“…Because the mass loading of the accelerometer could cause errors, particularly when testing light or small structures or highly damped nonlinear materials (D'Emilia et al, 1989;Castellini et al, 2006), true tissue vibration cannot be measured accurately with contact sensors (Terasaki et al, 2001). To overcome this drawback, alternative noncontact sensors, such as microphones (Mendoza et al, 2012;Costa et al, 2011) and the laser Doppler vibrometer (LDV), were introduced to provide a measurement of vibration response. The LDV measures the vibration velocity of the object via the Doppler shift of the laser beam frequency, which has a flatter response function over a wider range of frequencies compared with the microphone and shows superior performance over the microphone on its resistance to ambient noise (Taniwaki and Sakurai, 2010;Kondo et al, 2014).…”