Displacement Pickup for Measuring the Motion of a Loudspeaker Cone at Many Points
K.R. McLachlan · The Journal of the Acoustical Society of America · 1956
In the experimental investigation of the response of a loudspeaker a knowledge of the motion of the cone at many points is required. Such measurements present special difficulties, which arise from the shape of the loudspeaker and from the fact that the mechanical and acoustical loading of the cone, due to the measuring device, must be negligible. A method of measurement has been developed in which a beam of light is modulated by the motion of the cone and the result displayed on a cathode-ray tube. The sensing unit is automatically scanned over a radius of the cone and some 30 points on any radius may be sampled. The results are recorded and either the steady state or transient response at the selected points can be obtained. The equipment is calibrated statically and tests have shown that this calibration is valid up to at least 20 kc/sec. Vibration amplitudes from ±0.001 in. to ±0.125 in. can be measured with a linearity of better than 5%. In addition to the measurement of the response of loudspeakers, the equipment is useful in many vibration problems where the mechanical impedance of the vibrating system precludes the use of conventional vibration pickups.