Physics (0625) | Sound

Study Notes: Physics IGCSE (0625) - Sound Waves (Section 3.4)

Welcome to the fascinating world of sound! Sound is crucial to how we experience the world, from hearing music to communicating with friends. In your IGCSE Physics course, sound is studied as a specific type of wave, linking directly to the "Waves" section of your syllabus.

Don't worry if this chapter seems tricky at first; we will break down the production, properties, and uses of sound into simple, manageable steps!

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1. The Nature and Production of Sound

1.1 How is Sound Produced? (Core 1)

Sound waves are always produced by **vibrations**.

• When you speak, your vocal cords vibrate.
• When you hit a drum, the skin vibrates.
• When you pluck a guitar string, the string vibrates.

These vibrations push and pull on the surrounding medium (usually air), creating the wave that travels to your ear.

1.2 Sound Requires a Medium (Core 4)

Unlike electromagnetic waves (like light), sound waves are **mechanical waves**. This means they need something to travel through—a substance or a medium.

• Sound travels through solids, liquids, and gases.
• Sound cannot travel through a vacuum (empty space).

Analogy: Imagine a line of people holding hands. To transfer a message (energy), you have to push the first person, who pushes the second, and so on. If there's a gap (a vacuum), the energy transfer stops! This is why science fiction movies featuring loud explosions in space are incorrect!

1.3 Sound is a Longitudinal Wave (Core 2, Supplement 10)

Sound is categorised as a **longitudinal wave**.

Definition: In a longitudinal wave, the direction of vibration of the particles in the medium is **parallel** to the direction of energy transfer (propagation) of the wave.

When a vibrating source pushes the air, it creates regions of high pressure and regions of low pressure:

• Compression: A region where the particles are crowded together. This is a region of high pressure.
• Rarefaction: A region where the particles are spread far apart. This is a region of low pressure.

A sound wave is essentially a sequence of compressions and rarefactions travelling through the medium.

2. The Properties of Sound

2.1 Speed of Sound (Core 5, Supplement 11)

The speed of sound changes depending on the medium it travels through. Why? Because the particles in different media are packed together differently.

Speed Comparison (Supplement 11):

In general, sound travels fastest in Solids, slower in Liquids, and slowest in Gases.

• Solids: Particles are very close and strongly linked, allowing vibrations to pass quickly. (Example: Steel, \(v \approx 5000\text{ m/s}\))
• Liquids: Particles are close but not fixed, transmission is slower than solids. (Example: Water, \(v \approx 1500\text{ m/s}\))
• Gases: Particles are far apart, relying on collisions to transfer energy, resulting in the slowest speed. (Example: Air, \(v \approx 330 - 350\text{ m/s}\))

Key Fact (Core 5): The approximate speed of sound in **air** is 330 m/s to 350 m/s. (We often use 340 m/s for calculations unless specified otherwise.)

2.2 Frequency and Pitch (Core 7)

Pitch describes how high or low a sound is. Pitch is directly related to the **frequency** (\(f\)) of the sound wave.

• High Frequency = High Pitch (Think of a siren or a flute)
• Low Frequency = Low Pitch (Think of a deep bass drum or a foghorn)

2.3 Amplitude and Loudness (Core 7)

Loudness describes how intense or quiet a sound is. Loudness is related to the **amplitude** (\(A\)) of the sound wave.

• Large Amplitude = Loud Sound (More energy is being transferred)
• Small Amplitude = Quiet Sound (Less energy is being transferred)

Memory Aid: Amplitude is for All the noise (loudness).

2.4 The Audible Range (Core 3)

The range of frequencies that a typical human ear can detect is called the **audible range**.

• Human audible range is approximately 20 Hz to 20,000 Hz (or 20 kHz).

3. Measuring the Speed of Sound in Air (Core 6)

Since we know sound speed is given by the wave equation \(v = f\lambda\), we can also measure it directly using the simple formula for speed: \[v = \frac{\text{distance}}{\text{time}}\]

A common experimental method involves two observers standing a large distance apart (e.g., 500 meters).

Step-by-Step Method (Distance/Time):

1. Measure a large distance (\(d\)) between two points, Observer A and Observer B (e.g., 500 m).
2. Observer A creates a loud sound (like firing a starting pistol or clapping loudly). At the same moment, A starts a stopwatch.
3. Observer B, upon seeing the smoke or the clapping hands (since light is much faster than sound), immediately starts their own stopwatch.
4. Observer B stops the stopwatch when they hear the sound.
5. The time (\(t\)) recorded is the time it took for the sound to travel the distance \(d\).
6. The speed of sound is calculated using \(v = d/t\).

Why a large distance?

The time taken for sound to travel a short distance is very small (less than a second). Using a large distance minimizes the percentage error caused by human reaction time (the delay between seeing the clap and starting the timer).

4. Reflection and Ultrasound

4.1 Reflection of Sound: Echoes (Core 8)

When sound waves hit a hard, smooth surface, they can bounce back. This is called **reflection**.

• An **echo** is simply the name given to a reflected sound wave.
• For a clear echo to be heard, the distance to the reflecting surface must be large enough so that the reflected sound returns after the original sound has stopped.

4.2 Ultrasound (Core 9)

The sound frequencies we hear are the audible range (20 Hz to 20 kHz). Sound waves with frequencies higher than this are called **ultrasound**.

• Definition: Ultrasound is sound with a frequency higher than 20 kHz (20,000 Hz).

4.3 Uses of Ultrasound and Calculation (Supplement 12)

Because ultrasound has a very short wavelength (due to its high frequency), it can be highly directional and is useful for detecting small objects or fine details. This is the basis for many modern applications:

1. Medical Scanning (Imaging): Used to create images of soft tissues inside the body (e.g., scanning a foetus).
2. Sonar (Submarine/Ocean Depth): Used to measure the depth of the sea or locate underwater objects.
3. Non-Destructive Testing (NDT): Used in industry to detect tiny cracks or flaws in materials (like metal components) without damaging the object itself.

Calculating Distance using Ultrasound (Sonar)

The most common calculation involves Sonar, where a sound pulse is sent out and the time taken for the echo to return is measured.

Crucial Point: The measured time (\(t\)) is for the sound to travel out AND back (twice the distance to the object).

• Total distance travelled by the pulse: \(D = v \times t\)
• The distance to the object (\(d\)): \[d = \frac{\text{total distance}}{2} = \frac{v \times t}{2}\] Where \(v\) is the speed of sound in the medium (e.g., water).

Example: If a sonar pulse takes 4.0 seconds to return from the seabed, and the speed of sound in water is 1500 m/s, the depth (\(d\)) is:

\(d = (1500 \text{ m/s} \times 4.0 \text{ s}) / 2\)
\(d = 6000 \text{ m} / 2\)
\(d = 3000 \text{ m}\)

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