A method of measuring the velocity amplitude of sound waves propagated in hollow tubes is described. A conducting quartz fiber is stretched across a cross section of the tube where the latter is pervaded by the magnetic field of a permanent magnet. The potential difference across the oscillating fiber is recorded on an oscilloscope. Velocity amplitudes have been measured for frequencies up to about 20 000 cps. An approximate relation is derived for the velocity amplitude as a function of the generated potential. It has been found that the method is much freer from disturbances over the above frequency range than a similar method in which the cooling effect by the sound wave on a current heated wire or fiber is used. Some observations made during the production of quartz fibers sputtered with gold are reported.