Transcript Phy 103: Fundamentals of Physics III

Phy 103: Fundamentals of
Physics III
Chapter 20: Sound
Lecture Notes
Origin of Sound
• Sound is a longitudinal wave
• Sound is produced by the vibration of material objects
• The frequency range of sound we can hear is about 20
to 20,000 Hz
– Sound waves below 20 Hz are called infrasonic
– Sound waves above 20,000 Hz are called ultrasonic
– We cannot hear infrasonic or ultrasonic sound waves
• Sound is transmitted as a series of high pressure/low
pressure vibrations
– The high pressure region is called a compression
– The low pressure region is called a rarefaction
– The condensations and rarefactions travel (through the air)
along the direction of the sound wave
Media that Transmit Sound
• Any elastic material can transmit sound including:
– Solids (fastest)
In iron/steel: the speed of sound is ~5000 m/s
– Liquids
In water: the speed of sound is ~1400 m/s
– Gases (slowest)
In air: the speed of sound is ~ 340 m/s (at 20oC)
• The stiffer (more elastic) the substance the better it will
transmit sound
Speed of Sound in Air
• The speed of sound in air depends on temperature
– The colder the air the slower the sound wave
– The warmer the air the faster the sound wave
• At room temperature, the speed of sound is about 340
m/s
• To calculate the speed of sound (in air) at a particular
temperature (T in oC):
vsound  (331 + 0.60.T) m/s
Example:
At 20oC, the speed of sound is
vsound = [331 + (0.60)(20)] m/s = 343 m/s
What is the speed of sound at 30oC?
Reflection of Sound
• A reflected sound wave is called an echo
• Sound reflects from the surface of objects the same way
light does
– The smoother & flatter the surface, the greater the reflection
– When sound reflects off a smooth surface, the angle of
incidence is equal to the angle of reflection
This called the law of reflection
• When presenting sound (for instance, during a concert),
special care is made to control the reflective movement
of sound waves
• Acoustics is the study of sound properties
Refraction of Sound
• When sound travels through air of uneven
temperature (or in uneven winds) parts of the
wavefront will travel at different speeds
• The sound waves will tend to bend
– Sound waves bend away from warm air
– Sound waves bend toward cold air
• The bending of sound waves is called refraction
Interference & Beats
• Sound waves exhibit interference like other waves
• When 2 sound waves (with different frequencies) combine,
the resultant disturbance (wave pattern) exhibits both
constructive and destructive interference
• The combined effect of interference produces periodic
rises and drops in loudness called beats
• The frequency of the beats(fbeat) is equal to the
difference between the 2 sound frequencies:
fbeat = f1 - f2
where f1>f2
• Musicians often tune their musical instruments by
listening to beat frequency
Energy in Sound Waves
• All waves carry (transmit energy)
• Hearing is possible because energy from sound waves is
transferred to the ear (in the form of vibrations)
• Sound waves carry a very small amount of energy:
Example:
Consider a 100 W stereo, 100 J of energy are transmitted per
second from the speakers (50 J per speaker).
At a distance of 2 meters from the speakers, this is only 2 J of
energy per 1 m2 area (your eardrum has an effective area of
roughly 3x10-5 m2)
Energy at Ear = (2 J/m2) (3x10-5 m2) = 0.00006 J
So your ear on receives 0.00006 J (or 6x10-5 J) during 1 second
Your ear is a very sensitive instrument!