In the last lesson we spent time playing with our sound antenna’s, our ears. We have quite an amazing scientific instrument on our heads don’t you think? All of our senses do things naturally that are very difficult for science and technology to duplicate. The human being (and other living things for that matter) have remarkable ways of perceiving the world around them. In this lesson, we are going to spend time with the speed of sound.
Speed of Sound
Sound is a type of energy, and energy moves by waves. So sound moves from one place to another by waves; longitudinal waves to be more specific. So, how fast do sound waves travel? Well, that’s a bit of a tricky question. The speed of the wave depends on what kind of stuff the wave is moving through. The more dense (thicker) the material, the faster sound can travel through it. Remember that waves move because the particles bounce off one another? Well, the farther the particles are from one another, the longer it takes one particle to bounce off another. Think about a row of dominoes. If you put them all close together and push one over they all fall down pretty quick. If you spread them out a bit, the row falls much more slowly. (Be sure to watch the Domino movie in the members section of the website.) Sound waves move the same way. Sound moves faster in solid objects than it does in air because the molecules are very close together in a solid and very far apart in a gas. For example, sound travels at about 760 mph in air, 3300 mph in water, 11,400 mph in aluminum, and 27,000 mph in diamond! The temperature of the material also makes a difference. The colder the material, the faster the sound. This is why sound seems to be louder or clearer in the winter or at night. The air is a little cooler and since it’s cooler, the molecules are a little more tightly packed. Check out this movie for a video of this. Let’s play with this a bit with the following experiments.
Keeping Your Ear to the Ground
Spoon (or whatever is handy)
1. Sit at a table.
2. Have your partner sit at the other end of the table.
3. Have your partner very lightly scratch the table with the spoon.
4. Listen to see if you can hear it.
5. While your partner is scratching, put your ear on the table. Do you hear a
difference in the sound?
6. Switch roles so your partner gets a chance to try.
Did you notice how you could hear the soft spoon-scratching sound (I love a good alliteration) quite clearly when your head was on the table? The sound waves moved quickly through the table so they lost little of the loudness and quality of the original sound. When sound travels through the air, the sound energy gets dispersed (spread out) much more than through the table, so the sound does not travel as far nor as clearly. This next one is an oldie but a goodie!
A Couple Cups of Conversation
2 paper or foam cups
20 feet of string
Scissors or a nail
1. Using the scissors or a nail poke a hole in the middle of the bottom of both cups. Get an adult to help you with this. Since this isn’t biology, no bleeding allowed!
2. Thread an end of the string through the hole in the bottom of the cup and tie a big knot in it to keep it from sliding through the hole.
3. Do the same thing with the other cup so that when you are done you have a cup attached to both ends of the string.
4. Take one of the cups for yourself and hand the other cup to your partner. Walk apart from one another until the string is fairly taut.
5. Have your partner hold the cup up to his or her ear while you whisper into your cup.
6. Can your partner hear you? If not, see if you can stretch the string a little more.
7. Switch roles and try again.
The string being a solid and having tightly packed molecules allows the sound wave to move quickly and clearly through it. You can talk very quietly in one cup and yet your partner can still hear you fairly well. Let’s try one more.
The Voices in Your Head
1. Rub the spoon with your fingernail. Can you hear the sound? Is it very loud?
2. Put the bowl end of the spoon in your mouth and bite it (not too hard please).
3. Scratch the spoon again with your fingernail. Can you hear the sound better now than before?
Your head is also a solid (some of us have more air in our heads than others, but on the whole, heads are solids). The sound energy travels up the spoon, through our bones, and into our ears. This is why your voice sounds different to you than to other folks. Your voice vibrates through your bones to your ears.
Speed of Light and Sound
Have you ever experienced seeing something before you heard it? For example, have you ever been to a baseball game and seen the bat hit the ball a bit before you heard it hit? Or (and I’m sure you’ve experienced this) seen lightning and then heard thunder quite a bit later. The reason you hear things after you see them is because sound travels much more slowly than light. Don’t get me wrong, sound travels fast, but compared to light, it’s a snail! Sound in air travels at about 760 mph. Commercial airliners travel at about 550mph. If something travels at the speed of sound it is said to be going Mach 1. Mach 2 is twice the speed of sound and so on. Light, on the other hand travels at about 670,000,000 (that’s six hundred and seventy million!) miles an hour. It would take a sound a little less then a day and a half (32 hours) to go around the Earth (25,000 miles). It would take light less than the time it takes to snap a finger (a tenth of a second) to go around the Earth! Try this.
Like An Old Japanese Movie
Two hunks of wood or a pair of baseball bats
A neighborhood block
1. Give your partner the two bats or hunks of wood.
2. Have them walk a half a block away or at least 250 feet.
3. When they get there have them clack together the two pieces of wood (be careful not to smash fingers, you want to hear the wood, not the scream of you friend!).
4. Have them do it several times. Try to notice a difference between when you see the wood crashing together and when you hear it. If you don’t hear a difference, get farther apart from one another.
5. Trade places so that your partner can see the delay of sound (just like on one of those old Japanese movies).
Sound travels at about 760 mph. That’s the same as about 1000 ft/sec. If your friend was standing about 250 feet away from you, it took a quarter of a second for the sound to get from your partner to you. Here’s one more:
Thunder and Lightning
You may already know this but if not this is a good one.
A thunder storm
A timer if you wish
1. Wait until you see a lightning flash (do this indoors please!).
2. Now count “one Mississippi, two Mississippi”, etc. Or start your timer.
3. Stop counting when you hear the thunder.
4. Take whatever number you’ve reached and divide it by 5. That number is how many miles the lightning strike is away from you. So if you’ve counted for about 3 seconds the lightning strike is about a half mile away. If you’ve counted for 5 seconds the lightning is 1 mile away. If you’ve counted for 8 seconds the lightning is about 1.5 miles away.
Remember, sound travels at about 1000 ft/sec. A mile is a little over 5000 feet (5280 ft. to be exact). So it takes sound about 5 seconds to go 1 mile!
The next time you’re at a baseball game or a fireworks display try to time the difference between the time you see something and the time you hear something. Remember that sound travels 1000 ft/sec. If the distance is great enough you may be able to figure out how far away it is and amaze your friends!
Sound is a type of energy and moves by longitudinal waves.
Sound moves faster in solid objects than it does in air because the molecules are very close together in a solid and very far apart in a gas.
Sound travels at about 760 mph in air, about 1000 ft/s. Sound can travel a mile in 5 seconds.
Light travels much faster than sound.
1. Sound travels by waves. Transverse or longitudinal waves?
2. Sound travels faster in air, water, or solids?
3. Why does sound travel faster in that medium?
4. Would sound travel faster on a hot day or a cold day? Why?
5. Which travels faster, light or sound?
6. If you see a firework and hear the sound one second later, how far away is the firework?
7. If you see lightning and hear the lightning 10 Mississippi’s, uh I mean seconds, later, how far is the lightning?
1. Longitudinal. The waves travel with the medium.
3. The particles are close together. The closer the particles the faster sound travels.
4. A cold day, since the molecules are closer together.
5. Light is much faster.
6. Sound travels 1000 ft/sec, so that firework is 1000 feet away.
7. Take 10 seconds and divide it by 5. So the lightning is 2 miles away.