Lec16 - Today: Sound (Ch 20) Preliminaries What is the...

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Today: Sound (Ch 20)
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Preliminaries What is the origin of sound? Vibrations of objects. Eg. of a string, of a reed, of vocal cords. . Usually the small vibration stimulates vibration of a larger object eg. of the air, that then propagates through surroundings in form of longitudinal waves . Usually frequency of original vibration = frequency of sound waves = pitch High pitch means high frequency (eg a piccolo), whereas low pitch means low frequency (eg fog horn) Human ear can hear between 20 – 20 000 Hz. Infrasonic – below 20 Hz Ultrasonic – above 20 000 Hz
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How does sound travel in air? Longitudinal wave – air molecules vibrate to and fro along direction of wave Analogy with opening and shutting a door periodically: Open door inward: a compression travels across room (via molecules pushing neighbors) Close door: a rarefaction travels across room – some molecs are pushed out of room so leave lower pressure behind. Swing door open and shut periodically – get periodic compression-rarefaction wave across the room. Note again : medium (air molecules) are not transported across the room; rather the disturbance, and energy, are.
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How sound travels in air, continued… Tuning fork – is exactly this action on a smaller, faster scale: prong vibrating is like the door opening and shutting. Note: compression and rarefaction travel in the same direction Radio loudspeaker – cone that vibrates in synch with electric signal, causing neighboring air molecules to vibrate …eventually sound wave filling the room
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Sound waves cannot travel in A) air. B) water. C) steel. D) a vacuum. E) any of the above media Clicker Question Answer: D Sound is transmitted via periodic compression and rarefaction of the medium – if nothing there, this can’t happen.
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Speed of sound in air In dry air, at 0 o C, speed of sound ~ 330 m/s ~ 1200 km/h At sea-level and temp, speed ~ 340 m/s Increased speed if - air is moist - air is warmer: Speed goes up 0.6 m/s for every o C - if wind is blowing between source and receiver Speed does not depend on loudness (amplitude), nor on pitch (frequency). Speed of light is a million times as great - Hence, we see lightning before we hear thunder - Hence, we see a distant tree fall to the ground before we hear the thud… Note that low and high pitches all have the same speed – they differ in frequency and wavelength (long for low, short for high), such that the product freq x wavelength = v is same.
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Sound travels in other media too Doesn’t have to be air – just has to have an elastic property i.e be able to change shape in response to an applied force and then resume its original shape once force is removed. Eg. putty is not elastic but steel is
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This note was uploaded on 07/18/2011 for the course PHYS 10 taught by Professor Goldberg during the Fall '11 term at CUNY Hunter.

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Lec16 - Today: Sound (Ch 20) Preliminaries What is the...

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