Laser vibrometry

ISVR Consulting has a state-of-the art Laser Doppler Vibrometer (LDV) which is used to measure the vibration of a variety of structures where a non-contact method is preferable to the traditional use of an accelerometer. Such a system is highly sophisticated and can be used to assist in resolving noise and vibration problems in the design, manufacturing, research and development of structures.

The main advantage of an LDV is that the laser beam can be targeted to areas which are difficult to access, or which are simply too small to attach a typical accelerometer.  The LDV also has the advantage of avoiding the mass-loading effect of the accelerometer on the structure. Furthermore, it can be used to measure at very high, even ultrasonic, frequencies (up to 80 kHz in our in-house setup).

The LDV uses an optical transducer, working on the doppler principle, whereby the instrumentation measures a frequency shift of the back-scattered light from the vibrating surface.  Measurements can be carried out at fixed points on the surface, or a fully automated scanning measurement grid can be programmed into the LDV dedicated software.  Once the vibration (in terms of magnitude and phase) is mapped on the surface, then the operational deflection shapes, for instance, can be visualised at each frequency of interest.   The instrumentation also contains a high quality camera which provides an image on which to superimpose an animated version of the vibration.

A typical application of our LDV is illustrated below, where the vibration modes of a musical gong are revealed.  In this particular example, three typical out-of-plane vibration modes of the gong are shown as velocity contours in the still images, along with corresponding two-dimensional animated displays.  In the animations, green areas are moving away from, and red areas are moving towards, the camera.

 

Gong mode contours at 33.75 Hz

Gong velocity contours at 33.75 Hz

Gong vibration mode at 33.75 Hz

 

Gong mode contours at 88.75 Hz

Gong velocity contours at 88.75 Hz

Gong vibration mode at 88.75 Hz

 

Gong mode contours at 155 Hz

Gong velocity contours at 150 Hz

Gong vibration mode at 150 Hz

 

ISVR Consulting is capable of measuring the vibration response of many other types of structure, such as the  excursion of loudspeaker diaphragms, car door panels and other  surfaces that require a non-contacting measurement method.