Velocity of sound

Objective

Calculate the velocity of sound by measuring the pressure variation with distance. Sound travels as a series of compressions and rarefactions. Figure (b) shows the High and Low pressure regions along the direction of travel, along with output of a pressure sensor at corresponding positions.

We can display the pressure variation at any point with respect to the variation at the starting point. The phase of the microphone output changes as you change its distance from the Piezo. Moving by one wavelength changes the phase by 360 degrees. If the phase changes by X degrees for ΔD cm change in distance, the wavelength is given by λ = (360 × ΔD) ⁄ X. The velocity of sound can be calculated by multiplying the frequency with this.

schematics/sound-velocity.svg pics/sound-waves.png
Figure 5.1 (a) schematic (b)compressions et expansions along the direction
of sound.

Procedure

pics/sound-velocity180-screen.png

Discussion

At 3500 Hz, for a 2 cm change in distance the phase changed from 176 to 102. Using the equation, v = f × (360 × ΔD) ⁄ X, v = 3500 × (360 × 2) ⁄ (176102) = 34054cms1. It is important to keep the mic and the Piezo disc on the same axis, for accurate results.