Sound Intensity Nov 2012

7/27/2019 Sound Intensity Nov 2012

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The Doppler Effect

A source emits a sound of constant frequency. If theapparent frequency of the source is increased which ofthe following is true?

A. the source is moving toward you and you are stationary

B. the source is stationary and you are moving toward it

C. either A or BD. neither A nor B


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The Doppler Effect

A source emits a sound of constant frequency. If theapparent frequency of the source is increased which ofthe following is true?

A. the source is moving toward you and you are stationary

B. the source is stationary and you are moving toward it

C. either A or BD. neither A nor B


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Sound Intensity: Learning Goals

The student will be able to explain how theintensity of a sound wave is expressed and varieswith distance from a source. (E2.1, E3.6)


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Sound Intensity

3U Physics


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Energy and Amplitude

The amplitude of a wave depends upon theamplitude of the vibration at the source:

the more work done to displace the medium atthe source, the more energy the wave will carry.


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Intensity

The rate at which energy is transported past agiven area of the medium is known as the

intensity of the sound wave.

IntensityEnergy

Area Time


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Intensity

And since energy/time = power:

Intensity

Energy

Area Time

Power

Area


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Intensity

And since energy/time = power:

The units of intensity are W/m2.

Intensity

Energy

Area Time

Power

Area


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Intensity and Area

As a wave carries its energy through a 3D medium, theintensity of the wave decreases with increasing distancebecause energy is being distributed over a greater

surface area.(Energy, remember, is conserved.)


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Intensity and Area

The mathematical relationship betweenintensity and distance is an:

inverse square relationship.

i.e., if the distance from the source is

doubled (increased by a factor of 2),then the intensity is decreased by afactor of 22, or 4.


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Intensity and Hearing

Humans are equipped with very sensitive ears capable ofdetecting sound waves of extremely low intensity, as

faint as 110-12 W/m2.
http://en.wikipedia.org/wiki/Image:Studearring.JPG


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(This intensity corresponds to a pressure wave in which acompression increases the air pressure by a mere 0.3 billionths of

an atmosphere, or (in terms of amplitude) a wave in which the

particle displacement is a mere one-billionth of a centimetre.)


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(This intensity corresponds to a pressure wave in which acompression increases the air pressure by a mere 0.3 billionths of

an atmosphere, or (in terms of amplitude) a wave in which the

particle displacement is a mere one-billionth of a centimetre.)

This is known as the threshold of hearing (TOH).


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Intensity Scale

The most intense sound whichthe ear can safely detect

without suffering any

physical damage is morethan one billion times moreintense.


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Intensity Scale




Physicists therefore often use a

logarithmic scale (based onpowers of 10) for intensity:


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Intensity Scale




Physicists therefore often use a

logarithmic scale (based onpowers of 10) for intensity:the decibel (dB) scale.


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Decibels

The threshold of hearing (110-12 W/m2) is assigned asound level of 0 dB.


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Decibels


A sound (e.g. the sound of rustling leaves) which is 10

times more intense (1

10-11

W/m2

) is assigned a soundlevel of 10 dB.


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Decibels




10-11

W/m2


A sound (e.g. a whisper) which is 1010 or 100 timesmore intense ( 110-10 W/m2) is assigned a sound level

of __ db.


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Decibels




10

-11

W/m

2



of 20 db.


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Decibels




10

-11

W/m

2



of 20 db.A sound which is 101010 or 1000 times more intense(110-9 W/m2) is assigned a sound level of __ db.


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Decibels




10

-11

W/m

2



of 20 db.A sound which is 101010 or 1000 times more intense(110-9 W/m2) is assigned a sound level of 30 db.

Etc.


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Decibels

If one sound is 10xtimes more intense than anothersound, then it has a sound level which is 10xmoredecibels than the less intense sound.


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Decibels


Example: A mosquito's buzz is often rated with a decibelrating of 40 dB. Normal conversation is often rated at60 dB. How many times more intense is normalconversation compared to a mosquito's buzz?


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Decibels


Example: A mosquito's buzz is often rated with a decibelrating of 40 dB. Normal conversation is often rated at60 dB. How many times more intense is normalconversation compared to a mosquito's buzz?

Answer: 100 times.


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Decibels

Another Example: Someone standing 1 m awayfrom a busy street measures the decibel level ofthe traffic to be 70 dB. What level would they

measure if they were standing 10 m away?


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Decibels

Another Example: Someone standing 1 m awayfrom a busy street measures the decibel level ofthe traffic to be 70 dB. What level would they

measure if they were standing 10 m away?

Answer: 50 dB. (Ten times further away, theeffective area over which the sound must travel

is increased by 100, thus decreasing the intensityby a factor of 100, or 20 dB.)


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Loudness

Note that sound intensity is an objective quantity, butloudness is a subjective response which will vary with,for example, frequency.


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Intensity

High intensity sound wavesthat have largeamplitudes will result in

large-amplitudevibrations in the ear . . .which can damage theear, especially the

sensitive hair cells of thecochlea.


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How Loud is Too Loud?

Regular exposure to 110 dB formore than a minute -- or to 100dB for more than 15 minutes --

risks permanent hearing loss.Prolonged exposure to any noise

above 85 dB can cause gradualhearing loss.

Most MP3 players have amaximum volume setting equalto about 105 dB.


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Just Say No to Earbuds

Earbud headphones can be the most destructiveto hearing since

(a) they do not filter out external sounds, causingthe listener to increase the volume, and

(b) they are positioned very close to the eardrum.

Remember

IntensityPower

Area


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More Practice

Decibel Lab Activity

Homework: Sound Intensity

Sound Intensity Nov 2012

Documents

Transcript of Sound Intensity Nov 2012