NCERT Gist: Sound | Additional Study Material for UPSC PDF Download

- Sound waves are characterised by the motion of particles in the medium and are called mechanical waves.
- When a vibrating object moves forward, it pushes and compresses the air in front of it creating a region of high pressure. This region is called a compression
- When the vibrating object moves backwards, it creates a region of low pressure called rarefaction (R), It cannot travel through vacuum, a transverse wave is the one in which the individual particles of the medium move about their mean positions in a direction perpendicular to the direction of wave propagation.
- Light is a transverse wave but for light, the oscillations are not of the medium particles or their pressure or density - it is not a mechanical wave.
- Compressions are regions where density as well as pressure is high.
- Rarefactions are the regions of low pressure where particles are spread apart and are represented by the valley, that is, the lower portion of the curve.
- A peak is called the crest and a valley is called the trough of a wave.
- The loudness or softness of a sound is determined basically by its amplitude.
- The brain inteiprets the frequency of an emitted sound is called the pitch.
- The speed of sound remains almost the same for all frequencies in a given medium under the same physical conditions.
- The amount of sound energy passing each second through unit area is called the intensity of sound.
- The speed of sound depends on the properties of the medium through which it travels.
- The speed of sound decreases when we go from solid to gaseous state.
- In any medium as we increase the temperature the speed of sound increases.
- A sound created in a big hall will persist by repeated reflection from the walls until it is reduced to a value where it is no longer audible.
- The repeated reflection that results in this persistence of sound is called reverberation.
- To reduce reverberation, the roof and walls of the auditorium are generally covered with sound-absorbent materials like compressed fibreboard, rough plaster or draperies.
- Sounds of frequencies below 20 Hz are called infrasonic sound or infrasound.
- Rhinoceroses communicate using infrasound of frequency as low as 5 Hz.
- Whales and elephants produce sound in the infrasound range.
- Earthquakes produce low-frequency infrasound before the main shock waves begin which possibly alert the animals.
- Frequencies higher than 20 kHz are called ultrasonic sound or ultrasound.
- Ultrasound is produced by dolphins, bats and porpoises.
- Ultrasounds are high frequency waves.
- Ultrasounds are able to travel along well defined paths even in the presence of obstacles.
- Ultrasounds are used extensively in industries and for medical purposes.
- Ultrasound is generally used to clean parts located in hard-to-reach places.
- Ultrasounds can be used to detect cracks and flaws in metal blocks.
- Ultrasonic waves are made to reflect from various parts of the heart and form the image of the heart. This technique is called- echocardiography.
- Ultrasound scanner is an instrument which uses ultrasonic waves for getting images of internal organs of the human body.
- The acronym SONAR stands for Sound Navigation and Ranging.
- Sonar is a device that uses ultrasonic waves to measure the distance, direction and speed of underwater objects.