Step 1: Sound Needs a Medium

Unlike light, sound cannot travel through a vacuum. It needs particles (air molecules, water molecules, metal atoms) to vibrate and pass the energy along. No particles = no sound.

Step 2: Speed Depends on the Medium

Step 3: The Core Fact

Syllabus Note: You don't need to memorize the exact value, but know it's roughly 340 m/s.

The Problem

Sound moves fast (~340 m/s), so measuring it directly is tricky. You can't just time it over a few meters — the time would be too short (less than 0.01 seconds).

The Solution: Use Echoes

An echo is sound that has been reflected off a surface (like a wall or cliff).

Ultrasound is sound with a frequency above 20,000 Hz (20 kHz). Humans can't hear it, but it's incredibly useful.

How SONAR Works

Ships use ultrasound to measure the depth of water beneath them. A pulse is sent down, reflects off the seabed, and the echo is detected.

Why Ultrasound for Babies?

Ultrasound is used to scan pregnant women because:

How It Works

1. Ultrasound waves are sent into the body
2. They partially reflect at boundaries between different tissues (e.g., muscle/amniotic fluid/baby)
3. The reflected echoes are detected
4. A computer calculates the depth based on the echo time
5. An image is built up from thousands of pulses

What is NDT?

Non-Destructive Testing means testing materials (metal, concrete) for internal flawswithout breaking or damaging them. This is critical for:

• Aircraft wings (can't have cracks mid-flight!)
• Bridge supports (must detect weakness before collapse)
• Pipelines (find corrosion before leaks)

How Ultrasound Detects Cracks