SoundWaves. Lecture (2) Special topics Dr.khitam Y, Elwasife

Transcription

1 SoundWaves Lecture (2) Special topics Dr.khitam Y, Elwasife

2 VGTU EF ESK 2 Mode Shapes and Boundary Conditions,

3 VGTU EF ESK ELEKTRONIKOS ĮTAISAI 2009 Acoustic Waves and Mechanical Resonators 1.. The velocity of acoustic wave is dependent on the type of the wave. 2. The natural frequencies depend on the types of acoustic wave. 3

4 VGTU EF ESK 4

5 Sound Waves Sound waves are longitudinal waves They travel through any material medium The speed of the wave depends on the properties of the medium The mathematical description of sinusoidal sound waves is very similar to sinusoidal waves on a string

6 Categories of Sound Waves The categories cover different frequency ranges Audible waves are within the sensitivity of the human ear Range is approximately 20 Hz to 20 khz Infrasonic waves have frequencies below the audible range Ultrasonic waves have frequencies above the audible range

7 In order for sound waves to propagate there needs to be a medium to carry the disturbances produced by the vibrating object. In the case of sound waves in air the air molecules pass the disturbances on to adjacent air molecules. In water the water molecules act as the propagating medium and this is the case for any material i.e. glass, metals.

8 Wavelength: Distance between successive compressions or rarefactions. Compressions: Areas in the wave where the air molecules are pushed close together and so at a slightly higher pressure. Rarefaction: Areas in the wave where the air molecules are further apart and so at a slightly lower pressure.

9 Speed of Sound Waves Use a compressible gas as an example with a setup as shown at right Before the piston is moved, the gas has uniform density When the piston is suddenly moved to the right, the gas just in front of it is compressed Darker region in the diagram

10 Speed of Sound Waves, cont When the piston comes to rest, the compression region of the gas continues to move This corresponds to a longitudinal pulse traveling through the tube with speed v The speed of the piston is not the same as the speed of the wave

11 Speed of Sound Waves, General The speed of sound waves in a medium depends on the compressibility and the density of the medium The compressibility can sometimes be expressed in terms of the elastic modulus of the material The speed of all mechanical waves follows a general form: v elastic property inertial property

12 Speed of Sound in Liquid or Gas The bulk modulus of the material is B The density of the material is r The speed of sound in that medium is Speed of Sound in a Solid Rod v B r The Young s modulus of the material is Y The density of the material is r The speed of sound in the rod is v Y r

13 Speed of Sound in Air The speed of sound is greater in hot air than it is in cold air. This is because the molecules of air are moving faster and the vibrations of the sound wave can therefore be transmitted faster. The speed of sound also depends on the temperature of the medium, This is particularly important with gases For air, the relationship between the speed and temperature is v C (331 m/s) C The 331 m/s is the speed at 0 o C T C is the air temperature in Celsius T

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15 Speed of Sound in solid,liquids and Gas

16 Speed of Sound in an Aluminum Rod, An Example Since we need the speed of sound in a metal rod, v Y r kg 3 m Pa m 5090 This is smaller than the speed in a bulk solid of aluminum in Table 17.1, as expected The speed of a transverse wave would be smaller still s

17 Periodic Sound Waves A compression moves through a material as a pulse, continuously compressing the material just in front of it The areas of compression alternate with areas of lower pressure and density called rarefactions These two regions move with the speed equal to the speed of sound in the medium

18 Periodic Sound Waves, Example A longitudinal wave is propagating through a gas-filled tube The source of the wave is an oscillating piston The distance between two successive compressions (or rarefactions) is the wavelength Use the active figure to vary the frequency of the piston

19 Periodic Sound Waves, As the regions travel through the tube, any small element of the medium moves with simple harmonic motion parallel to the direction of the wave The harmonic position function is s (x, t) = s max cos (kx wt) s max is the maximum position from the equilibrium position This is also called the displacement amplitude of the wave

20 Periodic Sound Waves, Pressure The variation in gas pressure, DP, is also periodic DP = DP max sin (kx wt) DP max is the pressure amplitude It is also given by DP max = rvws max k is the wave number (in both equations) w is the angular frequency (in both equations)

21 Periodic Sound Waves, A sound wave may be considered either a displacement wave or a pressure wave The pressure wave is 90 o out of phase with the displacement wave The pressure is a maximum when the displacement is zero, etc.

22 Energy of Periodic Sound Waves Consider an element of air with mass Dm and length Dx The piston transmits energy to the element of air in the tube This energy is propagated away from the piston by the sound wave

23 Energy, The kinetic energy in one wavelength is K l = ¼ (ra)w 2 s max2 l The total potential energy for one wavelength is the same as the kinetic The total mechanical energy is E l = K l +U l = ½ (ra)w 2 s max2 l

24 Power of a Periodic Sound Wave The rate of energy transfer is the power of the wave DE El ravw smax Dt T 2 This is the energy that passes by a given point during one period of oscillation

25 Intensity of a Periodic Sound Wave The intensity, I, of a wave is defined as the power per unit area This is the rate at which the energy being transported by the wave transfers through a unit area, A, perpendicular to the direction of the wave I A

26 Intensity In the case of our example wave in air, I = ½ rv(ws max ) 2 Therefore, the intensity of a periodic sound wave is proportional to the Square of the displacement amplitude Square of the angular frequency In terms of the pressure amplitude, I DP 2 max 2rv

27 Intensity of a Point Source A point source will emit sound waves equally in all directions This results in a spherical wave Identify an imaginary sphere of radius r centered on the source The power will be distributed equally through the area of the sphere

28 Intensity of a Point Source, cont I av A av 2 4 r This is an inversesquare law The intensity decreases in proportion to the square of the distance from the source

29 Loudness and Intensity Sound level in decibels relates to a physical measurement of the strength of a sound We can also describe a psychological measurement of the strength of a sound Our bodies calibrate a sound by comparing it to a reference sound This would be the threshold of hearing Actually, the threshold of hearing is this value for 1000 Hz

30 example The faintest sounds the human ear can detect at a frequency of 1000 Hz correspond to an intensity of about 1.00x10-12 W/m 2, which is called threshold of hearing. The loudest sounds the ear can tolerate at this frequency correspond to an intensity of about 1.00 W/m 2, the threshold of pain. Determine the pressure amplitude and displacement amplitude associated with these two limits. Solution To find the amplitude of the pressure variation at the threshold of hearing. taking the speed of sound waves in air to be v =343 m/s and the density of air to be ρ = 1.20 kg/m3:

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32 Sound Level The range of intensities detectible by the human ear is very large It is convenient to use a logarithmic scale to determine the intensity level, b I b 10log Io

33 Sound Level I 0 is called the reference intensity It is taken to be the threshold of hearing I 0 = 1.00 x W/ m 2 I is the intensity of the sound whose level is to be determined b is in decibels (db) Threshold of pain: I = 1.00 W/m 2 ; b = 120 db Threshold of hearing: I 0 = 1.00 x W/ m 2 corresponds to b = 0 db

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35 Sound Level, Example What is the sound level that corresponds to an intensity of 2.0 x 10-7 W/m 2? b = 10 log (2.0 x 10-7 W/m 2 / 1.0 x W/m 2 ) = 10 log 2.0 x 10 5 = 53 db

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37 Sound Levels Loudnesshow loud or soft a sound is perceived to be. Pitch - description of how high or low the sound seems to a person Threshold of pain! -

38 Speed of Sound Medium velocity m/sec air (20 C) 343 air (0 C) 331 water (25 C) 1493 sea water 1533 diamond iron 5130 copper 3560 glass 5640

39 The Doppler Effect The Doppler effect is the apparent change in frequency (or wavelength) that occurs because of motion of the source or observer of a wave When the relative speed of the source and observer is higher than the speed of the wave, the frequency appears to increase When the relative speed of the source and observer is lower than the speed of the wave, the frequency appears to decrease Sounds from Moving Sources. A moving source of sound or a moving observer experiences an apparent shift of frequency called the Doppler Effect.

40 Doppler Effect, Observer Moving The observer moves with a speed of v o Assume a point source that remains stationary relative to the air It is convenient to represent the waves with a series of circular arcs concentric to the source These surfaces are called wave fronts

41 Doppler Effect, Observer Moving, cont The distance between adjacent wave fronts is the wavelength The speed of the sound is v, the frequency is ƒ, and the wavelength is l When the observer moves toward the source, the speed of the waves relative to the observer is v = v + v o The wavelength is unchanged

42 Doppler Effect, Observer Moving, final The frequency heard by the observer, ƒ, appears higher when the observer approaches the source v v ƒ' o ƒ v The frequency heard by the observer, ƒ, appears lower when the observer moves away from the source ƒ' v v o v ƒ

43 Doppler Effect, Source Moving Consider the source being in motion while the observer is at rest As the source moves toward the observer, the wavelength appears shorter As the source moves away, the wavelength appears longer

44 Doppler Effect When a source with a siren passes you, a noticeable drop in the pitch of the sound of the siren will be observed as the vehicle passes. This is an example of the Doppler effect. An approaching source moves closer during period of the sound wave so the effective wavelength is shortened, giving a higher pitch since the velocity of the wave is unchanged. Similarly the pitch of a receding sound source will be lowered

45 Doppler Wavelength Change The speed of sound is determined by the medium in which it is traveling, and therefore is the same for a moving source. But the frequency and wavelength are changed. The wavelengths for a moving source are given by the relationships below. It is sometimes convenient to express the change in wavelength as a fraction of the source wavelength for a stationary source

46 Doppler Effect, Source Moving When the source is moving toward the observer, the apparent frequency is higher ƒ' v v v s ƒ When the source is moving away from the observer, the apparent frequency is lower v ƒ' ƒ v vs

47 Doppler Effect, General Combining the motions of the observer and the source ƒ' v v v v o s ƒ The signs depend on the direction of the velocity A positive value is used for motion of the observer or the source toward the other A negative sign is used for motion of one away from the other

48 Doppler Effect Water Example A point source is moving to the right The wave fronts are closer on the right The wave fronts are farther apart on the left The word toward is associated with an increase in the observed frequency The words away from are associated with a decrease in the observed frequency The Doppler effect is common to all waves The Doppler effect does not depend on distance

49 Doppler Effect, Example A (source) travels at 8.00 m/s emitting at a frequency of 1400 Hz The speed of sound is 1533 m/s,b (observer) travels at 9.00 m/s What is the apparent frequency heard by the observer as the A,B approach each other? Then as they recede from each other?

50 Doppler Effect, Example Approaching each other: v v 1533 m s 9.00 m s o ƒ' ƒ (1400 Hz) v v s 1533 m s 8.00 m s 1416Hz Receding from each other: v v 1533 m s 9.00 m s o ƒ' ƒ (1400 Hz) v v s 1533 m s 8.00 m s 1385Hz

51 a wave source moving to the right at a speed less than the wave speed Doppler Effect think! When a source moves toward you, do you measure an increase or decrease in wave speed? Answer: Neither! It is the frequency of a wave that undergoes a change, not the wave speed.

52 Bow Waves A bow wave occurs when a wave source moves faster than the waves it produces.

53 Bow Waves When the speed of the source in a medium is as great as the speed of the waves it produces, something interesting happens. The waves pile up. If the bug swims as fast as the wave speed, it will keep up with the wave crests it produces. The bug moves right along with the leading edge of the waves it is producing.

54 Bow Waves The same thing happens when an aircraft travels at the speed of sound. The overlapping wave crests disrupt the flow of air over the wings, so that it is harder to control the plane when it is flying close to the speed of sound.

55 Bow Waves When the plane travels faster than sound, it is supersonic. A supersonic airplane flies into smooth, undisturbed air because no sound wave can propagate out in front of it. Similarly, a bug swimming faster than the speed of water waves is always entering into water with a smooth, unrippled surface.

56 Bow Waves When the bug swims faster than wave speed, it outruns the wave crests it produces. The crests overlap at the edges, and the pattern made by these overlapping crests is a V shape, called a bow wave. The bow wave appears to be dragging behind the bug. The familiar bow wave generated by a speedboat is produced by the overlapping of many circular wave crests.

57 Bow Waves v= speed of bug v w = wave speed The wave patterns made by a bug swimming at successively greater speeds change. Overlapping at the edges occurs only when the source travels faster than wave speed.

58 Shock Waves A shock wave occurs when an object moves faster than the speed of sound. A speedboat knifing through the water generates a two-dimensional bow wave. A supersonic aircraft similarly generates a shock wave. A shock wave is a threedimensional wave that consists of overlapping spheres that form a cone. The conical shock wave generated by a supersonic craft spreads until it reaches the ground.

59 Shock Waves The bow wave of a speedboat that passes by can splash and douse you if you are at the water s edge. In a sense, you can say that you are hit by a water boom. In the same way, a conical shell of compressed air sweeps behind a supersonic aircraft. The sharp crack heard when the shock wave that sweeps behind a supersonic aircraft reaches the listeners is called a sonic boom. We don t hear a sonic boom from a subsonic aircraft. The sound wave crests reach our ears one at a time and are perceived as a continuous tone

60 Shock Waves. Only when the craft moves faster than sound do the crests overlap and encounter the listener in a single burst. Ears cannot distinguish between the high pressure from an explosion and the pressure from many overlapping wave crests. A common misconception is that sonic booms are produced only at the moment that the aircraft surpasses the speed of sound.

61 Shock Waves In fact, a shock wave and its resulting sonic boom are swept continuously behind an aircraft traveling faster than sound. The shock wave has not yet encountered listener C, but is now encountering listener B, and has already passed listener A.

62 Shock Waves A supersonic bullet passing overhead produces a crack, which is a small sonic boom. When a lion tamer cracks a circus whip, the cracking sound is actually a sonic boom produced by the tip of the whip. Snap a towel and the end can exceed the speed of sound and produce a mini sonic boom. The bullet, whip, and towel are not in themselves sound sources. When they travel at supersonic speeds, sound is generated as waves of air at the sides of the moving objects.

63 Shock Wave The speed of the source can exceed the speed of the wave The envelope of these wave fronts is a cone whose apex half-angle is given by sin q v/v S This is called the Mach angle The sound source is traveling at 1.4 times the speed of sound, Since the source is moving faster than the sound waves it creates, it leads the advancing wavefront. The sound source will pass by a stationary observer before the observer hears the sound it creates.

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65 If the source is moving as fast or faster than the speed of sound, the sound waves pile up into a shock wave called a sonic boom. A sonic boom sounds very much like the pressure wave from an explosion

66 SHOCK WAVES CAN SHATTER KIDNEY STONES Extracorporeal shock wave 66 lith

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68 Shock Wave The conical wave front produced when v s > v is known as a shock wave This is supersonic The shock wave carries a great deal of energy concentrated on the surface of the cone There are correspondingly great pressure variations

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70 What is the Sound Ba sound barrier rrier? The sound barrier is not a faded out place, it s actually just a speed where you are going faster than the speed sound travels.

71 A sonic boom is the sound associated with the shock waves created by an object traveling through the air faster than the speed of sound. Sonic booms generate enormous amounts of sound energy, sounding much like an explosion. The crack of a supersonic bullet passing overhead is an example of a sonic boom

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