Physical Science - Introduction to Oscilloscope and Vibrations

Equipment

Introduction

Sound and music consist of vibrations of things.  Those things that vibrate can be the tines of tuning forks, stretched strings, air columns, organ pipes, disks, bells, ...  In order to observe the vibrations that make sound, a tool called the oscilloscope is used.  The oscilloscope permits the observations of very rapid vibrations characteristic of sounds.

The Oscilloscope is essentially a voltmeter.  Instead of displaying decimal digits or moving a needle, the oscilloscope displays voltages by moving an electron beam up and down on a phosphorescent screen.  Electrons travel in the vacuum inside the oscilloscope tube (a forerunner of a TV, but much more interesting to watch....)  Because electrons have such small mass, they can respond extremely quickly to rapid voltage changes, unlike the massive needle of a conventional voltmeter.  Hence the oscilloscope is a rapidly responding voltmeter.  As the input voltage increases and decreases, the spot on the display moves up and down.

The display of the oscilloscope usually is meant to move left to right at a constant speed.  As a result, the combined left to right motion with the vertical motion up and down, the oscilloscope displays a trace of voltage vs. time.  The rate of left-to-right travel is controlled by one of the many knobs to move as slow as one cm/sec and as fast as 1 cm/(.1 micro sec).

Mass-spring vibration - lecture demo

Connect the spring to the force transducer.  Hang the mass on the spring.  The force transducer generates a voltage whenever the spring pulls on it - the amount of voltage is proportional the the spring force.  The mass will be set to vibrate and the Oscilloscope will be set to move its spot very slowly.

Predict the behavior of the oscilloscope display when connected to the the output of the force transducer when the mass is moving up and down.

Do it.  Sketch the appearance in your notebook as the 'scope is set-up.

Complicated vibration.  A complicated vibration will be set up by pulling down in the middle of the spring and letting go - not just pulling down on the weight.  Observe this motion without the oscilloscope.  Predict the appearance of the 'scope display before observing the oscilloscope.

Observe the complicated vibration on the oscilloscope.  Draw the picture and describe the vibration in words.

Speed-up the time-base.  The complicated vibration is a bit fast for complete observing.  You may wish to speed-up the time base so the beam sweeps left-to-right faster in order to "open-up" the vertical vibrations.  Sketch the result in your notes and report.

Mass-spring vibration - computer data acquisition. - student experiment.

The trouble with the oscilloscope is that it does not record a permanent record.  In the old days Polaroid camera were used.  Now we can use the computer to record the voltages and record graphically.

Each of you will take a force transducer (the black box) set to +- 10 N.  The output is connected to DIN1 of the computer interface and Logger Pro software is used.  Once LoggerPro is launched, you should open the file called slowvibration.mbl.  Start the mass-spring vibrating (in the simple mode), make sure the amplitude is not too large, and record the voltage vs. time.  Since the system produces a graph of the spring stretch vs. time, you can calculate the frequency in cycles/sec by counting the cycles and measuring the time. 
  1. Record the frequency (cycles/sec).
  2. Calculate the period T in sec/cycle.  See text for formula.
  3. Repeat the measurements for a small amplitude and report the frequency and period.
  4. What can you conclude about the frequency dependence on the amplitude?
  5. Record a trace of the vibration when the spring is pulled down from the middle?  Include a sketch of the vibration curve in your report.
  6. Describe the motion on the basis of two simple motions.
  7. Measure and report on the frequency and period of the slowest motion.
  8. Discuss the results and their significance.
  9. Finally, instead of letting the mass-spring vibrate freely in the complicated motion, manually simulate the complicated by grabbing the weight and moving up and down in the complicated fashion, record the motion, and compare with number 5.

Oscilloscope and sound.

With the microphone connected to the oscilloscope, set the time-base very slow (about 1-2 cm/sec) and speak or sing into the microphone.  Describe what you see.

In order to see the rapid vibrations of sound better, speed-up the time display so that you see something similar to the complicated mass-spring vibrations, but at a much different time scale.

Next time we will explore many more sounds with the oscilloscope and measure the speed of sound.