In volume plethysmography, volume changes are converted into what kind of signal for recording?

• A. Digital audio signal
• B. Thermal signal
• C. Analog pressure signal
• D. Infrared signal

Answer: (C)

In volume plethysmography the movement of air and the resulting volume changes inside the sealed chamber cause small pressure fluctuations. A pressure transducer detects these changes and converts them into an electrical signal that is analog—typically a voltage proportional to pressure. This pressure-time signal is what gets recorded and later analyzed. It isn’t a digital audio signal, nor a thermal or infrared signal, because the fundamental measurable quantity here is pressure generated by gas movement, not sound, temperature, or light.