induced in focused Gaussian beams," J.Acoust.Soc.Am. 97, 2740-2746(1995). 

3. Parametric loudspeaker -- Amazing audio spotlight --

The parametric loudspeaker using the self-demodulation of finite-amplitude ultrasounds has a sharper directivity than a conventional loudspeaker of the same aperture size. This is because of the formation of the end-fire array with virtual sound sources which are generated in the ultrasound beam.....

Parametric loudspeaker. Airborne ultrasound waves of 40kHz are dynamically single-sideband-modulated with audio signals, being radiated from an ultrasound emitter. The inherent nonlinearity of the air works as a de-modulator. Thus de-modulated sounds having sharp beamwidth impinge on our ear drums. We can hear those sounds!

(1) "Dynamical SSB modulator for parametric loudspeaker," Trans. on IEICE. J91-A, 1166-1173 (2008).

(2) "Parametric sound fields by phase-cancellation excitation of primary waves," 18th International Symposium on Nonlinear Acoustics, 30-33 (2008).

(3) "Dynamic single sideband modulation for realizing parametric loudspeaker," 18th International Symposium on Nonlinear Acoustics, 613-616 (2008).

(4) "A highly directional audio system using a parametric array in air (Invited talk)," 9th WESPAC in Seoul (2006). Manuscript

(5) "Principle and applications of a parametric loudspeaker (Invited talk)," IEICE Technical report, US2005-116 (2006).

(6) "Improvement of the conversion efficiency for a parametric sound system," IEICE Technical Report, US2005-11, 25-28(2005).

(7) "Current study on a parametric speaker," IEICE Technical Report, US2004-94, 17-20(2005).

(8) "Parametric sound radiation from a rectangular aperture source," ACUSTICA, 80, 332-338(1994).

(9) "Parametric loudspeaker -- Applied examples --,"Electronics and Communications in Japan, 77(1), 64-74(1994).

(10) "Parametric loudspeaker -- Characteristics of acoustic field and suitable modulation of carrier ultrasound, "Electronics and Communications in Japan, 74(9), 76-82 (1991). 

(11) "Suitable modulation of the carrier ultrasound for a parametric loudspeaker," ACUSTICA, 73, 215-217 (1991).

(12) "Development of a parametric loudspeaker for practical use," Proceedings of 10th International Symposium on Nonlinear Acoustics, Kobe , 147-150 (1984). Manuscript

4. Acoustic radiation force acting on small particles

The acoustic radiation force acting on a small particle in a plane progressive wave is investigated including viscosity of a surrounding fluid. Two particles with different densities and sound speeds are chosen for numerical examples of the radiation force, i.e., an alminum particle and a hydrogen bubble. The theory predicts that small particles move towards the sound source, although the particles always must move to the direction of wave propagation in an inviscid fluid. Movement of hydrogen bubbles is experimentally observed in a 50kHz progressive wave in water. It is found that the bubbles whose radii are smaller than 35μm move actually toward the source. Theory is in qualitative agreement with experiment...


Bubble movement

Bubble movement. A quasi-plane wave emitted from a 50kHz ultrasound source in water is propagating from the left to the right. A hydrogen bubble whose radius is quite smaller than the resonance radius (65μm) is pulled back to the source while going up due to its own buoyancy, as the left movement in the figure illustrates. To the contrary, the bubble whose radius is comparable with the resonance radius is pushed along the wave propagation (the right locus). When there is no sound, the bubble only goes up vertically (the center locus).

(1) "Acoustic radiation force on a small particle in progressive waves," J.Acoust.Soc.Jpn. 55, 619-265 (1999).

(2) "Acoustic radiation force acting on a small bubble in progressive waves," 15th ISNA (AIP Conference Proceedings, 524).

5. Harmonic imaging

The ultrasonic diagnosis world has seen dramatic improvements in image quality so far using nonlinear harmonics in both tissue and contrast agents. A diagnostic ultrasonic pulse is composed of a group of frequencies which define their spectral contents. Harmonic frequencies occur at integer multiples of the fundamental frequency, just like the second harmonic occurring at twice the fundamental frequency. The second harmonic signals have the narrower beam widths and lower levels of the side-lobes than the fundamental signal. Furthermore, the third harmonic signal exhibits the narrower and lower side-lobe levels than those of the second harmonic signal...

B-mode imaging of a ball target of 3mm in diameter. The target is located at 40mm from a focused ultrasound source whose aperture radius is 3mm and is drived at frequency 6MHz in pulse mode. Imaging using the fundamental and third harmonic echo signal(a) and the third harmonic echo only(b).

(1) "Improvement of spatial resolution for third harmonic detection using a seperately arranged transmitting/receiving ultrasonic transducer," Jpn. J. Appl. Phys. 42, 305-310 (2003)

(2) "Highly sensitive detection of third harmonic signals using a separately arranged transmitter and receiver ultrasonic transducer," Acoust. Scie & Tech. 48, 53-56 (2002).

(3) "Detection of the second harmonic signals using an ultrasonic transducer with the separately arranged transmitter and receiver,"IEICE J84-A, 1549-1556 (2001).

6. Propagation of sound waves in a yielding tube

When sound waves are propagating in an elastic tube of a rubber-like material, the tube wall vibrates in response to the inner pressure, and the consequent motion causes the generation of velocity dispersion and sound energy dissipation. A simplified theory is presented for such geometrical dispersion under the assumption that the tube wall obeys a locally reacting model. In addition, for a finite-amplitude sound wave the waveform changes abruptly owing to the effects of the medium nonlinearity and dispersion. When the dispersion works as a counteractor against the waveform steeping due to the nonlinearity, acoustic solitary waves and/or solitons are possibly generated.

Velocity dispersion (a) and sound absorption (b) in a silicone rubber tube of inner diameter 25mm and thickness 4mm. Solid lines denote the theory based on a local-reacting model.

(1) "Propagation of sound waves in viscoelastic tubes with flattening," J.Acoust.Soc.Jpn. 49, 622-628 (1993).

(2) "On the propagation of sound waves in a viscoelastic tube," J.Acoust.Soc.Jpn. 48, 840-846 (1992).

(3) "Waveform distortions of finite-amplitude acoustic waves in an elastic tube," Proceedings of the 12th ISNA in Austin, Texas (1990).

7. Theoretical and experimental investigation of the near-field acoustic levitation

This study considers the effects of acoustic nonlinearity and leakage of acoustic energy on the near-field levitation. Sound fields in the small gap and radiation pressure acting on the disk are calculated numerically based on the hydrodynamic equations such as the Navier--Stokes equation. MacCormack's scheme is successfully employed to obtain the solutions. When the gap distance is comparable with the penetration depth of a viscous wave, acoustic nonlinearity affects sound fields considerably. On the other hand, when the gap distance is larger than the penetration depth, the energy leaks out of the gap greatly. Moreover, the levitation distance of the disk is predicted to be smaller than the distance derived from linear wave theory. Measurements of the levitation distance are made for some aluminium disks with different diameter and thickness by varying the velocity amplitude of a horn-type ultrasonic vibrator. Overall, experimental results agree well with theoretical results. It has been found that the energy leakage rather than the nonlinearity has to be included in theory to accurately evaluate the near-field levitator.

Snapshot of the nearfield acoustic levitation. As you can see, an aluminium disk of 35mm in diameter and 0.5mm in thickness is actually levitated just above a piston source. The source aperture is 40mm in diameter. The right figure shows a relationship between levitation distance L and velocity amplitude U1 of the sound source. Open circles are experimental results, and the dotted line(black) indicates an analytical solution based on linear theory. The dashed line(blue) is a numerical solution using MacCormack's numerical scheme when the source radius R1 and disk radius R2 are both 10mm. The solid line(Red) indicates the solution when R1 is 20mm and R2 is 10mm. The latter simulation rather than the former is more suitable for the actual configuration of the levitation system. The analytical solution does not include energy leakage out of the gap, while both the numerical solutions include the energy leakage.

(1) "Theoretical and experimental examination of near-field acoustic levitation," J. Acoust. Soc. Am. 111, 1578-1583 (2002)

(2) "Effects of acoustic nonlinearity and energy leakage on the near-field acoustic levitation," J. Acoust. Soc. Jpn. 57, 200-209 (2001).

(3) "Theoretical study on the near-field acoustic levitation," J. Acoust. Soc. Jpn. 56, 805-814 (2000).

8. Development of a diffusive hydrophone

Experimental performance of a newly developed diffusive hydrophone (DHP) isexamined with respect to its sensitivity and spatial resolution. Although conventional hydrophones such as a PVDF needle type hydrophone have been widely used in measurements of sound pressure, they have often failed to show sufficient sensitivity when their active elements being so small as to be $0.2$mm in diameter or below, for instance. The DHP allows precise use in sound field measurements with sufficient sensitivity. The basic operation of the DHP is discussed below, taking into account of the spatial averaging effect that occurs inevitably over the hydrophone aperture.


Diffusive hydrophones

(1) "Diffusive hydrophone for ultrasonic measurements," Jpn. J. Appl. Phys.40, 3570-3571 (2001).

9. Ultrasonic absorbing characteristics of polyurethane-foam sheets

Acoustic transmission losses are measured for some highly porous polyurethane-foam sheets in the frequency range of 1kHz to 120kHz, and their equivalent circuits with distributed elements are presented to explain physically the frequency responses of the transmission losses. Many research reports on sound transmission in porous materials have been published so far. Almost all the works are, however, limited to the treatments in the audio-frequency ranges. Modifying exisiting conventional equivalent circuits, we can extend such a network analysis to an ultrasonic frequency range...


Transmission losses of a two-film acoustic filter (circles; measured data, line; theory)

(1) "Propagation characteristics of airborne ultrasonic waves in porous materials," Acoust. Sci. & Tech.29, 82-85 (2008).

10. Underwater acoustic lenses


Sample source programs for predicting numerically harmonic generation in a finite-amplitude sound beam