1. Chapter 21 The Nature of Sound
Section 1
What is sound?

2. Sound is Produced by Vibrations
All sounds are created by vibrations.
A vibration is the complete back-and-forth motion of an object. (see figure 1 on p. 534)
A wave is a disturbance that transmits energy through matter or space.

3. Section 1 continued
Sound waves travel in all directions away from their source.
Air or other matter does not travel with the sound waves.
The particles of air only vibrate back & forth in place.

4. Section 1 continued
Sound can be created without being detected. (Can you hear the other teachers in the building now, or the students in the lunch room or gym?)
All sound waves require a medium.
The primary medium is air; other media include but not limited to metal, water, glass, wood.

5. Section 2 – Properties of Sound
Sound waves determine how high or low and how loud or soft they are.
The media determines the speed of sound. Therefore the speed of the sound waves will change when the medium changes.
The speed of sound also depends on the temperature.

6. Section 2 - continues
As a rule the cooler the medium, the slower the speed of sound. (Remember the temp determines the speed of the moles.)
The sound’s pitch is determined by the frequency of the sound wave.
The frequency of the wave is the number of waves produced in a given period of time.

7. Section 2 - continues
Frequency is expressed in hertz (Hz), where 1 Hz = 1 wave per second.
The average human ear can detect sounds that have frequencies between 20 Hz and 20,000 Hz.
Frequencies that humans can’t hear, lower than 20 Hz are called infrasonic and ultrasonic sounds have frequencies above 20,000 Hz.

8. Section 2 - continues
Ultrasonic waves are used to clean jewelry and to remove ice from metal.
The doppler effect is the apparent change in the frequency of a sound caused by the motion of either the listener or the source of the sound.

9. Section 2 - continues
Loudness is related to amplitude. By applying force you can change the loudness of the sound that is created. Force equals energy and amplitude.
Loudness is how loud or soft a sound is perceived to be.
The decibel (dB) is the unit used to express loudness.

10. Section 2 - continues
0 dB is the faintest sound an average human ear can hear. The level of 120 dB is sometimes called the threshold of pain. (Sounds this loud or louder can cause pain to the ears.)
Continued exposure to sounds about 85 dB causes gradual hearing loss by permanently damaging the hair cells in your inner ear. (see p. 543)

11. Section 2 - continues
To “see” sound an oscilloscope must be used.

12. Section 3 Interactions of Sound waves
Reflection is the bouncing back of a wave after it strikes a barrier.
Echoes are reflected sound waves.
Echolocation is the process of using reflected sound waves to find objects. (Echolocation is used by animals, bats & whales for ex.) to locate food. Sonar is a echolocation.

13. Section 3 - continued
Ultrasonography is used in medical procedures to use echoes to “see” inside a patient’s body.
Interference is the result of 2 or more waves overlapping.
A standing wave is a result of interference in which portions of the wave are at the rest position and other portions have a large amplitude.

14. Section 3 - continued
The sound barrier is “broken” when a jet plane, for instance, accelerates to the speed of sound which causes the sound waves in front of the plane to become compressed.
The sonic boom is the explosive sound heard when a shock wave reaches your ears.

15. Section 3 - continued
Resonance occurs when an object vibrating at or near a resonant frequency of a second object caused the second object to vibrate.
Diffraction is the bending of waves around barriers or through openings.

16. Section 4 – Sound Quality
Sound quality is the result of several pitches blending together through interference.
Noise can be described an any undesired sound, especially a nonmusical sound, that includes a random mix of pitches.
Noise pollution can be caused when noise reaches a level that can cause health problems.