Pattern Approval Tests of Sound Level Meters: The Influence of Mechanical Vibrations on their Performance

Mechanical vibrations of a base which, during measurement, affect the sound level meter (SLM) via a tripod can have a considerable influence on the measuring value displayed. Especially vibrations along the main measuring direction of the microphone (reference direction) cause a mechanical stimulation of the microphone and a considerable reduction of the linear measuring range. To check the vibrational sensitivity, an automatic measuring assembly has been set up in which two sound level meters are exposed simultaneously to a sound field of constant frequency. The displayed A-weighted values of both meters are compared with each other while one of the two meters is stimulated by an electro-dynamic shaker to vibrate at a band limited acceleration of 1 m/s². The stimulation is applied in two directions: (1) in the reference direction of the microphone and (2) turned 90^° in the direction perpendicular to the reference direction. The frequency of the stimulation varies between 31.5 Hz and 1 kHz. The result of the measurement shows that the lower limit of linearity under the specified conditions of mechanical vibration may be shifted to a level of up to more than 75 dB(A).