# Cabrillo College

Document Sample

```					Cabrillo College                                                                  Name __________________
Physics 10L

LAB 10
Music and Noise
Read Hewitt Chapters 20 and 21

What to learn and explore
Most of the sounds we hear are noises. Slamming doors, rustling of papers, creaking of floors, and
most of the sounds from traffic in city streets are noises. Noise corresponds to the irregular vibration
of the eardrum produced by some irregular vibration of some object. The sound of music, however,
tends to be regular and repeat itself. Of course, the line that separates music from noise is thin and
subjective.
Sound waves can be produced by many different kinds of oscillators: simple oscillators that produce
only a single frequency, standing wave oscillators that produce multiple but related frequencies
(fundamental and harmonics), and more complicated oscillators that produce many unrelated
frequencies—including the random frequencies we call noise. We will learn to distinguish and
describe these frequencies by looking at a sound's frequency spectrum, which is a graph that shows the
frequencies present in the sound and their corresponding amplitudes.

What to use
Computers (Spectrum and Data Studio), tuning forks, musical instruments, voices, and synthesizer.

What to do
Do the experiments starting on the next page to help answer the questions posed and other questions of
observations, and (b) discuss your predictions and observations with your lab partners to make sure
you all agree or agree to disagree. However, note that in this lab you will be making many discoveries
that do not lend themselves to prediction until after you have played with the equipment for a while.

When you finish the lab, please write any thoughts and comments you have on today’s experiment
here. Thank you.

75
1) Waveforms and Frequency Spectra
If you analyzed several different kinds of vegetable juice, you might make a graph to describe how
much of each kind of vegetable was in each one. Since most sound waves are made up of more than
one frequency, we create graphs to tell us how much of each separate frequency is included in the
sound we hear. This experiment will show you how it works.
a) Use the Spectrum application on the computer to study the relationship between the frequency,
amplitude and waveform of a wave. Shift-click on the frequency spectrum to make a wave with a
frequency of 4 cycles/sec and an amplitude of 50. Copy both the waveform graph and the frequency
spectrum graph into the space below.

waveform                                        frequency spectrum

b) Watch how the waveform changes when you use the up or down arrows to raise or lower the
amplitude. Try using the right or left arrows to raise or lower the frequency and see how that looks on
both graphs. Draw a couple of examples.

With a larger amplitude:
waveform                                        frequency spectrum

With a higher frequency:
waveform                                        frequency spectrum

76
Name

c) Now shift-click again to add a second wave to the first. Note that the waveforms of the two waves
add together, so it becomes difficult to determine the frequencies and amplitudes from the waveform
graph, but it is still easy to read frequencies and amplitudes from the frequency spectrum! Try many
combinations of two frequencies and amplitudes to get a feeling for how the waveform relates to the
frequency spectrum. Then try three waves together! How well can you predict one graph from the
other now?
Play for a while, then sketch a few of the easier ones to remind yourself how it works.

Two-frequency spectrum:
Waveform                                          frequency spectrum

Three-frequency spectrum:
waveform                                         frequency spectrum

It is much easier to draw and read a frequency spectrum than it is to draw and read a waveform! For
this reason, you will draw only the frequency spectra of the sounds you listen to in the rest of this
lab.

77
d) Shown below are two different waveforms. With your lab partners, try to reproduce each waveform
and then draw the corresponding frequency spectrum in the spaces below. Each waveform picture
represents 1 second of time.
Hints: The first waveform is a combination of two waves, the second is a combination of three waves.
Try to identify the individual waves and count the number of waves in the 1 second to find the
frequency. For example, if there were 5 full waves in the box, that would be 5 waves/second or 5 Hz.

78
2) Tuning Forks -
We use tuning forks to produce a fairly “pure” musical tone. Each one has a frequency stamped on it,
and one might assume that it produces only that single frequency. We’ll find out by using our
computer with Data Studio and the microphone to display the frequency spectrum of the sound.
a) Use DataStudio to study the frequency spectrum of a tuning fork. Hit the fork gently with the rubber
mallet. (You’ll notice that instead of seeing solid vertical lines like in Spectrum, here you see sharp
peaks with bases that roll off to low-level “noise” all around.) Sketch the frequency spectrum here.

frequency spectrum

b) Does the waveform of the tone look fairly ‘pure” (a smooth wave with only one major frequency)?

Does the frequency of the tallest peak agree with the frequency stamped on the fork?

c) Click “stop” in the computer program to turn off the sound analyzer. Now hit the tuning fork fairly
hard with the harder mallet. Does it sound different than it did with the rubber mallet?

Can you still hear the main tone (called the ‘fundamental’) that you heard before? Do you hear other
tones as well? Are they higher or lower than the fundamental tone?

With your lab partners, draw what you think the waveform and frequency spectrum of this sound
might look like. (don’t worry about getting it exactly right, just make an educated guess.)

Prediction
waveform                                        frequency spectrum

79
d) Now use the sound analyzer to see how close your guess is to the real thing.
Observation
waveform                                         frequency spectrum

e) Hit the fork again with the hard mallet and watch its frequency spectrum. How does the spectrum
change as the loudness of the fork dies off?

Do all the different frequencies die off at the same time? Which ones last the longest?

As the tuning fork dies off, can you hear the changes in its sound that correspond to the changes in the
frequency spectrum?

3) Musical Instruments

a) Play a note on a musical instrument. Can you identify the fundamental (lowest) frequency in its
spectrum? Sketch the frequency spectrum here.

frequency spectrum

b) Are harmonics (higher frequencies) present in the spectrum? How many? Which are loudest?

80
c) Play some other notes, varying the pitch and volume. Play and sketch a note with a higher pitch and
lower volume than the one above.

frequency spectrum – high note, low volume

d) You can recognize different instruments even when they are playing the same note. This is because
different instruments have differently shaped frequency spectra. Play the same note on 2 different
instruments and then draw and compare their spectra. (Hint: Get help if you need it to find the same
note on 2 different instruments. The guitar and the guitar tuner work well.)

Spectrum 1                                             Spectrum 2
Instrument:_____________________________                 Instrument:_________________________
Note:_________________                                   Note:_______________

What similarities are there between the two spectra? What are the main differences in the two spectra?
How would you describe the differences in sound between the two instruments?

4. Voice

Here you’ll try some things out with your voice to see what it looks like.
a) Have each partner sing a note they are comfortable with, one at a time. What was the fundamental
(lowest frequency) of your note? Did your note have a lot of harmonics? Which person’s voice had
stronger harmonics? Which voice was more like a single frequency?

b) Now try to sing the same note as your partner and see if the spectra look different? With practice, do you
think you could identify the person by their frequency spectrum, even when they were singing the same
note as someone else?

c) Try saying different vowel sounds, like “aaay” “eeee, “aahh” , “ooh”, or “uuuu”? Try to keep all the
sounds at the same “note”. Which vowel sound has the strongest harmonics? Which is the ‘purest’?

81
5) Music to our ears -- Synthesizer.

a) Simultaneously play the keys marked with red. Next play the keys marked with green. Which

Certain combinations of notes are called chords. These are groups of notes with specific relationships
between their frequencies. Some are pleasing to the ear, others may not be. Perhaps certain chords sound
good because they mimic natural harmonics. There is, as usual, a whole lifetime of study here. Use the
paper strips if you want to experiment with chords.

b) Play two or three keys at the same time on the keyboard. Can your ear pick out the individual tones?
Test you partner by having them close their eyes and tell you how many notes you are playing (1,2, or 3).
Ears are pretty cool, aren’t they? Remember this activity when you get to the lab about light. You’ll see if
our eyes work the same way, or not.

c)To play a musical scale on the keyboard, play the white keys from C to C. Can you tell how the
frequency of the high C compares to that of the low C? Use the sound analyzer if you want. In music, two
notes this far apart are called an ‘octave’. Can you tell why it’s called an octave from the keyboard?

d) Play a couple of other “octaves,” such as E to E or G to G. Use the computer to find the simple
relationship between the frequencies of two notes that are an octave apart.

6) Noise

a) What does the spectrum of a hiss look like? What about a “sh” sound? Rubbing hands together?
Clapping? Barking or snarling like a dog? Other ‘non-musical’ sounds? Sketch (and label) the
frequency spectrum of a couple of the sounds.

frequency spectrum of ___________________           frequency spectrum of ____________________

b) What do the spectra of these “noises” have in common? How are they different from the spectra of
musical sounds? Remember these noise spectra – we’ll talk about them again in the lab on light and color.

82
7) Canceling Noise Out / Hearing Test
Devices have been developed that can create pressure waves that are opposite to the noise in a room,
canceling it out to produce a very pleasant quiet. We have such a device. It is an electyronic box with

First turn the switch off, put on the headphones and listen to the room noises. Then turn on the
switch. After a pause, the device will turn on.
a) What happens? Cool?

b) Use the headphones to find out what frequencies you can hear. A book will tell you that “people can
hear in the range from 20 to 20,000 Hz”, but that is just a generalization. In this activity, you’ll find out
the highest frequency you can hear . Set the sound creater to 21,000 Hz and have your partner slowly
decrease the frequency until you can first hear it. Then (Be careful with the volume, so you don’t hurt
your ears. The volume will get louder as you get farther from the ends of the range.)

8) Wrap-up
Study the spectra you have drawn and see if you can answer the following questions. Please use
sketches!!
a) What is the essential difference between music and noise?

b) What characteristic in a waveform corresponds to loudness?

c) What characteristic in a waveform corresponds to pitch?

d) Did you notice any pattern about what instruments make what sounds? Would you say that bigger
instruments make higher-pitched sounds?

83

```
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
 views: 1 posted: 10/30/2012 language: Unknown pages: 9
How are you planning on using Docstoc?