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Sound Intensity
▪ Rate that energy flows through a given area– Intensity = (ΔE/Δt) = P .
area area
Intensity is Power ÷ area watt/m2
Equation for the area of a circle = 4πr2
Sound Intensity
▪ Vibrating objects do work on the air as they push against the molecules.
▪ Intensity is the rate of energy flow through an area.– What is “rate of energy flow”
called?▪ E/t is called power (P).
– Since the waves spread out spherically, you must calculate the area of a sphere. How?▪ A = 4r2
– So, what is the equation for intensity?
Sound Intensity
▪ SI unit: W/m2
▪ This is an inverse square relationship.– Doubling r reduces intensity by ¼.– What happens if r is halved?▪ Intensity increases by a factor of 4.
Intensity and Decibels
▪ An intensity scale based on human perception of loudness is often used.
▪ The base unit of this scale is the bel. More commonly, the decibel (dB) is used.– 0.1 bel = 1 dB,1 bel = 10 dB, 5 bels = 50 dB, etc.
– The lowest intensity humans hear is assigned a value of zero.
▪ The scale is logarithmic, so each increase of 1 bel is 10 times louder.– An increase in intensity of 3 bels is 1 000 times louder.
Threshold of Hearing
Source IntensityIntensity
Level# of Times
Greater Than TOH
Threshold of Hearing (TOH) 1*10-12 W/m2 0 dB 100
Rustling Leaves 1*10-11 W/m2 10 dB 101
Whisper 1*10-10 W/m2 20 dB 102
Normal Conversation 1*10-6 W/m2 60 dB 106
Busy Street Traffic 1*10-5 W/m2 70 dB 107
Vacuum Cleaner 1*10-4 W/m2 80 dB 108
Large Orchestra 6.3*10-3 W/m2 98 dB 109.8
Walkman at Maximum Level 1*10-2 W/m2 100 dB 1010
Front Rows of Rock Concert 1*10-1 W/m2 110 dB 1011
Threshold of Pain 1*101 W/m2 130 dB 1013
Military Jet Takeoff 1*102 W/m2 140 dB 1014
Instant Perforation of Eardrum 1*104 W/m2 160 dB 1016
Intensity
▪ Measured in decibels– Relative intensity- relating the intensityof a given sound to the intensity
a the threshold of hearing
– Dimensionless because it is a comparison unit
Audible Sounds
▪ The softest sound humans can hear is called the threshold of hearing.– Intensity = 1 10-12 W/m2 or zero dB
▪ The loudest sound humans can tolerate is called the threshold of pain.– Intensity = 1.0 W/m2 or 120 dB
▪ Human hearing depends on both the frequency and the intensity.
Vibration & Resonance
▪ Forced vibrations: The tendency of one object to force another adjoining or interconnected object into vibrational motion
▪ Sympathetic vibrations: a vibration produced in one body by the vibrations of exactly the same period in a neighboring body
Forced Vibrations
▪ Sympathetic vibrations occur when a vibrating object forces another to vibrate as well.– A piano string vibrates the sound board.
– A guitar string vibrates the bridge.
▪ This makes the sound louder and the vibrations die out faster.– Energy is transferred from the string to the sound board or bridge.
Vibration & Resonance
▪ Natural frequency: The frequency or frequencies at which an object tends to vibrate with when hit, struck, plucked, strummed or somehow disturbed
▪ Resonance:when one object vibrating at the same natural frequency of a second object forces that second object into vibrational motion.
Resonance
▪ The red rubber band links the 4 pendulums.
▪ If a blue pendulum is set in motion, only the other blue pendulum will have large-amplitude vibrations.– The others will just move a small amount.
▪ Since the vibrating frequencies of the blue pendulums match, they are resonant.
Resonance
▪ Large amplitude vibrations produced when the frequency of the applied force matches the natural frequency of receiver– One blue pendulum was the driving force and the other was the receiver.
▪ Bridges have collapsed as a result of structural resonance.– Tacoma Narrows in the wind
– A freeway overpass during an earthquake
Human Ear
▪ Transmits vibrations that cause nerve impulses
Pathway = Outer ear (ear canal)Eardrum3 bones (hammer, anvil, stirrup)CochleaBasilar membranes
Question 1
▪ When the decibel level of traffic in the street goes from 40 to 60 dB, how much louder does the traffic noise seem?
▪ How much greater is the intensity?
Question 2
▪ Of the following factors:– Intensity
– Speed of sound waves
– Frequency
– Decibel level
– Wavelength
– Amplitude
Which factors change the loudness of the sound?
Which factors change when pitch gets higher?