MET 125 Physical Meteorology

1. MET 125 Physical Meteorology Meteorological Acoustics Henry Bartholomew (M.S.) San Jose State University 1. Sound Propagation in the Atmosphere 2. Refraction of Acoustic Energy 3. Sounds of Meteorological Origin

2. 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

3. 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

4. Sound Waves Pressure Where are the regions of compression and rarefaction?

5. Sound Waves

6. Pitch is determined by frequency

7. Intensity is determined by amplitude

8. Appeal is determined by wave pattern

9. Sound Frequencies heard by Animals Elephants and moles can hear infrasound, including vibrations from earthquakes Longwave radio band: 150-550 kHz

10. 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)

11. High Pitch Sound vs. Low Pitch Sound http://www.cochlea.org/en/what-do-i-ear.html

12. Sound Intensity Sound Power per area Measured in dB

13. Sound Intensity

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. Speed of Sound for different materials Low Elasticity ? ? High Elasticity

22. Speed of Sound The speed of sound in the atmosphere is NOT a constant

23. 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)

24. 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)?

25. 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)

26. Speed of Sound at different temperatures

27. 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

28. 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)

29. 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?

30. 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

31. Speed of Sound vs. Altitude

32. 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?

33. Doppler Effect

34. Doppler Effect: Train Horn http://www.youtube.com/watch?v=O5rqMPdQMQ8

35. Doppler Effect f: observed frequency of waves f0: emitted frequency of waves c: emitted velocity of waves vr: velocity of receiver vs: velocity of source convention: vr is positive if receiver is moving toward source, while vs is positive if source is moving away from receiver

36. 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?

37. 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!

38. Sound Wave Refraction: Day and Night

39. Sound Wave Refraction: Day and Night

40. Sounds of Meteorological Origin Squeaking Snow Thunder

41. 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

42. 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

43. 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

44. Sonic Boom from a Navy F/18 Hornet

45. 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

46. Shock Waves and Sonic Boom

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

48. 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

49. 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

50. 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)