Velocity of Sound in Air

1. Aim

To determine velocity of sound in air using phase difference between reference waveform and microphone signal.

2. Apparatus / Components Required

SEELab3 unit
Piezo buzzer (sound source)
Microphone/pressure sensor
Scale/ruler
Stands and connecting wires

3. Theory & Principle

For a sinusoidal wave: \[v = f\lambda\]

If two positions differ by 180 degrees phase, separation equals $\lambda/2$.

Hence: \[v = 2f\Delta x\]

where $\Delta x$ is distance shift between in-phase and out-of-phase conditions.

4. Circuit Diagram / Setup

Drive piezo with WG at fixed frequency (around 3400 Hz).
Measure WG as reference and MIC as received signal.
Move microphone along the source axis and monitor phase relation.

5. Procedure

Set WG frequency near buzzer resonance.
Find microphone position where WG and MIC are in phase.
Find position where they are 180 degrees out of phase.
Measure displacement $\Delta x$ between these positions.
Compute velocity: $v = 2f\Delta x$.
Repeat multiple times and average.

Screen 1

Screen 2

6. Observation Table

Trial	Frequency $f$ (Hz)	In-phase position (cm)	180 degrees position (cm)	$\Delta x$ (m)	$v=2f\Delta x$ (m/s)
1
2
3

7. Results and Discussion

Measured velocity of sound: ____ m/s.
Value is affected by reflections and alignment errors.
Measurement quality improves in open space with fewer reflecting surfaces.

8. Precautions

Keep source and microphone collinear.
Minimize reflections from nearby walls/objects.
Use stable frequency and avoid touching setup during measurement.

9. Troubleshooting

Symptom	Possible Cause	Corrective Action
Phase relation unclear	Weak signal or noisy environment	Increase signal level and reduce ambient noise
Unrealistic velocity	Distance reading error	Re-measure positions carefully
Unstable waveform	Poor microphone placement	Fix sensor position and orientation

10. Viva-Voce Questions

Q1. Why does 180 degrees phase shift correspond to half wavelength?

Ans: A full wavelength corresponds to 360 degrees; half of it gives 180 degrees.

Q2. Why are results often slightly high/low?

Ans: Reflections and phase-reading uncertainty shift effective path difference.

Velocity of Sound in Air