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Sound Notes

CP Physics Ms. Morrison. Sound Notes. General Properties. Mechanical – needs medium Longitudinal Created by vibrations which disturb the medium and transmit the wave energy Sound pulses create compressions and rarefactions Compressions – high pressure, particles pressed together

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Sound Notes

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  1. CP Physics Ms. Morrison Sound Notes

  2. General Properties • Mechanical – needs medium • Longitudinal • Created by vibrations which disturb the medium and transmit the wave energy • Sound pulses create compressions and rarefactions • Compressions – high pressure, particles pressed together • Rarefactions – low pressure, particles expand out

  3. General Properties, pg 2 • Frequency = number of pressure oscillations per second (Hz) • Pitch = our impression of frequency • High pitch = high frequency and vice versa • Human range of hearing: 20 – 20,000 Hz • Infrasonic = below 20 Hz • Audible sound = 20 – 20,000 Hz • Ultrasonic = above 20,000 Hz

  4. General Properties, pg 3 • Loudness = our impression of sound intensity which is a measure of the wave’s energy (amplitude), measured in decibels (dB) • Barely heard = 0 dB • Sound 10x louder = 10 dB • Sound 100x louder = 20 dB

  5. General Properties, pg 4 • Sound waves have all the same properties as other waves • Refract • Reflect • Diffract • Interference • Reflection of sound = echo

  6. Speed of Sound • 331 m/s at 0 oC (32oF) • 343 m/s at 20 oC (68oF) • Equation: v = λf • As temperature of air increases, so does the speed of sound • Vsound = 331 m/s + 0.6(ΔT) • ΔT = change in temperature from 0 oC • Travels fastest through solids, then liquids, and slowest through gases • Travels fastest when medium is elastic and molecules close together

  7. Doppler Shift • Pitch higher when sound is approaching • Pitch lower when sound is moving away • Equation: f’ = f [(v + vd)/(v – vs)] • f’ = perceived frequency • f = actual sound frequency • v = speed of sound (assume 343 m/s, unless otherwise specified) • vd = velocity of detector (+ towards source, - away from source • vs = velocity of source (+towards detector, - away from dectector)

  8. Music – Sources of Sound • Human voice (singing) = vocal cords • Brass instruments (tuba, trumpet) = musician’s lips • Reed instruments (clarinet, saxophone) = reed • String instruments (piano, guitar, violin) = string against sounding board • Other instruments (flute, recorder) = air moving inside column

  9. Resonance • Forced vibration = vibration of an object causes another object or material to vibrate • Natural frequency = frequency at which a minimum amount of energy is needed to make object vibrate (need least amount of energy to continue vibrations

  10. Resonance, pg 2 • Resonance = forced vibration of object that matches its natural frequency • Results in a dramatic increase in the wave’s amplitude – sounds louder • Examples: swinging on swing, bridge vibrations, opera singer breaking glass • Can cause damage – Tacoma Narrows Bridge (1940) • Soldiers “break step” when crossing bridges to prevent resonance

  11. Resonance, pg 3 • Interference of sound waves (often occurs during resonance) • Constructive – waves are in phase, sound gets louder • Destructive – waves are out of phase, sound gets softer or no sound at all

  12. Resonance, pg 4 • Beats = difference in frequencies between two sounds heard simultaneously • Used to tune instruments – when frequencies are equal, then beat disappears • Find beat frequency by taking difference between two frequencies

  13. Applications of Sound • Sonar – uses echoes to determine the distance of objects • Bats – use echo location to determine distances of objects as they fly • Ultrasound – use sound waves to “see” inside humans and other materials • Relies on Doppler shift • See unborn babies • Examine heart (echocardiogram) and other organs

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