Sound is everywhere. It can travel through solids, liquids, and gases, but it does so at different speeds. It can rustle through trees at 770 MPH (miles per hour), echo through the ocean at 3,270 MPH, and resonate through solid rock at 8,600 MPH.
Sound is made by things vibrating back and forth, whether it’s a guitar string, drum head, or clarinet. The back and forth motion of an object (like the drum head) creates a sound wave in the air that looks a lot like a ripple in a pond after you throw a rock in. It radiates outward, vibrating it’s neighboring air molecules until they are moving around, too. This chain reaction keeps happening until it reaches your ears, where your “sound detectors” pick up the vibration and works with your brain to turn it into sound.
As long as there are molecules around, sound will be traveling though them. That’s why if you put an alarm clock inside a glass jar and remove the air, there’s no sound from the clock. There’s nothing to transfer the vibrational energy to. It’s like trying to get a ripple on a frozen pond. The rock just stops on the surface.
Sound can change its travel speed. Another word for sound speed is pitch. When the speed slows down, like with drumming, the sound speed is low. With clarinet reeds, it’s high. Guitar strings can do both, as they are adjustable. If you look carefully, you can actually see the low pitch strings vibrate back and forth, but the high pitch strings move so quickly it’s hard to see. But you can detect the effects of both with your ears. The range of your ears is about 20 – 20,000 Hz (cycles per second). Bats and dogs can hear a lot higher than we can!
Let’s try out some experiments with sonic vibrations:
Bobby Pin Strummer: Straighten three bobby pins. (A bobby pin, when straightened, has two different sides – a smooth side and a rippled side.) Wrap a rubber band tightly around the base of an empty tin can. Slip in a clothespin under the rubber band, jaw-end first. Add three clothespins, about 120 degrees apart. Clamp the rippled end of a bobby pin into each clothespin, so that your contraction low looks like a can with three legs. Strum each pin, one at a time. What happens if you clamp the pins at different heights?
String Test: Push the end of a length of heavy string and a length of light thread through a hole punched in the bottom of a can. Tie the ends inside the can to a paperclip so they stay put. The can should have two different strings coming out of the bottom. Place the can near your ear as you strum each strand. Can you make the pitch high and low? What other types of string, yarn, thread, fishing line, etc. can you use?
Mystery Pitch: Blow across the mouth of an empty soda or water bottle to make a whistling sound. Add a little water and try again. Add more water and try again. Add more water.. What happens if you use a glass bottle? Place an empty glass under the sink faucet and tap the side of it with a fork and listen to the sound. Slowly fill the glass with water while you continue to tap. What happens if you use a spoon? Knife? Whisk? Wooden spoon? Of these two experiments above, which one increases the pitch and which decreases the pitch?
Phonograph: Pierce the bottom of an empty plastic container (such as a butter tub or yogurt container) with a tack. Choose an old record that you don’t mind scratching and place it on a turntable. (In a pinch, you can place it on a pencil tip and spin, but this takes practice.) Use your new “stylus” and hear the music! Doe the size of the container matter? What if you use a pin instead of a tack?
Easy Guitars: Wrap a rubber band lengthwise around a ruler. Push a pencil crosswise (perpendicular) under the rubber band. Pluck the rubber band. What happens if you move the pencil? What if you use a different kind of rubber band? What if you use a different sized ruler?
Sneaky Clocks: Place an alarm clock (the kind that ticks) or a timer that is ringing on a table and listen. Now place your ear on the table. Fill a zipper bag of water and press it between you and the clock to hear the difference. Next, place the clock in a closed metal can (like a cookie tin or coffee can). What about a paper bag? A glass jar? A newspaper-filled shoebox?
Stryo-Phone: Make yourself a telephone by punching a small hole in the bottom of two cups (foam, paper, tin… is there a difference?) and threading string into each one. Tie the end of the string inside the cup to a paper clip so the string stays put. Cut the phones apart and tie each end to a slinky and try it out. What happens when you bang the slinky into different things, like walls, metal chairs, wood tables, or the floor? Untie the slinky and try a wire coat hanger, fork, spoon, or other kitchen utensil.