Kenshiro Takagi

Effect of moisture content on ultrasonic velocity and attenuation in woods

Abstract Ultrasonic velocity and attenuation are measured at 1 MHz in several kinds of wood as a function of the moisture content U . The results are interpreted in terms of water absorption mechanism of woods. The velocity decreases rapidly with increasing U up to a critical value U 1 at which the curve shows a sharp bend, and the decrease is very slow for U > U 1 . This critical point U 1 is in the range 20–50% depending on the species of wood and is regarded as the fibre saturation point of the wood, the moisture content at which the wood substance is saturated with absorbed water. Ultrasonic measurements thus provide us with an accurate way to determine the fibre saturation point. The attenuation curve also shows a critical change at U = U 2 , which is slightly lower than U 1 . The attenuation shows no change from U = 0 up to U 2 , but begins to increase rapidly thereafter. This point U 2 indicates the moisture content at which free water begins to enter the vacant space of wood cells.

A NEW LIGHT SCATTERING TECHNIQUE FOR A WIDE-BAND RIPPLON SPECTROSCOPY AT THE MHZ REGION

A new light scattering system was developed to investigate the propagation of liquid surface waves over a wide wave‐number range 4×104–3×106 m−1 (or 10 kHz–6 MHz in frequency). Thermal fluctuation of liquid surface displacement excites a surface tension wave called a ‘‘ripplon’’ whose behavior yields information on the dynamics of the liquid surface. The transmission diffraction method and some optical and electrical improvements are successfully applied to the light scattering system and the upper limit of the frequency measured was extended about two orders higher than the conventional one. The wave‐number dependence of the ripplon frequency and damping constant was measured for several kinds of liquids and the results agreed well with the classical hydrodynamic theory.

Ultrasonic spectroscopy in bovine serum albumin solutions

Ultrasonic absorption and velocity spectra in bovine serum albumin (BSA) aqueous solutions have been measured at 20 °C over the broad frequency range 0.1–1600 MHz in the pH range 1.5–13.2. Five different techniques were used: the plano–concave resonator, plano–plano resonator, pulse–echo overlap, Bragg reflection, and high‐resolution Bragg reflection methods. The absorption spectrum at neutral pH was well fitted to the relaxation curve assuming a distribution of relaxation frequency with a high‐frequency cutoff and long low‐frequency tail. The relaxation behavior was interpreted in terms of various degrees of hydration of BSA molecules. At acid pH’s, excess absorption over that at pH 7 was explained by double relaxation. The pH dependences of the relaxation frequency and maximum absorption per wavelength showed that the relaxation at about 200 kHz was related to the expansion of molecules and that at 2 MHz resulted from the proton transfer reaction of carboxyl group. At alkaline pH’s, the excess absorptio...

Acoustic phase conjugation by nonlinear piezoelectricity. II. Visualization and application to imaging systems

Phase conjugate waves of ultrasound were generated in PZT ceramics through nonlinear piezoelectric interaction between an incident ultrasonic field at ω and an electric field at 2ω. The amplitude reflectivity of the phase conjugator was 23% at 10 MHz. The behavior of the incident waves and the phase conjugate waves were visualized by stroboscopic schlieren technique. Time-reversal property and the automatic correction of wavefront distortion in the phase conjugate process were confirmed. A scanning ultrasonic imaging system with a PZT phase conjugator has been built. This system was used to visualize test samples composed of solid plates and phase disturbers. Images by phase conjugate reflection yielded clear figures of the solid plates in spite of the existence of phase disturbers made of agarose gel with rough surfaces, whereas conventional images showed serious distortion.

Hyper‐resolution Brillouin–Rayleigh spectroscopy with an optical beating technique

A system of optical beating spectroscopy has been newly developed for hyper‐resolution Brillouin scattering. Frequency resolution as high as 1 kHz was obtained, far beyond the limit of the classical Fabry–Perot interferometer which is commonly used in ordinary experiments. This system is useful over the frequency range from 104 to 109 Hz at present. Experiments for Rayleigh and/or Brillouin spectra were performed in water, toulene, and carbon disulfide to demonstrate the performance of this system.

Measurement of high viscosity with laser induced surface deformation technique

A technique for viscosity measurement was developed based on the principle of laser-induced surface deformation. Light incident into liquids increases its momentum due to the difference in refractive index and gives the surface an upward force as a reaction. The plane surface thus swells up and deforms, and the shape is determined so that the force is balanced with the surface tension and the gravity. On sudden laser irradiation, the deformation inevitably accompanies a viscous flow and exhibits a relaxational behavior with a delay time, which gives the viscosity. Theoretical prediction of the step-response function was given that takes surface tension waves excited by the laser into consideration. Nd–yttritium–aluminum–garnet laser with 0.6W output was focused to ∼200μm beam waist and used for the pumping. The deformation process was observed sensitively with another probe laser illuminating the activated area. This system was tested with the standard liquids for viscosity ranging from 1 to 106cSt. The r...

Vibrational double relaxation in liquid pyridine

Ultrasonic and hypersonic property of liquid pyridine has been studied at 20° and 30°C with three techniques: pulse‐echo‐overlap, high‐resolution Bragg reflection, and Brillouin scattering. The sound velocity observed over the frequency range from 3 MHz to 7 GHz suggests two relaxation regions, one centered at 762 MHz and the other at 3.9 GHz at 20°C. The velocity‐dispersions in the UHF and in the hypersonic ranges are 94 m/s and 65 m/s, respectively. The same type of vibrational double relaxation as observed in liquid benzene is hypothesized. Quantitative analysis of the relaxation strength indicates that the vibrational specific heat associated with both the lowest and the second lowest modes relaxes in the hypersonic range and all the rest, in the UHF range.

Vibrational–translational relaxation in liquid chloroform

Ultrasonic measurements were made in liquid chloroform over the frequency range from 3 MHz to 5 GHz by means of three experimental techniques, pulse‐echo overlap, high‐resolution Bragg reflection, and Brillouin scattering. The observed velocity dispersion revealed two relaxation processes, one at 650 MHz and the other at 5.1 GHz at 20 °C. They are interpreted in terms of vibrational–translational relaxation. Quantitative analysis of specific heat shows the lowest (261 cm−1) and the second lowest (366 cm−1) fundamental vibrational modes should have a common relaxation time at 50 ps and the group of all above the third mode (667 cm−1) at 290 ps. The present results are combined with recent data obtained by Laubereau et al. with the picosecond spectroscopy technique; a diagram illustrating V–T and V–V energy transfer is presented. A brief comment is given also on V–T and V–V processes in dichloromethane.

Ultrasonic and Hypersonic Studies of Relaxations in Ethanol-Water Mixtures

The sound velocity and absorption in ethanol and water mixtures are measured with a new technique utilizing high-resolution Bragg reflection in the frequency range from 150 MHz to 500 MHz. The absorption in the range 20?150 MHz as well as the hypersonic velocity in the range 3?7 GHz are also measured. Relaxation spectra over the range 10 MHz?10 GHz at the temperatures of 15? and 20?C suggest two relaxations, one at about 100 MHz and the other above 1 GHz. The relaxation strength of the former has a maximum at the ethanol concentration of about 40 wt%(?20 mole %), whereas the latter at 50 wt%. The two-state model A+Bm\rightleftarrowsABm is applied to interpret the concentration dependence of the relaxation strength at 100 MHz.

Acoustic phase conjugation by nonlinear piezoelectricity. I. Principle and basic experiments

The principle and the basic experimental results of the acoustic phase conjugation by nonlinear piezoelectricity are described. Acoustic phase conjugate waves at frequency ω are generated as a result of the interaction between incident acoustic waves at ω and pump electric fields at 2ω. An original explanation based on the modulation of sound velocity by the electric field together with the concept of time grating is given. Coupling equations for the PZT ceramics as a nonlinear material are derived. Experimental results of the sound velocity modulation and the phase conjugate generation in nonlinear piezoelectric PZT ceramics are shown. Amplitude reflectivity of the phase conjugation was 23% for the acoustic incidence at 10 MHz.