Edexcel A Level (IAL) Physics-2.6 Core Practical 4: Investigating the Speed of Sound- Study Notes- New Syllabus

CORE PRACTICAL 4: Determine the Speed of Sound in Air Using a 2-Beam Oscilloscope

This practical uses a signal generator, loudspeaker, microphone and a 2-beam oscilloscope to measure the speed of sound in air by comparing the phase difference between two waveforms.

Aim

To determine the speed of sound in air using the relationship:

$ v = f\lambda $

The wavelength \( \lambda \) is measured using phase changes on the oscilloscope.

Apparatus

• Signal generator
• Loudspeaker
• Microphone
• 2-beam (dual-trace) oscilloscope
• Ruler or measuring tape
• Connecting leads and stands

Theory

The signal generator produces a sound wave of known frequency \( f \). The speaker emits this sound. The microphone picks up the wave and displays it on the oscilloscope.

As the microphone is moved back and forth, the wave on the screen shifts in phase. A shift of one full cycle corresponds to moving the microphone by one wavelength.

1 full cycle shift ↔ microphone moved by \( \lambda \)

Then use:

\( v = f\lambda \)

Procedure

1. Connect the signal generator to the loudspeaker.
2. Set the signal generator to a frequency between 1–5 kHz.
3. Connect the microphone to the second channel of the oscilloscope.
4. Display both:
   • Signal generator output (reference wave)
   • Microphone signal (received wave)
   on the screen simultaneously.
5. Adjust oscilloscope to clearly see both sine waves.
6. Move the microphone slowly towards/away from the speaker.
7. Observe the movement of the microphone trace relative to the reference waveform.
8. Find two positions where the waves line up exactly in phase (peak over peak).
9. Measure the distance moved between these two positions → this is one full wavelength \( \lambda \).
10. Repeat to obtain several values and average them.

Data Analysis

• Calculate the wavelength:

  \( \lambda = \text{distance between in-phase positions} \)

• Use the known frequency from signal generator and compute:

  \( v = f\lambda \)

• Compare result with accepted value:

  Speed of sound in air ≈ \( 340\, \mathrm{m\,s^{-1}} \)

Sources of Error

• Difficulty in identifying exact phase alignment.
• Reflections causing standing waves.
• Background noise affecting microphone reading.
• Inaccurate distance measurement.
• Temperature variations affecting sound speed.

Improvements

• Use sound-damping materials to reduce reflections.
• Take multiple readings for better averaging.
• Use a digital oscilloscope for higher precision.
• Record temperature to adjust theoretical sound speed.