Oscillation and Waves

In a closed organ pipe filled with water up to 1/5th volume, the fundamental frequency:

Two air columns of 100 cm and 120 cm give 15 beats/sec in fundamental mode. Velocity of sound is:

Assertion: Sound wave has higher speed in solids than gases.
Reason: Gases have higher bulk modulus than solids.

Closed organ pipe (length = 10 cm) filled with gas of density ρ1​. Open pipe filled with gas of density ρ2=16ρ1 ​. If 9th harmonic of closed pipe = 4th harmonic of open pipe, length of open pipe is:

Fundamental frequency of closed pipe = first overtone of open pipe. If open pipe length = 60 cm, closed pipe length is:

Car P (20 m/s) sounds horn at 400 Hz. Car Q (40 m/s) behind hears frequency: (Speed of sound = 360 m/s)

Train moving at 10 m/s sounds whistle at 400 Hz. Frequency heard by passenger inside train is:

Two trains A and B move at 30 m/s towards and away from observer. Both emit 300 Hz. Frequency difference heard is: