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Sound, ultrasound and seismic waves
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Sound is a longitudinal wave. Frequencies above our hearing can see inside the body; earthquake waves reveal Earth's structure. Separate Physics only.
Hearing
Humans hear about 20 Hz to 20 000 Hz.
Sound makes the eardrum vibrate; small bones pass the vibrations on to the cochlea.
Sound makes the eardrum vibrate; small bones pass the vibrations on to the cochlea.
Ultrasound and infrasound
Ultrasound: above 20 000 Hz. Used in prenatal scans, sonar and finding cracks. Partly reflected at boundaries; the echo time gives the distance.
Infrasound: below 20 Hz. Detected from earthquakes, volcanoes and some animals.
Infrasound: below 20 Hz. Detected from earthquakes, volcanoes and some animals.
Seismic waves
P-waves: longitudinal, faster, travel through solids and liquids.
S-waves: transverse, slower, travel only through solids.
S-waves do not pass through the outer core, which shows it is liquid.
S-waves: transverse, slower, travel only through solids.
S-waves do not pass through the outer core, which shows it is liquid.
An ultrasound pulse returns from an organ after 0.0002 s. Sound travels at 1500 m/s in tissue. How deep is the organ?
- Total distance = 1500 × 0.0002 = 0.3 m
- Halve it: 0.15 m
Answer: 0.15 m
Hearing is about 20 Hz to 20 kHz. Ultrasound (above 20 kHz) uses echoes to measure distance. P-waves are longitudinal and pass through liquids; S-waves are transverse and do not, revealing a liquid outer core.
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