PHYS 140
Frequency Spectrum Lab
A frequency spectrum is a graph that has amplitude on the y-axis and frequency on the x-axis. A pure
tone—one with a single frequency and zero overtones—would look like a spike. For example, if you had
a pure tone of 150 Hz, a frequency spectrum would show a spike at the 150 Hz mark on the x-axis.
You will need a Vernier microphone connected to the LabPro interface box. (Plug it into Channel 1).
Check that your interface box is plugged in to a power outlet and also check that its USB cable is
connected to your computer.
Once you are sure everything is plugged in, open LoggerPro (the icon should be on the desktop—if it
isn’t, look for Vernier Software under Programs.). Go to File -> Open -> Physics with Vernier ->
Mathematics of Music. This will open a window that displays pressure versus time, and below that you
will see the corresponding frequency spectrum (ie, the Fourier transform).
A. Frequency Spectra of Music vs. Noise Earlier in this course, you saw that a noise (like
knocking on the table or snapping your fingers) makes a non-repeating pattern on an
oscilloscope, unlike music, which requires a repeating pattern of definite frequency.
PREDICTIONS

What should the Fourier transform of a pure note (a perfect wave with only one frequency)
look like?

What should the Fourier transform of a noise (a sudden burst of sound) look like? (A spike?
Multiple spikes? Something more spread out rather than spiky?)
TEST

Press the green “Collect” button and make a noise that you know is not musical. [Note: The
microphone captures the sound over a very short period of time, so you need to repeatedly produce the sound,
and even then the mic might not pick it up. If you get a flat line for your upper graph, repeat the experiment
until the mic captures the sound.]

Now repeat with a musical note (generated any way you like).

Describe how the two graphs of pressure vs. time differed.

Describe how the Fourier transforms differed. Were your predictions correct?
B. Tuning Fork vs. Human Voice

Which do you predict would be a purer note (fewer or smaller overtones)—a tuning fork or
the human voice?

Collect data for the sound of the tuning fork. If overtones appear:
1)
Are they harmonics (integer multiples of the fundamental)?
2) Which overtone(s) are the strongest? (give numbers—“the first overtone,” “the third
overtone,” etc.)

Now collect data for the human voice. Do this however you think would give the purest
wave (humming, singing, etc). If overtones appear:
3)
Are they harmonics?
4) Which overtone(s) are the strongest?

What differences do you observe between the tuning fork frequency spectrum and the
vocal spectrum?
C. Vowel Sounds: What makes them sound different from each other?
In this experiment, you’ll be comparing vowel sounds that have similar pitch and loudness. Try singing
the same note several times, but vary the vowel sound. Try at least 3 different vowels.

What do you find? If the pitch and loudness are similar, what makes different vowel sounds
different from each other?
D.
The world’s cheapest musical instrument, The Straw Oboe
Are you in for a treat or what. In this activity, you will be making a straw oboe.
Snip off the end of a straw to make the “reed.” Use the instructor’s demo straw as a guide to
see how it needs to be shaped. Make sure to clean up any straw scraps when you’ve made your
oboe; don’t leave any trash lying around.

What frequency is your straw playing? I don’t expect you to have perfect pitch (I don’t
either), but you can figure out the pitch using the Vernier microphones.

What note should your straw be playing? Measure the length of your straw. Use the
appropriate formula (and what you know about the speed of sound in air) to determine
the frequency. (Hint: once your mouth is over your straw, do you think your instrument
has two open ends, or one open and one closed end? If you get an answer that’s wildly
off from your actual frequency, you might have chosen the wrong formula!)
Another hint: remember that when a musical instrument produces a note, the pitch that
you hear coming from it is dominated by the fundamental frequency.

Blow into the wrong end of your straw, with your mouth about a centimeter from the
end. Why is this note much softer than when you blow into the other end?

While blowing into the reed end (making a nice loud noise in the process), snip off little
bits of your straw. How does the pitch change, and why?

When you’re done snipping, measure the new frequency. Record it below.

Re-measure your straw and calculate the theoretical frequency.

How close is your calculation to the actual frequency?
E. Spectral Lines Observation: White Light vs. Discharge Lamps
Finally, before you leave, make sure you use the spectroscope to look at (a) white light, and (b) the gasfilled glass tube (called a discharge lamp). What is the difference between the frequency spectrum you
observe for the white light and the gas?
.