Adhi Harmoko S
Thunder
USG
Sound waves
Adhi Harmoko S
Thunder
y You 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?
USG
y An ultrasound image of a human fetus in the womb after 20 weeks of development, showing the head, body, arms, and legs in profile.
Sound
Sound waves - three frequency ranges
y Audible 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.
y 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.
y 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 sound
y Wave characteristics:
y Wave length – l (m)
y Frequency – f(Hz) ‐ pitch
y Wave velocity ‐ v=l f, m/s y Wave speed – property of
material Æ one – to – one correspondence of frequency and wave length in a given medium:
λ
f = vρ
v = B
(
T)
m sv ≈ 331+ 0.60 /
Intensity of sound
y Loudness : a sensation in the consciousness of a human being y The energy transported by a wave per unit time across unit
area perpendicular to energy flow (W/m2)
y The human ear : Min 10‐12 W/m2 and Max 1 W/m2 y Measure of loudness/sound level β in Decibel
y Io : the intensity of some reference level (usually the minimum intensity audible to an average person)
( )
Io
dB I
in = 10log
β
2
12 /
10 0
.
1 W m
Io = × −
Intensity of various sound
Auto focusing with sound waves
y Auto 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 Representation
y A one‐dimensional sinusoidal wave traveling along the x axis
y D is along the direction of wave propagation
(
kx t)
D
D = M sin −
ω
Pressure waves
y The point view of variations in pressure
y Wave compression
y Molecules are closest together
y The pressure is higher than normal
y Expansion /Rarefaction
y The pressure is less than normal
Pressure waves
y From definition of The Bulk modulus
y Pressure variation
y Pressure amplitude V B V P = − Δ Δ
(
kx t)
P
P = −Δ M −
ω
Δ cos
f D v PM = 2
π ρ
M ΔExample – 1
y Two identical machines are positioned the same distance from a worker. The intensity of sound delivered by each machine at the location of the worker is 2.0 × 10‐7 W/m2. Find the sound level heard by the worker
(a) when one machine is operating
(b) when both machines are operating.
Solution – 1
y The sound level at the location of the worker with one machine operating is
y When both machines are operating, the intensity is doubled to 4.0 × 10‐7 W/m2; the sound level now is
(
2.0 10)
53dBlog 10 10
0 . 1
10 0
. log 2
10 12 5
7
1 ⎟⎟ = × =
⎠
⎜⎜ ⎞
⎝
⎛
×
= × −−
β
(
4.0 10)
56dBlog 10 10
0 . 1
10 0
. log 4
10 12 5
7
2 ⎟⎟ = × =
⎠
⎜⎜ ⎞
⎝
⎛
×
= × −−
β
Doppler Effect
y The change in wavelength due to motion of the source y "Wheeeeeeeeeeee…….Oooooooooooooo”
y Examples:
y moving cars and trains
y rotating whistle
Doppler Effect
y At rest
y Emitting sound in all direction
y Depends only on the medium
y Independent of the velocity of the source and observer
Doppler Effect
y The fire truck is moving
y The siren emit sound at the same frequency
y The sound wavefront are close together (toward)
y The observer will detect more wave crest passing per second Æ frequency heard is higher
Frequency change
y If source and observer moving
y 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
v v
v f v
f' m
Shock waves
y The 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 waves
y If the object’s velocity is less than the velocity of sound : Doppler effect
y If the object’s velocity is greater than the velocity of sound : a shock wave
y The angle of shock wave cone
obj snd
v
= v sin
θ
Example – 2
y As 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 – 2
y Taking 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
y As the vehicles recede from each other, the person hears the frequency
Hz v f
v v f v
S
O 400 475
5 . 33 343
6 . 24
' 343 ⎟ =
⎠
⎜ ⎞
⎝
⎛
−
= +
⎟⎟⎠
⎜⎜ ⎞
⎝
⎛
−
= +
Hz v f
v v f v
S
O 400 338
5 . 33 343
6 . 24
' 343 ⎟ =
⎠
⎜ ⎞
⎝
⎛
+
= −
⎟⎟⎠
⎜⎜ ⎞
⎝
⎛
−
= +
Latihan Soal – 1
y A 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 – 2
y On 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.
y (a) With what frequency do the trucks pass him?
y (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?
