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The Doppler Effect

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A source emits a sound of constant frequency. If the apparent frequency of the source is increased which of the 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 B

D. neither A nor B

A source emits a sound of constant frequency. If the apparent frequency of the source is increased which of the 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 B

D. neither A nor B

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

Sound Intensity

3U Physics

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

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

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

And since energy/time = power:

And since energy/time = power:

The units of intensity are W/m2.

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

(Energy, remember, is conserved.)

The mathematical relationship between intensity 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 a factor of 22, or 4.

Humans are equipped with very sensitive ears capable of detecting sound waves of extremely low intensity, as faint as 1×10-12 W/m2.

Humans are equipped with very sensitive ears capable of detecting sound waves of extremely low intensity, as faint as 1×10-12 W/m2.

(This intensity corresponds to a pressure wave in which a compression 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.)

Humans are equipped with very sensitive ears capable of detecting sound waves of extremely low intensity, as faint as 1×10-12 W/m2.

(This intensity corresponds to a pressure wave in which a compression 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).

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

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

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

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

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

The threshold of hearing (1×10-12 W/m2) is assigned a sound level of 0 dB.

The threshold of hearing (1×10-12 W/m2) is assigned a sound level of 0 dB.

A sound (e.g. the sound of rustling leaves) which is 10 times more intense (1×10-11 W/m2) is assigned a sound level of 10 dB.

The threshold of hearing (1×10-12 W/m2) is assigned a sound level of 0 dB.

A sound (e.g. the sound of rustling leaves) which is 10 times more intense (1×10-11 W/m2) is assigned a sound level of 10 dB.

A sound (e.g. a whisper) which is 10×10 or 100 times more intense ( 1×10-10 W/m2) is assigned a sound level of __ db.

The threshold of hearing (1×10-12 W/m2) is assigned a sound level of 0 dB.

A sound (e.g. the sound of rustling leaves) which is 10 times more intense (1×10-11 W/m2) is assigned a sound level of 10 dB.

A sound (e.g. a whisper) which is 10×10 or 100 times more intense ( 1×10-10 W/m2) is assigned a sound level of 20 db.

The threshold of hearing (1×10-12 W/m2) is assigned a sound level of 0 dB.

A sound (e.g. a whisper) which is 10×10 or 100 times more intense ( 1×10-10 W/m2) is assigned a sound level of 20 db.

A sound which is 10×10×10 or 1000 times more intense (1×10-9 W/m2) is assigned a sound level of __ db.

The threshold of hearing (1×10-12 W/m2) is assigned a sound level of 0 dB.

A sound (e.g. a whisper) which is 10×10 or 100 times more intense ( 1×10-10 W/m2) is assigned a sound level of 20 db.

A sound which is 10×10×10 or 1000 times more intense (1×10-9 W/m2) is assigned a sound level of 30 db.

Etc.

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

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

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

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

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

Answer: 100 times.

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

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

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

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

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

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

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

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

Earbud headphones can be the most destructive to hearing since

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

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

Remember

Decibel Lab Activity

Homework: Sound Intensity