Takashi Honda

Ultrasonic attenuation and grain‐size evaluation using electromagnetic acoustic resonance

Electromagnetic acoustic resonance is applied for determining the frequency dependence of the ultrasonic attenuation and the average grain size of low‐carbon steels. Use of a noncontacting electromagnetic acoustic transducer (EMAT) makes it possible to isolate the attenuation within the plate specimens. The method relies on the Lorentz force mechanism to couple the EMAT to the specimen surfaces and then eliminates the other losses, which may otherwise occur with the contacting piezoelectric transducers. The measurement is independent of the EMAT used, the specimen thickness, the surface condition, the lift‐off, etc., and is stable because of the noncontacting nature. First, the resonant frequencies are measured, to the accuracy of 10 Hz, by sweeping the operating frequency and obtaining the amplitude spectrum over a band in the 0.5‐ 20‐MHz range. The ringing signals are excited and received by a shear wave EMAT and then processed with a superheterodyne receiver. Second, the attenuation coefficient as a fu...

Ultrasonic diffraction from a transducer with arbitrary geometry and strength distribution

The exact solution to the Helmholtz equation with a Dirichlet boundary condition is obtained to study three‐dimensional ultrasonic diffraction phenomena and derive the numerical data of amplitude loss and the phase shift for correcting induced errors. Calculation is made for near‐field diffraction, for the rectangular transducers, and for the transducers with strength distribution on the radiating area. In the near field, where the wavelength and the propagation distance are comparable with each other, the longitudinal and shear waves undergo different diffraction. For transducers having a noncircular shape and a strength distribution on the area, both the amplitude loss and the phase shift experience different tendencies from the classical work on the circular piston source. Use of diffraction data specific to each measurement condition is then necessary to correct the errors. The calculated results are verified for pulse‐echo measurements using a shear‐wave electromagnetic acoustic transducer.

An excitation synchronous pitch waveform extraction method and its application to the VCV-concatenation synthesis of Japanese spoken words

A novel pitch waveform extraction method has been proposed. Being different from conventional pitch mark decision algorithms, such as the peak search method, this new algorithm decides excitation points based on the phase-equalized residual excited linear prediction (PE-RELP) model. A pitch waveform is extracted from two adjacent excitation intervals by using the asymmetrical Hanning window. The new pitch waveform extraction method takes advantage of being free from the extraction errors caused by the formant resonance and being fully automatic. Therefore, no manipulation is required and no roughness is heard in the pitch-modified speech sound. The superiority of the new method has been ensured by means of spectral distortion measurement and subjective quality evaluation. Finally, spoken word generation by means of VCV (vowel-consonant-vowel) waveform concatenation is demonstrated. Consequently, it has been shown that the generation of very natural-sounding spoken words is possible.

A study on natural‐sounding Japanese phonetic word synthesis based on the pitch waveform concatenation

A speech output system in which messages are composed by embedding some phonetic words in the prerecorded sentence patterns requires very natural‐sounding phonetic words to be embedded. Especially in the case where a very large or an unlimited vocabulary is required, synthesis of very natural‐sounding phonetic words would be desired. Therefore, a novel Japanese VCV‐concatenation synthesis method based on the pitch waveform concatenation has been studied. The VCV waveforms are extracted from the VCV‐balanced spoken word database and pitch waveforms are extracted by means of the excitation synchronous pitch waveform extraction method [Y. Arai et al., ICSLP 96, 1437–1440 (1996)]. The wavelength of each pitch waveform is modified depending on the pitch modification rate, and the global pitch contour of each VCV waveform is modified along with the target pitch pattern, while the micro‐prosody and the small perturbation in pitch frequency inherent in that VCV waveform are retained. The auditory tests confirmed ...