Passive and Active Sonar — Listening versus Measuring Echoes
Compare the two modes by the sound they use and the path that sound follows. By the end, you will be able to distinguish sound radiated by a target from an echo.
Passive sonar listens to sound radiated by the target
Passive sonar does not transmit. It receives sound radiated by the target, such as machinery noise, propeller noise, marine-animal vocalizations, or flow noise. Because it remains silent, passive sonar is well suited to monitoring without revealing your position through a transmission and to long-term observation of underwater sound.
The tradeoff is that a target producing little sound provides little to hear. A single hydrophone listening passively also cannot determine the target's absolute range directly.
Tell passive and active apart by following the sound path
The Chapter 2 equation range = c × t / 2 uses the time from transmission until a reflected sound returns. It corresponds to the active-sonar round trip transmitter → target → receiver. Passive sonar instead uses the one-way path target → receiver.
This difference carries directly into the signal-to-noise ratio (SNR) equations in the next chapter. Passive sonar includes one one-way transmission loss (TL). Active sonar includes one TL on the outbound leg and one on the return leg, for a total of 2TL.
Active sonar is also classified by transmitter and receiver placement
Active sonar is commonly divided into two configurations according to where the transmitter and receiver are located.
- Monostatic: the transmitter and receiver are at nearly the same location. Hull-mounted sonar and echo sounders are familiar examples: they receive the echo of a sound transmitted from the same location.
- Bistatic: the transmitter and receiver are separated. This geometry can view the target from different positions, but it requires timing synchronization and calibration between the transmitting and receiving sites.
Unless stated otherwise, this course explains SNR and transmission loss for a monostatic configuration.
Active sonar measures the echo
Active sonar transmits a sound and receives the echo returning from the target. The round-trip time gives range, while echo strength provides clues about the target.
An important quantity here is target strength (TS), which describes how readily the target reflects incident sound back toward the sonar. The transmitted sound also experiences one one-way transmission loss (TL) outbound and another on return, which is why 2TL appears in the basic active-sonar equation.
Choose the mode to match the job
For an introductory rule of thumb, use passive sonar when you want to keep listening without transmitting, and active sonar when you want to measure range from an echo. Real systems combine array processing, tracking, frequency analysis, and observations from multiple sites to extract much more information from either mode.
Passive
Listens to sound radiated by the target. It does not transmit, and a single sensor cannot determine range directly.
Active
Transmits sound and measures the echo. TS matters, and round-trip time makes range easier to determine.
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Chapter 3 / Practice 1
Unanswered
Q11. Listening to sound radiated by the target
Which mode detects a target by receiving sound that the target radiates?
Show hint
This mode sends no sound of its own; it only listens to what arrives.
Show reasoning
Passive sonar detects and classifies targets by listening to machinery noise, propeller noise, vocalizations, and other sound radiated by the target.
Chapter 3 / Practice 2
Unanswered
Q12. Where target strength appears
In which mode's basic equation does target strength (TS), a measure of how readily a target reflects sound, appear directly?
Show hint
Think about the mode that uses the strength of the returning echo.
Show reasoning
TS describes how strongly a target reflects incident sound back toward the sonar, so it appears in the active-sonar equation that uses the echo.
Chapter 3 / Practice 3
Unanswered
Q13. Why the active equation contains 2TL
Why does
2TL appear in the simplified active-sonar equation?Show hint
Follow the path transmitter → target → receiver.
Show reasoning
The transmitted sound experiences one one-way transmission loss (TL) on its way to the target, and the echo experiences another TL on the return path.
Chapter 3 / Practice 4
Unanswered
Q14. Detecting a quiet but reflective target
A target radiates almost no sound but returns a strong enough echo when sound reaches it. Which mode can detect it more directly?
Show hint
Passive sonar needs sound radiated by the target that it can hear.
Show reasoning
Passive detection becomes difficult when the target radiates almost no sound. Active sonar is more direct because it transmits a sound and uses the reflection.
Chapter 3 / Practice 5
Unanswered
Q15. What one passive hydrophone cannot determine directly
When you observe passively with only one hydrophone, which quantity is the hardest to determine directly?
Show hint
Active sonar can measure round-trip time, but one passive sensor has no equivalent clue.
Show reasoning
A single passive observation cannot determine absolute range directly. It needs additional geometric information, such as observations from multiple locations or target motion.
Key takeaways from this chapter
- Passive sonar listens to sound radiated by the target; active sonar measures the echo of a sound it transmits.
- TS describes how readily a target reflects sound and appears in the active-sonar equation.
- Passive sonar includes one one-way transmission loss (TL); active sonar includes the outbound and return losses as
2TL.