“Hear”
We
Go
Again
“Hear” We Go Again
Adapted from “OPPS” written by Louisiana State University
Overview
Brain-based strategies engages the student in making connections between
waves and sound. The concepts of pitch, frequency, sound travel through
different materials are developed through interactive models, appropriate visual
representations, and alternative assessment. The real world application of music
is highlighted throughout the learning series.
National Science Education Standards:
As a result of activities in grades k-4, all students should develop
Content Standard A: Science as Inquiry
Abilities necessary to do scientific inquiry - asking simple questions about objects,
organisms and events in the environment, plan and conduct investigations using simple
equipment and tools to collect data and extend senses, use data to give an explanation to
observations, and communicate their ideas and results to others.
Content Standard B: Physical Science
Give students a chance to increase their understanding of the characteristics of objects
and materials that they encounter daily. Through the observation, manipulation, and
classification of common objects children will develop the understanding that sound is
produced by vibrating objects. The pitch of the sound can be varied by changing the rate of
vibration.
Louisiana Benchmarks/GLE#
Science as Inquiry
1.
2.
3.
Ask questions about objects and events in the environment
Pose questions that can be answered by using students’ own observations, scientific
knowledge, and testable scientific investigations
Use observations to design and conduct simple investigations or experiments to answer
testable questions
4. Predict and anticipate possible outcomes
5. Identify variables to ensure that only one experimental variable is tested at a time.
6. Use a variety of methods and materials and multiple trials to investigate ideas (observe,
measure, accurately record data)
7. Use five senses to describe observations
10. Express data in a variety of ways by constructing illustrations, graphs, charts, tables,
concept maps, and oral and written explanations as appropriate
12. Use a variety of appropriate formats to describe procedures and to express ideas about
demonstrations or experiments (e.g., drawings, journals, reports, presentations,
exhibitions, portfolios)
13. Identify and use appropriate safety procedures and equipment when conducting
investigations (e.g., gloves, goggles, hair ties)
15. Distinguish between what is known and what is unknown in scientific investigations
18. Base explanations and logical inferences on scientific knowledge, observations, and
scientific evidence.
21. Use evidence from previous investigations to ask additional questions and to initiate
further explorations
Physical Science
3rd Grade
27. Use the words high/low to compare the pitch of sound and the words loud/soft to
compare the volume (amplitude) of sound
4th Grade
28. Explain the relationship between volume (amplitude) of sound and energy required to
produce the sound
29. Compare the rates at which sound travels through solids, liquids, and gases
30. Explain the relationship between frequency (rate of vibration) and pitch
Making Sound
Students Explore How Sound is Produced
Getting Started:
1. Organize material so the each cooperative group will be given a variety of
items to explore.
2. Based on student needs and previous experience, plan effectively in order
to provide the adequate time needed
3. Copy student sheets.
4. Obtain materials for group presentation.
5. Practice with your own set of items in order to be familiar with the student
results and experience.
Materials (per group)
Rubber bands of different sizes strung over a shoe box or tacked to a piece of
cardboard
Balloon stretched over a tin can
Rice or beads placed into a film canister or baby food jar
Plastic bottles of different sizes (test tubes, pipes of different sizes may also be
used)
** If time is limited you can give each group a different item to explore and report
their observations to the group.
Materials for demonstrations:
**See “visual vibrations” demonstrations handout for list of materials
Procedure:
1. Assemble students in cooperative groups.
2. Cooperative groups will complete “Making Sound” probe. Have each group
briefly share their answers. Do not discuss the answers with them at this
point; the groups will come back to the probe and re-evaluate their
answers.
3. Inform groups that they will be engaged in an exploration to determine
how sound is made. Point out that each group will have a specific set of
materials to work from but each group is to come up with as many different
observations as they can, recording each in learning logs with a description
of the process.
4. Have materials manager collect materials and challenge groups to explore
as instructed.
5. Following the exploration: have groups share findings, creating a master
list on the board, overhead, or newsprint based on group discussion.
6. Perform “visual vibration” demonstrations as part of the concept
development process.
** Do at least two of the demonstrations
7. Revisit the “Making Sound” Probe and have students re-evaluate their
answers and make corrections as needed. Have each group discuss any
changes they have made to their answers based on their new knowledge.
Safety Notes:
Make sure to instruct students on expected behavior before starting the
exploration. You may want to tape the rubber bands to the box to prevent
rubber bands being used as a weapon. Make sure the film canisters are sealed
tightly (may want to glue top to bottom) to prevent a mess with the rice. Use
caution when using electrical equipment. If students blow across the pipes or
bottles, wash pipes with alcohol or a solution of 2 tsp. of Clorox per gallon of
water.
Making Sound
Probe
Purpose:
The purpose of this assessment probe is to elicit students’ ideas about sound production. The
task specifically probes to determine whether students recognize that sounds result from
vibrations produced by an object or by objects or materials in contact with the object listed.
Explanation
Sound is a form of energy caused by back-and-forth vibrations. All the objects on the list
involve the production of sound as a result of vibration of the object itself or the material it
comes in contract with, such as air. Some vibrations are obvious, such as watching the strings
plucked on a guitar. Other vibrations are so small that you can’t see them. Some vibrations
that cannot be seen can be felt. For example, when you put your fingers over your vocal cords,
you can feel the vibrations created when you speak. The loudness of a sound depends on the
size of vibrations. The size of vibrations is called their amplitude. Increasing or decreasing the
amplitude changes the loudness of a sound. Leaves rustling in a gentle wind create sound with
a low amplitude of vibrations, whereas a blaring radio speaker creates sound with a high
amplitude. Vibrations also affect the pitch of a sound. Pitch describes how high or low the
notes are that are produced by the vibrations of the object. Pitch is affected by changing the
frequency of the vibrations-how quickly or slowly the object vibrates. The more vibrations that
occur per second, the higher the frequency and the higher the pitch of the sound.
Elementary Instruction
By the end of the elementary grades, most students have had opportunities to learn about
sound and how sound is made. Early ideas about sound are connected to position and motion
of objects. Teaching and learning about sound is primarily observational and includes having
students identify different types of sound and their sources, observe vibration of sound-making
objects, and relate loudness and pitch to different types of sound production. The emphasis at
this developmental stage is on the objects, even though in some cases it is the air that is in
contact with the object that is the source of vibration. Because the notion of vibrating air is a
more abstract idea at the elementary level and not directly observable, the national standards
focus on vibrating objects at the elementary level. Ideas about vibration and pitch are gradelevel expectations described in the national standards. Students at this level often learn about
sound through the context of musical instruments. As a results students may become contextbound in their understanding of how sound is produced and may fail to generalize across
different examples.
Making Sound
(Student Sheet)
Directions: All of the objects listed below make sounds. Put an X next to the
objects you thing involve vibrations in producing sound.
___ guitar strings
____ clapped hands
___ singer
____ snapped fingers
___ drum
____ flute
___ barking dog
____ piano
___ popped balloon
____ crumpled paper
Explain your thinking: What “rule” did you used to decide which objects involve
vibrations in producing sound?
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
Making Sound
Student Sheet
Using the objects on your tray, make as many different
sounds as possible.
Write your observations of what you do to the
instrument, what sound you hear and what you see the
object doing
Making Sound
Student Sheet
Using the objects on your tray, make as many different
sounds as possible.
Write your observations of what you do to the
instrument, what sound you hear and what you see the
object doing
“Visual Vibrations” Demonstrations
It is often difficult to see that an object or material that is making a sound is
vibrating. The following demonstrations, all of which are suitable to use
with children, make these vibrations visible. These observations can also
be used to demonstrate that vibrations move through air and the concept
of frequency of vibrations.
Bubble Machine
Materials: 4 straws, string, liquid detergent, 1 Tsp glycerin, pie pan
Make bubble solution in the pie pan by mixing four parts water to one part liquid
detergent. Add 1 Tsp. glycerin to bubble solution.
Make a bubble blowing frame by running a long string of about 4 straw lengths
through 4 straws and tying the ends of the string together. Dip the frame into the
frame into the bubble solution until a soap film forms.
Hold the frame so the soap film hangs vertically just in front of the loudspeaker of
a radio that is playing loud music. You can also create a variety of sounds (from
the “Making Sound” probe) behind the film bubble.
Observation: What happens to the film bubble when sound is produced?
Dancing Beads
Materials: Radio with exposed speakers, tiny plastic beads, small bits of
Styrofoam or grains of rice
Lay radio down so that the speaker faces upward. Turn the radio on. Put a few
beads (or grains) on the speaker. If possible, change the treble/bass setting (more
bass the better the beads will vibrate.)
Observation: What are the beads (grains) doing when sound is produced?
Layer Pen Vibrations
Materials: Laser pen ( or flashlight with a narrow beam), Large tin can with ends
removed and taped for safety, small mirror, balloon, masking tape, radio
Place a tightly stretched rubber balloon around one end of the tin can and secure
it with tape. Attach a small mirror to the middle of the balloon on the outside of
the can. Aim the laser pin at the mirror and have it reflect on a screen, ceiling, or
white board. You may need to attach the pin or flashlight to an object to keep it
stationary.
Have someone talk into the open end of the can or aim the radio loud speakers
(with good bass) toward the open end of the tin can.
Observation: What happens to the light dot as sound is produced?
Water Waves
Materials: pie pan, water, tuning fork
Fill the pie pan ½ full of water. Strike the tuning fork and place the tip of the fork
in the water.
Observations: What did the tuning forks vibration do to the water?
Making Music
Students Explore Relationships between frequency/pitch and
sound/amplitude
Getting Started:
1. Cut wire coat hangers to get a long strip of wire.
2. Make copies of student handouts.
Materials:
Metal rod
Procedure:
1. Review terms: frequency, pitch, amplitude, and loudness
2. Demonstrate how to make the metal rod vibrate.
3. Let the students explore the relationships.
4. Discuss the relationships between frequency/pitch and
amplitude/loudness
Safety Note:
Make sure to tape one end of the metal rod to prevent injury to the students
hands.
Teacher Note:
It would be a good idea to mark the areas on the metal rod with tape that well get
the best results to prevent confusion.
Frequency/Pitch and Sound and Amplitude Relationships
(Student Sheet)
A. Frequency/Pitch
1. Place metal rod so that about 5 cm of it extend over the edge of a table.
2. Place your hand over the part of the rod lying on the table. (Push it tight against the table)
3. Pluck the rod to make it vibrate.
4. Listen carefully to the sound generated and watch the vibration of the steel rod.
5. Now extend the rod outward.
6. Pluck the rod again and listen carefully to the sound generated and watch the vibration of
the steel rod.
7. Extend the rod outward several times listening carefully to the sound generated.
Questions:
1. How did the rods vibration (frequency) change as the rod became longer?
2. How did the Pitch (high and low) change as the rod became longer?
3. What relationship is there between the pitch (high or low)of sound and frequency (the
vibration of an object)?
B. Amplitude/sound
1.
2.
3.
4.
5.
Place metal rod so that about 5 cm of it extend over the edge of a table.
Place your hand over the part of the rod lying on the table.(Push it tight against the table)
Pluck the rod and release the rod to make it vibrate.
Listen carefully to the sound generated.
Now pluck the rod harder and carefully listen to the sound.
Questions:
1. When you plucked the rod harder how did the sound change?
2. Which rod took more energy to pluck?
3. Based on question 2, which wave had more amplitude the first wave or second wave
produced?
4. What is the relationship between the amplitude and the loudness of a sound produced?
Speed of Sound in a solid, liquid and Gas
Purpose:
These activities allow students to determine what types of matter sound can
travel through. Once students understand that sound can travel through solids,
liquids, and gas they can compare the speed of sound in these types of matter.
Getting Started:
1. Fill bags with sand, water, and air.
2. Copy student handouts.
Materials: ** each group will get a bag of sand, water, and air
Sand
Water
Air
Zip lock bags
Procedure:
1. Give students the three bags. Allow them to determine if sound can travel
through the different states of matter.
2. Discuss what the students observed.
3. Let the students graph the data for the speed of sound and determine how
the speed changes in different media.
4. Discuss the students’ results.
Teachers notes:
Make sure to remove the air from the sand and water bags to prevent
confusion. If you are to make a premade graph for the students to paste into
their journals, let the students label their own axis to practice determining
independent and dependent variables.
Sound in a Solid, Liquid and Gas
(Student Sheet)
Problem: Can Sound travel through a solid, liquid, and gas?
Hypothesis: Write your prediction.
Procedure:
1. Hold the bag of sand (Solid) up to your ear.
2. Carefully put your finger in the other ear to block all other sound.
3. Have a partner lightly flick the bag that is held up to your ear.
4. Write your observations of the sound you hear or write no sound if
you did not hear any sound.
5. Repeat with the bag of water (liquid) and air (gas).
Conclusion:
I know that sound can travel through _________because___________.
Speed of Sound
(student sheet)
Problem: Does sound travel faster in a solid, liquid, or gas?
Hypothesis: Write your prediction.
Use the data below to draw a graph. Below each type of material write
whether it is a solid, liquid, or gas.
Type of material
State
of
matter
Speed of Sound
(m/s)
Water
Air ( 20o C)
Steel
Helium Gas
Glass
Alcohol
Liquid
Gas
Solid
Gas
Solid
Liquid
1,437
343
5,000
927
5,640
1,143
Questions:
1. Which state of matter does sound travel through the
fastest?
2. Which state of matter does sound travel through the
slowest?
Music Sticks
Experiencing a Real World Application of Concepts Learned
Getting Started:
1. Cut the 7 ft ½”PCV Pipe according to the chart below.
Note
C
D
E
F
G
A
Bb
C
Length(cm)
32.8
29.2
26.0
24.6
21.9
19.5
18.6
16.4
Frequency(Hz)
262
294
330
349
392
440
466
524
Color
yellow
Green
Blue
Purple
Brown
Red
Orange
yellow
2. Sand the ends to make them straight, write the corresponding note and
draw a corresponding color line on each pipe.
3. Determine the concepts that will be developed or reviewed while using the
music sticks.
4. Make transparencies or handouts of music to be played.
Materials Per Cooperative Group:
4 music sticks of different lengths
Learning logs (optional)
Graphing paper
Procedure:
1. Give student graphing paper or a pre printed graphing paper. Instruct the
students to make a Length Vs. Frequency graph. Have them write the
relationship between the length of the stick and the frequency of vibration
it produces. Discuss.
2. Give each group 4 music sticks of different lengths.
3. Instruct students on the proper way to play the sticks.
4. Have the groups predict the order of pitch sound from lowest pitch to
highest pitch based on the information from the graph.
5. Students can test their hypothesis by playing their music sticks.
Extension: Have students play songs with their music sticks. Discuss how
music notes are used to correspond with different pitch sounds.
Safety Notes:
Un-sanded ends of the tubes tend to hurt the palms of the hands. Younger
children might have a tendency to swing (and hit) others with these
lightweight batons. If students blow across the pipes, wash pipes with alcohol
or a solution of 2 tsp. of Clorox per gallon of water.
Teacher Notes:
The source of any sound is a vibrating object. Almost any object can vibrate
and thus be a source of sound. In a PVC pipe, the source (air) is set into
vibration by hitting the end of the pipe with the palm of the hand. Once
disturbed, air within the tube vibrates creating a certain frequency
corresponding to the wavelength of the standing wave. A long wavelength has
a low the frequency creating a low pitch.
Music Sticks
Student Sheet
1. Use the data below in the data table to make a line graph in your journal.
How does length of a music stick change
the frequency of the vibrating air in the stick?
Length(cm) Frequency(Hz)
33
262
29
294
26
330
25
349
22
392
20
440
19
466
16
524
2. Write in your learning log the answer to the following question: How Does
length of a music stick change the frequency of the vibrating air in the
stick?
3. In your learning logs, predict the order of the sticks: Start with the stick
with the lowest pitch and end with the stick with the highest pitch.
(You can use the color on the music stick to write your order.)
4. Check to see if your prediction is supported by playing your music sticks.
How does the length of a music stick change
the frequency of the vibrating air in the stick?
Frequency (Hz)
550
525
500
475
450
425
400
375
350
325
300
275
250
225
200
175
150
125
100
75
50
25
0 2 4
6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
Length of Music Stick (cm)
Background Information
What is Sound?
Sound and music are parts of our everyday sensory experience. Just as humans have eyes for
the detection of light and color, so we are equipped with ears for the detection of sound. We
seldom take the time to ponder the characteristics and behaviors of sound and the mechanisms
by which sounds are produced, travels, and detected. The basis for an understanding of sound,
music and hearing is the physics of waves. Sound is a wave which results from the back and
forth vibration of the particles of the medium through which the sound wave is moving. If a
sound wave is moving from left to right through air, then particles of air will be displaced both
rightward and leftward as the energy of the sound wave passes through it. The motion of the
particles are parallel to the direction of the energy transport. This is what characterizes sound
waves in air as longitudinal waves.
Sound waves require a Medium
Another characteristic of sound is that all sound waves require a medium. Sound waves need
particles to vibrate to travel and be detected. Most of the sounds that you hear travel through
air at least part of the time. But sound waves can also travel through other materials, such as
water, glass, and metal.
Properties of Sound
Imagine you are swimming in a neighborhood pool. You hear many different sounds as you
float on the water. Some are high, like the laughter of small children, and some are low like the
voices of men. Some sounds are loud, like the BOING of the diving board, and some are soft,
like the sound of water lapping on the sides of the pool. The differences between the soundshow high or low and how loud and soft they are—depend on the properties of the sound
waves.
The speed of Sound
A sound wave is a disturbance which travels through a medium by means of particle-to-particle
interaction. As one particle becomes disturbed, it exerts a force on the next adjacent particle,
thus disturbing that particle from rest and transporting the energy through the medium. Like
any wave, the speed of a sound wave refers to how fast the disturbance is passed from particle
to particle. Since the speed of a wave is defined as the distance which a point on a wave (such
as a compression or a rarefaction) travels per unit of time, it is often expressed in units of
meters/second (abbreviated m/s). In equation form, this is
speed = distance/time
The faster a sound wave travels, the more distance it will cover in the same period of time. If a
sound wave is observed to travel a distance of 700 meters in 2 seconds, then the speed of the
wave would be 350 m/s. A slower wave would cover less distance - perhaps 660 meters - in the
same time period of 2 seconds and thus have a speed of 330 m/s. Faster waves cover more
distance in the same period of time.
The phase of matter has a tremendous impact on the speed at which sound travels. In general,
solids have the strongest interactions between particles, followed by liquids and then gases. For
this reason, longitudinal sound waves travel faster in solids than they do in liquids than they do
in gases.
vsolids > vliquids > vgases
The density of a medium also influences the speed of sound waves. The greater the density of
individual particles of the medium, the less responsive they will be to the interactions between
neighboring particles and the slower that the wave will be. As stated above, sound waves travel
faster in solids than they do in liquids than they do in gases. However, within a single phase of
matter, the property of density tends to be the property which has a greatest impact upon the
speed of sound. A sound wave will travel faster in a less dense material than a more dense
material. Thus, a sound wave will travel nearly three times faster in Helium as it will in air. This
is mostly due to the lower mass of Helium particles as compared to air particles.
The temperature of a medium will also influence the speed of sound. In general, the cooler the
medium, the slower the speed of sound. This happens because particles in cool materials move
slower than particles in warmer materials. When the particles move slower, they transmit
energy more slowly. Therefore sound travels more slowly in cold air than in hot air.
Pitch and Frequency
A sound wave, like any other wave, is introduced into a medium by a vibrating object. The
vibrating object is the source of the disturbance which moves through the medium. The
vibrating object which creates the disturbance could be the vocal
chords of a person, the vibrating string and sound board of a guitar or
violin, the vibrating tines of a tuning fork, or the vibrating diaphragm
of a radio speaker. Regardless of what vibrating object is creating the
sound wave, the particles of the medium through which the sound moves is vibrating in a back
and forth motion at a given frequency. The frequency of a wave refers to how often the
particles of the medium vibrate when a wave passes through the medium. The frequency of a
wave is measured as the number of complete back-and-forth vibrations of a particle of the
medium per unit of time. If a particle of air undergoes 1000 vibrations in 2 seconds, then the
frequency of the wave would be 500 vibrations per second. A commonly used unit for
frequency is the Hertz (Hz), where 1 Hertz = 1 vibration/second
As sound wave moves through a medium, each particle of the medium vibrates at the same
frequency. This is sensible since each particle vibrates due to the motion of its nearest
neighbor. The first particle of the medium begins vibrating, at say 500 Hz, and begins to set the
second particle into vibrational motion at the same frequency of 500 Hz. The process continues
throughout the medium; each particle vibrates at eh same frequency. And of course the
frequency at which each particle vibrates is the same as the frequency of the original source of
the sound wave. Subsequently, a guitar string vibrating at 500 Hz will set the air particles in the
room vibrating at the same frequency of 500 Hz which carries a sound signal to the ear of the
listener which is detected as a 500 Hz sound wave.
The ears of a human (and other animals) are sensitive detectors capable of detecting the
fluctuations in air pressure which impinge upon the eardrum. It is sufficient to say that the
human ear is capable of detecting sound waves with a wide range of frequencies, ranging
between approximately 20 Hz to 20 000 Hz. Any sound with a frequency below the audible
range of hearing (i.e., less than 20 Hz) is known as an infrasound and any sound with a
frequency above the audible range of hearing (i.e., more than 20 000 Hz) is known as an
ultrasound. Humans are not alone in their ability to detect a wide range of frequencies. Dogs
can detect frequencies as low as approximately 50 Hz and as high as 45 000 Hz. Cats can detect
frequencies as low as approximately 45 Hz and as high as 85 000 Hz. Bats, being nocturnal
creature, must rely on sound echolocation for navigation and hunting. Bats can detect
frequencies as high as 120 000 Hz. Dolphins can detect frequencies as high as 200 000 Hz. While
dogs, cats, bats, and dolphins have an unusual ability to detect ultrasound, an elephant
possesses the unusual ability to detect infrasound, having an audible range from approximately
5 Hz to approximately 10 000 Hz.
The sensation of frequencies is commonly referred to as the pitch of a sound. A high pitch
sound corresponds to a high frequency sound wave and a low pitch sound corresponds to a low
frequency sound wave.
Loudness Is Related to Amplitude
A vibrating guitar string forces surrounding air molecules to
be compressed and expanded, creating a disturbance. The
disturbance then travels from particle to particle through
the medium, transporting energy as it moves. The energy
which is carried by the disturbance was originally
transferred to the medium by the vibrating string. The
amount of energy which is transferred to the medium is
dependent upon the amplitude of vibrations of the guitar
string. If more energy is put into the plucking of the string
(that is, more work is done to displace the string a greater
amount from its rest position), then the string vibrates with a greater amplitude. The greater
amplitude of vibration of the guitar string thus imparts more energy to the medium, causing air
particles to be displaced a greater distance from their rest position. Subsequently, the
amplitude of vibration of the particles of the medium is increased, corresponding to an
increased amount of energy being carried by the particles.
Another example is beating of a drum. If you gently tap a bass drum, you will hear a soft
rumbling. But if you strike the drum with a large force, you will hear a loud BOOM. By
changing the force you use to strike the drum, you change the amount of energy transferred to
the drum. The drum moves with a larger vibration and transfers more energy to the
surrounding air. This increase in energy causes air particles to vibrate farther from their rest
positions (increase in amplitude).
The amount of energy which is transported past a given area of the medium per unit of time is
known as the intensity of the sound wave. The greater the amplitude of vibrations of the
particles of the medium, the greater the rate at which energy is transported through it, and the
more intense that the sound wave is.
While the intensity of a sound is a very objective quantity which can be measured with sensitive
instrumentation, the loudness of a sound is more of a subjective response which will vary with
a number of factors. The same sound will not be perceived to have the same loudness to all
individuals. Age is one factor which effects the human ear's response to a sound. Quite
obviously, your grandparents do not hear like they used to. The same intensity sound would not
be perceived to have the same loudness to them as it would to you. Despite the distinction
between intensity and loudness, it is safe to state that the more intense sounds will be
perceived to be the loudest sounds.
Doppler Effect
The Doppler effect is a phenomenon observed whenever the source of waves is moving with
respect to an observer. The Doppler effect can be described as the effect produced by a moving
source of waves in which there is an apparent upward shift in frequency for the observer and
the source are approaching and an apparent downward shift in frequency when the observer
and the source is receding.
The Doppler effect can be observed to occur with all types of waves - most notably water
waves, sound waves, and light waves We are most familiar with the Doppler effect because of
our experiences with sound waves. Perhaps you recall an instance in which a police car or
emergency vehicle was traveling towards you on the highway. As the car approached with its
siren blasting, the pitch of the siren sound (a measure of the siren's frequency) was high; and
then suddenly after the car passed by, the pitch of the siren sound was low. That was the
Doppler effect - a shift in the apparent frequency for a sound wave produced by a moving
source.
The Doppler effect is observed because the distance between the source of sound and the
observer is changing. If the source and the observer are approaching, then the distance is
decreasing and if the source and the observer are receding, then the distance is increasing. The
source of sound always emits the same frequency. Therefore, for the same period of time, the
same number of waves must fit between the source and the observer. If the distance is large,
then the waves can be spread apart; but if the distance is small, the waves must be compressed
into the smaller distance. For these reasons, if the source is moving towards the observer, the
observer perceives sound waves reaching him or her at a more frequent rate (high pitch). And if
the source is moving away from the observer, the observer perceives sound waves reaching
him or her at a less frequent rate (low pitch). It is important to note that the effect does not
result because of an actual change in the frequency of the source. The source puts out the
same frequency; the observer only perceives a different frequency because of the relative
motion between them.