What it shows:
Two loudspeakers, separated about 1.7 meters emit the same tone of frequency 500 Hz and produce a pattern of constructive and destructive interference.
How it works:
At this frequency, the successive positions of constructive interference (maximum intensities of sound) occur approximately every two meters at a distance of 10 meters (which is roughly the middle of the lecture hall). The separation of maxima would be about 2.3 meters at 440 Hz. One way to make the interference pattern evident to the students is to place the two loudspeakers of a board and slowly rotate the board. This has the effect of sweeping the interference pattern through the audience. Students will hear successive maxima and minima of sound.
Alternatively, have the students get up and walk around to hear the maxima and minima. Then have them stop when they hear a maximum. The interference pattern will become evident from the clustering of students along the paths of constructive interference — a nice visual touch.
Setting it up:
Set the two bookshelf speakers on a 2-meter-long board. Place the PASCO function generator in the middle of the board — it is powerful enough to drive the two speakers in parallel. The board can easily be rotated on top of the lecture bench if you choose to perform the demo the first way described.
The calculation to determine the position of the maxima is the same as the derivation for Young's double-slit experiment, so it's a nice introduction to the mathematical analysis the students will encounter in the second semester. Also, if you show the ripple tank interference pattern later on, you can remind students of this experiment.