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š” Beyond our hearing
The sounds we cannot hear turn out to be some of the most useful — imaging babies, cleaning parts, finding submarines.
What are ultrasound and infrasound used for?
- Infrasonic (<20 Hz): detecting earthquakes, volcanic eruptions and severe storms over long distances.
- Ultrasonic (>20 kHz): ultrasonography, breaking kidney stones, welding and cleaning delicate parts.
- Ultrasound also detects defects in metal blocks and locates objects (SONAR).
| Frequency range | Some applications |
|---|---|
| Infrasonic (< 20 Hz) | Detecting earthquakes and volcanic eruptions; detecting severe storms across long distances. |
| Audible (20 Hz – 20 kHz) | Speech, music and everyday communication. |
| Ultrasonic (> 20 kHz) | Ultrasonography; breaking kidney stones; welding and cleaning parts; detecting defects in metals; SONAR and locating objects. |
š Bridging Science and Society
Humans use ultrasound in SONAR (Sound Navigation and Ranging): ultrasonic waves are sent into water and the reflected waves reveal the distance, direction and speed of underwater objects like submarines and shipwrecks. Drones and aircraft can also be tracked by the low-frequency hum of their engines using sound sensors (audio surveillance).
Example 10.6 — A naval sonar signal sent
Signal returns after 0.90 s, so time to reach the object \(=\dfrac{0.90}{2}=0.45\ \text{s}\); with \(v=1530\ \text{m/s}\), distance \(=1530\times 0.45=688.5\ \text{m}\).
NCERT Question 10 — A ship searching for a
A ship searching for a sunken ship sent a sonar signal and detected an echo after 5 s. If ultrasonic wave travels at 1525 m sā»¹ in seawater, approximately how far down in the ocean is the wreckage of the sunken ship located?
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