Physics Dept. Demonstration Manual

DVW20.1 - Longitudinal Wave Blaster

Objective:

To mechanically generate longitudinal waves of very high amplitude.

 
Apparatus:  
  • Solid aluminum rod, 3.66 metres long, 1.25" diameter
  • Support blocks, triangular, two
  • Powdered resin
  • 'Large' piece of chamois
  • 12 inch steel ruler, narrow
  • One entire lab bench
  • Scrap rod of aluminum

Method:

Set up blocks and rod on the Lab Bench. Position the blocks at the marks which identify the nodes. Apply resin to the chamois.

Tap the end of the rod with the small aluminum sample to start resonance. While holding the rod at the nearest block, pull the chamois towards one end, until the original vibration is greatly amplified.

Hold the steel ruler by the fingertips and allow it to contact the vibrating end.

Notes:

  1. This demo is only to be used in the physics lab.
  2. This demo requires one full lab bench.
  3. The sound level can reach painful levels.
  4. The steel ruler illustrates the power contained in the longitudinal waves,
  5. The ruler bounces away from the end of the long aluminum rod.

f  =    /

  =  5000 m/s

  =  3.66 m

f = 5000 m/s / 3.66 m = 1366 Hz

Additional Work:

  1. Measurement of Frequency, Oscilloscope Method

    Additional Apparatus:

  • Tektronix model 2445 scope
  • Microphone

Scope Settings:

Y
=
20 volts/div.
(varies with amplitude setting, microphone sensitivity and setup geometry)
X
=

0.2 ms for 1.37 kHz (f1)

=
= 50   s for 4.11 kHz (3f1)

Set the rod into resonant vibration as described in the procedure. While viewing the wave pattern on the scope, adjust the cursors' positions such that the cursorsare separated by one wavelength. The scope screen will display the frequency.

Points to Note:

For a rod supported at two points, 1/4 L from each end, resonant frequencies occur at f1 (   = L), 3f1(3   = L), 5f1 (5   = L).... It is only at the fundamental and odd multiples of the fundamental that nodes are positioned at the support points.

The demonstrator can induce vibrations at f1 and at 3f1. Persons with a musical ear will hear a change in sound quality as the rod is stroked to stronger vibration at 3f1. Each frequency is readily displayed on the scope. The complex wave pattern from summed vibrations at f1 and 3f1 can also be seen. (A little practice is needed to produce these effects competently. Dieter can help you learn the technique.)

  1. .Beats Method

    Additional Apparatus:

  • Signal generator (Wavetek function generator)
  • Speaker

    Set the rod into vibration as described in the procedure. Set the volume of sound from the speaker to be about the same as the sound from the rod. Find the generator frequency for which slight +/- adjustments yields beats.