For an ideal gas at the same temperature, sound speed depends on temperature and molecular properties, not on pressure, so it remains essentially constant.
Among the given gases, sound travels fastest in the lightest molecule (hydrogen) due to higher molecular speed.
924
The ratio of velocity of sound in a monoatomic gas to that in a linear triatomic gas having same molar mass under similar conditions of temperature and pressure is approximately
Speed ∝ sqrt(γ); for monoatomic γ≈5/3 and for linear triatomic γ≈7/5, so ratio ≈ sqrt(1.6667/1.4) ≈1.12.
925
The intensity level of a sound wave is said to be 4 decibel. If the intensity of the wave is doubled then the intensity level of the sound as expressed in decibels would be
Doubling intensity increases level by 10 log10(2) ≈ 3.01 dB, so 4 dB + 3.01 dB ≈ 7 dB.
926
A train approaching a hill at a speed of 60 km/hr sounds a whistle of frequency 590 Hz when it is at a distance of 2 km from a hill. A wind with a speed of 40 km/hr is blowing in the direction of motion of the train. If the velocity of sound in air is 1200 km/hr, then the frequency of the whistle heard by an observer on the hill in Hz is
Account for wind by transforming to air frame: source speed relative to air = 60-40 = 20 km/hr and observer moves toward source in air frame with 40 km/hr; using Doppler f' = f( c+v_obs)/(c-v_source ) gives ~620 Hz.
927
Beats can be heard when difference of frequency is not more than:
OCR shows a marker next to 10 Hz; beats are audible when frequency difference is small (commonly up to about 10 Hz).
928
A tube closed at one end and containing air produces, when excited, the fundamental note of frequency 512 Hz. If the tube is open at both ends, the fundamental frequency that can be excited is (in Hz)
Longitudinal waves like sound have oscillations along the direction of propagation and thus cannot be polarized, unlike transverse electromagnetic waves.
