PURPOSE: To demonstrate that sound waves cause the air to vibrate longitudinally.

DESCRIPTION: 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.

Below are photographs of the candle flame with the frequency of the oscillator set to the loudspeaker resonance frequency to obtain the largest effect: looking backwards along the direction of propagation (left), and looking across the direction of propagation to see that the motion of the air is longitudinal (right).

Videos of this effect can be seen by clicking your mouse on the appropriate link below:

1. Side view at speaker resonance frequency, then setup rotated from side view to direct view toward speaker then back to side view: mpg video (5.6 MB) or wmv video (942 kB)
2. Front view at speaker resonance frequency, video zooms in on flame, and speaker is rotated from front to side view: mpg video (3.9 MB) or wmv video (715 kB)

SUGGESTIONS: See Question of the Week #242 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA 3B30.40)

EQUIPMENT: Loudspeaker, oscillator, and mounted candle.

SETUP TIME: 5 min.