This topic accounts for approximately 7% of your exam marks.
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Stable7%
Speed of sound calculations, echo timing and ultrasound uses are standard shorter questions.
The audible range
A healthy young human ear responds to sound waves with frequencies roughly 20 Hz to 20 000 Hz (20 kHz)
This range narrows with age; older adults often cannot hear above 12–15 kHz, while children may still hear close to the full upper limit
Many other animals have very different ranges:
dogs hear up to about 45 kHz (which is why high-frequency dog whistles work)
bats hear and emit sounds up to roughly 120 kHz for echolocation
elephants and whales can produce and detect frequencies well below 20 Hz
Below the range: infrasound
Infrasound is sound with a frequency below 20 Hz, too low for the human ear
Sources of infrasound include earthquakes, volcanic activity, large explosions, and slow-rotating industrial machinery
The body sometimes detects infrasound as a feeling of vibration in the chest rather than as a heard tone
Above the range: ultrasound
Ultrasound is sound with a frequency above 20 000 Hz, too high for the human ear
Common uses of ultrasound include medical scanning (prenatal scans, scans of internal organs), industrial cleaning of delicate parts, sonar in ships and submarines, and animal-distance sensors
Ultrasound is covered in more detail in later topics