SOUND

 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 interprets 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.