Learn in the textbookDetection Devices (Distance and Environmental-Quantity Sensors)
DefinitionAn ultrasonic sensor (超音波センサー) sends out sound at a frequency too high for human ears (ultrasound) and works out distance from the time it takes to hit an object and come back.
ExplanationSound travels through air at a nearly constant speed (about 340 m per second at room temperature), so multiplying the round-trip time by that speed and halving it gives the distance. Because it does not rely on light, it can easily detect transparent things such as glass, or objects in the dark. It is used for parking assistance in cars and for measuring the liquid level inside tanks.
AnalogyIt is like shouting at a mountain and judging how far away it is by how long the echo takes to come back.
Memory hook"Ultra" means beyond the range of sound people can hear. Bats dodge walls while flying in the dark using the same echo principle.