Sound · Grade 8
Sound is made by vibrations, which pass through a material as a longitudinal wave of squashes (compressions) and stretches (rarefactions). It needs particles, so it cannot cross empty space. Bigger vibrations sound louder; faster vibrations sound higher. Echoes and ultrasound let us measure distances – from cliffs and sea beds to babies in the womb.
Key facts
- All sounds come from vibrations; sound needs a medium (solid, liquid or gas).
- Loudness depends on amplitude; pitch depends on frequency (Hz).
- Speed of sound: about 340 m/s in air, 1,500 m/s in water, 5,000 m/s in steel.
- Echo: distance = speed × time ÷ 2 (the sound goes there and back).
- Humans hear about 20 Hz to 20,000 Hz. Above that is ultrasound; below is infrasound.
- v = fλ works for sound too; going up an octave doubles the frequency.
Worked examples
Example 1
You shout towards a cliff and hear the echo 1.5 s later. Sound travels at 340 m/s. How far away is the cliff?
Answer: 255 m
Example 2
An ultrasound pulse in soft tissue (1,540 m/s) returns from an organ after 65 µs. How deep is the organ?
Answer: about 50 mm