Frequency response of various materials on REAL
In addition to the mask and throat surface, REAL can measure the vibration of plastic bags, papers etc. The optical characteristics and vibration characteristics of the surfaces directly determine the REAL measurement results. The scattering characteristics of the surfaces will determine the collected laser power and hence the SNR of REAL. The vibration characteristics including frequency-dependent damping of the surfaces are key to the quality of REAL audios. The frequency response of mask, plastic bag and paper (Figure S2b) on REAL is studied (Figure S2a). These three surfaces are excited by a loudspeaker playing chirp audio (0.1~7 kHz). In Figure S2c, the frequency responses measured by REAL on three respective surfaces is shown. It can be seen that there are sharp peaks and valleys at some frequencies, which are related to the resonances of sound waves on these surfaces. Both plastic bag and paper perform better than mask, possibly due to the more elastic nature.