Properties and Detection of Sound The human ear is a detector that receives pressure waves and...

Properties and Detection of Sound

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.

The Human Ear

Section

15.1


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.

The Human Ear

Section

15.1


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.

The Human Ear

Section

15.1


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.

Perceiving Sound – Loudness

Section

15.1


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

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

Section

15.1

Click image to view movie.


The Doppler Effect

Section

15.1

For both a moving source and a moving observer, the frequency that the observer hears can be calculated using the equation below.

dd s

s

v vf = f v v

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.


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.

The Doppler Effect

Section

15.1


The Doppler Effect

Section

15.1

VD (+) Detector moves away from source

VD (-) Detector moves towards source

VS (+) Source moves towards detector

VS (-) Sources moves away from detector


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

Show the velocities of the source and the detector.

Properties and Detection of SoundSection

15.1

The Doppler Effect

Identify the known and unknown variables.


15.1

Known:

V = +343 m/s

Vs = +24.6 m/s

Vd = 0 m/s

fs = 524 Hz

Unknown:

fd = ?

The Doppler Effect


15.1

Substitute v = +343 m/s, vs = +24.6 m/s, and fs = 524 Hz.

Use with vd = 0 m/s.v v

f = fv v

dd s

s

–

–

1f = f

v1 -

v

d ss

1= 524 Hz

24.6 m/s1-

343 m/s

= 564

The Doppler Effect

The steps covered were:


15.1

The Doppler Effect

Step 3: Evaluate the Answer

Step 2: Solve for the Unknown

v vf f

v v

dd s

s

– Use with vd = 0 m/s

Section Check

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)?

Question 3

Section

15.1

vf f

v

sd s= 1–

vf f

v

sd s= 1+

f fv

v

d ss

1=

1–

A.

B.

C.

D.

f f

v

v

d ss

1=

1–

Section Check

Answer: C

Answer 3

Reason:

Section

15.1

v vf f

v v

v

f fv

v

dd s

s

d

d ss

According to Doppler's effect

–=

–

The person is standing, = 0.

1=

1–

Properties and Detection of Sound The human ear is a detector that receives pressure waves and...

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