CORE Physics (9223) | Sound and ultrasound

*** Study Notes: CORE Physics (9223) - Sound and Ultrasound ***

Hello future physicist! Welcome to the fascinating world of sound waves. Sound is everywhere, and in this chapter, we will learn how this energy travels, why some sounds are high, and how doctors use waves we can't even hear! Don't worry if the concepts seem tricky at first; we’ll break them down step-by-step.

Let's dive in and understand the physics behind every noise you hear!

1. The Nature of Sound Waves

What Exactly Is Sound?

Sound is a form of energy that travels through a medium (like air or water) as vibrations. When something vibrates—like a speaker cone or your vocal cords—it pushes the particles around it, causing a chain reaction that travels outward.

Key Concept: Sound waves are a type of mechanical wave, meaning they need particles to bump into to travel.

Sound is a Longitudinal Wave

In the "Waves" section, you learned about transverse and longitudinal waves. Sound is definitely longitudinal.

What does Longitudinal mean?
The particles in the medium vibrate parallel to the direction the energy is travelling.

Analogy: Imagine a line of dominoes. When you push the first one (the vibration source), the others fall in the same direction—but they don't move across the room, they just push their neighbor forward. Sound works the same way.

• Compressions: These are regions where the particles are crowded together (high pressure).
• Rarefactions: These are regions where the particles are spread apart (low pressure).

A sound wave is a constant pattern of alternating compressions and rarefactions moving through the medium.

2. Travelling Sound: Speed and Medium

Does Sound Need to Travel? (Yes, it does!)

Because sound waves rely on particles bumping into each other, they absolutely must have a medium (a substance) to travel through.

Crucial Point: Sound cannot travel through a vacuum (empty space).

Example: If an explosion happens in space, you wouldn't hear anything, even if you were nearby, because there are no air particles to carry the vibrations to your ear.

The Speed of Sound in Different Materials

The speed of sound depends entirely on how quickly the particles can transfer the vibrations.

In general, sound travels fastest in materials where the particles are closest together and tightly bound:

Speed Order: Solids > Liquids > Gases

• Solids (Fastest): Particles are packed very closely. The vibrations are passed along almost instantly. (e.g., about 5000 m/s in steel)
• Liquids (Medium Speed): Particles are closer than in gas but can slide past each other. (e.g., about 1500 m/s in water)
• Gases (Slowest): Particles are far apart. It takes time for them to bump into the next particle. (e.g., about 340 m/s in air at room temperature)

Did you know? This is why you hear a train coming sooner if you press your ear to the rail (solid) than if you just listen through the air (gas).

Common Mistake to Avoid

Do not confuse the speed of sound with the speed of light. Light travels vastly, vastly faster than sound. This is why you see lightning flash before you hear the thunder clap!

3. Describing Sound: Pitch, Loudness, and the Wave Equation

We use two main properties of the wave to describe what a sound sounds like:

A) Frequency and Pitch

The pitch of a sound (how high or low it is) is determined by the frequency of the wave.

• Frequency (\(f\)): The number of complete vibrations (cycles) passing a point every second. Measured in Hertz (Hz).
• High Frequency = High Pitch (a whistle or a tiny mosquito buzz)
• Low Frequency = Low Pitch (a deep rumble or a foghorn)

Memory Aid: Think of a fast-moving guitar string (high frequency) creating a high note.

B) Amplitude and Loudness

The loudness (or volume) of a sound is determined by the amplitude of the wave.

• Amplitude: The maximum displacement of a particle from its rest position. In sound, this relates to the pressure difference in the compressions and rarefactions.
• Large Amplitude = Loud Sound (a shout or an explosion)
• Small Amplitude = Quiet Sound (a whisper)

Analogy: If you gently tap a drum (low amplitude), it’s quiet. If you hit it hard (high amplitude), the skin moves far more, and the sound is loud.

C) The Wave Equation

Like all waves, sound obeys the fundamental wave equation, linking speed, frequency, and wavelength.

$$ v = f \lambda $$

• \(v\) = Velocity or speed of the wave (m/s)
• \(f\) = Frequency (Hz)
• \(\lambda\) = Wavelength (m)

In simple terms: if the speed (\(v\)) remains constant (which it does in one medium), then if the frequency (\(f\)) increases (higher pitch), the wavelength (\(\lambda\)) must decrease (shorter wave).

4. Hearing Limits and Ultrasound

Not all sound waves are audible to humans. Our ears are only sensitive to a specific range of frequencies.

The Human Hearing Range (Audible Range)

The average healthy human ear can typically hear sounds between:

20 Hz (very low pitch) and 20,000 Hz (20 kHz, very high pitch)

As we age, the upper limit usually drops significantly.

Infrasound and Ultrasound

Frequencies outside this 20 Hz to 20,000 Hz range are classified as:

1. Infrasound: Sound waves with frequencies below 20 Hz.
Example: Elephants communicate using infrasound; large earthquakes produce infrasound waves.

2. Ultrasound: Sound waves with frequencies above 20,000 Hz (20 kHz).
We cannot hear ultrasound, but it has extremely useful practical applications.

Uses of Ultrasound (CORE Curriculum Focus)

Ultrasound is used because high-frequency waves (short wavelength) are good for detailed imaging and targeting, and they are non-ionising (safer than X-rays).

A) Medical Imaging (Scans)

Ultrasound is used to create images of internal body parts (like scanning a pregnant woman’s baby).
Step-by-Step Process:

1. A transducer sends short pulses of high-frequency ultrasound waves into the body.
2. When the waves hit a boundary between tissues (e.g., muscle and bone), some waves are reflected (echo).
3. The transducer receives these reflected echoes.
4. The time taken for the echo to return is measured. Since the speed of sound in the body is known, a computer calculates the distance to the reflecting surface, building up a 2D image.

B) Industrial Uses (Non-destructive testing)

Ultrasound can be used to check for flaws (like cracks or bubbles) inside metal structures, concrete, or pipes without damaging them. If the wave hits a crack, it will reflect back early, alerting technicians to the defect.

C) Echolocation / SONAR

This technique is used to measure the depth of the sea or locate objects (like submarines or shoals of fish). A pulse is sent downwards, and the time delay of the returning echo calculates the distance (depth).

$$ \text{Distance} = \text{Speed} \times \frac{\text{Time}}{2} $$ (We divide the time by 2 because the wave has to travel down and back up.)

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You have successfully navigated the world of Sound and Ultrasound! You now understand how vibrations turn into the music you listen to and the tools doctors use. Keep practicing the key terms, and you'll master this chapter!