# Sound waves

##### Demonstration

A microphone shows the standing wave pattern in the space between a loudspeaker and an acoustically ‘hard’ reflective wall.

#### Apparatus and materials

Loudspeaker

Signal generator, low frequency

Microphone

#### Health & Safety and Technical notes

A video showing the use of an oscilloscope is freely available at the National STEM Centre eLibrary. There is also a video showing the use of a signal generator.

#### Procedure

a Connect the loudspeaker to the signal generator and drive it at a suitable frequency (2000 to 4000 Hz). Connect the microphone to the input of the oscilloscope by long leads. Set the AC–DC switch to AC, the time-base to l00 micro-seconds per cm and the gain to maximum (or whatever is appropriate for the microphone used).

b Slowly move the microphone away from the speaker to show the change in amplitude with distance.

c Set the loudspeaker and the microphone facing a ‘hard’ wall, and move the microphone back and forth in the space between the speaker and the wall to show amplitude maxima and minima. You may need to adjust the frequency carefully to produce a standing wave pattern.

#### Teaching notes

1 You may need to revise with students the key features of sound waves, or to explain what part each piece of apparatus plays.

2 Reflections from walls and bench tops can be troublesome in a lab. Sound experiments are much better if done in the open.

3 How Science Works Extension: If your students are familiar with standing waves, nodes and anti-nodes, they could be set the task of investigating the pattern of standing waves described above. Can they use it to find the speed of sound? How accurate is their final result?

Students will have to move the microphone along the line from the loudspeaker perpendicular to the reflecting surface. They should measure across as many nodes or anti-nodes as possible. Which is easier to detect, a node or an antinode? A microphone is quite a large object, so how can they define its precise position?

Measurements of wavelength (= 2 x distance between adjacent nodes) at different frequencies will allow students to calculate the speed of sound (= frequency x wavelength), but a graphical method is better. Discuss how this reduces the uncertainty in the final value.

This experiment was safety-checked in February 2006