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SOUND WAVESP H Y S I C S F O R 1 2 T H G R A D E R S
Chapter Overview
• Properties of Sound Waves
• Speed of Sound Waves
• Doppler Effect
• Beat
• Sound Waves on Strings and Pipes
• Intensity and Intensity Level of Sound Waves
• Standing Waves in Air Columns
• The Ear
12 Lesson Hours
Prop. of Sound Waves
Is there any sounds that cannot be detected by our hearing sense? How
to create regular sounds? What are the benefits of sound wave in our
daily lives? We will learn them in this chapter.
Sound waves are longitudinal waves traveling through a
medium. Solid, Liquid or Gas
Prop. of Sound Waves
The sound wave propagates energy.
Energy is propagated through the collisions of particles
inside the medium.
In solid medium where the particle structures are denser
than in liquid and gas mediums, the collisions among
particles are easier and faster.
Doppler’s EffectAs this ambulance moves to the
left, Observer A hears the siren at
a higher frequency than the driver
does, while Observer B hears a
lower frequency.
an observed change in frequency when
there is relative motion between the source
of waves and an observer
Because frequency
determines pitch, the
observer in front of the
ambulance hears a higher
pitch, while the observer
behind the ambulance
hears a lower pitch.
Doppler’s Effect
SourceObserver
+𝑣𝑠−𝑣𝑠+𝑣𝑜−𝑣𝑜
𝑓𝑠𝑓𝑜
𝑣𝑠 (+) jika sumber bunyi bergerak menjauhi pendengar
𝑣𝑠 (–) jika sumber bunyi bergerak mendekati pendengar
𝑣𝑠(+) jika pendengar bergerak mendekati sumber bunyi
𝑣𝑜 (–) jika pendengar bergerak menjauhi sumber bunyi
𝑓𝑠
𝑓𝑜=
𝑣 ± 𝑣𝑠
𝑣 ± 𝑣𝑜𝑣 = speed of sound in the air = 340 m/s
𝑓𝑜 = frequency of the observer (Hz)
𝑓𝑠 = frequency of the source (Hz)
𝑣𝑜 = velocity of the observer (m/s)
𝑣𝑠 = velocity of the source (m/s)
Doppler’s Effect
A police is being chased by a robber (*lho koq). The scared police sounds a
serene with frequency of 720 Hz. The velocity of the police is 20 m/s, while
the robber chase at the velocity of 40 m/s. With the speed of sound of 340
m/s, what is the frequency heard by the robber?
𝑣𝑠 = 20 𝑚/𝑠𝑣𝑜 = 40 𝑚/𝑠
𝑓𝑠 = 720 𝐻𝑧𝑓𝑜
𝑓𝑠
𝑓𝑜=
𝑣 ± 𝑣𝑠
𝑣 ± 𝑣𝑜
720 𝐻𝑧
𝑓𝑜=
340 𝑚/𝑠
340 𝑚/𝑠
+ 40 𝑚/𝑠
+ 20 𝑚/𝑠
𝑓𝑜 = (720 𝐻𝑧)380 m/s
360 m/s
𝑓𝑜 = 760 Hz
Example 1
Doppler’s Effect
Dua buah mobil polisi A dan B bergerak searah masing-masing
kecepatannya 36 km/jam dan 144 km/jam. Mobil A yang berada di depan
mobil B membunyikan sirine pada frekuensi 1400 Hz. Jika cepat rambat bunyi
di udara 340 m.s-1, maka frekuensi yang didengarkan pengendara mobil B
setelah menyalip mobil A adalah….
𝑣𝑠 = 36 𝑘𝑚/ℎ𝑣𝑜 = 144 𝑘𝑚/ℎ
𝑓𝑠 = 1400 𝐻𝑧
Example 2
𝑣𝑠 = 10 m/s
𝑓𝑜
𝑣𝑠 = 40 m/s
Doppler’s Effect
Sebuah mobil ambulans bergerak dengan kecepatan 72 km.jam-1 sambil
membunyikan sirine dengan frekuensi 1500 Hz. Pengendara sepeda motor
bergerak dengan kecepatan 20 m.s-1 berlawanan arah dengan mobil
ambulans. Jika cepat rambat bunyi di udara 340 m.s-1, maka perbandingan
frekuensi yang didengar oleh pengendara sepeda motor sewaktu mendekat
dan menjauh mobil ambulans adalah ....
𝑣𝑠 = 72 𝑘𝑚/ℎ𝑣𝑜 = 20 𝑚/𝑠
𝑓𝑠 = 1500 𝐻𝑧𝑓𝑜2
Example 3
𝑣𝑠 = 20 m/s
𝑓𝑜1
Doppler’s Effect
Seorang pemain sepak bola berlari dengan
kecepatan vo menuju wasit yang diam sambil
membunyikan peluit yang frekuensi fs. Jika
kecepatan udara di tempat tersebut v, maka
besar frekuensi yang didengar pemain
tersebut dirumuskan ....
Review
Doppler’s Effect
Mobil pemadam kebakaran yang sedang bergerak dengan kecepatan
25 m/s membunyikan sirene pada frekuensi 700 Hz. Mobil menjauhi
seorang pengamat yang berdiri di tepi jalan. Jika cepat rambat bunyi
325 m/s, pengamat tersebut akan mendengar frekuensi sirine sebesar
....
A. 800 Hz
B. 750 Hz
C. 720 Hz
D. 650 Hz
E. 600 Hz
Review
BEATS
If two sound waves having a little bit
different frequencies are present at the
same time, then we will hear some
loud and weak sounds
interchangeably.
If both the little bit different
frequencies are known, the beat
frequency can be calculated by the
formula:
Sound Intensity
The sound intensity is
inversely proportional
to the square of the
distance
𝐼0 = 1 × 10−12
Sound Intensity Level
Decibel (dB)
Sound Intensity Level
Taraf intensitas bunyi sebuah mesin rata-rata 40 dB.
Jika 100 mesin dihidupkan bersama-sama taraf intensitas
yang dihasilkan sebesar ....
A. 400 dB
B. 140 dB
C. 60 dB
D. 50 dB
E. 20 dB
Sound Intensity Level
Sebuah motor yang sedang digunakan memiliki intensitas
bunyi 10−8 W/m2. Jika 10 motor yang identik dibunyikan secara
bersama-sama, taraf intensitas bunyi yang terdengar sebesar
....
A. 40 dB
B. 50 dB
C. 80 dB
D. 100 dB
E. 120 dB
Sound Intensity Level
Sebuah motor yang sedang digunakan memiliki intensitas bunyi
10−8 W/m2. Jika 10 motor yang identik dibunyikan secara bersama-
sama, taraf intensitas bunyi yang terdengar sebesar ....
A. 40 dB
B. 50 dB
C. 80 dB
D. 100 dB
E. 120 dB
Sound Intensity Level