Ultrasound — Properties and Applications

Ultrasound is sound with frequency above 20,000 Hz (20 kHz) — beyond the upper limit of human hearing.

Animals that use ultrasound: bats, dolphins, whales, dogs (limited range).

1. High frequency → short wavelength → can detect very small objects
2. Travels in straight lines (like all sound)
3. Reflected by surfaces → enables echo-based detection
4. Can penetrate soft tissue (human body) but reflects at boundaries (e.g., tissue-fluid, tissue-bone)
5. High energy at high frequencies → can cause localized heating/vibration

1. SONAR (Sound Navigation and Ranging)

Used to measure ocean depth and detect submarines/fish/icebergs.

Principle: Emit ultrasound pulse → wait for echo → calculate distance.

Distance = (Speed of sound in water × Time for echo) / 2

Example: Echo returns in 3 s, speed = 1500 m/s

Depth = (1500 × 3) / 2 = 2250 m

(Divide by 2 because sound travels to the bottom AND back.)

2. Medical Ultrasonography (Ultrasound Scan)

• Probe emits high-frequency ultrasound into body
• Sound reflects at tissue boundaries (different densities)
• Reflected signals processed into a real-time image
• Used for: monitoring foetal development, detecting tumours, examining organs (liver, kidney, heart)
• Safe: no ionising radiation (unlike X-rays)

3. Echolocation in Bats

• Bats emit ultrasound pulses (~20,000–100,000 Hz) through mouth or nose
• Echoes from insects, walls, and obstacles return to their large ears
• Brain calculates distance, direction, and size of objects from echo delay and intensity
• Allows navigation and hunting in complete darkness

4. Industrial Uses

• Flaw detection: ultrasound sent through metal; cracks/voids reflect it → locate internal defects in machines, railway tracks
• Cleaning: ultrasound vibrations in liquid dislodge dirt from delicate objects (watches, surgical instruments) — ultrasonic cleaning
• Welding: high-frequency vibrations create heat at the joint of two plastic or metal pieces

5. Ultrasonic Range Finder

• Used in parking sensors in cars
• HC-SR04 sensor (common in school robotics) emits and detects ultrasound → measures distance to obstacle

Distance to object = (Speed of sound × Time for echo to return) / 2

Always divide by 2 — the sound travels to the object AND back.

• Ultrasound: frequency > 20,000 Hz
• SONAR: depth = (v × t) / 2
• Medical: safe imaging without radiation
• Bats: natural echolocation using ultrasound
• Industrial: flaw detection, ultrasonic cleaning
• Cannot be heard by humans; can be heard/produced by bats, dolphins

1. Define ultrasound. What is its frequency range?
2. A SONAR pulse returns after 4 seconds. Speed of sound in water = 1500 m/s. Find the ocean depth.
3. Why is ultrasound preferred over X-rays for imaging a foetus?
4. How do bats use ultrasound to catch insects?
5. Stretch: Why does ultrasound imaging work at tissue boundaries but not within a uniform tissue?