A new method for measurements of the dispersion of waves is described. The experiment gives directly the value of D = d2k/dω2(k, wave number; ω, angular frequency). The measurements of D is based on the transformation of phase modulation in amplitude modulation and vice versa, when a modulated sine wave travels through a dispersive medium. The sensitivity of the method depends only on the noise level in the receiver and it is independent of the unavoidable small random variations of the sound velocity in the medium under test. The results of experiments on ultrasonic waves in thin wires (longitudinal waves where used) and in water at 1 Me/sec (modulation frequency: 100 kc/sec) are given. The experiments were undertaken for determining the possibilities of the method in acoustics. In the case of longitudinal waves in a thin wire the dispersion can be calculated from the theory of elasticity. The value of D obtained experimentally is in a good agreement with the calculated one. The relative change of the phase velocity corresponding to the frequency interval of 100 kc/sec was about 10−4 In water, no dispersion was observed that agrees with the theory. A variation of velocity of 1 cm/sec in a frequency interval of 100 kc/sec was detectable when the receiver band was about 1 kc/sec. In order to prevent reflected waves an optical indicator of ultrasonic waves was used.

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