Quick Revision Notes & Formula Sheet

Class 9 Science - Chapter: Sound

MP Board

Sound is a form of energy that produces a sensation of hearing in our ears. It is produced by vibrating bodies.
Vibration: A rapid to-and-fro or back-and-forth motion of an object.
Propagation of Sound: Sound travels through a medium (solid, liquid, or gas) from the source to the listener in the form of waves. Sound cannot travel through a vacuum (it requires a material medium).

Longitudinal Wave: A wave in which the particles of the medium vibrate in the direction parallel to the direction of propagation of the wave.
Compression (C): Regions of high pressure and high density where particles are crowded together.
Rarefaction (R): Regions of low pressure and low density where particles are spread apart.

Wavelength ($\lambda$): The distance between two consecutive compressions (C) or two consecutive rarefactions (R).
- SI Unit: metre (m).
Frequency ($f$ or $\nu$): The number of oscillations or vibrations per unit time.
- SI Unit: Hertz (Hz) or $s^{-1}$.
Time Period ($T$): The time taken to complete one full oscillation or vibration.
- SI Unit: second (s).
Amplitude ($A$): The maximum displacement of the particles of the medium from their mean position.
- SI Unit: metre (m).
Speed of Sound ($v$): The distance travelled by a wave per unit time.
- SI Unit: metre per second ($m/s$).

Relationship between Frequency and Time Period: $$f = \frac{1}{T}$$ (Frequency is inversely proportional to Time Period)
Wave Equation (Speed, Frequency, and Wavelength): $$v = f \times \lambda$$ Where:
- $v$ = Speed of sound wave ($m/s$)
- $f$ = Frequency ($Hz$)
- $\lambda$ = Wavelength ($m$)
Speed of Sound Formula (Pulse/Echo method): $$Speed (v) = \frac{\text{Distance travelled }(d)}{\text{Time taken }(t)}$$ For an echo: If sound travels to a wall and back, total distance is $2d$, so: $$v = \frac{2d}{t}$$

Loudness (प्रबलता): Depends on the Amplitude of the sound wave. Greater the amplitude, louder the sound. Measured in decibels (dB).
Pitch / Shrillness (तारत्व): Depends on the Frequency of the sound wave. Higher the frequency, higher the pitch (e.g., a woman's voice has higher pitch than a man's).
Quality or Timbre (गुणवत्ता): The characteristic of sound that enables us to distinguish one sound from another having the same pitch and loudness.

Sound bounces back when it strikes a hard surface, following the laws of reflection:
1. The angle of incidence is equal to the angle of reflection.
2. The incident sound wave, the reflected sound wave, and the normal all lie in the same plane.
Echo (प्रतिध्वनि): The repetition of sound caused by the reflection of sound waves from a rigid obstacle.
- Condition for hearing a distinct echo: The minimum time gap between the original sound and the reflected sound must be $0.1$ seconds (persistence of hearing).
- Minimum distance: For normal air conditions, the minimum distance from a reflecting surface to hear an echo is $17.2$ metres.
Reverberation: Persistence of sound in a big hall due to repeated reflections from the walls, ceiling, and floor. (Reduced by using sound-absorbing materials like curtains, compressed fibreboard, etc.)

Audible Range: Sounds of frequencies between $20\text{ Hz}$ and $20,000\text{ Hz}$ ($20\text{ kHz}$). Humans can hear this range.
Infrasonic Sound (अध्वर/अवश्रव्य ध्वनि): Frequencies below $20\text{ Hz}$ (produced by earthquakes, whales, elephants).
Ultrasonic Sound (पराश्रव्य ध्वनि): Frequencies above $20,000\text{ Hz}$ (produced by bats, dolphins, porpoises).

SONAR (Sound Navigation and Ranging): Used to measure the depth of oceans and locate underwater objects like submarines or shipwrecks.
- Formula: $2d = v \times t$ (where $d$ is depth, $v$ is speed of sound in water, $t$ is total time).
Medical Diagnostics: Used in Ultrasonography (e.g., echocardiography, examining foetus during pregnancy).
Industrial Cleaning: Used to clean hard-to-reach parts of objects like spiral tubes, electronic components.
Detecting Flaws: Used to detect cracks and flaws in metal blocks used in construction.