Science 8th grade
UNIT 11: Sound 11.4 Sounds on a screen
Science 8th grade
UNIT 11: Sound 11.4 Sounds on a screen
A sound wave is a vibration travelling through the air, or through another material. We cannot see a sound wave.
A microphone can detect sound waves. It changes them into electrical vibrations.
Question
$A+I$: 1) Who might use a microphone in their work? What would they use it for?
Seeing a trace
If you connect a microphone to an oscilloscope, it will allow you to see a picture of the sound wave, as a line on the screen. The line is called a trace.
Changing amplitude
The pictures on the right show what happens to the trace on the Oscilloscope screen when the loudness of the sound changes.
* When there is no sound (silence), the trace is a flat line. The air molecules are not vibrating.
* With a quiet sound, the trace goes up and down a bit in the shape of a wave. It has a small amplitude.
* With a loud sound, the trace goes up and down much more.
Its amplitude is greater.
Remember that we measure the amplitude from the middle of the trace to the peak (the highest point).
Question
2) Draw a diagram to show what is meant by the amplitude of a trace on an oscilloscope screen.
Changing pitch
The pitch of a sound tells you how high or low it is. The two traces shown here are for a high-pitched sound and for a lowpitched sound.
• You can see that, for a high-pitched sound, the waves shown by the trace are closer together. This is because the sound has a higher frequency. The molecules of the air are vibrating more times each second.
• For a low-pitched sound, the waves are more spread out.
They don't go up and down so often because the sound has a lower frequency.
Question
3) Draw two oscilloscope traces, side-by-side, to show the following:
a: a quiet sound with a low pitch.
b: a loud sound with a high pitch.
Activity 11.4 (Screen sounds)
SE: Your teacher will show you a microphone connected to an oscilloscope. Your task is to predict:
• what you will hear
• what you will see on the screen
when you try each of the following.
1) A quiet sound gets louder and then quieter again.
2) The frequency of a sound is increased and then decreased.
3) The microphone is moved closer to the loudspeaker, and then further away again.
4) Someone plays a note on a musical instrument.
