1: INTRODUCTION TO SOUND SIGNALS
INTRODUCTION
Most of the labs will use various tools to visualize sounds. For example, we will record sound
with computers that store the recording as a list of numbers (similar to the way a CD represents
sound).
Today, you will talk into a microphone, which converts the sound pressure into an electrical
signal, and the computer will measure the strength of that electrical signal about 22,000 times
each second. The result is that each second of recorded sound is represented by a time-series
of 22,000 numbers that we will call a "sound signal". Once the sound data is stored on the
computer, there are many things that the computer allows us to do to analyze or modify the
sound recording. One thing the computer can do is to make a graph of this data. Here is how
the computer graphs a (very small) part of the recording of the word "hello:"
The vertical axis is the strength of the electrical signal and the horizontal axis is time. The
computer connects the points on the plot to help us to see the patterns.
RECORDING SOUND
A. Let's try it out.
1. Turn on the computer by pressing the button with the arrow label at the top of the
keyboard. After a short time, the computer monitor will light up and you will see the
"desktop." Using the mouse, point the cursor arrow at the symbol in the lower left that is
labeled "voicescope." Then, quickly click the mouse button twice [from now on we will say
"Double-click"]. This automatically starts up the program called SoundScope that
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records sound from the microphone and graphs it for us. Wait a minute for the program to
load itself.
2. Record a sound: hold down the "apple" key on the keyboard ("? ") and press the numeral
zero key ("0") once [from now on we will say "type ? -0"] . The program is set to record
one second of sound from the microphone, which is enough time to say your name (or part
of it). You need to say something into the microphone as soon as you press the zero key
(one second is not very much time!).
3. Play back your recorded sound by typing "? -6" If you do not hear anything, one of several
things may have happened:
•you did say anything during the one second that the program was recording, or
• you were not close enough to the microphone, or
• you clicked the mouse button when the cursor was pointing outside of the SoundScope
window (if so, the word "voicescope" in the top border of the window will be gray instead
of black)—simply click somewhere inside the SoundScope window and try recording your
sound again, or
• something is not working.
Try repeating the previous steps (and speak up!). If you still do not hear anything when you
play the sound, alert your instructor.
B. Understanding the display
The display shows a graph of one second's worth of sound data (the computer graphs all
22,000 data points rather quickly). The data starts at the left side and ends on the right side.
You cannot see all the details of the sound data; it is even difficult to see that the sound signal
oscillates up and down. You should be able to see the louder parts of the recording (where the
sound signal oscillated the most) and the quieter parts (where the sound signal remains almost
constant).
1. Find the loudest and softest parts of your recording. Later, when you print out your
recording, indicate and label these parts on the printout.
Introduction to Sound - 2
7.8
H
Volt
5.2
2.6
0
-2.6
-5.2
0
100
200
300
400
500
600
700
800
msec
100 msec /Div
The display above shows exactly one second of recorded sound (note: 1000 msec means
1000 milliseconds = 1 second). To see more of the details of the sound signal, use the upand down- arrows toward the bottom right corner of the voicescope display. Point the
cursor arrow at the down-arrow and click once on the arrow. The display will "zoom" in on
the first part of the sound recording.
2. To see the remainder of the sound recording point the cursor arrow at the right-pointing
arrow at the bottom of the display and hold the mouse button down. The display should
"scroll" to reveal more of the graph of the sound data. Another way to move horizontally is
to click in the border between the left-arrow and the right-arrow. You can "zoom-in" more
by clicking with the mouse on the down-arrow again or "zoom-out" by clicking on the uparrow. To see the full recording again, click on the "H" in the upper-right-corner of the
display. Does the wave look different if you zoom in far enough?
3. Play back parts of your recording. Try selecting less than an entire word. To do this, point
the cursor at some part of the sound signal, click and hold the mouse button down, and drag
to another part of the sound signal. Now type ? -9 to play the selected part of your
recording. Can you still understand what is being said?
4. Try saying (or singing) different things into the microphone and recording the results. The
different sounds often look different. We hope to develop the tools to analyze the
differences systematically during the semester.
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C. Print your recording
Once you have a good recording on the display, try printing the display. The easiest way to
accomplish this is to type "?-p". A panel will pop up (if the computer insists that you have not
chosen anything to print yet, alert your instructor). All you have to do is click on the button
marked "Print", and about a minute later the page comes out of the printer.
Introduction to Sound - 4
VIEWING A SOUND SIGNAL ON THE OSCILLOSCOPE
Our other tool this semester is the oscilloscope. It is useful for looking at how the sound signal
changes at the same time as the sound signal is being made (which is not possible with
SoundScope). We will also use the oscilloscope to measure simple sound signals (but it is not
so useful when we need to analyze complicated sounds, but for this purpose SoundScope
excels).
Using the Oscilloscope
1. Make any sustained sound into the small black microphone that is connected to the
oscilloscope. Make it louder, then softer: how does the appearance of the sound signal
change?
2. Then try a musical sound: make it higher in pitch, then lower: how does the appearance
of the sound signal change?
WHAT DO WE MEAN BY "SOUND?"
When people talk about “sound,” they may mean one of several things. Usually, they mean
either the sensation people have when vibrations of the air hit their ears, or they mean the actual
vibration of the air, whether or not anyone is there to hear it.
The relationship between vibrations and sensual perceptions is complicated, largely because
perception itself is so complicated. How loud a particular sound seems to you may change
depending on your mood. Nevertheless, the three most basic qualities that are often used to
describe perceived sound—loudness, pitch, and timbre—do correspond in a fairly
straightforward way to physical properties of vibration that can be easily measured.
The purposes of the next part are: (1) to give you some experience with the relationship
between the qualities of perceived sound and the properties of physical sound, and (2) to
introduce you to another of the tools we will use for the rest of the semester: the Function
Generator, a device that can be used to make several different kinds of electrical oscillations
that can be perceived as sound. Most usefully, the function generator allows you to
independently control loudness, pitch, and timbre. "Independently" means that you can change
one of these three while leaving the other two the same, which helps you to identify how the
sound changes when you change that property.
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LOUDNESS
Vibrations can be large or small. Large vibrations are said to have a large amplitude. In the
world of sound, amplitude corresponds to loudness—the greater the amplitude, the louder the
sound.
A.
Setting the amplitude
1.
Turn on your function generator (the little grey box) by pushing the red POWER ON
button. Choose a sine wave button, rather than a square or triangle wave. Find the knob
called AMPLITUDE and make sure it is turned all the way down (counter-clockwise). Put
on the headphones and then slowly turn up the amplitude. Notice that the sound gets
louder.
That’s all there is to it. [not really—our perception of loudness is also influenced by pitch and
timbre]
PITCH
Vibrations can happen quickly or slowly. This property of vibration is called frequency.
Frequency tells how often something happens in a given unit of time. For example, the sound
you just heard caused your eardrum to move back and forth about 1000 times in each second,
so the frequency was about 1000 per second. The unit “per second” has an abbreviation, Hz,
named after Heinrich Hertz and pronounced “hurts.” So a vibration that happens 1000 times
each second is said to have a frequency of 1000 Hz.
The physical property frequency corresponds to the sensation of pitch. Try this out for
yourself: put on the headphones, turn up the amplitude to a comfortable volume, and then turn
the knob on the function generator that is labeled FREQUENCY. As you move the knob
clockwise (toward the number .2) both the frequency and the pitch go down. Twist the knob
the other way and the pitch goes up.
B.
Setting the frequency
The function generator can produce any frequency from less than 1 Hz up to more than
20,000,000 Hz. This wide range is obtainable by use of the bank of grey buttons labeled
FREQUENCY MULTIPLIER–Hz. The 1K button is currently pushed in. The k is an abbreviation for
kilo meaning 1000 (as in kilometer or kilogram). So the generator is now set to generate
frequencies near 1000 Hz. To find the frequency more accurately, you multiply this number by
the number on the frequency dial, which goes from 0.2 to 2.0. So when the 1k button is pushed
in and the dial is set to 2.0, the frequency is 2.0 x 1000 Hz = 2000 Hz.
1.
What is the frequency when the 1k button is in and the dial is set to 0.2?
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The dial is not very precise. Later in the semester we will learn how to measure the frequency
more exactly.
To get higher or lower pitches you push a different FREQUENCY MULTIPLIER button. Push in the
10k button. This puts you in the frequency range from 2000 to 20,000 Hz, and is at the top of
the human hearing range. Be sure you understand the relationship between the dial and button
settings on the generator and the frequency.
2.
With the multiplier set to 10k, what range of frequencies can we obtain?
a) Use the generator and counter to determine the highest frequency you can hear:
b) Without changing the amplitude knob, turn the dial from its lowest setting (0.2) to its
highest (2.0). Does the loudness you perceive change?
What is the dial setting when the sound seems loudest?
3.
To get low frequencies, set the multiplier to 100. What range of frequencies can you get
with this setting?
4.
What is the lowest frequency you can hear that sounds like a pitch?
(Nore: If you only hear clicking, you are not hearing a frequency—because the headphones
are small, they do not produce low sounds very efficiently, so you will need to turn the
amplitude all the way up on both the generator and on the volume control on the headphone
box. You will probably want to turn it down again later, or it the sounds will be annoyingly
loud.)
TIMBRE (pronounced “tamber” )
You have seen (and heard) that sounds can be loud or soft, depending on amplitude, and they
can be high or low, depending on frequency. But that is not all there is to sound. You can
distinguish sounds that have the same pitch and the same volume. For example, you can sing the
vowel sound “a” and then the vowel “e”, and they sound different, even though they have the
same frequency and amplitude. The sounds a and e have different tone qualities or timbres.
Timbre much more complicated than loudness or pitch, and you will spend much of the
semester studying it.
C.
Setting the function
To begin your investigation of the timbre, listen to the effect of changing the “shape” of the
vibration. The function generator can produce three different types of oscillation, called sine,
triangle, and square. So far, you have been listening to the smooth sine function. You can
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choose one type of oscillation or another by pushing one of three MODE buttons, labeled
for sine,
for triangle, and
for square.
1.
First, set the frequency to around 250 Hz. Then push the triangle button and listen to the
sound. Typically, the triangle function is said to be “brighter” or “sharper” than the sine
function. You should avoid calling it “higher,” since that implies a higher pitch. Do you
agree that the sound is “brighter”?
2.
Now try the square function. It is much brighter, possibly even “buzzy.” It will probably
also sound louder to you, so you may want to turn down the volume to make a good
comparison.
3.
Switch back and fourth, listening to the three timbres. Try listening to the different shapes
at widely differing frequencies.
4.
Is it easier to distinguish the sounds at low or at high frequency?
5.
Is there any frequency at which you cannot tell the three apart?
The smooth sine function is the simplest, purest, cleanest, least complex sound there is.
Every other sound is, at least to some degree, brighter, sharper, or more harsh than the sine
sound.
LOOKING AT THE SOUNDS
To see why these sounds are described by shapes, such as “square” and “triangle,” turn on the
oscilloscope. The oscilloscope makes a picture of the electrical signal that the function
generator produces.
D.
Electronic signals on the oscilloscope
1.
Set the generator to the sine function and the frequency to 1000 Hz and look at the
oscilloscope. Sketch the appearance of the sound signal in the space below:
2.
Increase the frequency using the FREQUENCY knob and look at the signal
again.Sketch it in the space below:
Introduction to Sound - 8
3.
How does the appearance of high pitches on the screen differ from low pitches?
4.
How could you use the screen image to help you measure the pitch?
5.
Change the frequency multiplier and observe the sound signal again. How does the
picture on the screen change when you increase or decrease the multiplier?
6.
Now leave the frequency at a comfortable pitch and vary the amplitude. What is the
difference between the way loud and soft sounds look on the screen?
7.
The volume control on the headphone box has no effect on the signal, which goes to the
oscilloscope.) What happens if you push in the button on the function generator that is
labeled ATT 30 dB? (Make sure you leave it in the out position when you are done.)
What happens on the screen? How does the sound you hear change?
8.
Now leave the frequency and amplitude fixed but change the function to the "square"
wave or the "triangle" wave. How do the different sound signals appear on the screen?
9.
What does it mean for both the amplitude and frequency of the sound to be the same but
the timbre to be different (what is different on the scope)?
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OTHER SOUNDS
Turn off the function generator.
E.
Looking at natural sounds
1.
Strike the tuning fork with the rubber mallet and listen to it. Would you say the sound is
bright, like a square function, or pure, like a sine function?
You can check by looking at the sound on the scope, but to do that you will need to use a
microphone. If you need help, ask your instructor to plug in the microphone for you. Strike the
tuning fork and hold the microphone very close to one of the vibrating tines.
2.
What shape is the signal? Draw it. Were you right?
3.
Now try whistling. What do you conclude?
4.
Now try singing into the mike, holding the mike very close to your mouth. Sing all
different types of vowel sounds. Which vowel (a, e, i, o, oo, ah, uh) makes the most
sine-like pattern?
5.
How does a very bright e look (sketch it in the space below)?
Note that even though they are more complicated, musical sounds and vowel sounds (and any
other "steady" sound) are periodic just like the sounds produced by the function generator.
That means that approximately the same sound signal is repeated over and over. We can use
loudness, pitch, and timber to describe all periodic sounds, and we use amplitude, frequency,
and shape to measure the physical properties of those sounds.
F. Looking at more complicated sounds
Introduction to Sound - 10
1.
Some sounds are different: look at various noisy sounds like ssssss and shhhhhhh..
How do these sounds differ from the simpler sounds you looked at earlier?
Be sure to turn everything off before you leave the lab.
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