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Power 3.846×10 26 Watts of light Light bulb 5 Watts of light.

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Presentation on theme: "Power 3.846×10 26 Watts of light Light bulb 5 Watts of light."— Presentation transcript:


2 Power 3.846×10 26 Watts of light

3 Light bulb 5 Watts of light

4 A speaker

5 Intensity ie. Loudness and brightness


7 Intensity of a uniform spherical wave

8 Intensity Ratio


10 iclicker time A concert loudspeaker suspended high above the ground emits 35w of sound power. A small microphone with a 1.0cm 2 area is 50 m from the speaker. – What is the sound intensity at the position of the microphone? a)0.035 W/m 2 b)0.0011 W/m 2 c)0.056 W/m 2 d)0.0045 W/m 2

11 Decibels Sound Intensity Level

12 Decibels -The Basics β = (10dB)log 10 (I / I o ) For the calculations that we are dealing with, I o = 1.0 x 10 -12 W/m 2 This is also known as the threshold of hearing The symbol β is the Greek letter beta, computed as a base-10 logarithm rather than a natural logarithm

13 Decibels The decibel is named after the inventor of the telephone, Alexander Graham Bell Decibels are only a name to remind us that we are dealing with an intensity level rather than true intensity Sound intensity level is actually dimensionless because it’s formed from the ratio of two intensities

14 How to calculate Decibels At the threshold of hearing, I = I o, where the sound intensity level is: β = (10dB)log 10 (I o / I o ) = 0 dB dB is just a way to measure the sound level, and is completely arbitrary; it is not in any way a unit 0 dB does not mean no sound is produced, but rather means that to most people no sound is heard

15 Decibels (cont.) Since dogs have more sensitive hearing than humans, a 0 dB sound is easily heard by them As we all know, sound at a very high level can cause pain and ear damage The sound intensity level at the threshold of pain is β = (10dB)log 10 (10 W/m 2 ÷ 10 -12 W/m 2 ) = 130 dB


17 Decibels The sound intensity level increases by 10 dB each time the true intensity increases by a factor of 10 Studies indicate that when the intensity is increased by a factor of 10, the sound is perceived to be “twice as loud” The loudness of a sound is doubled with each increase in 10 dB

18 How Loud Is Too Loud? Even though the threshold of pain is 130 dB, quieter sounds can also cause serious ear damage Regular exposure of more than 1 minute of 110 dB risks permanent hearing loss 100 dB – no more than 15 minutes of unprotected exposure is recommended Prolonged exposure to any sound above 85 dB can cause gradual hearing loss

19 Just so you know Decibels 150 120 110 105 100 95 90 85 60 40 30 0 Sound Source Firecracker Ambulance siren Chain saw, rock concert Ipod at max volume Wood shop, snowmobile Motorcycle Power mower Heavy city traffic Normal conversation Refrigerator humming Whispered voice Threshold of human hearing

20 For teh lulz vmc vmc yGQ yGQ Gjs Gjs

21 Sources Images – n/photostream/ n/photostream/ – _Imaging_Assembly_of_NASA%27s_Solar_Dynamics_Observato ry_-_20100819.jpg _Imaging_Assembly_of_NASA%27s_Solar_Dynamics_Observato ry_-_20100819.jpg – – – vg vg Sources – Pearson. Physics for scientists and Engineers. 2008. Nov 2011. –

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