the doppler effect a source emits a sound of constant frequency. if the apparent frequency of the...

The Doppler EffectThe Doppler Effect

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

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

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

C. either A or BC. either A or B

D. neither A nor BD. neither A nor B

Sound Intensity: Sound Intensity: Learning GoalsLearning Goals

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

Sound IntensitySound Intensity3U Physics3U Physics

Energy and AmplitudeEnergy and Amplitude

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

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

IntensityIntensity

The rate at which energy is The rate at which energy is transported past a given area of the transported past a given area of the medium is known as the medium is known as the intensityintensity of of the sound wave. the sound wave.

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IntensityIntensity

And since energy/time = power:And since energy/time = power:

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IntensityIntensity

And since energy/time = power:And since energy/time = power:

The units of intensity are W/mThe units of intensity are W/m22..

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Intensity and AreaIntensity and Area

As a wave carries its energy through a 3D As a wave carries its energy through a 3D medium, the intensity of the wave medium, the intensity of the wave decreases with increasing distance decreases with increasing distance because energy is being distributed over a because energy is being distributed over a greater surface area. greater surface area.

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

Intensity and AreaIntensity and Area

The mathematical relationship The mathematical relationship between intensity and distance between intensity and distance is an:is an:inverse square relationshipinverse square relationship. .

i.e., if the distance from the i.e., if the distance from the source is doubled (increased by source is doubled (increased by a factor of 2), then the intensity a factor of 2), then the intensity is decreased by a factor of 2is decreased by a factor of 222, or , or 4. 4.

Intensity and HearingIntensity and Hearing

Humans are equipped with very sensitive Humans are equipped with very sensitive ears capable of detecting sound waves of ears capable of detecting sound waves of extremely low intensity, as faint as 1extremely low intensity, as faint as 1××1010--

1212 W/m W/m22..


Humans are equipped with very sensitive Humans are equipped with very sensitive ears capable of detecting sound waves of ears capable of detecting sound waves of extremely low intensity, as faint as 1extremely low intensity, as faint as 1××1010--

1212 W/m W/m22. .

(This intensity corresponds to a pressure (This intensity corresponds to a pressure wave in which a compression increases wave in which a compression increases the air pressure by a mere 0.3 billionths the air pressure by a mere 0.3 billionths of an atmosphere, or (in terms of of an atmosphere, or (in terms of amplitude) a wave in which the particle amplitude) a wave in which the particle displacement is a mere one-billionth of a displacement is a mere one-billionth of a centimetre.)centimetre.)


Humans are equipped with very sensitive ears Humans are equipped with very sensitive ears capable of detecting sound waves of capable of detecting sound waves of extremely low intensity, as faint as 1extremely low intensity, as faint as 1××1010-12-12 W/mW/m22. .

(This intensity corresponds to a pressure wave (This intensity corresponds to a pressure wave in which a compression increases the air in which a compression increases the air pressure by a mere 0.3 billionths of an pressure by a mere 0.3 billionths of an atmosphere, or (in terms of amplitude) a atmosphere, or (in terms of amplitude) a wave in which the particle displacement is a wave in which the particle displacement is a mere one-billionth of a centimetre.)mere one-billionth of a centimetre.)

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

Intensity ScaleIntensity Scale

The most intense sound The most intense sound which the ear can which the ear can safely detect without safely detect without suffering any suffering any physical damage is physical damage is more than one billion more than one billion times more intense.times more intense.


The most intense sound The most intense sound which the ear can which the ear can safely detect without safely detect without suffering any physical suffering any physical damage is more than damage is more than one billion times more one billion times more intense.intense.

Physicists therefore often Physicists therefore often use a logarithmic scale use a logarithmic scale (based on powers of (based on powers of 10) for intensity:10) for intensity:


The most intense sound The most intense sound which the ear can safely which the ear can safely detect without suffering detect without suffering any physical damage is any physical damage is more than one billion more than one billion times more intense.times more intense.

Physicists therefore often Physicists therefore often use a logarithmic scale use a logarithmic scale (based on powers of 10) (based on powers of 10) for intensity: the for intensity: the decibel (dB) decibel (dB) scale.scale.

DecibelsDecibels

The threshold of hearing (1The threshold of hearing (1××1010-12-12 W/m W/m22) is ) is assigned a sound level of 0 dB.assigned a sound level of 0 dB.

DecibelsDecibels

The threshold of hearing (1The threshold of hearing (1××1010-12-12 W/m W/m22) is ) is assigned a sound level of 0 dB. assigned a sound level of 0 dB.

A sound (e.g. the sound of rustling leaves) A sound (e.g. the sound of rustling leaves) which is 10 times more intense (1which is 10 times more intense (1××1010-11-11 W/mW/m22) is assigned a sound level of 10 dB.) is assigned a sound level of 10 dB.

DecibelsDecibels


A sound (e.g. the sound of rustling leaves) A sound (e.g. the sound of rustling leaves) which is 10 times more intense (1which is 10 times more intense (1××1010-11-11 W/mW/m22) is assigned a sound level of 10 dB. ) is assigned a sound level of 10 dB.

A sound (e.g. a whisper) which is 10A sound (e.g. a whisper) which is 10××10 or 10 or 100 times more intense ( 1100 times more intense ( 1××1010-10-10 W/m W/m22) is ) is assigned a sound level of __ db.assigned a sound level of __ db.

DecibelsDecibels



A sound (e.g. a whisper) which is 10A sound (e.g. a whisper) which is 10××10 or 10 or 100 times more intense ( 1100 times more intense ( 1××1010-10-10 W/m W/m22) is ) is assigned a sound level of 20 db.assigned a sound level of 20 db.

DecibelsDecibels



A sound (e.g. a whisper) which is 10A sound (e.g. a whisper) which is 10××10 or 10 or 100 times more intense ( 1100 times more intense ( 1××1010-10-10 W/m W/m22) is ) is assigned a sound level of 20 db. assigned a sound level of 20 db.

A sound which is 10A sound which is 10××1010××10 or 1000 times 10 or 1000 times more intense (1more intense (1××1010-9-9 W/m W/m22) is assigned a ) is assigned a sound level of __ db.sound level of __ db.

DecibelsDecibels



A sound (e.g. a whisper) which is 10A sound (e.g. a whisper) which is 10××10 or 10 or 100 times more intense ( 1100 times more intense ( 1××1010-10-10 W/m W/m22) is ) is assigned a sound level of 20 db. assigned a sound level of 20 db.

A sound which is 10A sound which is 10××1010××10 or 1000 times 10 or 1000 times more intense (1more intense (1××1010-9-9 W/m W/m22) is assigned a ) is assigned a sound level of 30 db.sound level of 30 db.

Etc. Etc.

DecibelsDecibels

If one sound is 10If one sound is 10xx times more intense than times more intense than another sound, then it has a sound level another sound, then it has a sound level which is 10which is 10xx more decibels than the less more decibels than the less intense sound.intense sound.

DecibelsDecibels

If one sound is 10If one sound is 10xx times more intense than times more intense than another sound, then it has a sound level another sound, then it has a sound level which is 10which is 10xx more decibels than the less more decibels than the less intense sound.intense sound.

ExampleExample: A mosquito's buzz is often rated with : A mosquito's buzz is often rated with a decibel rating of 40 dB. Normal a decibel rating of 40 dB. Normal conversation is often rated at 60 dB. How conversation is often rated at 60 dB. How many times more intense is normal many times more intense is normal conversation compared to a mosquito's buzz?conversation compared to a mosquito's buzz?

DecibelsDecibels

If one sound is 10If one sound is 10xx times more intense than times more intense than another sound, then it has a sound level which another sound, then it has a sound level which is 10is 10xx more decibels than the less intense more decibels than the less intense sound.sound.

ExampleExample: A mosquito's buzz is often rated with a : A mosquito's buzz is often rated with a decibel rating of 40 dB. Normal conversation decibel rating of 40 dB. Normal conversation is often rated at 60 dB. How many times more is often rated at 60 dB. How many times more intense is normal conversation compared to a intense is normal conversation compared to a mosquito's buzz?mosquito's buzz?

AnswerAnswer: 100 times.: 100 times.

DecibelsDecibels

Another ExampleAnother Example: Someone standing : Someone standing 1 m away from a busy street 1 m away from a busy street measures the decibel level of the measures the decibel level of the traffic to be 70 dB. What level traffic to be 70 dB. What level would they measure if they were would they measure if they were standing 10 m away?standing 10 m away?

DecibelsDecibels

Another ExampleAnother Example: Someone standing 1 m : Someone standing 1 m away from a busy street measures the away from a busy street measures the decibel level of the traffic to be 70 dB. decibel level of the traffic to be 70 dB. What level would they measure if they What level would they measure if they were standing 10 m away?were standing 10 m away?

AnswerAnswer: 50 dB. (Ten times further away, : 50 dB. (Ten times further away, the effective area over which the sound the effective area over which the sound must travel is increased by 100, thus must travel is increased by 100, thus decreasing the intensity by a factor of decreasing the intensity by a factor of 100, or 20 dB.)100, or 20 dB.)

LoudnessLoudness

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

IntensityIntensity

High intensity sound High intensity sound waves that have waves that have large amplitudes large amplitudes will result in large-will result in large-amplitude amplitude vibrations in the vibrations in the ear . . . which can ear . . . which can damage the ear, damage the ear, especially the especially the sensitive hair cells sensitive hair cells of the cochlea.of the cochlea.

How Loud is Too Loud?How Loud is Too Loud?

Regular exposure to 110 dB Regular exposure to 110 dB for more than a minute -- for more than a minute -- or to 100 dB for more or to 100 dB for more than 15 minutes -- risks than 15 minutes -- risks permanent hearing loss.permanent hearing loss.

Prolonged exposure to any Prolonged exposure to any noise above 85 dB can noise above 85 dB can cause gradual hearing cause gradual hearing loss.loss.

Most MP3 players have a Most MP3 players have a maximum volume setting maximum volume setting equal to about 105 dB.equal to about 105 dB.

Just Say No to EarbudsJust Say No to Earbuds

Earbud headphones can be the most Earbud headphones can be the most destructive to hearing since destructive to hearing since

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

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

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Decibel Lab ActivityDecibel Lab Activity

Homework: Sound IntensityHomework: Sound Intensity

the doppler effect a source emits a sound of constant frequency. if the apparent frequency of the...

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