1. Properties and Detection of Sound
Section
Properties and Detection of Sound
15.1
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.

2. Properties and Detection of Sound
Section
Properties and Detection of Sound
15.1
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.

3. Properties and Detection of Sound
Section
Properties and Detection of Sound
15.1
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.

4. Properties and Detection of Sound
Section
Properties and Detection of Sound
15.1
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.

5. Properties and Detection of Sound
Section
Properties and Detection of Sound
15.1
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.

6. Properties and Detection of Sound
Section
Properties and Detection of Sound
15.1
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.

7. Properties and Detection of Sound
Section
Properties and Detection of Sound
15.1
The Doppler Effect
Click image to view movie.

8. Properties and Detection of Sound
Section
Properties and Detection of Sound
15.1
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.

9. Properties and Detection of Sound
Section
Properties and Detection of Sound
15.1
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.

11. Properties and Detection of Sound
Section
Properties and Detection of Sound
15.1 VD
(+)
Detector moves away from source
(-)
Detector moves towards source VS
Source moves towards detector
Sources moves away from detector
The Doppler Effect

12. Properties and Detection of Sound
Section
Properties and Detection of Sound
15.1
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.

13. Properties and Detection of Sound
Section
Properties and Detection of Sound
15.1
The Doppler Effect
Show the velocities of the source and the detector.

14. Properties and Detection of Sound
Section
Properties and Detection of Sound
15.1
The Doppler Effect
Identify the known and unknown variables.
Known:
V = +343 m/s
Vs = m/s
Vd = 0 m/s
fs = 524 Hz
Unknown:
fd = ?

15. Properties and Detection of Sound
Section
Properties and Detection of Sound
15.1
The Doppler Effect
Substitute v = +343 m/s, vs = m/s, and fs = 524 Hz.
Use with vd = 0 m/s.

16. Properties and Detection of Sound
Section
Properties and Detection of Sound
15.1
The Doppler Effect
The steps covered were:
Step 2: Solve for the Unknown
Use with vd = 0 m/s
Step 3: Evaluate the Answer

17. 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)?

18. Section
Section Check
15.1
Answer 3
Answer: C
Reason: