PURPOSE: To demonstrate the phase of pulses reflected from open ends and closed ends of a tube.
DESCRIPTION: A pulse is created by the leading edge of a low-frequency square wave input into a loudspeaker at one end of a plastic tube. This pulse travels along the tube and is reflected back and forth at the ends. The phase of the pulse as it reflects back and forth is observed using a microphone inserted into the center of the tube. Several reflections can be observed. It is easy to see the difference between reflection at the open end and reflection at a closed end, obtained by placing a metal plate against the end of the tube opposite the loudspeaker.
Above is a close-up view of the oscilloscope. The bottom trace is the square wave applied to the speaker (at the left end of the tube), and the top trace shows the pulse as it makes its first two passes past the microphone (at the center of the tube). Because the metal plate at the right end of the tube is a fixed end, the initial pulse reflects off the right end back to the microphone with NO phase change.
REFERENCES: (PIRA unavailable.)
EQUIPMENT: Oscillator, small speaker, plastic tube, reflecting screen, and microphone mounted on optical rail, and oscilloscope.
SETUP TIME: 10 min.
Go back to Lecture-Demonstration Home Page.