# Engineering Physics Questions and Answers – Acoustics

This set of Engineering Physics Multiple Choice Questions & Answers (MCQs) focuses on “Acoustics”.

1. Standing waves are produced in 10m long stretched string. If the string vibrates in 5 segments and wave velocity is 20m/s, its frequency is?
a) 2Hz
b) 4Hz
c) 5Hz
d) 10Hz

Explanation: 5× ʎ/2=10 or ʎ=4m
v=20m/s-1
γ=v/ʎ=20/4=5Hz.

2. If vibrations of a string are to be increased by a factor 2, tension in the string must be made ____________
a) Half
b) Twice
c) Four times
d) Eight times

Explanation: γ=1/2π×√(T/m)
To double γ, the tension must be made 4 times the original tension.

3. The tension in the piano wire is 10N. What should be the tension in the wire to produce a note of double the frequency?
a) 5N
b) 20N
c) 40N
d) 80N

Explanation: T=4T=4×10=40N.

4. A string in a musical instrument is 50cm long and its fundamental frequency is 800Hz. If a frequency of 1000Hz is to be produced, then required length of string is?
a) 62.5cm
b) 50cm
c) 40cm
d) 37.5cm

Explanation: γ∝1/l
γ/γ=l/l
l=γ/γ × l = 800/1000×50=40cm.

5. The frequency of a tuning fork is 256. It will not resonate with a fork of frequency?
a) 256
b) 512
c) 738
d) 768

Explanation: Tuning fork of frequency 256Hz will resonate with forks of integral multiple frequencies such as 256, 768, 1024 etc. It will not resonate with a fork of frequency 738Hz as it is not an integral multiple of 256Hz.
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6. An organ pipe closed at one end has a fundamental frequency of 1500Hz. The maximum number of overtones generated by this pipe, which a normal person can hear is?
a) 12
b) 9
c) 6
d) 4

Explanation: Human year can hear frequencies upto 20000Hz. For a closed organ pipe,
v=n×v/4L
=n×fundamental frequency
20000=n×1500
n≅13
The possible harmonics are: 1, 3, 5, 7, 9, 11, 13
Maximum number of overtones=7-1=6.

7. A tube closed at one end containing air produces a fundamental note of frequency 512Hz. If the tube is open at both ends, the fundamental frequency will be __________
a) 256Hz
b) 768Hz
c) 1024Hz
d) 1280Hz

Explanation: Fundamental frequency of a closed pipe,
γ=v/4L=512Hz
Fundamental frequency of open pipe,
γ=v/2L=2γ=2×512=1024Hz.

8. A closed organ pipe and an open pipe of the same length produce four beats per second when sounded together. If the length of the closed pipe is increased, then the number of beats will __________
a) Increase
b) Decrease
c) Remain the same
d) First decrease then remain the same

Explanation: γopenclosed=γ/(2Lopen)-γ/(4Lclosed)
=Beat frequency
Clearly, if the length of the closed pipe is increased, then the beat frequency will increase.

9. At resonance air column of length 20cm resonates with a tuning fork of frequency 450Hz. Ignoring end correction, the velocity of sound in air is?
a) 720m/s
b) 820m/s
c) 920m/s
d) 360m/s

Explanation: ʎ/4=20cm, ʎ=80cm=0.80m,
γ=450Hz
v=γʎ=450×0.80=360m/s.

10. Statement: Sound wave cannot propagate through a vacuum but light can
Reason: Sound wave cannot be polarised but light can
a) Both statement and reason are true and the reason is the correct explanation of the statement
b) Both statement and reason are true but the reason is not the correct explanation of the statement
c) Statement is true but the reason is false
d) Both statement and reason are false

Explanation: Both the statement and reason are true but the reason is not a correct explanation of the statement. Sound waves, being mechanical waves, cannot travel through a vacuum. Light waves being electromagnetic waves, can travel through a vacuum.

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