Sound · revision map · IGCSE 0625, Topic 3.4
Sound is made by vibrations and travels as a longitudinal wave of compressions and rarefactions. The particles vibrate along the direction of travel. Sound needs a medium, so it cannot travel through a vacuum.
Amplitude sets the loudness; frequency sets the pitch.
Speed in air is roughly 330 to 340 m/s. Sound travels faster in liquids and fastest in solids.
Closer, more tightly bound particles pass the vibration on faster. In a vacuum there are no particles, so no sound is heard.
A healthy human hears from about 20 Hz to 20 000 Hz (20 kHz).
- Below 20 Hz is infrasound; above 20 kHz is ultrasound.
- The upper limit falls with age.
An echo is sound reflected from a hard surface. Timing an echo measures the speed of sound.
the distance twice
Ultrasound is sound above 20 000 Hz, beyond human hearing. It is sent as pulses and the echoes are timed.
- Prenatal and medical scanning.
- Cleaning delicate items and teeth.
- Detecting flaws and cracks in metal.
A ship times an ultrasound echo from the seabed. The pulse travels the depth twice.
t = echo time