Describe the uses of ultrasound in nondestructive testing of materials, medical scanning of soft tissue and sonar including calculation of depth or distance from time and wave speed

3.4 Sound – Ultrasound & Sonar

Ultrasound in Nondestructive Testing (NDT)

🔍 What it does: Ultrasound waves (20 kHz–10 MHz) are sent into a material.

They bounce back from internal flaws (cracks, voids) and are detected by a receiver.

The time it takes for the echo to return tells us the depth of the flaw.

This is like listening to a stone echo to find hidden cracks.

• High frequency → better resolution but less penetration.
• Used in aerospace, automotive, and construction to check welds and castings.
• Non‑destructive: the part stays intact.

Ultrasound in Medical Scanning

🩺 How it works: A transducer emits pulses that travel through soft tissue.

Different tissues reflect sound at different strengths, creating an image.

Think of it as a medical “echo” camera that can see inside the body without cutting.

1. Place the transducer on the skin.
2. Send short sound bursts.
3. Receive echoes and convert them into a picture.

Sonar – Depth & Distance Calculations

🐟 What sonar does: Sends sound waves into water and measures the time until the echo returns.

Because sound travels at a known speed in water, we can calculate how far the target is.

Exam Tip: Remember the formula – distance = speed × time ÷ 2 because the sound travels to the target and back.

Formula: \$d = \frac{v \cdot t}{2}\$

where d = distance to target, v = speed of sound, t = round‑trip time.

Example: If v = 1500 m s⁻¹ (speed of sound in seawater) and the echo returns after t = 4 s, then

\$d = \frac{1500 \times 4}{2} = 3000\,\text{m}\$

So the target is 3 km away.

Exam Tip: When calculating depth with sonar, always divide by 2 because the sound travels to the target and back. Check the units – speed in m s⁻¹ and time in s gives distance in metres.