A parametric loudspeaker which utilizes the ultrasound can transmit the audible sound to a particular area. However, the sound reproduced by a parametric loudspeaker contains the harmonic distortions because the sound is demodulated by the nonlinearity in the air. Thus, measuring the harmonic distortions is required to evaluate the sound quality of a parametric loudspeaker. Measurement with a sine wave has used as the method to measure harmonic distortions. The harmonic distortion is measured by analyzing the integral multiplication frequency of a reproduced sine wave. Many measurements by using sine waves with each different frequency are required to measure the wideband harmonic distortions. Therefore, measuring the wideband harmonic distortions with sinusoidal wave method requires much more time. Recently, using Log-TSP (Logarithmic-Time-Stretched Pulse) signal was proposed to measure the wideband harmonic distortions in a short time for electrodynamic loudspeakers. Thus in this paper, we attempt to measure harmonic distortion of a parametric loudspeaker by using Log-TSP signal. We carried out an objective evaluation experiment in a soundproof room. The result by using Log-TSP signal was compared with that by using sinusoidal wave method. As a result, we confirmed the result with Log-TSP is equivalent to that with sinusoidal wave method.
Recently, parametric loudspeakers have been utilized to reproduce an audible sound in a particular area by an ultrasound. However, parametric loudspeakers have lower sound quality than general electrodynamic loudspeakers because it is difficult to reproduce an audible sound at lower frequency. Moreover, the reproduced sound includes harmonic distortions causing deterioration of sound quality. Thus, in this paper, we attempt to improve sound quality of parametric loudspeakers. It depends on a modulation technique for designing the amplitude modulated wave. Therefore, we propose a new modulation method to reproduce audible sound with high quality. The proposed method controls a weight of double sidebands for reducing harmonic distortions at higher frequency and emphasizing power at lower frequency. We carried out evaluation experiments to confirm the effectiveness of the proposed method. The results confirmed that the proposed method could improve sound quality of parametric loudspeakers.
Noise reduction methods have been proposed for various large noises. However, we often perceive unpleasant feeling even if the small noise arises in quiet indoor environments. To overcome this problem, we focus on auditory masking which is phenomenon that human can't hear a target noise by composite sound. In this paper, we propose the unpleasantness reduction method based on auditory masking for indoor noise. In the proposed method, we discuss how to design the artificial sound for noise control. Furthermore, we develop an active unpleasantness reduction system based on the proposed method. As a result of evaluation experiments, we could confirm the effectiveness of the development system.
Parametric loudspeaker, which utilizes an ultrasonic of non-linear interaction, is developed to achieve audio spot. The parametric loudspeaker has sharper directivity, but reflections and intercepts by emitted sounds become severe problems. This is because reflections and intercepts lead to an invasion of privacy, and become noise to other listeners except a target listener. Principle of the parametric loudspeaker can formulate as non-linear interaction of carrier and sideband waves in emitted ultrasonic sounds on air. This suggests that we can design audio spot by individually emitting the carrier and sideband waves. In the present paper, therefore, we propose the design method of audio spot with the separating emission of the carrier and sideband waves. More specifically, the audible sound is demodulated at an area where the carrier and sideband waves individually emitted from each parametric loudspeaker are overlapped. We carried out evaluation experiments to measure sound pressure level (SPL) of demodulated audible sound. In addition, we evaluated the speech articulation of the demodulated audible sound with the proposed method. As a result of evaluation experiments, we confirmed the effectiveness of the proposed method.
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