Light and Sound » Practical: Measuring Sound Frequency

The idea of the practical

This content is examined in Paper 2 only.

In Oscilloscopes, Pitch and Loudness you saw that more waves across the screen means a higher frequency. This practical goes one step further: you use the trace to measure the frequency in hertz.

The plan is simple. Time runs across the oscilloscope screen, so the width of one complete wave tells you the time period of the sound. Once you have the time period, the frequency comes from frequency = 1 / time period.

Key terms:

• Division: one square of the grid on the oscilloscope screen.
• Time base: the oscilloscope setting that fixes how much time each division across the screen stands for, for example 2 ms per division (2 ms/div).
• Gain: the oscilloscope setting that fixes how many volts each division up the screen stands for. It changes the height of the trace.
• Tuning fork: a metal fork with two prongs that vibrates at one fixed frequency when struck. The frequency is marked on it.
• Signal generator: a device that produces an electrical signal of a frequency you choose with a dial. Connected to a loudspeaker, it gives a steady note.

Apparatus

You need a microphone connected by a cable to the input of an oscilloscope, and a source of a steady, pure note. There are two common choices.

To draw the apparatus in the exam, draw the source on the left (a loudspeaker joined to a box labelled 'signal generator', or a tuning fork). Draw a microphone a short distance in front of it. Then draw a wire from the microphone to a box with a screen, labelled 'oscilloscope'.

Method

1. Connect the microphone to the input of the oscilloscope and switch the oscilloscope on. With no sound, check that the trace is a flat line on the zero line across the middle of the screen.
2. Make the sound. Strike the tuning fork on a cork or rubber block and hold the prongs close to the microphone. Or switch on the signal generator and loudspeaker and place the microphone in front of the loudspeaker.
3. Adjust the gain until the trace is as tall as possible while still fitting on the screen.
4. Adjust the time base until the trace is steady and shows a few complete waves across the screen.
5. If the oscilloscope has a button that freezes the trace, press it. This is very helpful with a tuning fork.
6. Count the number of divisions across the screen for one complete wave, for example from one crest to the next crest. Better still, count the divisions for several complete waves.
7. Write down the time base setting.
8. Calculate the time period, then the frequency.
9. Repeat and find a mean. With a signal generator, repeat for other frequencies.

Why adjust the settings? If the waves are squashed together or the trace is tiny, you cannot read the divisions accurately. A tall trace with a few wide waves is the easiest to measure.

From the trace to the frequency

There are two steps.

Step 1: time period = number of divisions for one complete wave × time base setting

Step 2: frequency = 1 / time period, f = 1 / T

Remember, f = 1 / T is given on the formulae page, and it only gives hertz if the time period is in seconds. Time base settings are usually in milliseconds, so divide by 1000 before step 2.

Measuring across several waves

You can only read the screen to about the nearest 0.1 of a division. If one wave is only 2 divisions wide, an error of 0.1 of a division is a large fraction of your reading. If you measure across several waves instead, the same small reading error is shared out between all of them, so the error in the time for one wave is much smaller.

Count carefully. From the first crest to the sixth crest is 5 complete waves, not 6. Call the first crest zero.

Results and checking

With a signal generator you can test several notes and compare each answer with the dial. You change the time base each time so that a few waves fill the screen.

Frequency on the dial in Hz	Time base in ms/div	Divisions for one wave	Time period in ms	Measured frequency in Hz
125	2	4.0	8.0	125
320	1	3.1	3.1	323
800	0.5	2.5	1.25	800
1000	0.2	5.1	1.02	980

The measured values are close to the dial values but not always exactly the same. Small differences come from reading the divisions on the screen, and the dial on the signal generator may not be perfectly accurate either.

Variables: when you test several notes, the frequency set on the signal generator is the independent variable. The time period measured from the trace is the dependent variable. Keep the same microphone, loudspeaker and oscilloscope throughout.

Safety: this is a low-risk practical. Keep the loudspeaker at a comfortable volume to protect your hearing. Strike the tuning fork on a cork or rubber block, never on a hard bench or near your eyes. The equipment runs from the mains, so keep liquids away and cables tidy.

Exam-style question

A student investigates the frequency of the sound from a tuning fork. She holds the vibrating fork near a microphone connected to an oscilloscope.

The trace shows 4 complete waves in 7.8 divisions across the screen. The time base setting is 1 ms/div.

(a) Describe how the student should adjust the oscilloscope to obtain a trace that is easy to measure. [2 marks]

(b) Calculate the frequency of the sound. Give your answer to 3 significant figures. [4 marks]

(c) Explain why the student measured across 4 waves instead of 1 wave. [2 marks]

(d) The tuning fork is marked 512 Hz. Comment on the student's result. [1 mark]

(e) Suggest one advantage of using a signal generator and loudspeaker instead of the tuning fork. [1 mark]