Download Properties and Detection of Sound

Section
15.1
Properties and Detection of Sound
The Human Ear
The human ear is a detector that receives pressure waves and
converts them into electrical impulses.
Sound waves entering the auditory canal cause vibrations of the
tympanic membrane.
Section
15.1
Properties and Detection of Sound
The Human Ear
Three tiny bones then transfer these vibrations to fluid in the
cochlea. Tiny hairs lining the spiral-shaped cochlea detect
certain frequencies in the vibrating fluid. These hairs stimulate
nerve cells, which send impulses to the brain and produce the
sensation of sound.
Section
15.1
Properties and Detection of Sound
The Human Ear
The ear detects sound waves over a wide range of frequencies
and is sensitive to an enormous range of amplitudes.
In addition, human hearing can distinguish many different
qualities of sound.
Section
15.1
Properties and Detection of Sound
Perceiving Sound – Loudness
Most people perceive a 10-dB increase in sound level as about
twice as loud as the original level.
In addition to pressure variations, power and intensity of sound
waves can be described by decibel scales.
Section
15.1
Properties and Detection of Sound
Perceiving Sound – Loudness
Exposure to loud sounds, in the form of noise or music, has
been shown to cause the ear to lose its sensitivity, especially to
high frequencies.
The longer a person is exposed to loud sounds, the greater the
effect.
A person can recover from short-term exposure in a period of
hours, but the effects of long-term exposure can last for days or
weeks.
Long exposure to 100-dB or greater sound levels can produce
permanent damage.
Section
15.1
Properties and Detection of Sound
Perceiving Sound – Loudness
Hearing loss also can result from loud music being transmitted
through stereo headphones from personal radios and CD players.
Cotton earplugs reduce the sound level only by about 10 dB.
Special ear inserts can
provide a 25-dB reduction.
Specifically designed earmuffs
and inserts as shown here in
the figure can reduce the
sound level by up to 45 dB.
Section
15.1
Properties and Detection of Sound
The Doppler Effect
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Section
15.1
Properties and Detection of Sound
The Doppler Effect
For both a moving source and a moving observer, the frequency
that the observer hears can be calculated using the equation
below.
The frequency perceived by a detector is equal to the velocity of
the detector relative to the velocity of the wave, divided by the
velocity of the source relative to the velocity of the wave,
multiplied by the wave’s frequency.
Section
15.1
Properties and Detection of Sound
The Doppler Effect
In the Doppler effect equation,
v is the velocity of the sound wave,
vd is the velocity of the detector,
vs is the velocity of the sound’s source,
fs is the frequency of the wave emitted by the source, and
fd is the frequency received by the detector.
Section
Properties and Detection of Sound
15.1
The Doppler Effect
VD
(+) Detector moves away from source
VD
(-) Detector moves towards source
VS
(+) Source moves towards detector
VS
(-) Sources moves away from detector
Section
15.1
Properties and Detection of Sound
The Doppler Effect
A trumpet player sounds C above middle C (524 Hz) while traveling
in a convertible at 24.6 m/s. If the car is coming toward you, what
frequency would you hear? Assume that the temperature is 20°C.
Section
15.1
Properties and Detection of Sound
The Doppler Effect
Show the velocities of the source and the detector.
Section
15.1
Properties and Detection of Sound
The Doppler Effect
Identify the known and unknown variables.
Known:
Unknown:
V = +343 m/s
fd = ?
Vs = +24.6 m/s
Vd = 0 m/s
fs = 524 Hz
Section
Properties and Detection of Sound
15.1
The Doppler Effect
Substitute v = +343 m/s, vs = +24.6 m/s, and fs = 524 Hz.
Use
 v – vd 
fd = fs 

v
–
v

s 
with vd = 0 m/s.
Section
15.1
Properties and Detection of Sound
The Doppler Effect
The steps covered were:
Step 2: Solve for the Unknown
–Use
 v  vd 
with
vd = 0 m/s
fd  fs 

 v  vs 
Step 3: Evaluate the Answer
Section
Section Check
15.1
Question 3
A person is standing on a platform and a train is approaching toward
the platform with a velocity vs. The frequency of the train’s horn is fs.
Which of the following formulas can be used to calculate the
frequency of sound heard by the person (fd)?
A.
C.
B.
D.
Section
Section Check
15.1
Answer 3
Answer: C
Reason: