Sounds travel along longitudinal waves. It can travel through gases, liquids, and solids.
Demonstrating Sound Waves lab - For this, the students filled Zip-Lock bags with air and sealed them tightly. They held the bag over one ear while covering their other ear with their hand. A second student tapped on the bag with the eraser end of a pencil.
We then put water in the bags and tried this again. As a final test, the students held a wooden block against their ear while a friend tapped on the block with the pencil eraser.
This demonstrated how sounds can travel through gases, liquids, and solids but that sound waves differ when traveling through each.
Good Vibrations lab - Each student blew up a balloon and held the balloon next to one of their ears. A partner then pressed their lips against the balloon and spoke. We rotated around the room so everyone had a chance to play the roles of listener and speaker. This allowed the speaker to "feel" their voice as the balloon vibrated against their lips. The listeners were able to hear and feel the vibrations that created the sound waves.
Instruments
Different instruments made sounds in different ways. We spent some time looking at wind and string instruments and discussing how they make sounds.
Wind instruments such as a flute or trumpet use vibrating air to make sounds. A flutist blows across the hole at the top of a flute. Their breath then creates waves inside the flute's tube.
Guitars are string instruments. When a guitarist plucks a guitar string, they create rarefactions and compressions as the string vibrates back and forth. Shortening a guitar string makes it sound higher (it gives it a higher pitch).
Human Voices
When a person speaks or sings, air is forced forced from their lungs up through their voice box or larynx. The voice box is made up of two folds (vocal chords) that vibrate as the air from the lungs rushes past them. When the chords move together they form compressions; as they move apart they create rarefactions.
Pitch
Pitch describes the highness or lowness of a sound or musical note.
Frequency describes how often something happens. In waves, frequency refers to the number of waves that pass a certain point each second.
Higher pitch sounds have higher frequencies while lower pitch sounds have lower frequencies. High pitch sound waves are more compressed; low pitch sound waves are more rarefied.
The students watched this animation of pitch and frequency:
http://www.physics.org/explorelink.asp?id=4562&q=sound%20wave¤tpage=1&age=0&knowledge=0&item=1
(you may need to go to "What is Sound?" from the menu on the left and then find page 4 [you can click on the "4" at the top of the page.])
Straw Flute lab - We couldn't get this one to work very well despite trying straws of two different sizes.
Each student was given a drinking straw and pair of scissors. They made a "reed" by cutting a small section (about 1/2-in.) off each side of the top of the straw. The students then placed the straw in their mouths. They blew while pushing on the reed and tried to make a sound. We tried shortening the length of the straw which should have produced a higher pitch.
The point of this was to see that shortening the length of the medium that the wave is traveling in will increase the wave's pitch. Earlier we mentioned that a shorter guitar string will produce a higher sounding note. This is the same idea. The longer the column or tube of vibrating air, the lower the pitch. The shorter the column, the higher the pitch.
Twang lab - This one did allow us to hear differences in pitch caused by changing the length of the medium.
Each student was given a wooden ruler. They placed the rulers on the table with about 10 inches of the ruler hanging off the end of the table. They held down on the ruler with one hand and then pushed and released the free end of the ruler with their other hand. The students listened to the sound produced then tried this again while sliding the ruler further onto the table.
The shorter the ruler became, the higher the pitch it created.
You can see an animation of this lab ("Vibrations") on the University of Salford/Physics.org "Sounds Amazing" site. Click on "What is Sound?" on the left-hand side menu then click on page 2 from the menu at the top of the page. Make sure to hold down on the mouse button to hear the twangs (and make sure the volume on your computer is not up too high!)
String Music lab - This one was another lab that demonstrated that changing lengths of the medium changes the pitch of the sound.
We cut a length of string a little longer than the length of the table and then tied each end to a bucket. We then placed several rocks in each of the two buckets to act as weights. I placed a pencil under the string close to each bucket and then invited the students to pluck the string. They listened to the pitch of the sound created and then moved the pencils closer to each other. They tried plucking again and listening to the sounds. We noticed that as the pencils moved closer (shortening the length of the medium), the pitch became higher.
Loudness
The amplitude of a wave is the distance between the trough and the crest. It's the vertical length of the wave or how tall the wave is. I like to think of "altitude" (how high above sea level you are) to remember this because that word sounds a bit like "amplitude."
Loudness is related to amplitude. A wave with a higher amplitude will produce a louder sound.
Check out page 5 from "What is Sound?" on the University of Salford site: "Loudness and Decibels"
http://www.physics.org/explorelink.asp?id=4562&q=sound%20wave¤tpage=1&age=0&knowledge=0&item=1
Loudness is measured in decibels (dB). A higher decibel means something is louder. We went over the decibels of some common sounds:
*Rustling leaves - 10 dB
*Whisper - 20 dB
*Loud conversation - 60-70 dB
*Loud music - 90-100 dB
*Rock concert - 115-120 dB
*Jet engine - 120-170 dB
*Space shuttle engine - 200 dB
We also spent a bit of time talking about loud noises and the potential for hearing loss.
The Speed of Sound
Sound waves actually change speeds as they travel through different media or different forms of matter. The students are all aware of how the particles in solids, liquids, and gases are related so I built on that knowledge to show them how the speed of sound can change.
Solids - The particles in solids are very tightly packed. These particles vibrate next to each other but they don't move around. Because the particles are so close together, sounds travel quickly through solids. This is the type of matter that sounds travel fastest through.
Liquids - The particles in a liquid are a little more loose than those in a solid. They get to move around and over each other. This means sounds travel a little slower through liquids since sound waves have a bit further to go between particles.
Gases - Gases are the least tightly packed. The particles in gases are able to buzz all over and move a lot. Sounds travel slowest through gases.
When scientists talk about the "speed of sound" they are referring to the speed of sound through air (gas) at room temperature (~76 degrees F). This is about 340 meters/second (about twice as fast as a jet plane). Here's how the speed of sound differs in different materials:
*Fresh water - 1,490 m/s.
*Plastic - 1,800 m/s.
*Steel - 5,200 m/s.
*Glass - 4,540 m/s.
The students seemed pretty interested in this so we spent a bit of time talking about the speed of sound, sonic booms, and Concorde/supersonic aircraft. We even looked up the speed of light (299,792,458 m/s) and compared that to the speed of sound. :)
Sounds and Temperature
When I was little, my parents would tell my siblings and I to "keep it down" when we were outside at night since "your voices carry at night." Here's the physics behind that.
It's all related to temperature. During the day, sound waves are able to spread out due to a higher air temperature. At night, the cool air sinks closer to the ground while the warmer air rises. This cooler air causes the sound waves to spread out and bend (this is known as diffraction). The sound waves, then, travel further at night or in cooler temperatures.
If you go to the University of Salford site and click on Lesson 3: Wave Behaviour then on page 6 at the top you'll get the "Refraction" animation. This shows how the train sounds differently at night and during the day.
http://www.physics.org/explorelink.asp?id=4562&q=sound%20wave¤tpage=1&age=0&knowledge=0&item=1
Diffraction also allows you to hear sounds around corners.
Page 7 of Wave Behaviour from the U. of Salford site shows this.
http://www.physics.org/explorelink.asp?id=4562&q=sound%20wave¤tpage=1&age=0&knowledge=0&item=1
Just like light waves, sound waves can also be reflected. This is what happens in an echo when sound waves bounce off a mountain or the walls of a cave.
Echolocation and sonar also use reflected sound waves.
We played around on Page 3 of Wave Behaviour (slides 1 and 2) to view animations of echoes and echolocation.
http://www.physics.org/explorelink.asp?id=4562&q=sound%20wave¤tpage=1&age=0&knowledge=0&item=1
The Doppler Effect
To finish up class we spent a little time talking about the Doppler Effect. The Doppler Effect relates to pitch and frequency of sound waves. We know higher frequency equals higher pitch.
I asked the class if they've ever listened to a police, fire, or ambulance siren (pay attention to this next time you hear one). As the vehicle with the siren moves toward you, the pitch increases. As it goes by and moves away, the pitch decreases. You would expect, given what you know about frequency and pitch, that the frequency of the waves is changing as the car moves. The frequency actually isn't changing. This apparent change is frequency is known as the Doppler Effect.
Christian Doppler was an Austrian scientist. In 1845 he conducted an experiment with a train and a group of musicians. He put the band on a flatbed train car and instructed them to play while the train moved. Doppler stood on the ground near the tracks and listened. As the train approached, he noticed the band seemed to be playing at a higher pitch than they were as they moved past and away from him. Doppler then tried standing on the train car while the musicians played on the ground next to the tracks. He noticed the same thing.
Scientists believe this change in pitch occurs not because of a change in frequency, but because of the motion of the source of the sound and the motion of the observer/listener.
Think about passing a ball to a friend. If you and your friend stand still and you throw one ball to your friend every second, your friend will receive one ball every second. The frequency is 1 ball per second.
However, if you move toward your friend while throwing the ball, your friend will receive more than one ball per second. The frequency of the balls being caught increases because the wavelength (the distance between the crests of two waves or, in this case, between yourself and your friend) decreases. This is similar to the experiments we conducted earlier - "Twang" and "String Music." In those, a shorter length created a higher pitch.
We had one more lab that we didn't get to. Here's a link with the details so you can try it at home.
Reminder:
No classes the next two weeks due to STAR Testing (April 12-14) and Spring Break (April 18-22).
We will begin a two-week study of electricity when we come back to class on April 28.
References:
The animations we used are all from the University of Salford (that's where my dad went to university) in Manchester, England. Those web pages were designed for high school students getting ready to take the physics portions of their university entry exams - A levels and GCSE.
There's tons of great information and the little clips really make it easy to understand.
http://www.physics.org/explorelink.asp?id=4562&q=sound%20wave¤tpage=1&age=0&knowledge=0&item=1
The labs "String Music," "Twang," and "Straw Flute" are all from Physics for Every Kid.
VanCleave, J. (1991). Physics for Every Kid: 101 Easy Experiments in Motion, Heat, Light, Machines, and Sound. San Francisco: Jossey-Bass.
The lab "Demonstrating Sound Waves" was found on this website (lots of others to try at home):
The lab "Good Vibrations" was found here:
Thank you for sharing your lessons. I learned so much from your background information. Great ideas!
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