Transcript Sound Wave Properties

SOUND WAVE
PROPERTIES
Sound
• Sound is a longitudinal
(Mechanical)wave caused by a
vibrating object
• Molecules collide, producing
sound
• Examples: Vocal chords, guitar or
piano strings, tuning fork, etc.
Longitudinal Wave
• Referred to as a PRESSURE WAVE
• A sound wave has high pressure and low pressure
regions moving through a medium
• The high pressure regions are called compressions,
molecules are compressed
• The low pressure regions are called rarefactions,
molecules are spread out
Frequency
• The frequency of a sound wave (or any
wave) is the number of complete vibrations
per second.
• The frequency of sound determines its
pitch
• The higher the frequency, the higher the
pitch
• The lower the frequency, the lower the pitch
Wavelength
• Wavelength is the distance between two
high pressures or two low pressures
• Wavelength and frequency are inversely
related
• A short wavelength (high frequency) results
in a high pitch
• http://phet.colorado.edu/en/simulation/sound
Frequency and the human ear
• Humans can hear a range of frequencies from 20 Hz to
20,000 Hz
• The older you get, the hearing range shrinks
• Sound waves with frequencies below 20 Hz are called
infrasonic
• Sound waves with frequencies above 20,000 Hz are
called ultrasonic
Amplitude
• The AMPLITUDE of a sound wave determines it’s
loudness or softness
• This means the more energy in a sound wave, the louder
the sound
• Sound intensity is a measure of how much energy passes
a given point in a time period
• Intensity is measured in decibels
Sound Behaviors: Reflection
• Reflection of sound results in an echo
• Sound waves leave a source, travel a distance, and
bounce back to the origin
• Animals, like bats, uses echoes to locate prey
• Other uses include determining distance between objects,
echocardiograms
• The distance the sound travels to get back to the origin is
2x the distance between the sound source and boundary
Sound Behavior: Refraction
• Refraction occurs when sound moves from one medium
to another
• The wave bends, and the speed changes
• Even when
sound moves
from warmer
areas to
cooler areas,
refraction
occurs
Sound Behavior: Diffraction
• Diffraction occurs when sound waves pass through an
opening or through a barrier
• Low pitched sound waves travel farther than high pitched
sound waves
• Animals use diffraction for communication
Velocity
• Velocity of sound depends on the medium it travels
through and the phase of the medium
• Sound travels faster in liquids than in air (4 times faster in
water than air)
• Sound travels faster in solids than in liquids (11 times
faster in iron than in air)
• Sound does not travel through a vacuum (there is no air
so sound has no medium)
Velocity and Temperature
• In air at room temperature, sound travels at 343 m/s (at
20°C). This is about 766 mph.
• As temperature increases, the velocity of sound increases
v=331 + (0.6)T
v= velocity of sound in air
T=temperature of air in °C
Wave Equation
• The basic wave equation is also applied to sound
𝑣 = λ𝑓
• V= velocity, measured in m/s
• λ= wavelength, measured in meters
• f= frequency, measured in hertz
Bellwork
1. What type of wave is a sound wave?
2. What is a compression? Rarefaction?
3. Based on yesterday’s class, state the
relationship between wavelength and the
pitch of a sound wave.
4. Explain in one sentence how blowing
across a straw produces a sound.
5. How can you change the loudness of a
sound you produce?
Example Problems:
1. Sound waves travel at approximately 340 m/s. What is
the wavelength of a sound wave with a frequency of 20
Hz?
2. What is the speed of sound traveling in air at 20º C?
3. If the above sound wave has a frequency of 261.6 Hz,
what is the wavelength of the wave?
What is the Doppler Effect?
• http://molebash.com/doppler/home.htm
Doppler Effect
• Sound waves move out in all directions
Definition
• The Doppler effect is a change in the
apparent frequency due to the motion
of the source or the receiver
• Example: As an ambulance with sirens
approaches, the pitch seems high. As
the ambulance moves by the pitch
lowers.
Doppler Effect
• As the wave travels outward, the front of
the wave bunches up, producing a shorter
wavelength
• We hear a higher
frequency
• The back of the wave spreads out,
producing a longer wavelength
• We hear a lower frequency
• http://www.sounddogs.com/searchresu
lts.asp?Keyword=Doppler
•Observer A hears a low pitch (lower
frequency)
•Observer B hears the correct pitch (no
change in frequency)
•Observer C hears a high pitch (high
frequency)
When the source goes faster, the wave fronts
in the front of the source start to bunch up
closer and closer together, until...
The object actually starts to go faster than the
speed of sound. A sonic boom is then created.
Uses of the Doppler Effect
• Police use Doppler to measure your speed with radar
• A frequency is sent out with a radar gun
• The sound wave hits your car and bounces back to the police car
• Speed can be determined based on the frequency changes
received
• Radar can be used to determine the speed of baseballs
• Astronomers can determine the distance to other galaxies
• Bats use Doppler to locate prey
• If the bat is catching the prey, the frequency is high
• If the prey is moving away from the bat, the frequency is low
Doppler Equation
𝑣 ± 𝑣𝑑
𝑓𝑑 = 𝑓𝑠
𝑣 ± 𝑣𝑠
• 𝑓𝑑 = frequency detected by observer
• 𝑓𝑠 = frequency produced by the source
• 𝑣 = velocity of the sound wave
• 𝑣𝑑 = velocity of the detector (observer)
• 𝑣 𝑠 = velocity of the source
Things to remember
• The velocity detected by the observer (vd) is negative if
the observer moves away from the source
• The velocity detected by the observer (vd) is positive is the
observer moves toward the source
• The velocity of the source (𝑣𝑠) is negative if the source
moves toward the observer
• The velocity of the source (𝑣 𝑠) is positive if the source
moves away from the observer
Example
A trumpet player plays a C note of 524 Hz while traveling
in a convertible at 24.6 m/s. If the car is coming toward you,
what frequency should you hear? Assume the temperature
is 20°C.
Homework
p.405 # 1-5 (use table 15-1 for the speed of sound in
various media)
p. 409 #6-8 Read Carefully!
Hearing Range Frequencies
• http://www.movingsoundtech.com/
• http://www.noiseaddicts.com/2009/03/can-you-hear-this-
hearing-test/