An oscilloscope displays an electrical signal versus time. In an acoustic wavelength measurement, the microphone trace supplies the tone's frequency and oscillation amplitude as the detector changes position.
A connection that couples a measured electric potential difference into an oscilloscope. Its electrical resistance and capacitance contribute to instrument loading.
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FNIRSI 1014D review by Kerry Wong (2022)
Source. One of the cheapest oscilloscopes available at the time.