Vibrations: Seeing and Feeling Sound
Sounds are created from vibrations, whether it’s a tuning fork, the reed of a musical instrument, or sound coming from someone’s voice box.
The vibrations make the air molecules move backwards and forwards, creating something called a compression wave. The first set of air molecules pushes against the next set of air molecules, and the next, and the next, carrying the sound.
Slower vibrations produce lower notes and faster vibrations produce higher notes. The speed of the vibrations is called the frequency, and the highness or lowness of the note is called the pitch.
Making Sounds Using VibrationsLoop a rubber band around a door handle, and then pull it tight. Pluck the rubber band like the strings on a guitar and watch them vibrate. What happens when the rubber band is stretched more to make the ‘strings’ longer? Blow up a balloon and let the air out slowly by pulling the sides of the neck. What happens when the sides of the neck are pulled further apart?
Stretching the elastic band or the neck tighter makes the note higher because it vibrates faster. When tuning a violin or guitar, turning the tuning pegs makes the strings looser or tighter, so lower or higher.
Put a ruler over the edge of a table, hold one end firmly on the table, push the other end down and let it go. Make the overhanging part longer or shorter – how does this change the note?
The longer the ruler, the slower the vibrations so the lower the note.
Feeling SoundTap a tuning fork and listen to the sound. Gently touch the ends of the tuning fork to feel the vibrations. Tuning forks always produce the same note, so they are used to help tune pianos. Touching the tuning fork stops the vibrations, so stops the sound.
Gently lay a finger on someone’s voice box (larynx) when he or she is making low-pitched and high-pitched sounds. Do the low and high sounds feel different?
In the voice box, the vibrations are made by air passing over the vocal chords and making them vibrate. High-pitched sounds come from high frequency vibrations, and low-pitched sounds from low frequency vibrations.
Watching Sound Waves Using WaterTap a tuning fork against the edge of the table and then put the prongs about 5 centimetres under the surface of a glass of water – what happens to the water? What happens with a lower or higher pitch tuning fork?
The vibrating tuning fork makes waves in the water. A lower tuning fork produces slower vibrations, so fewer waves.
Watching Sound Waves Using LightCut a hole, about 8-10 cm across, in the side of a plastic milk container and cover it with tissue paper. Tape the edges down to hold the tissue paper taut, lay the milk container down flat, and rest a small mirror on the tissue. Turn the lights off and shine a torch on the mirror at an angle so that the light reflects onto the wall. Get someone to talk into the open neck of the container and watch what happens to the light. Is there any difference when they talk quietly or loudly, or in a high or deep voice?
The sound vibrations from the voice travel through the air, making the tissue and the mirror vibrate.