Speaker And Candle
To demonstrate that sound waves cause the air to vibrate longitudinally.
Loudspeaker, oscillator, and mounted candle.
A lighted candle is placed directly in front of the center of a large loudspeaker, which is operating in the 10 Hertz range. The motion of the candle flame is longitudinal, following the motion of the air, illustrating the longitudinal nature of sound waves.