A pulse of sound gets "inverted" when reflecting off the open end of a pipe, but does not get inverted when reflecting off the closed end.

What It Shows

Due to an impedance mismatch, a sound pressure pulse traveling down the length of a pipe suffers a reflection when it reaches the end. The nature of that reflection depends on whether or not the pipe is open or closed at the end—an open end results in an inverted reflection and a closed end produces a non-inverted reflection. The initial pulse and its reflection are detected by a microphone and are displayed on an oscilloscope.

How It Works

A short pulse of sound (200-500 μs long) is generated by a loudspeaker driven by a function/pulse generator. The sound pressure pulse enters an 8-ft long (2.45 m), 4-inch diameter, PVC pipe. A microphone placed at the mouth of the pipe detects the pressure pulse as it enters the pipe as well as its reflection from the opposite end. The opposite end is either open or capped closed.

The signal from the microphone is displayed on an oscilloscope. The first photo, below, shows the pulse (reflected from the end cap) arriving about 14 ms after the initial pulse has passed the mike. It is a positive pressure pulse followed by an overshoot. That negative overshoot is the reflection from the open end of the pipe where the microphone is located. You can demonstrate that that is so by moving the microphone further into the pipe. The two reflections superimposed in the display will separate. The second photo clearly shows the inversion of the reflected pressure pulse (the condensation becomes a rarefraction) when the end cap is removed and the pipe is open. Again, the overshoot is due to the reflection from the open end where the microphone is located.

By measuring the time delay between the two pulses (14.2 ms) and the round-trip travel distance (4.9 m), you can also use this opportunity to determine the speed of sound. If you wish, you can increase the time scale on the scope and see multiple reflections as the pulse keeps bouncing back and forth inside the tube (if you do this, push the microphone into the tube as far as it will go).

Setting It Up

The pulse generator (Wavetek model 801), bookshelf speaker, and oscilloscope (Tek TDS 3014B) reside on one cart and the long PVC pipe rests on another.

The pulse generator can drive the loudspeaker directly from its 50-Ω output. The sound is not very loud, but loud enough. If you desire a better signal to noise ratio than what you see in the photographs above, you can amplify the signal with a power amp. Adjust the pulse length to be between 200 and 500 μs long and a repetition rate between 2 and 5 HZ. Trigger the oscilloscope with the TTL output of the function generator. Not only is that going to give you reliable triggering, but you can also show the change in temporal pulse position on the scope display as you move the microphone closer or further from the speaker. (If you measure the actual distance between microphone and speaker, you can then determine the speed of sound.)

The microphone of choice is the Earthworks model M30 used with the Yamaha MG10/2 mixer. Other microphone and mixer combinations will do, but be sure it's not a directional microphone (it won't be sensitive to the initial pulse coming from behind).

Comments:

It's difficult for students to conceptualize the nature of reflections from closed and open pipes—especially open pipes. "How can the sound reflect off something that isn't there?" This is a convincing demonstration of the phenomenon.