text: ap physics help: frequency and resonance? what is the lowest frequency that will resonate in an organ pipe 2.00 m in length, closed at one end? the speed of sound in air is 340 m/s.   a) 42.5 hz b) 85.0 hz c)170 hz d) 680 hz  simply explaining how to go about doing the problem would be appreciated. to have resonance, the sound wave must complete a whole number of cycles when travelling through the pipe.   for instance, sound travels 340 meters in one second. if you had an organ pipe 340 meters long, sound with a frequency of 1 hz would resonate the pipe because it completes exactly 1 cycle in 1 second, which is how long it takes the sound to travel 340 meters.  first find how long it will take the sound to travel through the pipe:  l / v = t where l is the length of the pipe and v is the speed of sound.  next, find the frequency that would allow just one full cycle to be completed while in the pipe:  t * f = r where t is the time sound takes to travel through the pipe, f is the frequency with which the pipe will resonate, and r is the number of cycles the sound wave will complete. to obtain the frequency f, set r=1 and solve.  below is the problem solved: t = l / v = 2 / 340 = 1/170 s f = r / t = 1 / (1/170) = 170 hz  i hope this helps!