Physical Science - Introduction to
Oscilloscope and Vibrations
- Force transducer and strain gauge
- spring and weights
- Microphone and pre-amp
- Computer and data acquisition system
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
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
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
- Record the frequency (cycles/sec).
- Calculate the period T in sec/cycle. See text for
- Repeat the measurements for a small amplitude and report the
frequency and period.
- What can you conclude about the frequency dependence on the
- Record a trace of the vibration when the spring is pulled
down from the middle? Include a sketch of the vibration curve in
- Describe the motion on the basis of two simple motions.
- Measure and report on the frequency and period of the
- Discuss the results and their significance.
- 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.