The directivity of an ultrasonic transducer is defined as the angle at which ultrasound intensity decayed -3dB at specific distance from the transducer, with respect to the maximum intensity defined as zero dB in the main emitting direction. The directivity angle of an ultrasonic transducer is affected by the wavelength of the ultrasound and the dimension of the emitting surface. The higher the frequency, the smaller wavelength and the narrower directivity; the larger the dimension of the emitting surface, the smaller directivity. For example: if the measurement distance is 30 cm and the ultrasound directivity is 8°, this translates to a beam width of 8.7 cm.