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SOUND Longitudinal waves (e.g sound waves) Sound is produced by any vibrating body. Sound waves: (1) are progressive waves, propagate energy. (2) vibrations are parallel to the direction of propagation. (3) cannot travel in vacuum
Longitudinal nature of sound. • Sound waves travel in the form of longitudinal waves, i.e, in the form of compressions and rarefactions.
Longitudinal wave • A wave in which the particles of the media vibrate along the same line as the direction of motion of the wave.
Direction of propagation Particle of medium Oscillation of particle
Wave profile λ λ C C R R R λ λ
(b) Compression (C) • Region where the layers of the medium are compressed (position of high concentration of molecules) • Region of high density • Region of high pressure (gas)
(c ) Rarefaction (R ) • Region where the layers of the medium are depressed (position of low concentration of molecules) • Region of low density • Region of low pressure
(d) Wavelength, : the distance below 2 consecutive compression or rarefaction (distance for one cycle) (e) Frequency , f : the number of cycle which cross a fixed point per second. (f) Velocity of propagation, v : the distance travelled per second in the direction of propagation. (g)The periodic time, T : the time taken to complete one cycle. T = 1 / f
EXPERIMENT TO SHOW THAT SOUND REQUIRE A MATERIAL MEDIUM FOR ITS PROPAGATION To electrical circuit Bell jar gong hammer To vacuum pump
Press the bell push, the sound of the ringing bell is heard and the hammer is seen to strike the gong • Switch on the vacuum pump. As the air is removed out of the jar slowly, the intensity of the sound decreases, till finally, no sound is heard, though the hammer is still striking the gong. • The experiment shows that sound requires a material medium.
Speed of sound • Speed of sound in air (gas) 330m/s - 340m/s (except for hydrogen =1400m/s) • Speed of sound in water (liquid) 1500 m/s • Speed of sound in steel (solid) 5200 m/s Speed of sound in solid > speed of sound in liquid > speed of sound in gas
Audible frequency: • Least audible frequency = 20Hz • Highest audible frequency = 20 000 Hz ( 20 kHz) • Range of audible frequency: 20 Hz - 20 000 Hz
Echo (Reflected sound) : Sound reflected by hard surfaces such as cliff, mountain, walls, etc.. QUESTION: A ship transmit sound pulses. The sound pulses are reflected back by the ocean bed. If the time between transmitting and receiving the sound is 4.0 s and the speed of sound in water is 1500 m/s. What is the depth of the ocean at that point?
PROPERTIES OF SOUND • LOUDNESS • PITCH • QUALITY (TIMBRE)
LOUDNESS • Loudness or intensity of sound depend on the energy possessed by the sound wave. Loud sound will have high amplitude. Loudness Amplitude • Small amplitude low sound • High amplitude loud sound
Loud sound a a Low sound
PITCH OF SOUND • The pitch of the sound depends on the frequency of the sound wave. Pitch of sound Frequency of sound • (bass) Low pitch: Low frequency of sound • (sharp, treble) high pitch: High frequency of sound
Low pitch High pitch
QUALITY OF SOUND • Quality of sound is related to musical notes. • The quality of sound depends on the musical instrument emitting the notes. • e.g: A musical note played in a guitar will sound different from that played in a piano. i.e, the quality of sound is different • Therefore, each instrument has a typical mixture of frequencies with the fundamental frequency to produce the same note.
Note: • Sound is a vibration • Sound is a mechanical wave • Speed of sound in air at normal temperature is 340 m/s. • As the temperature rises, speed of sound in air increases. • Loudness Amplitude • Pitch Frequency • Range of audible sound ( 20 Hz - 20 KHz)