Introduction to Sound, Ultrasound, and Infrasound

Welcome to one of the most "vibrant" chapters in Physics! Sound is all around us, but there is much more to it than just what we can hear. In this chapter, we will explore the sounds that are too high or too low for human ears, how we use them to see inside the human body, and even how we use them to map the center of the Earth. This topic is part of your Paper 1 studies, building on what you have already learned about wave properties.

1. The Basics: Human Hearing

Before we dive into the "hidden" sounds, let's look at what we can actually hear. Sound waves are longitudinal waves that require a medium (like air, water, or a solid) to travel through. They transfer energy, not matter.

The frequency of a sound wave determines its pitch. Humans have a specific auditory range:

  • The lowest frequency we can hear is roughly \(20 \text{ Hz}\).
  • The highest frequency we can hear is roughly \(20,000 \text{ Hz}\) (or \(20 \text{ kHz}\)).

Quick Tip: If the frequency is below \(20 \text{ Hz}\), it is called infrasound. If it is above \(20,000 \text{ Hz}\), it is called ultrasound.

2. Ultrasound (Higher Tier & Physics Only)

Ultrasound is defined as sound with a frequency higher than \(20,000 \text{ Hz}\). Even though we can't hear it, it is incredibly useful because of a property called partial reflection.

How Ultrasound Works

When an ultrasound wave meets a boundary between two different materials (like the boundary between a baby’s skin and the fluid in the womb), some of the wave is reflected back, and some is transmitted (passes through). By measuring how long it takes for these reflections to return, we can calculate how far away the boundary is.

Common Uses of Ultrasound

  • Foetal Scanning: Doctors use ultrasound to create images of a baby in the womb. It is much safer than X-rays because it does not use ionising radiation.
  • Sonar (Sound Navigation and Ranging): Boats and submarines use ultrasound to find the depth of the ocean or to locate shoals of fish. They send a pulse down and wait for the echo to return.
  • Industrial Imaging: Checking for hidden cracks or flaws inside metal pipes or machinery.

3. Infrasound (Higher Tier & Physics Only)

Infrasound is sound with a frequency lower than \(20 \text{ Hz}\). These waves can travel very long distances and pass through the Earth.

Exploring the Earth's Structure

Scientists use infrasound, specifically seismic waves produced by earthquakes, to investigate the internal structure of our planet. There are two main types you need to know:

1. P-waves (Primary waves): These are longitudinal waves. They are very fast and can travel through both solids and liquids. Because they can travel through the Earth's liquid outer core, they can be detected all over the world after an earthquake.

2. S-waves (Secondary waves): These are transverse waves. They are slower than P-waves and can only travel through solids. Because they cannot travel through the liquid outer core, they create "shadow zones" where no S-waves are detected. This is how we proved that the Earth's outer core is liquid!

Key Takeaway: Ultrasound helps us see small things nearby (like babies or fish), while Infrasound helps us see huge things far away (like the Earth's core).

4. Reflection and Refraction (Physics Only)

Just like light waves, sound waves can be reflected and refracted.
- Reflection: This is what creates an echo. Hard, smooth surfaces reflect sound best.
- Refraction: Sound waves change speed when they move from one medium to another (e.g., from air into water). This change in speed causes the wave to change direction.

5. Calculations: Finding Distance (Higher Tier)

In your exam, you might be asked to calculate the depth of the sea or the distance to an object using ultrasound. You will use the standard wave speed formula:

\(v = \frac{x}{t}\)

Where:
- \(v\) is wave speed (in \(\text{m/s}\))
- \(x\) is distance (in \(\text{m}\))
- \(t\) is time (in \(\text{s}\))

Common Mistake Alert! In "echo" questions (like Sonar or foetal scans), the sound travels to the object and back again. If the question asks for the depth or the distance to the object, you must divide the total distance by 2, or divide the total time by 2 before calculating.

6. Core Practical 4.17: Measuring Wave Speed

You must know how to investigate the speed of sound in both fluids (like air) and solids.

Measuring Speed in Air

One simple method is to stand a long distance away from a tall wall (e.g., \(100 \text{ m}\)). Clap two wooden blocks together and listen for the echo. Time how long it takes for 10 echoes to return, then use:

\(\text{Speed} = \frac{2 \times \text{distance to wall}}{\text{average time for one echo}}\)

Measuring Speed in Solids

To measure the speed of sound in a solid (like a metal rod):
1. Suspend a metal rod and tap one end with a hammer.
2. Use a smartphone app or oscilloscope to find the peak frequency of the sound produced.
3. The wavelength (\(\lambda\)) in the rod is twice the length of the rod (\(\lambda = 2L\)).
4. Calculate speed using \(v = f \times \lambda\).

Chapter Summary

  • Human Hearing: \(20 \text{ Hz}\) to \(20,000 \text{ Hz}\).
  • Ultrasound: \(> 20,000 \text{ Hz}\). Used for foetal scans and sonar via partial reflection.
  • Infrasound: \(< 20 \text{ Hz}\). Used to study the Earth’s core (P-waves and S-waves).
  • P-waves: Longitudinal, travel through solids and liquids.
  • S-waves: Transverse, travel through solids only.
  • Calculations: Always check if you need to "halve the distance" for echo/sonar problems!