Physics Year 8 Sound

Sound is a form of energy.
Produced by vibrations of a source such as vocal cords or a vibrating tuning fork.
Sound waves are longitudinal waves.

Definition: The transmission of vibrations occurs in the form of sound waves.
Media for Transmission: Solids, liquids, and gases can transmit sound waves because they contain particles.
- Sound waves cannot be transmitted in a vacuum due to the absence of particles.
Mechanism of Transmission:
- Back and forth movement of particles creates regions of high pressure (compressions) and regions of low pressure (rarefactions).

Production: Sound waves are produced by vibrating objects.
- Example: Hitting a tuning fork produces sound; the larynx vibrates to create sound when we talk and can be felt by touching the throat.
Spreading: Sound wave can spread in various media.
Requirement of a Medium: Sound requires a medium to travel through and cannot travel in a vacuum.
Travel through Matter: Sound waves can travel through solids, liquids, and gases; most sound waves reaching ears travel through air. Sound can also be heard underwater and through walls, windows, doors, and ceilings.
Energy Form: Sound represents a type of energy.
Reflection: Sound waves can reflect off surfaces, contributing to audible echoes.

For sound to be heard:

Based on its frequency, sound is classified into three categories:
1. Infrasound: Frequencies less than 20 Hertz; inaudible to humans but heard by some animals (e.g., crickets, elephants, and doves).
2. Audiosound: Frequencies from 20 Hertz to 20,000 Hertz; audible to humans.
3. Ultrasound: Frequencies greater than 20,000 Hertz; inaudible to humans but heard by some animals (e.g., bats, dogs, and dolphins).

Experimentation by French physicist Marsenne (1588 – 1648) led to the formulation of laws governing natural frequency of a string or wire:
1. Frequency depends on string length: Longer strings have lower frequency; shorter strings have higher frequency.
2. Frequency depends on the cross-sectional area of the string: Thicker strings have lower frequency; thinner strings have higher frequency.
3. Frequency depends on string tension: Tighter strings have higher frequency; looser strings have lower frequency.
4. Frequency depends on the density of the string: Strings with lower density have higher frequency; those with higher density have lower frequency.
Formula to Calculate Frequency: $f = rac{A}{l}$ where:
- $A = ext{section area of the string (m²)}$
- $l = ext{length of the string (m)}$
- Other factors include force tension of the string ( $F$ ) and string density ( $<br>ho$ ).

Tone (Regular Sound):
- Defined by a constant frequency; often produced by musical instruments. Examples include guitar, piano, flute, and violin.
- Higher frequency results in higher tone.
- Musical Notes and Frequencies:
  - Note C: 264 Hz
  - Note D: 297 Hz
  - Note E: 330 Hz
  - Note F: 352 Hz
  - Note G: 396 Hz
  - Note A: 440 Hz
  - Note B: 495 Hz
  - Note c': 528 Hz
Fricative: Sounds with irregular frequencies, e.g., rustling leaves and flowing rivers.
Bang (dentum): A very loud sound such as cannon fire or thunder.

The loudness is influenced by the amplitude of the sound wave.
- Larger amplitude results in louder sound; smaller amplitude results in quieter sound.

The highness or lowness of sound is influenced by frequency.
- Higher frequencies result in higher pitches; lower frequencies result in lower pitches.

Definition: The combination of two sounds of the same frequency perceived differently due to the fundamental tone and overtone producing a unique sound wave.
- E.g., the same frequency tone c in guitar and piano will sound different due to their timbre.

Definition: The phenomenon where a body vibrates in response to another body with a matching vibrational frequency.
Resonance Formula:
- $n = rac{1}{2}$ for harmonic frequencies in air columns.
- Practical use in musical instruments to amplify sound.
Examples of instruments using resonance: Guitar, Violin, Gamalan, and Drums.

Factors Affecting Speed:
- Temperature of air influences the speed of sound: higher temperatures increase speed, while lower temperatures decrease speed.
- The speed does not depend on air pressure.
- Speed differs in different media: fastest in solids, slower in liquids, and slowest in gases.

Example Formula: $v = rac{d}{t}$ where:
- $d = ext{distance to listener (m)}$
- $t = ext{time for sound to propagate (s)}$

$v_t = v_0 + 0.6t$ where:
- $v_t$ = speed of sound at temperature $t$ .
- $v_0$ = speed of sound at 0°C.

Laws of Sound Reflection:
- The incident sound, reflected sound, and normal fall in the same plane.
- Angle of incidence (i) equals angle of reflection (r).
Types of Reflected Sound:
- Sound amplification occurs when distance to the reflective surface is small, making the reflected sound simultaneous with the original, thus louder.
- Echo: Reflected sound heard after the original; noticeable when the sound source is far from the surface.
Use of Reflection:
- Determining depth of water or location of fish using sonar.
- Geophysical research and detecting metal defects\n - Ultrasonography (USG) for viewing fetuses and other applications.

Definition: The perception of sound changes as the observer moves towards or away from the sound source.
- Approaching the source increases frequency; moving away decreases frequency.
Doppler Effect Formula:
- $f_o = rac{(f_s imes (v + V_o))}{(v + V_s)}$ where:
- $f_o$ = frequency heard by observer (Hz)
- $f_s$ = frequency of source (Hz)
- $V_o$ = speed of observer (m/s)
- $V_s$ = speed of source (m/s)
- $V$ = speed of sound (m/s)
Exercise Examples:
1. Sirens at different speeds, calculating the frequency heard by observers, etc.
2. Various scenarios involving moving trucks and ambulances, demonstrating the Doppler effect with calculations.