ThunderYou can estimate the distance to an approaching storm by listening carefully to the sound of the thunder. How is this done? Why is the sound that follows a lightning strike sometimes a short, sharp thunderclap and other times a longlasting rumble?
USGAn ultrasound image of a human fetus in the womb after 20 weeks of development, showing the head, body, arms, and legs in profile.
Sound waves - three frequency rangesAudible waves are waves that lie within the range of sensitivity of the human ear. They can be generated in a variety of ways, such as by musical instruments, human vocal cords, and loudspeakers.
Infrasonic waves are waves having frequencies below the audible range. Elephants can use infrasonic waves to communicate with each other, even when separated by many kilometers.
Ultrasonic waves are waves having frequencies above the audible range. You may have used a “silent” whistle to retrieve your dog. The ultrasonic sound it emits is easily heard by dogs, although humans cannot detect it at all. Ultrasonic waves are also used in medical imaging.
Speed of soundWave characteristics:
Wave length – l (m)
Frequency – f(Hz) ‐ pitch
Wave velocity ‐ v=l f, m/s
Wave speed – property of material one – to – one correspondence of frequency and wave length in a given medium:
λv
f =ρB
v =
( ) smTv /60.0331+≈
Intensity of soundLoudness : a sensation in the consciousness of a human being
The energy transported by a wave per unit time across unit area perpendicular to energy flow (W/m2)
The human ear : Min 10‐12 W/m2 and Max 1 W/m2
Measure of loudness/sound level β in Decibel
Io : the intensity of some reference level (usually the minimum intensity audible to an average person)
( )oII
dBin log10=β
212 /100.1 mWIo−×=
Auto focusing with sound wavesAuto focusing cameras emit a pulse of very high frequency (ultrasonic) sound that travels to the object being photographed and include a sensor that detects the returning reflected sound
Mathematical RepresentationA one‐dimensional sinusoidal wave traveling along the x axis
D is along the direction of wave propagation
( )tkxDD M ω−= sin
Pressure wavesThe point view of variations in pressure
Wave compression
Molecules are closest together
The pressure is higher than normal
Expansion /Rarefaction
The pressure is less than normal
Pressure wavesFrom definition of The Bulk modulus
Pressure variation
Pressure amplitude
VV
BPΔ
−=Δ
( )tkxPP M ω−Δ−=Δ cos
fDvP MM ρπ2=Δ
Example – 1Two identical machines are positioned the same distance froma worker. The intensity of sound delivered by each machine atthe location of the worker is 2.0 × 10‐7 W/m2. Find the soundlevel heard by the worker
(a) when one machine is operating
(b) when both machines are operating.
Solution – 1The sound level at the location of the worker with one machine operating is
When both machines are operating, the intensity is doubled to 4.0 × 10‐7 W/m2; the sound level now is
( ) dB53100.2log10100.1100.2
log10 512
7
1 =×=⎟⎟⎠
⎞⎜⎜⎝
⎛××
= −
−
β
( ) dB56100.4log10100.1100.4
log10 512
7
2 =×=⎟⎟⎠
⎞⎜⎜⎝
⎛××
= −
−
β
Doppler EffectThe change in wavelength due to motion of the source "Wheeeeeeeeeeee…….Oooooooooooooo”Examples:
moving cars and trainsrotating whistle
Doppler EffectAt rest
Emitting sound in all direction
Depends only on the medium
Independent of the velocity of the source and observer
Doppler EffectThe fire truck is moving
The siren emit sound at the same frequency
The sound wavefront are close together (toward)
The observer will detect more wave crest passing per second frequency heard is higher
Frequency changeIf source and observer moving
the upper signs (+ vO and (– vS) refer to motion of one toward the other, and the lower signs (– vO and + vS) refer to motion of one away from the other.
⎟⎟⎠
⎞⎜⎜⎝
⎛ ±=
S
O
vvvv
ffm
'
Shock wavesThe V‐shaped bow wave of a boat is formed because the boat speed is greater than the speed of the water waves. A bow wave is analogous to a shock wave formed by an airplane traveling faster than sound.
Shock wavesIf the object’s velocity is less than the velocity of sound : Doppler effect
If the object’s velocity is greater than the velocity of sound : a shock wave
The angle of shock wave cone
obj
snd
vv
=θsin
Example – 2As an ambulance travels east down a highway at a speed of 33.5 m/s, its siren emits sound at a frequency of 400 Hz. What frequency is heard by a person in a car traveling west at 24.6 m/s
(a) as the car approaches the ambulance
(b) as the car moves away from the ambulance?
Solution – 2Taking the speed of sound in air to be 343 m/s. As the ambulance and car approach each other, the person in the car hears the frequency
As the vehicles recede from each other, the person hears the frequency
Hzfvvvv
fS
O 4754005.333436.24343
' =⎟⎠⎞
⎜⎝⎛
−+
=⎟⎟⎠
⎞⎜⎜⎝
⎛−+
=
Hzfvvvv
fS
O 3384005.333436.24343
' =⎟⎠⎞
⎜⎝⎛
+−
=⎟⎟⎠
⎞⎜⎜⎝
⎛−+
=
Latihan Soal – 1A stone is dropped into a deep canyon and is heard to strike the bottom 10.2 s after release. The speed of sound waves in air is 343 m/s. How deep is the canyon? What would be the percentage error in the calculated depth if the time required for the sound to reach the canyon rim were ignored?
Latihan Soal – 2On a Saturday morning, pickup trucks carrying garbage to the town dump form a nearly steady procession on a country road, all traveling at 19.7 m/s. From this direction, two trucks arrive at the dump every three minutes. A bicyclist also is traveling toward the dump at 4.47 m/s.
(a) With what frequency do the trucks pass him?
(b) A hill does not slow the trucks but makes the out‐of‐shape cyclist’s speed drop to 1.56 m/s. How often do the noisy trucks whiz past him now?