Physics Study Notes: Sound Waves

Hey everyone! Get ready to explore the amazing world of sound. From your favourite music to the annoying buzz of a mosquito, sound is everywhere. In these notes, we'll break down exactly what sound is, how it travels, and how we describe it. We'll even look at the difference between music and noise. Don't worry if waves seem tricky – we'll use simple examples to make everything crystal clear. Let's dive in!


1. The Nature of Sound

What is a Sound Wave?

At its core, sound is a vibration. When you pluck a guitar string, it vibrates. When you speak, your vocal cords vibrate. These vibrations travel through a medium (like air) to reach our ears. Here are the most important things to remember:

  • Sound needs a medium: Sound waves must have something to travel through, like a solid, liquid, or gas. They can't travel through a vacuum (empty space). That's why in space, nobody can hear you scream!
  • Sound is a longitudinal wave: This is a super important term! It means the particles of the medium vibrate parallel to the direction that the wave energy is travelling.
Analogy: The Slinky Spring

Imagine pushing and pulling one end of a Slinky. You'll see a wave of squashed-up parts and stretched-out parts move along the spring. The bits of the spring itself are just moving back and forth, not travelling down the whole length. That's exactly how sound works!

  • The squashed-up parts are called compressions (areas of high pressure).
  • The stretched-out parts are called rarefactions (areas of low pressure).

A sound wave is a travelling series of compressions and rarefactions.

Quick Comparison: Sound Waves vs. Light Waves

It's helpful to compare sound with light, another type of wave we experience every day. They have some key differences:

  • Type of Wave:
    Sound: Longitudinal wave (vibrations are parallel to wave direction).
    Light: Transverse wave (vibrations are perpendicular to wave direction).
  • Medium Required?:
    Sound: Yes, it needs a solid, liquid, or gas.
    Light: No, it can travel through a vacuum.
  • Speed (in air):
    Sound: Around 340 m/s (relatively slow). This is why you see lightning before you hear thunder.
    Light: Around 300,000,000 m/s (incredibly fast!).
Key Takeaway

Sound is a longitudinal wave that transfers energy through the vibration of particles in a medium. It cannot travel in a vacuum and is much slower than light.


2. Properties of Sound Waves

Because sound is a wave, it exhibits all the classic wave behaviours. You've already seen these in your daily life!

How Sound Behaves

  • Reflection: This is when a sound wave bounces off a surface. The reflection of sound is called an echo. Hard, smooth surfaces like walls and cliffs are great at reflecting sound.
    Example: Shouting into a canyon and hearing your voice come back a moment later.
  • Refraction: This is the bending of a sound wave as it passes from one medium to another, or when conditions (like temperature) change.
    Example: On a cold night, sound can travel further across a lake because the sound waves bend downwards in the cooler air near the water.
  • Diffraction: This is the ability of sound waves to bend around obstacles and spread out through openings. This is why sound can "get around corners".
    Example: You can hear people talking in another room even if you can't see them because the sound waves diffract through the doorway.
  • Interference: When two sound waves meet, they can combine.
    • Constructive interference: The waves add up to make a louder sound.
    • Destructive interference: The waves cancel each other out, creating a quieter spot (or even silence!). This is the principle behind noise-cancelling headphones.
Key Takeaway

Sound behaves like other waves. It can reflect (echoes), refract (bend), diffract (spread around corners), and interfere (combine with other sound waves).


3. Describing Sound: Pitch, Loudness & Quality

When we listen to music or any sound, our brain notices different characteristics. In physics, we have precise terms for these.

Pitch: How High or Low?

Pitch is directly related to a wave's frequency (the number of vibrations per second).

  • High Frequency = High Pitch (e.g., a bird's chirp, a whistle)
  • Low Frequency = Low Pitch (e.g., a deep voice, a bass drum)

Frequency is measured in Hertz (Hz). One vibration per second is 1 Hz.

Loudness: How Strong is the Sound?

Loudness is directly related to a wave's amplitude (the maximum displacement of the vibrating particles).

  • Large Amplitude = Loud Sound
  • Small Amplitude = Quiet Sound
Common Mistake Alert!

Don't mix up pitch and loudness! A sound can be high pitch and quiet (a soft whisper), or low pitch and loud (a booming drum). Frequency determines pitch, and amplitude determines loudness. They are independent properties.

Quality (or Timbre): Why Instruments Sound Different

Quality (or timbre) is the characteristic that allows us to distinguish between two different instruments playing the same note at the same loudness. A piano and a violin playing a 'middle C' have the same fundamental frequency (pitch) and amplitude (loudness), but they sound completely different. This is because most sounds are not pure tones but a complex mix of the main frequency and other higher frequencies called overtones. The unique combination of these overtones gives an instrument its specific quality.

Key Takeaway

We describe sound using three terms: Pitch (determined by frequency), Loudness (determined by amplitude), and Quality/Timbre (determined by the complexity of the waveform).


4. The Range of Hearing & Beyond

What Can We Actually Hear?

Humans can't hear all frequencies. The typical audible frequency range for a healthy young person is from 20 Hz to 20,000 Hz (or 20 kHz). This range tends to shrink as we get older, especially at the higher frequencies.

Beyond Hearing: Ultrasound

Sound with a frequency above 20,000 Hz is called ultrasound. It's too high-pitched for the human ear to detect, but many animals can hear and use it!

Real-world uses of ultrasound:

  • Medical Imaging: Used to safely create images of organs and unborn babies inside the body.
  • Animal Communication & Navigation: Bats and dolphins use ultrasound for echolocation—they emit high-frequency squeaks and listen for the echoes to "see" their surroundings and find prey.
Did you know?

Dogs can hear frequencies up to 45,000 Hz, which is why they can hear special dog whistles that are completely silent to humans!

Key Takeaway

The human hearing range is about 20 Hz to 20,000 Hz. Sounds above this range are called ultrasound and have important uses in medicine and nature.


5. Noise and Sound Level

What is Noise?

In everyday terms, noise is simply any sound that is considered unwanted, unpleasant, or disruptive. From a physics perspective, noise is often a sound with an irregular and non-repeating waveform, unlike the regular, periodic waveform of a musical note.

Measuring Loudness: The Decibel (dB) Scale

While amplitude tells us if a sound is loud or quiet, we use a special scale to measure the sound intensity level in a way that relates to human hearing. This unit is the decibel (dB).

The decibel scale is a logarithmic scale. This means that a small increase in decibels corresponds to a huge increase in sound intensity. For example, a sound at 70 dB is 10 times more intense than a sound at 60 dB!

Here are some typical sound levels:

  • 0 dB: The quietest sound a human can hear.
  • 60 dB: Normal conversation.
  • 85 dB: The level where prolonged exposure can begin to cause permanent hearing damage.
  • 100 dB: A noisy factory or a rock concert.
  • 120 dB: Pain threshold; a jet engine nearby.

Noise Pollution and Acoustic Protection

Noise pollution is the presence of excessive and harmful noise in the environment. It can have serious health effects, including:

  • Permanent hearing loss
  • Stress and anxiety
  • Sleep disturbances

It's crucial to protect our hearing. This is called acoustic protection. If you are in a very loud environment (like working with power tools, at a construction site, or attending a loud concert), you should always use protection like earplugs or earmuffs to reduce the sound level reaching your ears.

Key Takeaway

Sound level is measured in decibels (dB). Prolonged exposure to levels above 85 dB can cause hearing damage. Noise pollution is a serious issue, and we must use acoustic protection in loud environments.