Sound interference illustrated with two speakers connected to a single signal generator
Sound waves will add in some places and cancel in others.
NCEA & Science Curriculum
Connect the speakers to a single signal generator using a BNC T connector or suitable banana cables. Point the speakers forward. Turn the table back and forth. Nodes and antinodes will pass over the students in the classroom or lecture theatre.
As an alternative, get a couple of long coax cables (5 meters each) and have two students hold the speakers far apart facing each other. Have students walk between the speakers finding loud and soft spots.
This does not work as well as one might guess. Sound reflects off walls, floors, and ceilings so cancellation is incomplete. The non-linear response of the human ear results in the incomplete cancellation sounding not much less than the incomplete addition! Works best in a large room with carpet and/or soft seats. Anyway, it works well enough to make the point. Try a range of frequencies around 100-300 Hz.
The apparatus shown above is meant to show spatial interference and beats on the same day.
Bring an extension cord so you can move the trolley around!
Individual teachers are responsible for safety in their own classes. Even familiar demonstrations should be practised and safety-checked by individual teachers before they are used in a classroom.
Combining Sounds Software
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