Hideyuki Nomura

Theoretical and experimental examination of near-field acoustic levitation

A planar object can be levitated stably close to a piston sound source by making use of acoustic radiation pressure. This phenomenon is called near-field acoustic levitation [Y. Hashimoto et al., J. Acoust. Soc. Am. 100, 2057-2061 (1996)]. In the present article, the levitation distance is predicted theoretically by numerically solving basic equations in a compressible viscous fluid subject to the appropriate initial and boundary conditions. Additionally, experiments are carried out using a 19.5-kHz piston source with a 40-mm aperture and various aluminum disks of different sizes. The measured levitation distance agrees well with the theory, which is different from a conventional theory, and the levitation distance is not inversely proportional to the square root of the surface density of the levitated disk in a strict sense.

Application of the split-step Padé approach to nonlinear field predictions

We herein propose a new theoretical approach for analyzing the nonlinear propagation of directive sound beams emitted from a planar piston source with a circular aperture. The proposed approach relies on the split-step Padé approximation, which is an efficient method for obtaining wide-angle one-way wave equations, especially in underwater acoustics. Despite including only two Padé terms in the expansion, the theory was applicable to a beam angle of up to ±40° relative to the main propagation direction, the angle of which is approximately twice that of the Khokhlov-Zabolotskaya-Kuznetsov equation, which is based on parabolic approximation. In order to demonstrate the effectiveness of the newly proposed theoretical approach, we performed an experiment using an airborne ultrasonic emitter with a circular aperture of 7.5cm in radius. We drove the emitter powerfully at a 36-kHz and 40-kHz bi-frequency signal and measured the beam patterns of the primary and secondary waves, such as parametric sounds within wide propagation angles. Excellent agreement between measured data and the corresponding numerical simulations supports the validity of the proposed model equations and the computational methods for their numerical solutions.

Parametric audible sounds by phase‐cancellation excitation of primary waves

An ultrasound source with a simple configuration is considered as a theoretical model. The source with a circular aperture consists of two coaxially arranged planar emitters: i.e., one is an inner disc emitter and the other is an outer ring emitter. The active areas of these emitters are the same. The outer diameter of the source is 20 cm. Both the emitters are driven individually at the same frequencies of 40 and 42 kHz but different phase angles. Especially, we focus on two extreme cases of the usual in‐phase driving and out‐of‐phase driving. Numerical computation using the KZK equation demonstrates that when the driving signals are in phase the difference frequency beam of a 2‐kHz wave has a candle‐flame‐like directivity. The beam has a similar directivity when the signals are out‐of‐phase by 180 degrees, although the peak of the sound pressure level decreases by few decibels. Interestingly, the second harmonic pressure level of the difference frequency reduces by ten decibels and more. Needless to say...

The feasibility of pulse compression by nonlinear effective bandwidth extension

Chirp-encoded excitation has been utilized for increased signal-to-noise ratio (SNR) in both linear and harmonic imaging. In either case, it is necessary to isolate the relevant frequency band to avoid artifacts. In contrast, the present study isolates and then combines the fundamental and the higher harmonics, treating them as a single, extended bandwidth. Pulse-inverted sum and difference signals are first used to isolate even and odd harmonics. Matched filters specific to the source geometry and the transmit signal are then separately applied to each harmonic band. Verification experiments are performed using up to the third harmonic resulting from an underwater chirp excitation. Analysis of signal peaks after scattering from a series of steel and nylon wires indicates increased compression using the extended bandwidth, as compared to well-established methods for fundamental and second harmonic chirp compression. Using third harmonic bands, a mean pulse width of 56% relative to fundamental compression and 48% relative to second harmonic compression was observed. Further optimization of the compression by altering the transmission indicated 17% additional reduction in the pulse width and a 47% increase in peak-to-sidelobe ratio. Overall, results establish the feasibility of extended bandwidth signal compression for simultaneously increasing SNR and signal resolution.

Parametric Sound Fields Formed by Phase-Inversion Excitation of Primary Waves

Two planar ultrasound projectors having identical rectangular apertures were placed side by side. Both projectors radiated bifrequency primary waves in air. The frequencies were 26 and 28 kHz, and the initial phases were different. Two driving modes were considered, namely, conventional in-phase driving and phase-inversion driving. The spatial profiles of sound pressure fields were measured along and across the sound beam axis for the primary waves and for the difference frequency wave of 2 kHz. The second and third harmonic components of the difference frequency waves were also measured. The pressure levels of the primary waves were considerably suppressed near the beam axis owing to phase cancellation when the driving signals were phase-inversed, i.e., 180 degrees out of phase. The beam pattern of the difference frequency was, however, almost the same as that for the case in which the signals were in phase. Interestingly, the harmonic pressure amplitudes of the difference frequency were reduced by more than 10 dB. The validity of the experimental results were confirmed based on their good agreement with the theoretical predictions based on the Khokhlov-Zabolotskaya-Kuznetsov equation.

Ultrasound field measurement using a binary lens

Field characterization methods using a scattering target in the absence of a point-like receiver have been well described, in which scattering is recorded by a relatively large receiver located outside the field of measurement. Unfortunately, such methods are prone to artifacts caused by averaging across the receiver surface. To avoid this problem while simultaneously increasing the gain of a received signal, the present study introduces a binary plate lens designed to focus sphericallyspreading waves onto a planar region having a nearly-uniform phase proportional to that of the target location. The lens is similar to a zone plate, but modified to produce a bi-convexlike behavior, such that it focuses both planar and spherically spreading waves. A measurement device suitable for characterizing narrowband ultrasound signals in air is designed around this lens by coupling it to a target and planar receiver. A prototype device is constructed and used to characterize the field of a highly-focused 400-kHz in-air transducer along 2 radial lines. Comparison of the measurements with numeric predictions formed from nonlinear acoustic simulation showed good relative pressure correlation, with mean differences of 10% and 12% over the center 3-dB full-width at half-maximum drop and 12% and 17% over the 6-dB drop.

Approximation of linear gain slope equalizer using Bernstein-Stancu polynomials

This paper presents a design of linear gain slope equalizer for correcting the linear gain distortion. It is based on the Bernstein-Stancu polynomials, where the well-known and readily available approximated of the desired transfer function are applied. As it is known that the Bernstein-Stancu polynomials have the several advantages. For example, there is a flexible parameter as a that can be used to adjust the magnitude response for the best result. If the parameter a equals to zero, it becomes to the classical Bernstein polynomial. In addition, the phase response is linear. As the results, the proposed method is capable of designing linear gain slope equalizer which is also shown to be efficient performance without degrading its phase characteristics. The stability of the approximated of the transfer function can guarantee with Mihailovs criterion.

Feasibility of low-frequency ultrasound imaging using parametric sound

The penetration depth of high-frequency ultrasound is limited, since the ultrasound at high frequency is much attenuated by medium viscosity. In this study, to resolve this problem, we propose low-frequency ultrasound imaging using parametric sound sources as a low-frequency directive sound. In order to verify the proposed imaging method in water, a ring type transducer with the center hole was used to transmit modulated primary ultrasounds with center frequency of 2.8 MHz, and a hydrophone placed within the hole of transmitter was used to receive chirp-modulated parametric sound echoes with center frequency of 300 kHz and a bandwidth of 400 kHz. After receiving parametric sound echo signals from a target with dimensions of several centimeters, a pulse compression technique was applied to the signals in order to improve the range resolution and signal-to-noise ratio. The obtained B mode images reveal the feasibility of low-frequency ultrasound imaging using compressed parametric sounds.

Development of a steerable stereophonic parametric loudspeaker

The parametric loudspeaker is a type of directional loudspeakers making use of the nonlinear acoustic effects. The past studies to reproduce the three-dimensional audio contents with a pair of the parametric loudspeakers have demonstrated satisfactory performance. In this paper, the steerable parametric loudspeakers are proposed to relocate the sweet spot to follow the head movement of the listener. Although the spatial aliasing effects are observed in the steerable parametric loudspeaker, they can be converted to generate multiple sound beams simultaneously. A new case of the grating lobe elimination, namely the over elimination, is studied to extend the controllable level difference between the two sound beams. The simulation results to compare the equal and Chebyshev weights are also presented in this paper.

Wavelet‐based neural networks applied to automatic detection of road surface conditions using tire noise from vehicles.

The detection of road surface conditions is an important process in efficient road management. In particular, in snowy seasons, prior information about the road conditions such as an icy state, helps road users or automobile drivers to obviate serious traffic accidents. This paper proposes a novel approach for automatically detecting the states of the road surface from tire noises of vehicles. The method is based on a wavelet transform analysis, artificial neural networks, and the mathematical theory of evidence. The proposed method employs the wavelet transform using multiresolution signal decomposition techniques. The proposed classification is carried out in sets of multiple neural networks using learning vector quantization networks. The outcomes of the networks are then integrated using the voting decision‐making scheme. It seems then feasible to detect passively and readily the states of the surface, i.e., as a rule of thumb, the dry, wet, snowy, and slushy state, automatically.