Laser Vibrometry

A Laser Vibrometer is an essential tool in any Loudspeaker Research and Development operation. The education of a loudspeaker designer is incomplete without access to this ‘dynamic microscope’, which allows the user to view microscopic movements on a vibrating surface. In principle, we can measure ‘anything that moves’ (such as the human tympanic membrane, or eardrum, and instruments such as violins) but we usually concentrate on loudspeakers! A beam of laser light is focused upon the vibrating surface. This beam has a carrier frequency, typically tens of MHz. The light reflected from the surface is Doppler frequency-shifted by an amount proportional to the velocity of the surface.

By guiding the light via a pair of orthogonal optical-grade mirrors the beam can be scanned across the entire vibrating surface, be it a diaphragm or a cabinet panel, and the Engineer can study the mechanical behaviour of the entire surface (the system detects velocity, so the shape of the loudspeaker is inherently removed from the display). By gaining an understanding of the nature of the vibration he or she can, if necessary, diagnose and take remedial action at an early stage of the driver or cabinet development.