L35-s1,12

Physics 114 – Lecture 35• Chapter 12 Sound• Parameters associated with our sense of hearing: note

sound → longitudinal waves propagated in a medium• Source, transmission (longitudinal waves), detector• §12.1 Characteristics of Sound• Wave motion• Speed of sound in air at room temperature is roughly,• v ≈ (331 + 0.60 T) m/s• At room temp, T = 200C, v = (331 + 0.60 X 20) m/s

= 343 m/s

L35-s2,12

Physics 114 – Lecture 35• Study Example 12.1, distance from a lightning strike• Loudness is related to intensity (energy/unit time/unit

area• Pitch – high, like flute or low, like a string bass. The

physical quantity specifying this is the frequency, f = v/λ• Audible range roughly 20 Hz to 20,000 Hz – c.f., 1Hz =

1 cycle/s• Sound waves with wavelengths, λ > 20,000 Hz are said

to be ultrasonic, c.f., dogs, bats, … can hear such sounds

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Physics 114 – Lecture 35• Applications of ultrasound – medicine, autofocusing

devices, …• Study Example 12.2• Infrasonic waves – f < 20 Hz, e.g., earthquakes,

thunder, volcanoes, heavy equipment, … can cause injuries

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Physics 114 – Lecture 35• §12.2 Intensity of Sound: Decibels• Human ear can hear sounds with an intensity range

from 10-12 W/m2 to about 1 W/m2

• Sound above the latter intensity becomes painful• The human ear does not perceive loudness to be

proportional to the intensity, e.g., a sound that is increased in intensity by a factor of 10 is perceived by the ear to be only about twice as loud

• It is therefore more realistic to give sound intensity levels on a logarithmic scale

L35-s5,12

Physics 114 – Lecture 35• The resulting unit is known as the bel• The commonly used unit is the decibel (db) which is

one tenth of a bel and is defined as follows,

• Where I0 = 1.0 X 10-12 W/m2 , the minimum intensity audible to a good ear

• Thus for I = 1.0 X 10-10 W/m2

010 I

Ilog 10 db)(in β

db 20 100 log 10 W/m10 X 0.1

W/m10 X 0.1log 10 β

10212-

2-10

10

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Physics 114 – Lecture 35• Study examples 12.3, 12.4, 12.5• §12.4 Sources of Sound: Vibrating Strings and Air

Columns

• String instruments,

standing waves

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Physics 114 – Lecture 35• Open tubes or pipes

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Physics 114 – Lecture 35• Tubes or pipes closed at one end

L35-s9,12

Physics 114 – Lecture 35• §12.7 Doppler Effect• The pitch or frequency of a sound is affected by

motion of the source or/and of the detector relative to the medium in which the sound is transmitted

L35-s10,12

Physics 114 – Lecture 35

• Source moving towards observer

• T = 1/f = λ/v

• λ´ = λ – ds = λ – vsT = λ – vs λ/v

• λ´ = λ ( 1 – vs/v)

• v/f ´ = (v/f) ( 1 – vs/v)

•

• Note that if vs > 0, f´ > f

v

v-1

1f f

s

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Physics 114 – Lecture 35• Moving observer, velocity, v, stationary source

• f´ = f ( 1 + vo/v ) for motion approaching source

• f´ = f ( 1 - vo/v ) for motion receding from source

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Physics 114 – Lecture 35

• In general for motion of both the source, vs, and the observer, vo ,

•

• where the upper sign indicates motion of the source approaching the observer or of the observer approaching the source

v

v1

v

v 1

f fs

o