You must firstly know the frequency of the drone. I guessed around 125 Hz by ear. Secondly you must calculate the wavelength of the sound using the formula:
wavelength = speed of sound / frequency.
Using 1100 feet/second as an approximate speed of sound, plug the numbers into the formula.
wavelength = 1100 / 125 = 8.8 feet.
The length of the tube should be one quarter wavelength or 2.2 feet = 26 inches.
The exhaust sound approaches the "T". Some of it turns and heads into the tube. It travels the length of the tube, reflects off the solid wall at the end, then returns back to the "T". Because the sound has travelled two quarter wavelengths in total inside the tube, it gets reversed in polarity just like if you swapped the positive and negative wires in a speaker. Once this shifted sound combines with the original sound at the "T", they cancel each other. :nod: