Core practical 4: speed of sound in air

Method

• Connect a signal generator to a loudspeaker and to channel 1 of the oscilloscope.
• Connect a microphone in front of the speaker to channel 2. Both traces have the same frequency.
• Move the microphone directly away from the speaker along a metre rule until the two traces are in phase. Note the position.
• Keep moving it until the traces have come back into phase several times (say n times) and measure the total distance moved. Each return to in phase means the microphone has moved one wavelength.

Calculation

λ = total distance / n, and v = fλ, with f read from the signal generator (or found from the oscilloscope: T = number of divisions for one cycle x time-base setting, f = 1/T). Repeating for several frequencies and plotting λ against 1/f gives a straight line of gradient v.

Improving accuracy

• Measure over many wavelengths to reduce the percentage uncertainty in λ.
• Reduce reflections (work away from walls, or use soft materials) so the microphone picks up the direct sound.
• Check the frequency on the oscilloscope rather than trusting the generator dial.

Worked example

At 2.0 kHz the microphone is moved until the traces have come back into phase 5 times, a total distance of 0.85 m. Find the speed of sound.

1. λ = 0.85 / 5 = 0.17 m
2. v = fλ = 2000 x 0.17

Answer: v = 340 m/s

Check yourself

1. Sound of frequency 1500 Hz has a measured wavelength of 0.228 m. Find the speed of sound in m/s.
   Show answer

   342 m/s. v = fλ = 1500 x 0.228 = 342 m/s.

2. At 2000 Hz, the microphone moves 0.68 m as the traces come back into phase 4 times. Find the speed of sound in m/s.
   Show answer

   340 m/s. λ = 0.68 / 4 = 0.17 m; v = 2000 x 0.17 = 340 m/s.

3. On the oscilloscope, one cycle spans 2.5 divisions with the time-base at 0.20 ms per division. Find the frequency in Hz.
   Show answer

   2000 Hz. T = 2.5 x 0.20 ms = 0.50 ms; f = 1 / 0.00050 = 2000 Hz.

4. The microphone is moved until the two traces are back in phase for the first time. How far has it moved?
   • One wavelength
   • Half a wavelength
   • A quarter of a wavelength
   • Two wavelengths
   Show answer

   One wavelength. Points one wavelength apart oscillate in phase.

5. Why is the microphone moved through several wavelengths rather than one?
   • To make the sound louder
   • To change the frequency
   • To reduce the percentage uncertainty in the wavelength
   • To remove reflections
   Show answer

   To reduce the percentage uncertainty in the wavelength. The uncertainty in reading positions stays the same, but the measured distance is larger.

6. A graph of λ (y-axis) against 1/f (x-axis) is a straight line through the origin. What is its gradient?
   • The speed of sound
   • The frequency
   • The period
   • 1 / speed of sound
   Show answer

   The speed of sound. λ = v x (1/f), which has the form y = mx with m = v.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Physics specification (XPH11/YPH11), Issue 3
• Pearson qualifications: International Advanced Levels Physics (2018)
• Cognito: Edexcel International A level Physics specification and core practicals

Last updated 7 November 2026. Spotted a mistake? Tell us.