Light and Sound » Oscilloscopes, Pitch and Loudness

Turning a sound into a picture

This content is examined in Paper 2 only.

You cannot see a sound wave. The compressions and rarefactions in the air are invisible, and they pass far too quickly to follow. To study a sound we turn it into a picture on a screen. That takes two pieces of equipment: a microphone and an oscilloscope.

Remember, a microphone turns a sound into an electrical signal. Here is the whole chain, one step at a time:

1. A source vibrates and sends a sound wave through the air.
2. The sound wave reaches the microphone. The changing pressure of the air makes a part inside the microphone vibrate in step with the sound.
3. The microphone produces a small voltage that changes in the same pattern as the sound. This is the electrical signal.
4. Wires carry the signal to the input of the oscilloscope.
5. The oscilloscope draws a line on its screen that shows how the voltage changes with time. This line is the trace.

Key terms:

• Oscilloscope: an instrument that displays, as a line on a screen, how a voltage changes with time.
• Pitch: how high or low a sound seems to a listener.
• Loudness: how loud or quiet a sound seems to a listener.

What the trace shows

The oscilloscope screen is covered by a grid of squares. Think of it as a graph that draws itself.

• Up and down the screen is the voltage from the microphone.
• Across the screen, from left to right, is time.

With no sound, the microphone gives no signal and the trace is a flat line across the middle of the screen, on the zero line. When a steady, pure note reaches the microphone, the voltage rises and falls again and again, so the trace becomes a smooth, regular wave, equally above and below the zero line.

To draw a trace in the exam, rule a horizontal zero line across the middle of the grid. Then draw a smooth wave along it, with every crest the same height above the line and every trough the same depth below it, and every wave the same width.

Pitch and frequency

The pitch of a sound depends on the frequency of vibration of the source. A source that vibrates more times each second makes a sound wave of higher frequency, and we hear a higher pitch.

• Higher frequency of vibration = higher pitch.
• Lower frequency of vibration = lower pitch.

A xylophone shows this well. The short bars vibrate at a higher frequency than the long bars, so the short bars give the high notes. In the same way, the keys at the right-hand end of a piano make their strings vibrate at higher frequencies than the keys at the left-hand end, so the notes get higher as you move to the right.

On the oscilloscope, a higher frequency means each wave takes less time. Time is across the screen, so each wave is narrower and more complete waves fit across the screen. A lower pitch gives fewer, wider waves.

Loudness and amplitude

The loudness of a sound depends on the amplitude of vibration of the source. A source that vibrates through a bigger distance pushes the air harder and makes a sound wave of bigger amplitude, and we hear a louder sound.

• Bigger amplitude of vibration = louder sound.
• Smaller amplitude of vibration = quieter sound.

Press a piano key gently and then strike the same key hard. The string vibrates at the same frequency both times, so the pitch is the same. But the hard strike makes the string vibrate with a bigger amplitude, so the note is louder.

On the oscilloscope, a louder sound gives a bigger voltage from the microphone, so the trace is taller: the crests are further above the zero line and the troughs are further below it. A quieter sound gives a shorter trace.

Comparing traces

Exam questions often show two or three traces and ask you to compare the sounds. You can only compare traces fairly if the oscilloscope settings are the same for each one. Pitch and loudness are separate: one can change without the other.

Change to the sound	Change at the source	Change to the trace
Louder, same pitch	Bigger amplitude, same frequency	Taller, same number of waves across the screen
Quieter, same pitch	Smaller amplitude, same frequency	Shorter, same number of waves across the screen
Higher pitch, same loudness	Higher frequency, same amplitude	More waves across the screen, same height
Lower pitch, same loudness	Lower frequency, same amplitude	Fewer waves across the screen, same height
Higher pitch and louder	Higher frequency, bigger amplitude	More waves across the screen and taller

Exam-style question

A student plays notes on a recorder in front of a microphone. The microphone is connected to an oscilloscope. The oscilloscope settings are not changed.

Trace X shows 3 complete waves across the screen. Each crest is 2 squares above the zero line.

Trace Y shows 6 complete waves across the screen. Each crest is 2 squares above the zero line.

Trace Z shows 3 complete waves across the screen. Each crest is 4 squares above the zero line.

(a) State what the microphone does. [1 mark]

(b) State the quantity shown (i) up the screen and (ii) across the screen. [2 marks]

(c) Compare the sound that gives trace Y with the sound that gives trace X. [2 marks]

(d) Compare the sound that gives trace Z with the sound that gives trace X. Give a reason for each point you make. [3 marks]

(e) Another student says, 'The trace proves that sound is a transverse wave.' Explain why this student is wrong. [2 marks]