Download - MET 125 Physical Meteorology
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MET 125 Physical Meteorology
Meteorological AcousticsHenry Bartholomew (M.S.)
San Jose State University
1. Sound Propagation in the Atmosphere2. Refraction of Acoustic Energy3. Sounds of Meteorological Origin
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Sound
Sound is a longitudinal wave, made up of molecules
It can travel through solid, liquid, or gas, but not vacuum
The sound wave represents differences in pressure
Regions of higher pressure on sound wave are called compression
Regions of lower pressure on sound wave are called rarefaction
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Sound Waves
Exist as disturbances in a medium that transfer energy from one place to another, without permanent displacement.
Created by vibration of object, which causes surrounding air to vibrate
As a result, the human eardrums to vibrate, and we hear sound
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Sound Waves
Where are the regions of compression and rarefaction?
Pressure
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Sound Waves
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Pitch is determined by frequency
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Intensity is determined by amplitude
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Appeal is determined by wave pattern
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Sound Frequencies heard by Animals
Elephants and moles can hear infrasound, including vibrations from earthquakes
Longwave radio band: 150-550 kHz
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Whales and Dolphins
Can hear sounds from about 150 Hz to 150 kHz (very large range!)
Shorter frequencies (longer wavelengths) can travel very far (whale songs)
Longer frequencies (shorter wavelengths) are used for echolocation (animal sodar)
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High Pitch Sound vs. Low Pitch Sound
http://www.cochlea.org/en/what-do-i-ear.html
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Sound Intensity
Sound Power per area
Measured in dB
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Sound Intensity
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Speed of Sound
How fast does sound travel through dry air? How about liquid water?
Is the speed of sound in the atmosphere a constant?
To answer these questions, let’s first examine properties that affect the speed of sound
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Speed of Sound
Remember, sound is a wave
The speed of any wave depends on two main categories of properties of the medium through which it travels – Elastic Properties– Inertial Properties
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Elastic Properties
Refers to the tendency for the material to maintain shape and not deform
A measure of the flexibility of a material
A material with higher elasticity will experience a smaller change of shape when a force is applied to it– Due to stronger bonds between molecules
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Speed of Sound vs. Elasticity
The higher the elasticity of a medium, the faster the waves will travel through it
This is because when bonds are stronger between molecules, energy will be transferred faster between them, resulting in a higher phase speed
Solids are more elastic than liquids, which are more elastic than gases
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Inertial Properties
Inertia is the tendency of a material to resist a change in velocity
One example is density
Higher density mediums have higher inertia
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Speed of Sound vs. Inertia
The greater the density of molecules in medium, the greater the inertia, and the slower the reactions between molecules– This causes a slower speed of sound
For a given state of matter, as temperature increases, density decreases (all else being constant), and so the speed of sound increases
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Speed of Sound vs. State of Matter
In general, the elastic properties have a greater influence on the speed of a wave than the inertial properties
Therefore, if v is speed of sound,– vsolid > vliquid > vgas
Nonetheless, in a particular phase, the inertial properties are important
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Speed of Sound for different materials
??
Low Elasticity
High Elasticity
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Speed of Sound
The speed of sound in the atmosphere is NOT a constant
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Speed of Sound
The speed of sound in dry air, at sea level, can be approximated as a function of temperature using the following equation:
v = 331 m s-1 + (0.6 m s-1 °C-1)*T, where v is speed of sound (m s-1), and T is temperature (°C)
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Class Activity
v = 331 m s-1 + (0.6 m s-1 ° C-1)*T, where v is speed of sound (m s-1), and T is temperature (°C)
1. At sea level, how fast does sound travel when the air temperature is at the freezing point (0°C, 32°F)?
2. At sea level, how fast does sound travel when the air temperature is 20°C (68°F)?
3. At sea level, how fast does sound travel when the air temperature is 40°C (104°F)?
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Class Activity Answers
1. 331 m s-1 (740.4 mph)
2. 343 m s-1 (767.3 mph)
3. 355 m s-1 (794.1 mph)
– Thus, speed of sound in dry air at sea level is about 7% greater at 40°C (record/near record high temperature on a summer day in San Jose) than 0°C (low temperature during a cold clear winter night in San Jose)
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Speed of Sound at different temperatures
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Speed of Sound
In warmer air, the molecules are faster moving and have more energy associated with them, and leading to quicker transfers of energy– Results in faster moving sound waves
Another way to explain the increase in speed of sound with temperature is that as air warms, it becomes less dense, and its thermal inertia decreases– Faster reactions between molecules
Temperature is not the only variable that affect the speed, however– Humidity is another
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Speed of Sound
A higher dew point will cause a very slight increase in the speed of sound– No more than 0.5%
Hence, the equation for speed of sound in dry air is usually used (humidity effect is ignored)
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Review Questions
1. What characteristics of a sound wave determine a) pitch, b) intensity, and c) appeal?
2. What two types of properties determine the speed at which a wave propagates through a material?
3. What is the speed of sound in dry air at sea level with a temperature of 20°C?
4. As the temperature of air increases, what happens to the speed of sound?
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Speed of Sound vs. Altitude
In the troposphere (the lowest layer of the atmosphere, with a depth of about 8 km at the poles and up to 15 km in the tropics), what generally happens to temperature as altitude increases?– It decreases!
Hence, sound waves typically travel slower with increasing height in this layer–Exception: Temperature Inversion
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Speed of Sound vs. Altitude
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Doppler Effect
It is named after Charles Doppler, who first suggested it in 1842
With respect to sound, it represents the change in frequency (and hence wavelength) that occurs when source moves with respect to observer
What property of sound does this change?
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Doppler Effect
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Doppler Effect: Train Horn
http://www.youtube.com/watch?v=O5rqMPdQMQ8
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Doppler Effect
f: observed frequency of wavesf0: emitted frequency of waves c: emitted velocity of wavesvr: velocity of receivervs: velocity of source
convention: vr is positive if receiver is moving toward source,while vs is positive if source is moving away from receiver
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In Class Problem
For this problem, the temperature and humidity of the air constitute a speed of sound of 350 m s-1.
You are on the freeway driving at 30 m s-1. An ambulance is approaching you at 40 m s-1, emitting a frequency of 450 Hz. What is your observed frequency of the siren?
You and the other drivers slow and move to the right to let the ambulance pass. After it does so, you resume your prior speed; the ambulance continues at 40 m s-1. Now what is the observed frequency of the siren?
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Refraction of Acoustic Energy
In the atmosphere, as sound travels from more dense air to less dense air, it will refract (bend) and slow down
During the day, the earth’s surface heats up faster than the air above it– This creates a temperature decrease with height near the
surface
At night, as the surface emits Infrared radiation upward, the earth’s surface cools faster than the air above it– Radiation Inversion often develops, especially on clear night
Due to refractive properties, in the air next to the ground, sound usually travels FARTHER at night than during day!
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Sound Wave Refraction: Day and Night
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Sound Wave Refraction: Day and Night
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Sounds of Meteorological Origin
Squeaking Snow
Thunder
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Quiet Day after a Snowfall
After a recent snowfall, it may appear quieter than normal– Fresh snow absorbs sound
Absorption is proportional to depth
As the snow becomes older, sound absorption decreases
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Sound of Snow
At air temperatures near freezing, stepping on snow can cause it to partially melt– No sound
When the air temperature is below -10°C (14°F), stepping on snow will not melt it– Instead, ice crystals are crushed under
weight of foot and shoe/boot This produces squeaking sound
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Sonic Boom
Occurs when shock waves move faster than speed of sound
Results in loud noise, due to large amount of sound energy generated
Examples: thunder, jets breaking sound barrier
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Sonic Boom from a Navy F/18 Hornet
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Sonic Boom from a Navy F/18 Hornet
Because jet travels faster than speed of sound, sound waves don’t precede jet, but pile up behind it
Listener hears sonic boom
Cloud forms (still uncertainties as to why)– Could be due to drop in pressure causing
condensation of moist air
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Shock Waves and Sonic Boom
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Video
http://www.youtube.com/watch?v=-d9A2oq1N38
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Thunder
Sound heard as a result of lightning
Lightning is an electrical discharge
Peak temperature of lightning bolt is around 30,000 K (about 55,000°F)!
Due to this intense heating of the lightning “channel,” air spreads out, and sound travels faster than it would in cooler surrounding air
Outward moving pulse causes shock wave
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Sound of Thunder
When lightning is nearby, thunder often sounds like clap
Farther away, it may sound more like a rumble– Can be caused by sound originating from
different locations of stroke, and highlighted when sound wave reflects off obstacles, such as buildings and mountains
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Determining the distance from lightning
You can determine your distance from lightning by counting the number of seconds between when you see the flash and hear the thunder
The speed of sound is approximately 1 mile per 5 seconds
Distance = Time*Speed
Thus, multiply time (in seconds) by speed of sound (1 mile/5 seconds) to get distance from lightning (in miles)
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Thunderstorm
http://www.youtube.com/watch?v=2Ey4KSnoReo