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