Learning Objectives
- Understand how sound is produced by vibrations
- Learn how sound travels through different media
- Understand frequency, amplitude, loudness, and pitch
- Learn about noise pollution and its prevention
Key Concepts
Production of Sound
Sound is produced by vibrating objects. When an object vibrates, it creates disturbances in the surrounding medium, which travel as sound waves. Examples: vibrating strings (guitar, sitar), vibrating membranes (tabla, drum), vibrating air columns (flute).
In humans, sound is produced by the voice box (larynx). It contains two vocal cords stretched across it. When air passes through the gap between them, the vocal cords vibrate to produce sound.
Propagation of Sound
Sound needs a material medium (solid, liquid, or gas) to travel. Sound cannot travel through a vacuum. Sound travels fastest in solids, then in liquids, and slowest in gases.
Speed of sound: In air โ 340 m/s (at room temperature), in water โ 1500 m/s, in steel โ 5000 m/s.
Characteristics of Sound
Amplitude: The maximum displacement of a vibrating object from its rest position. Greater amplitude means louder sound. Loudness is measured in decibels (dB).
Frequency: The number of vibrations (oscillations) per second. Measured in hertz (Hz). 1 Hz = 1 vibration per second.
Pitch: The shrillness of a sound. Higher frequency means higher pitch. A thin string vibrates faster and produces a higher pitch. A thick string vibrates slower and produces a lower pitch.
Time period: Time taken for one complete vibration. Time period = 1/Frequency.
Audible and Inaudible Sounds
The human ear can hear sounds in the frequency range of 20 Hz to 20,000 Hz. Sounds below 20 Hz are called infrasound (e.g., earthquakes, whale communication). Sounds above 20,000 Hz are called ultrasound (e.g., bat navigation, medical imaging - sonography).
Noise Pollution
Unpleasant and loud sounds that are disturbing are called noise. Prolonged exposure to noise above 80 dB can cause hearing problems.
Causes: Loud music, traffic, construction, factory machines, bursting crackers.
Effects: Hearing impairment, sleeplessness, hypertension, anxiety.
Prevention: Planting trees (absorb sound), using silencers on vehicles, setting up noise barriers, maintaining a distance from loudspeakers.
Summary
Sound is produced by vibrations and requires a medium to travel. It is characterised by amplitude (loudness), frequency (pitch), and speed. Humans can hear 20 Hz to 20,000 Hz. Noise pollution from excessive loud sounds causes health problems and must be controlled through various measures.
Important Terms
- Vibration: Rapid back-and-forth motion of an object
- Amplitude: Maximum displacement from the rest position; determines loudness
- Frequency: Number of vibrations per second (Hz); determines pitch
- Pitch: How shrill or flat a sound is
- Loudness: How loud or soft a sound is, measured in decibels (dB)
- Ultrasound: Sound with frequency above 20,000 Hz
- Infrasound: Sound with frequency below 20 Hz
Quick Revision
- Sound = vibration; needs a medium; cannot travel through vacuum
- Speed: solids > liquids > gases (steel > water > air)
- Amplitude โ loudness (dB); Frequency โ pitch (Hz)
- Human hearing range: 20 Hz to 20,000 Hz
- Ultrasound (>20,000 Hz): bats, sonography; Infrasound (<20 Hz): earthquakes
- Noise pollution: above 80 dB is harmful; reduce with trees, silencers, barriers