Applications of reflection of sound
Reflection of sound is used in whispering galleries, stethoscopes, megaphones and sound boards, and as echoes in SONAR, ultrasound scans and bats' hunting.
Uses of repeated reflection
| Device | How reflection helps |
|---|---|
| Whispering gallery | A whisper near the curved wall of a dome reflects again and again along the wall and is heard clearly on the far side. Example: Gol Gumbaz in Vijayapura, Karnataka. |
| Stethoscope | The sound of the heartbeat enters the chest piece and is reflected many times along the rubber tube to the doctor's ears. |
| Megaphone and horn loudspeaker | The cone-shaped tube sends sound forward by repeated reflection, so it travels far in one direction without spreading out. |
| Ear trumpet (simple hearing aid) | A wide funnel collects sound and reflects it into the narrow end placed in the ear. |
| Sound board | A concave board placed behind a speaker on stage. The speaker stands at its focus, and the sound reflects evenly towards the audience. |
| Curved ceilings of halls | Spread the reflected sound evenly to every part of an auditorium. |
SONAR
SONAR stands for SOund Navigation And Ranging. A transmitter on a ship sends ultrasonic waves into the water. They reflect from the seabed, a submarine or a shoal of fish. A detector (receiver) picks up the echo. If v is the speed of sound in water and t is the time for the echo, the distance is d = vt/2. Ultrasound is used because it can be sent as a narrow beam in one direction and gives sharp echoes, even from small objects. It is also not confused with the ordinary noises of the sea and the ship.
Other uses of echoes
- Ultrasound scanning (sonography): echoes from organs build a picture of the inside of the body, such as the liver or a growing baby.
- Bats fly and hunt in the dark by echolocation: they send out ultrasonic squeaks and judge distances from the echoes. Dolphins do the same in water.
- Echo sounders and fish finders on fishing boats measure the depth of the sea and find shoals of fish.
- Echoes are used to measure the speed of sound and to find hidden cracks in metal blocks (ultrasonic testing).
Worked example: depth of the sea by SONAR
A ship's SONAR sends an ultrasonic pulse. The echo from the seabed returns after 4 s. The speed of sound in sea water is 1500 m/s. Find the depth of the sea.
- Formula: d = vt/2.
- Given v = 1500 m/s, t = 4 s.
- d = (1500 m/s × 4 s) ÷ 2 = 6000 ÷ 2 = 3000 m.
3000 m (3 km)
How to draw SONAR
- 1Draw a ship on the water surface at the top.
- 2Draw the seabed (or a submarine) below as a wavy line.
- 3Draw a downward arrow from the ship to the seabed and label it 'ultrasonic waves sent (transmitter)'.
- 4Draw an upward arrow back to the ship and label it 'reflected waves, echo (detector)'.
- 5★Mark the depth d between the ship and the seabed, and write d = vt/2.
★ key step
Fishing boats from Rameswaram and Thoothukudi use fish finders, a small SONAR, to locate shoals of fish. Pregnant women get ultrasound scans to check the baby's growth safely, without X-rays.
2 marks: “What is SONAR? Why does it use ultrasonic waves?” SOund Navigation And Ranging: it finds depth and distance under water using echoes. Ultrasound can be sent as a narrow beam and gives sharp echoes.
4 marks: “Explain how a stethoscope, a megaphone and a sound board use the reflection of sound.” Stethoscope: repeated reflection in the tube carries heart sounds to the ears. Megaphone: the cone reflects sound forward. Sound board: concave board with the speaker at its focus reflects sound to the audience.
4 marks: “A SONAR echo from a submarine returns in 1.2 s. The speed of sound in water is 1500 m/s. How far away is the submarine?” d = vt/2 = (1500 × 1.2) ÷ 2 = 1800 ÷ 2 = 900 m.
SONAR = SOund Navigation And Ranging: send, bounce, receive, halve (d = vt/2).