Amplify with Air

FIELD: Physical Science DURATION: 45 min.
OBJECTIVE: Students will observe the effects of different volumes of air on the amplification of a sound.
METHOD: Students observe the loudness of a tuning fork while varying the volume of a container amplifying it.
KEY VOCABULARY: Sound wave, amplification, loudness. GROUP SIZE: Any

BACKGROUND:

Sound waves travel through the air from where they are created, such as a by school bell, clapping hands, or tuning fork, to our ears where we hear them as sounds (or noises). Oscillating sound waves are created by musical instruments or anything with a steady tone.

We can describe oscillating sound waves pretty well with two main qualities: frequency and wavelength, or the pitch and the 'shape'. Frequency is the pitch of the sound and wavelength is the physical length of one full cycle of the wave. All sound waves travel at the same speed in the same air (approximately 340 meters per second).

Because oscillating sound waves have a particular shape, they will fit well into spaces that have the same shape. The wavelength of the sound wave determines the shape. The better the fit, the louder the sound. A cello is a large instrument because low notes fit better inside it and thus will be naturally louder for deep sounds. A violin on the other hand is much smaller and can generally produce louder, richer high notes.

We can amplify any sounds by providing a space they fit well in. The sound wave from an "A" tuning fork has a very particular wavelength: 77 cm (30 in). It will sound loudest being played into this length container.

In this activity you will find the shape of the sound waves created by an "A" tuning fork by listening for loudness. When the length of the tube equals the wavelength or half the wavelength of the wave, you'll here the loudest amplification.

MATERIALS (per group):

Can opener - shared
Packaging tape
3 empty cans - 19oz (540mL)
pail or large container
an "A" tuning fork (440 Hz)
Tap water

PROCEDURE:

1. Remove both ends of all the cans.
2. Tape the cans together, end-to-end, to make one long tube.
3. Fill the pail with water.
4. Strike the tuning fork on a hard surface an listen to the tone. Notice how loud it is.
5. Put one end of the long tube in the water.
6. Strike the tuning fork again and hold over the open end of the long tube. Is it louder or softer?
7. Try again but now move the tube up or down in the water while listening to the fork. What makes it louder and what makes it softer?
8. Find the optimum height of the tube for the loudest sound.

EXTENSIONS:

1. MEASURING WAVELENGTH: When the optimal height of the tube is found, measure the distance from the water's surface to the top of the tube. This height is close to half the wavelength of the sound wave for "A".
 
2. MULTIPLE TONES: Use tuning forks for different notes and compare the relative heights of the tube that give the loudest sound. Does a higher pitch need a higher tube or lower?


Created April 24, 1997 by Danica Nuccitelli.
Maintained by
danica@mills.edu.
Last updated April 24, 1997.