Acoustics – Sound-modifying means
Patent
1986-04-28
1989-04-25
Fuller, Benjamin R.
Acoustics
Sound-modifying means
181 30, 181148, 181151, 181155, 381158, 381160, E04B 199
Patent
active
048239089
DESCRIPTION:
BRIEF SUMMARY
FIELD OF TECHNOLOGY
The present invention relates to a parametric loudspeaker system utilizing the nonlinearity of air relative to an ultrasonic wave for reproducing audible sounds having a super directivity and is intended to provide, in the first place, a method for intercepting powerful ultrasonic waves, secondly a method for minimizing the depth by the use of a reflective plate, thirdly a method for obtaining an arbitrary directivity by providing an ultrasonic wave radiator or the reflective plate with a movable mechanism, and fourthly a directional loudspeaker system wherein a parametric loudspeaker is combined with any other loudspeaker.
BACKGROUND ART
In the field of public address systems, one of the most important problems is to freely control the directivity of sound. In particular, as noise pollution has recently become a social problem, demands have increased for a direction variable, or direction controlled, loudspeaker system. However, since the wavelength of a sound wave is extremely long as compared with light, it has been difficult to realize a loudspeaker system having super directivity like a spot-light while a wide directivity can be readily realized.
Hitherto, a horn loudspeaker has been mainly used to sharpen the directivity, but there is a drawback in that a gigantic horn is necessitated to sharpen the directivity to low frequencies such as the voice band.
On the other hand, a loudspeaker system utilizing the nonlinear interaction between finite amplitude ultrasonic waves by the nonlinearity of a medium (parametric loud-speaker) has recently drawn attention because it can give super directivity as compared with the conventional system (Japanese Laid-open Patent Publication No. 58-119293). However, chiefly for the following reasons, the parametric loudspeaker has not long been used in practice.
(1) Because of a low conversion efficiency, an extremely powerful ultrasonic wave is required to reproduce an audible sound of practically acceptable level, and when listeners are subjected directly to this powerful ultrasonic sound, harm such as hearing impairment will occur.
(2) Since a space which is called a parametric array is required to reproduce an audible sound from the ultrasonic wave, the loudspeaker system is increased in length and the space for installation is limited.
(3) Because of the low conversion efficiency, the use of a very bulky and expensive ultrasonic wave radiator is required in order to cover a large listening area.
(4) As is the case with the conventional loudspeaker, the directivity cannot be freely controlled.
In order to control the directivity of a loudspeaker, a loudspeaker having super directivity is necessary in the first place. This is because, if the super directivity is realized, any directional characteristic can be realized by combination therewith. Hitherto, as a loudspeaker having super directivity, a horn loudspeaker has been used chiefly. This is, as shown in FIG. 1, a version wherein an acoustic tube 2 having its cross-sectional area varying gradually, which is called a horn, is fitted frontwardly to a dynamic electroacoustic transducer 1 which is called a driver. However, the directional characteristic of the horn loudspeaker depends mainly on the shape of a horn side wall 3 and the length of the horn, and there is a problem in that an extremely long horn is necessary in order to have super directivity at a low frequency. It is to be noted that 3a represents a movable side wall.
On the other hand, the parametric loudspeaker, which is a sound reproducing system utilizing a nonlinear effect, is capable of realizing super directivity comparable to the conventional loudspeaker utilizing a linear phenomenon, even though it has a radiating surface area of a size equal to one tenth of that of the conventional loudspeaker. Hereinafter, the fundamental principle of the parametric loudspeaker will be described with reference to FIG. 2.
In FIG. 2, 4 represents a source of an audio signal to be reproduced, 5 represents a high frequency oscillator used in a
REFERENCES:
patent: 2064911 (1936-12-01), Hayes
patent: 3908095 (1975-09-01), Jinsenji
patent: 4233477 (1980-11-01), Rice et al.
Iwasa Mikio
Kimura Youichi
Nakamura Akira
Tanaka Tsuneo
Fuller Benjamin R.
Matsushita Electric - Industrial Co., Ltd.
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