Ultrasound and infrasound
Sound waves are longitudinal waves that must pass through a medium. Ultrasound waves have a frequency above the normal range of human hearing. They can be used to scan for birth defects in unborn babies and for defects in manufactured equipment. Infrasound has a frequency below normal hearing. Infrasound can be used to track animals and monitor seismic activity.
Sound waves are longitudinal waves. Their vibrations occur in the same direction as the direction of travel. Sound waves can only travel through a solid, liquid or gas.
When an object or substance vibrates, it produces sound. The bigger the vibrations, the greater the amplitude [amplitude: The maximum height of a wave, measured from the mid-point of its vibration. ] and the louder the sound.
These diagrams show snapshots from oscilloscope traces of three sounds.
Diagrams 1 and 2 show two sounds with the same frequency but different amplitude (the height of the trace). The trace on 1 comes from a sound with a smaller amplitude than on 2. Sound 1 is quieter than sound 2.
Diagrams 2 and 3 show two sounds with the same amplitude but different frequencies. The faster the vibrations, the higher the frequency [frequency: The number of repetitions per second of a wave. The unit of frequency is the hertz, 'Hz'. ] and the more highly pitched the sound.
The trace on 3 comes from a sound with a higher frequency than the one on 2. So sounds 2 and 3 are the same volume (loudness), but 3 is higher pitched.
The normal range of human hearing is between about 20Hz and 20kHz, but the range becomes less as we get older. Sounds with frequencies above about 20kHz are called ultrasound.
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