III. Procedure: Longitudinal Waves in Air

Standing waves in air, quite analogous to the standing waves on a string, can be produced in the air column inside a cylindrical tube. An aluminum plug may be moved along the tube and the presence of the standing wave detected by listening for nodes and anti-nodes in the standing wave pattern. In this case, the excitation is provided by a small speaker driven by a function generator. The arrangement is sketched below. 

 

 

 

 

 

 

Set the frequency of the output signal within DataStudio  to 1000 Hz.  Starting with the movable piston near the speaker, move the piston away from the end until the sound reaches a minimum.  Record the position of the piston when there is a minimum intensity.  Continue to move the piston along the tube, recording all the rest of the points along the tube where the sound is weakest.  Average the distances between these successive points; then find the corresponding wavelength. Use Eq.(1) to calculate the speed of sound in air.

The velocity of sound in air is known to vary with temperature as:

(3)                         

where               R = gas constant = 8.314 joules/mole K,
M = molecular weight = .0288 kg/mole for air,
T = absolute temperature,
and                   g = ratio of specific heats = 1.40 for air.

Compare your value of v measured for the standing wave with the one found from Eq.(3) using the % difference.