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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

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

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

Sound Waves

Where are the regions of compression and rarefaction?

Pressure

Sound Waves

Pitch is determined by frequency

Intensity is determined by amplitude

Appeal is determined by wave pattern

Sound Frequencies heard by Animals

Elephants and moles can hear infrasound, including vibrations from earthquakes

Longwave radio band: 150-550 kHz

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)

High Pitch Sound vs. Low Pitch Sound

http://www.cochlea.org/en/what-do-i-ear.html

Sound Intensity

Sound Power per area

Measured in dB

Sound Intensity

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

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

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

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

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

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

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

Speed of Sound for different materials

??

Low Elasticity

High Elasticity

Speed of Sound

The speed of sound in the atmosphere is NOT a constant

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)

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)?

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)

Speed of Sound at different temperatures

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

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)

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?

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

Speed of Sound vs. Altitude

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?

Doppler Effect

Doppler Effect: Train Horn

http://www.youtube.com/watch?v=O5rqMPdQMQ8

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

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?

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!

Sound Wave Refraction: Day and Night

Sound Wave Refraction: Day and Night

Sounds of Meteorological Origin

Squeaking Snow

Thunder

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

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

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

Sonic Boom from a Navy F/18 Hornet

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

Shock Waves and Sonic Boom

Video

http://www.youtube.com/watch?v=-d9A2oq1N38

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

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

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)

Thunderstorm

http://www.youtube.com/watch?v=2Ey4KSnoReo