Production of sound
Sound is a result of vibrating objects that cause a vibration in air molecules that lead to a series of compressions and rarefactions.
Recall the sound is a longitudinal wave. Compressions are when air molecules are closest together and rarefactions are when they are furthest apart.
We hear sound when the sound waves reach our ear and cause our ear drums to vibrate. We can hear frequencies from about 20 Hz to 20 kHz – Any more or any less is very difficult for our ears to pick up.
Remember that all waves (including sound) have a frequency and an amplitude:
- The frequency of a wave is the number of waves that pass a fixed spot per second (unit=Hz). The higher the frequency, the higher the pitch of the sound.
- The amplitude of the wave is the maximum displacement of the vibrating particles. The larger the amplitude, the louder the sound will be.
Speed of sound
Speed of sound through various mediums
Sound waves cannot travel through vacuum. They must be transmitted through vibrations of particles within a medium. The closer the particles are within the medium, the faster sound will travel. For example, the speed of sound is:
- 330 m/s in gases i.e. air
- 1500 m/s in liquids i.e. water
- 5000 m/s in metals i.e. metals
Air is a gas so particles are very spread out, which is why sound does not travel very fast. Metals on the other hand are generally solids, and particles are very closer together allowing fast transmission of sound waves.
Determining the speed of sound in air
Above we state that the speed of sound is approximately 330 m/s in air. We can experimentally prove this by using this set-up:
Two microphones are separated by exactly 1m. They are connected to a digital timer that starts when it gets signal from microphone 1 and stops when it gets signal from microphone 2.
A hammer is used to hit a metal block which generates sound. The sound waves will hit microphone 1 first which starts the timer, and then hit microphone 2 which stops the timer.
Since the microphones are separated by 1m, the sound has traveled 1m in 0.003 seconds.
Since speed = distance / time, 1/0.003 = 330 m/s
Echoes
Just like light waves can be reflected off a boundary, so can sound waves. When sound gets reflected off a surface, it generates an echo.
