1. Chapter 1 (Hall)
The Nature of Sound
Dr. Jie Zou
PHY 1071

2. Outline Introduce some of the basic concepts of sound.
Acoustics and music
Organizing our study of sound
The Physical nature of sound
The speed of sound
Pressure and sound amplitude
Dr. Jie Zou
PHY 1071

3. Acoustic and music
Acoustics: The study of the physical nature of sound.
Audible sound:
Music: Includes those intentional combinations of sounds that we choose to hear for esthetic enjoyment and usually depends on an orderly pattern of sounds for its pleasing effect.
Speech: Much in common with music but differs in the purpose. Speech sounds communicate the entire range of human ideas through word symbols rather than by conveying emotions directly.
Noise: Unorganized, unpleasant, or unwanted sounds.
Dr. Jie Zou
PHY 1071

4. Organizing our study of sound
Study sound in three main areas:
(1) Production: How is it created?
Sound originates from vibration.
(2) Propagation: How does it travel from one place to another?
Sound waves need a medium to propagate.
(3) Perception: How does it affect the senses and emotions of a listener?
Pitch: The sensation of how “high” or “low” a sound is.
Treble: High-pitched sounds. Bass: Low-pitched sounds.
Loudness: The sensation of strength or weakness in a sound.
Dr. Jie Zou
PHY 1071

5. The physical nature of sound
What is sound in physics?
Sound entails a disturbance (vibration) of the air through which it moves.
Sound in air consists of longitudinal waves carrying energy outward from their source.
Vibration: A rapid back-and-forth movement of a single object or of a single small piece of some large object.
Wave: A disturbance traveling outward from a vibrating source.
Connection between vibrations and waves:
The passage of a wave through any region causes each little piece of material in that region, in turn, to vibrate.
For a sound wave, it is not the air that travels across the room; it is the signal (the disturbance) in the air.
Dr. Jie Zou
PHY 1071

6. Representation of a sound wave
Ray
Crest 
A simplified, stylized representation of sound waves
Ray: The rays indicate the direction in which the wave is moving.
Crests: Points of greatest compression.
Wavelength : The distance from one crest to the next along the direction of travel. For audible sound   2 cm to 20 m.
Corresponding more literal representation (see lower left picture)
Dr. Jie Zou
PHY 1071

7. The speed of sound
The speed of sound in dry afternoon temperature (T = 20°C) is v = 344 m/s or 1130 ft/s, or 770 mi/hr.
All sounds, whether high or low in pitch (frequency), travel through the air at the same speed.
If the air temperature changes, so does the speed of sound.
If the temperature is higher, the random molecular motions are faster, neighboring molecules collide more often, and they can pass the sound disturbance faster from one region to another. The sound speed increases about 0.6 m/s for each degree of temperature rise on the Celsius scale.
Dr. Jie Zou
PHY 1071

8. Example#1
How far does sound travel during second if the temperature is 20°C?
Dr. Jie Zou
PHY 1071

9. Pressure and sound amplitude
Displacement amplitude: The distance each bit of air moves to either side of its normal position during its vibration.
For ordinary sound waves, the displacement amplitude is on the order of millionths of a meter or less – displacement is very difficult to measure.
Pressure amplitude: The maximum increase of air pressure (above normal atmospheric pressure) in a sound wave compression.
Pressure = Force/Area, 1 atm = 1.01 x 105 N/m2.
Pressure amplitudes are also typically very small, but they are easily measured because microphone diaphragms respond directly to the pressure fluctuations.
The pressure amplitudes of sound waves at comfortable listening levels range from about 0.01 to 1 N/m2, or 10-7 to 10-5 atm.
Dr. Jie Zou
PHY 1071

10. Example #2
Suppose a particular sound wave momentarily creates an extra pressure of 10-4 atm upon a microphone diaphragm that has an area of 1 cm2. What total force in newtons (N) does this make on the diaphragm?
Dr. Jie Zou
PHY 1071

11. Homework (Hall) Ch. 1, P. 15, Exercises: #3, 11, 12. Dr. Jie Zou
PHY 1071