Sound and Sound Waves

Science, Grade 6


Table Of Contents: Sound and Sound Waves

1. Sound
Sound is a longitudinal wave created by vibrations, the back-and-forth motion of an object. Longitudinal waves are made of compressions and rarefactions and can only travel through a medium. Waves that must have a medium to transfer energy are called mechanical waves. The medium can be a solid, liquid, or gas. If there is no medium, such as in a vacuum, then the energy of sound will not be transferred. In the vacuum of outer space, an astronaut cannot hear any sounds when he is outside his spaceship. On Earth the air around us transfers sounds to our ears. We hear sound when the disturbance in the air reaches our ears.
2. Wave Interactions and Sound
We call the reflection of sound an echo. We do not always hear an echo but sound reflects off any barrier. We also know sound diffracts. If people are talking in the hallway we can hear them in the class room because sound diffracts around the opening of the doorway.
3. Speed of Sound
When two people are talking to you at once, you hear the words at the same time because each sound travels at the same speed. The speed of sound depends on the physical properties of the medium and its temperature. Sound generally travels faster in solids. The molecules of a solid are closer together and the energy transfers more easily. The speed of sound is also affected by temperature. Speed increases as the temperature of the medium increases. Higher temperatures mean the molecules are vibrating faster which transfers energy more easily.
4. Loudness and Amplitude
Loudness depends on the amplitude of the wave and the distance between the source and the observer. Sounds with higher amplitudes are louder. When the observer is closer to the source, sound is louder. The unit for measuring loudness is decibels (dB). Normal conversation is about 50 dB. A car horn is 100 dB. Sounds above 120 dB can be painful and may damage a person’s hearing.
5. Pitch and Frequency
Pitch is how high or low a sound seems to the listener. The pitch of a sound depends on its frequency. Higher pitched sounds have a higher frequency while lower pitched sounds have a lower frequency. All animals have a range of frequencies they can hear. Humans can hear sound in a range from 20 Hz to 20,000 Hz. You cannot hear sounds that are 50,000 Hz, but your cat can. Sound waves above 20,000 Hz are called ultrasonic.
6. Doppler Effect
Think about the sound of the siren on a fire truck as it approaches and passes you. What happens to the pitch of the siren? The Doppler effect occurs when the source of a sound is moving. As the source of a sound approaches the listener, the sound waves get pushed together. The frequency and pitch will seem higher to the observer. After the source passes the observer, the sound waves get pulled farther apart. The frequency and pitch will seem lower to the observer.