Sonar Essentials: 18 MCQs with answers

Sound Navigation and Ranging

SONAR is an acronym for Sound Navigation and Ranging, which describes its function of using sound to navigate and determine distances.

Ultrasonic waves

Sonar uses ultrasonic waves (high-frequency sound) because they travel well through water and can be directed in narrow beams.

Measuring ocean depth

Sonar is widely used to measure ocean depth (bathymetry) by sending sound pulses to the seafloor and timing their return.

The sound pulse emitted by an active sonar

A 'ping' is the term used for the short burst of sound emitted by an active sonar system.

True

Sonar sends sound pulses to the seafloor and measures the time for the echo to return, allowing depth calculation.

Reflection of sound waves

Sonar works by emitting sound pulses and measuring the time taken for the echoes (reflected sound) to return from objects.

d = (v × t) / 2

The distance is half the product of speed (v) and total time (t) because the sound travels to the object and back, so the time is for the round trip.

Echo sounder

An echo sounder is a type of active sonar that measures depth and detects objects, but it typically does not provide detailed images.

Active sonar emits sound pulses; passive sonar only listens for sounds

Active sonar generates sound pulses and listens for echoes, while passive sonar simply listens for sounds emitted by other objects (like ships or marine life).

Temperature, salinity, and pressure

The speed of sound in seawater depends on temperature, salinity, and pressure (depth), which affect the density and elasticity of the medium.

Sound waves travel farther in water than electromagnetic waves

Electromagnetic waves (like radar) are rapidly absorbed in water, whereas sound waves propagate efficiently, making sonar effective for underwater detection.

Medical imaging of bones

Medical imaging uses ultrasound (similar principle) but not sonar; sonar is used for underwater applications like fish finding, navigation, and mapping.

It is affected by temperature gradients and salinity variations

Sonar performance can be degraded by variations in water temperature, salinity, and pressure, which cause sound waves to bend or scatter.

Listening for submarine engine noise

Passive sonar listens for sounds emitted by other objects, such as the engine noise of a submarine or ship, without emitting any sound itself.

False

Passive sonar only listens for sounds from other sources; it does not emit any sound pulses. Active sonar emits pulses.

To create detailed images of the seafloor

Side-scan sonar is towed behind a vessel and emits fan-shaped sound pulses to produce detailed images of the seafloor, useful for mapping and object detection.

1 kHz to 500 kHz

Sonar frequencies typically range from about 1 kHz to 500 kHz, with lower frequencies for long-range and higher frequencies for higher resolution.

Higher resolution images

Higher frequency sound waves have shorter wavelengths, which allow them to resolve smaller details, but they are absorbed more quickly, reducing range.