Quantifying the complex transmission of substrate-borne vibrations with scanning laser vibrometry

Substrate-borne vibrations are ubiquitous in nature and are used by diverse taxa to communicate and to obtain information about their environments. However, substrate-borne vibrations remain understudied compared with other sensory and signaling modalities, in part due to human sensory biases. In addition, understanding and quantifying the transmission of vibrations remains a challenging task due to it being dependent on both signal properties and properties of the substrates that the signals transmit through. Here, we provide methods for playing back and measuring the transmission of vibrations throughout a substrate. Using linear resonant actuators, we conducted playbacks of pure tones and frequency sweeps on wooden dowels and on the stems of potted Ptelea trifoliata L. (Rutaceae) plants. We used scanning laser Doppler vibrometry to measure the signals at multiple locations along the length of the dowels and plant stems. We demonstrate that playback of a frequency sweep yields more data in a shorter amount of time than multiple playbacks and measurements of pure tone signals. Our results are also consistent with previous findings showing that signals produce frequency and location specific minima and maxima (nodes and antinodes) throughout the substrates, rather than simply attenuating with distance. This results in filtering of signals, such that their spectra are unique at any given measurement location—illustrating the importance of measuring vibrations at multiple locations. We discuss the implications of such filtering phenomena for vibrationally signaling animals and the biotremologists that study them.