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971
The unit for measuring intensity of noise is
A. Ohm
B. Decibel
C. Knot
D. Calorie
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Explanation
Noise intensity is measured in decibels (dB).
972
If λ1, λ2 and λ3 are the wavelengths of the waves which give resonance in the fundamental, first and second overtones respectively in an organ pipe closed at one end (neglecting end correction), the ratio λ1 : λ2 : λ3 is
A. 1 : 2 : 3
B. 3 : 2 : 1
C. 15 : 5 : 3
D. 1 : 3 : 5
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Explanation
A closed pipe has odd harmonics (1,3,5); wavelengths are inversely proportional to harmonic number, so λ1:λ3:λ5 = 1:(1/3):(1/5) → 15:5:3.
973
The velocity of sound in any gas depends upon
A. Intensity of sound waves only
B. Density and elasticity of gas
C. Amplitude and frequency of sound
D. Wavelength of sound only
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Explanation
Sound speed in a gas is determined by the gas's elastic properties and density (e.g., v = √(elastic modulus/density)).
974
An increase in intensity level of one decibel implies an increase in intensity of
A. 3.01%
B. 0.1%
C. 26%
D. 1%
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Explanation
A 1 dB increase corresponds to intensity ratio 10^(1/10) ≈ 1.2589, i.e., about a 25.9% increase, ≈26%.
975
A sonic measuring tape is used to measure the length of a room. It measures a time interval of 0.06 s between transmitting a sound pulse and receiving the echo. The speed of sound in air is 330 m s^-1. What is the length of the room?
A. 3.3 m
B. 9.9 m
C. 9.0 m
D. 6.0 m
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Explanation
Round-trip distance = v × t = 330 × 0.06 = 19.8 m, so one-way (room length) = 19.8 / 2 = 9.9 m.
976
When wind blows in the same direction in which the sound travels, the sound velocity
A. remains unchanged
B. becomes zero
C. increases
D. decreases
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Explanation
Wind in the direction of sound adds to the effective propagation speed relative to ground, increasing observed sound velocity.
977
During a thunderstorm, an observer sees a lightning flash. Six seconds later he hears the thunder. The speed of sound is 330 m s^-1. Approximately how far away is the observer from the lightning?
A. 1/3 km
B. 1/20 cm
C. 2 km
D. 1/2 km
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Explanation
Distance ≈ speed × time = 330 m/s × 6 s ≈ 1980 m ≈ 2 km.
978
The energy flowing per second through a unit area held perpendicular to the direction of wave is
A. the intensity of sound waves
B. the pitch of the sound waves
C. the sum of the frequencies of two tuning forks
D. the intensity level of sound waves
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Explanation
Energy flow per second per unit area is the definition of intensity (power per unit area).
979
Pitch of a sound determines
A. quality of sound
B. loudness of sound
C. its shrillness
D. intensity level
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Explanation
Pitch refers to the perceived highness or lowness (shrillness) of a sound, which is related to frequency.
980
Velocity of sound in vacuum (in m/sec) is
B. 1000
C. 156
D. 330
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Explanation
Sound requires a material medium and cannot propagate in vacuum, so its speed there is zero.
