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CBSE | Class 9 | science
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Summary Note
Key concept recap
Sound is a form of energy that produces a sensation of hearing. It is generated by vibrating objects and travels through a medium.
When you clap or speak, mechanical energy is used to produce sound, converting motion into auditory energy.
Sound cannot travel through vacuum; it needs a medium like air, liquid, or solid to propagate.
Summary Note
Key concept recap
Sound is produced by vibrating objects. Common methods of producing vibrations include striking, plucking, blowing, or shaking.
These vibrations cause the surrounding medium’s particles to oscillate, creating a wave that carries sound energy forward.
The wave propagates via compressions (high pressure) and rarefactions (low pressure), forming a longitudinal wave.
Summary Note
Key concept recap
Sound waves are longitudinal waves, as the medium’s particles vibrate parallel to the direction of wave propagation.
Important characteristics of a sound wave include frequency (number of oscillations per second), wavelength (distance between two compressions), amplitude (maximum displacement), and speed (rate of travel through a medium).
Pitch is determined by frequency, loudness by amplitude, and quality by the waveform pattern.
Summary Note
Key concept recap
Speed of sound depends on the medium and its temperature. It is fastest in solids, slower in liquids, and slowest in gases.
In air at 25°C, sound travels at approximately 346 m/s. Sound travels faster in water (about 1498 m/s) and much faster in steel (about 5960 m/s).
The relation is given by: Speed = Wavelength × Frequency (v = λν).
Summary Note
Key concept recap
Sound reflects from surfaces and obeys the laws of reflection like light. The angle of incidence equals the angle of reflection.
Reflection causes echoes, which occur when the time gap between the original and reflected sound is at least 0.1 seconds.
Uses include megaphones, stethoscopes, auditorium acoustics, and curved ceilings or boards to direct sound.
Summary Note
Key concept recap
Humans can hear sound frequencies between 20 Hz and 20,000 Hz. Sounds below 20 Hz are called infrasound; those above 20,000 Hz are ultrasound.
Animals like elephants and whales use infrasound, while bats and dolphins use ultrasound.
Ultrasound is used in medicine (ultrasonography), industry (detecting flaws), and cleaning hard-to-reach parts.
Summary Note
Key concept recap
Sound – Vibrational energy transmitted through a medium.
Vibration – Rapid back and forth motion of an object.
Longitudinal Wave – A wave where particles move parallel to wave direction.
Compression – Region of high pressure in a sound wave.
Rarefaction – Region of low pressure in a sound wave.
Amplitude – Maximum displacement from the mean position.
Frequency (ν) – Oscillations per second, measured in hertz (Hz).
Wavelength (λ) – Distance between two compressions or rarefactions.
Time Period (T) – Time for one complete oscillation.
Pitch – Perceived frequency of sound.
Loudness – Perceived strength of sound, linked to amplitude.
Echo – Reflected sound heard after a delay.
Ultrasound – Sound with frequency above 20 kHz.
Reverberation – Persistence of sound due to multiple reflections.
Intensity – Energy carried by sound per unit area per second.
