I made an experiment in a tube, 75 centimeters. There was sand in it so I could distinguish one wave length. the biggest one I got was 0.335 meters at a frequency of 480Hz and the smallest was 0.07 meters at a frequency of 2000 Hz. I calculated the resonance in the tube as 1.5 meters as the biggest and 0.3 meters as the smallest. I'm now wondering why this differs so much from my measuring in the tube?