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== Age ==
{| class="wikitable" style="color:black; background-color:#ddd; margin-left: auto; float:right"
| [[Physics]], [[Astronomy]]:
| Sound, Waves, Doppler Effect
|-
| Grade Range:
| [[Elementary School]], [[Middle School]], [[High School]]
|-
| Format:
| [[Stage]]
|}


Elementary School, Middle School
This demonstration uses a foam ball and a noisemaker to demonstrate the Doppler Effect. This demonstration easily connects to everyday life for most students, and they will enjoy knowing an answer to one of the most common phenomenon they experience!
 
== Format ==
 
Hands-on


== Materials ==
== Materials ==


    Doppler Ball
* Doppler Ball
    A working 9V battery + backup
* A 9V battery  


== Safety Precautions ==
== Safety Precautions ==


Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience. Depending on the age, judge how hard you should be throwing the ball. A simple underhand toss will suffice for most participants.
Please see the General Safety Precautions in the [[Demonstration Safety]] Page.
 
== Preparation ==
 
Ensure that the Doppler ball has a working battery (9V). Make sure there is a large enough space available for tossing the ball (preferably a location quiet enough to hear the ball).


== Demonstration ==
== Demonstration ==


Explain the Doppler effect (see script and scientific explanation below) Toss the Doppler ball back and forth and point out the change in pitch. If the audience is of an appropriate age, you can choose members of the audience to throw to. If they are very young, you should only toss the ball between presenters.
# Show the Doppler Ball to the students. Turn it on briefly to let them hear the sound, and explain that they need to listen to that sound as you throw the ball back and forth with your volunteer.
 
# Turn on the ball and start tossing it back and forth from across the room. If students are in bleachers or seats, toss it back and forth in front of them. If they are sitting on the ground, have your volunteer go behind the crowd and throw it over their heads.
== What to Say ==
# Ask students to describe how it sounded:
 
## While it was coming '''towards them'''.
Start with asking the participants if they have ever heard the sound a train makes when it whizzes past them really fast (try to imitate the sound with your mouth).
## While it was moving '''away from them'''.
# Ask for ideas on why the pitch of the sound increased as it came towards them and decreased as it went away from them.


Sound travels as a series of compressions of air. When a sound source is moving, the compressions pile up in front of the object and spread out behind the object. The compression or spreading of sound waves changes the pitch of the sound thus creating the familiar phenomena we hear when a train goes by.
== Why This Works ==


Depending on the age of the audience, you should incorporate material from the "Why It Is" section in your demonstration.
===Short Explanation===


== Why It Is ==
Sound is a type of '''''Wave'''''. Sound waves are compression waves, which means they travel through matter, like air, by making pockets of high and low pressure. Sound waves travel pretty fast, but we can hear the difference in how fast an object is moving by listening to how the sound coming from it changes. When the ball is traveling towards you, the sound waves are ''slightly closer together'', which increases the pitch you hear. The opposite is also true; when the ball travels away from you, the sound waves are ''slightly farther apart'', which decreases the pitch. This change in pitch is known as the '''''Doppler Effect''''', which states that the sound you hear from a source can have a different pitch if the source is getting closer or farther from you.


Sound is a wave that travels through a medium (solid, liquid, or gas) by a series of compressions. Since the compressions occur at a variety of regular time intervals, we perceive a variety of frequencies or "pitch" of sound. (the human ear can hear from about 20Hz to about 20,000Hz or 20 compressions per second to 20,000 compressions per second).
The Doppler Effect also applies to Astronomy, since it is an effect that happens with all waves! With light, we call it the '''''Redshift/Blueshift Effect'''''. If you are observing a distant star, it might appear red even if the star isn't red. This means that the star is moving away from us, so the light waves coming from it are ''slightly farther apart'', making it appear more red! likewise, if the star is moving towards us, the light waves might appear ''slightly closer together'', which would make it look more blue!


When an object is moving and emitting sound, the compressions in front of the object in a sense get piled up. They occur more frequently, which raises the frequency of the sound and makes the sound higher in pitch.
===Full Explanation===


When an object is moving away from the observer and emitting sound, then the waves are allowed to spread out (opposite of what is happening in front of the object). The compressions are spread out and are then heard less frequently which makes the pitch of the sound lower.
Sound is a wave that travels through a medium (solid, liquid, or gas) by a series of compressions. Since the compression waves occur at a variety of regular time intervals, we can perceive a variety of frequencies, or "pitches" of sound. The human ear can hear from about 20Hz to about 20,000Hz or 20 compression waves per second to 20,000 compression waves per second! When an object is moving and emitting sound, the distance between the compression waves can change, resulting in a change of what pitch we hear. This change in pitch is what we call the '''''Doppler Effect''''', and we can calculate the change by use of the following equation:


A simple equation explaining the change in the frequency is as follows.
{| class="wikitable" style="color:black; background-color:#ddd; text-align: center; margin: auto"
| Frequency heard (receiver), '''f''' =
| [('''v + v<sub>r</sub>''') / ('''v + v<sub>s</sub>''')] * '''f<sub>0</sub>'''
|-
| Frequency heard (thrower), '''f''' =
| [('''v + v<sub>s</sub>''') / ('''v + v<sub>r</sub>''')] * '''f<sub>0</sub>'''
|}


In this equation, f0 is the initial frequency of the emitted sound, v is the velocity of sound in the medium (e.g. air), and vs is the velocity of the object emitting the sound. (Note: This equation assumes that the receiver (you) are standing still relative to the medium (i.e. air).) We can see that if the velocity of the object is positive (moving away) then the frequency will be a fraction of the original frequency and if the velocity of the object is negative (moving towards observer), then the frequency will be larger than the original frequency.
In this equation, '''v''' is the speed of sound, '''v<sub>r</sub>''' is the velocity of the ball moving towards the receiver, '''v<sub>s</sub>''' is the velocity of the ball moving towards the thrower, and '''f<sub>0</sub>''' is the original frequency. Since the thrower is, well, throwing the ball to the receiver, the '''v<sub>r</sub>''' value will be positive, while the '''v<sub>s</sub>''' value will be negative. This means that in the first equation, we will have a multiplication factor greater than 1.0, resulting in an increased frequency. Likewise, the second equation will have a multiplication factor of less than 1.0, resulting in a decreased frequency.


== Real Life Examples ==
This effect applies to all types of waves, including light waves, and is a known pheonomena in Astronomy called the '''''Redshift/Blueshift Effect'''''. If astronomers are observing a distant object, such as a star, they will make calculations based on their observations to determine the mass, direction of movement, and color of the star. However, they sometimes will look at a star and find that, instead of it looking yellow, it might look red. This tells the scientist that the star must be moving ''away'' from us, and they can calculate the speed of the star by using what they know (it should be yellow) and what they see (it looks red). Likewise, the opposite can happen where the star might look blue instead, meaning that it is moving towards us. They can then calculate using what they know (it should be yellow) and what they see (it looks blue) to determine how fast the star is moving.


Often we hear this effect when we listen to a train whistle as the train passes by or when a police car with its siren blaring approaches and then passes and moves away. Another example is a race car that "vrooms" past. The effect happens whenever there is a sound emitting source moving in a radial direction from the observer.
==Tips and Tricks==


This effect can also occur with light. Light can be Doppler shifted and change color. This is useful in astronomy when calculating the motion of stars and galaxies.
* Often we hear this effect when we listen to a train whistle as the train passes by or when a police car with its siren blaring approaches and then passes and moves away. Another example is a race car that "vrooms" past. The effect happens whenever there is a sound emitting source moving in a radial direction from the observer.
* This demonstration is a part of the [[Astronomy Show]].

Latest revision as of 15:47, 25 March 2016

Physics, Astronomy: Sound, Waves, Doppler Effect
Grade Range: Elementary School, Middle School, High School
Format: Stage

This demonstration uses a foam ball and a noisemaker to demonstrate the Doppler Effect. This demonstration easily connects to everyday life for most students, and they will enjoy knowing an answer to one of the most common phenomenon they experience!

Materials

  • Doppler Ball
  • A 9V battery

Safety Precautions

Please see the General Safety Precautions in the Demonstration Safety Page.

Demonstration

  1. Show the Doppler Ball to the students. Turn it on briefly to let them hear the sound, and explain that they need to listen to that sound as you throw the ball back and forth with your volunteer.
  2. Turn on the ball and start tossing it back and forth from across the room. If students are in bleachers or seats, toss it back and forth in front of them. If they are sitting on the ground, have your volunteer go behind the crowd and throw it over their heads.
  3. Ask students to describe how it sounded:
    1. While it was coming towards them.
    2. While it was moving away from them.
  4. Ask for ideas on why the pitch of the sound increased as it came towards them and decreased as it went away from them.

Why This Works

Short Explanation

Sound is a type of Wave. Sound waves are compression waves, which means they travel through matter, like air, by making pockets of high and low pressure. Sound waves travel pretty fast, but we can hear the difference in how fast an object is moving by listening to how the sound coming from it changes. When the ball is traveling towards you, the sound waves are slightly closer together, which increases the pitch you hear. The opposite is also true; when the ball travels away from you, the sound waves are slightly farther apart, which decreases the pitch. This change in pitch is known as the Doppler Effect, which states that the sound you hear from a source can have a different pitch if the source is getting closer or farther from you.

The Doppler Effect also applies to Astronomy, since it is an effect that happens with all waves! With light, we call it the Redshift/Blueshift Effect. If you are observing a distant star, it might appear red even if the star isn't red. This means that the star is moving away from us, so the light waves coming from it are slightly farther apart, making it appear more red! likewise, if the star is moving towards us, the light waves might appear slightly closer together, which would make it look more blue!

Full Explanation

Sound is a wave that travels through a medium (solid, liquid, or gas) by a series of compressions. Since the compression waves occur at a variety of regular time intervals, we can perceive a variety of frequencies, or "pitches" of sound. The human ear can hear from about 20Hz to about 20,000Hz or 20 compression waves per second to 20,000 compression waves per second! When an object is moving and emitting sound, the distance between the compression waves can change, resulting in a change of what pitch we hear. This change in pitch is what we call the Doppler Effect, and we can calculate the change by use of the following equation:

Frequency heard (receiver), f = [(v + vr) / (v + vs)] * f0
Frequency heard (thrower), f = [(v + vs) / (v + vr)] * f0

In this equation, v is the speed of sound, vr is the velocity of the ball moving towards the receiver, vs is the velocity of the ball moving towards the thrower, and f0 is the original frequency. Since the thrower is, well, throwing the ball to the receiver, the vr value will be positive, while the vs value will be negative. This means that in the first equation, we will have a multiplication factor greater than 1.0, resulting in an increased frequency. Likewise, the second equation will have a multiplication factor of less than 1.0, resulting in a decreased frequency.

This effect applies to all types of waves, including light waves, and is a known pheonomena in Astronomy called the Redshift/Blueshift Effect. If astronomers are observing a distant object, such as a star, they will make calculations based on their observations to determine the mass, direction of movement, and color of the star. However, they sometimes will look at a star and find that, instead of it looking yellow, it might look red. This tells the scientist that the star must be moving away from us, and they can calculate the speed of the star by using what they know (it should be yellow) and what they see (it looks red). Likewise, the opposite can happen where the star might look blue instead, meaning that it is moving towards us. They can then calculate using what they know (it should be yellow) and what they see (it looks blue) to determine how fast the star is moving.

Tips and Tricks

  • Often we hear this effect when we listen to a train whistle as the train passes by or when a police car with its siren blaring approaches and then passes and moves away. Another example is a race car that "vrooms" past. The effect happens whenever there is a sound emitting source moving in a radial direction from the observer.
  • This demonstration is a part of the Astronomy Show.
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