I've been spending a rather large amount of time looking into the relationship between a pipe's diameter and its fundamental frequency. As a result, I discovered the phenomenon of End Correction, and discovered that f = v / 2(L + 0.6d) in an open-open pipe.
I designed an experiment where I could find the fundamental frequencies of four different pipes with different diameters but equal lengths, and conducted said experiment. My experiment consisted of placing a phone speaker at one end of a pipe and a microphone at the other end of the pipe. I would play different frequencies from the speaker and measure the amplitude of the sound wave with the microphone. Using this information, I would then determine the fundamental frequency of the pipes. At the end of the experiment, my data showed that f actually equals v / 2(L + 1.2d).
At first, I assumed that error from uncertainty was at play here. However, after spending a couple hours calculating my uncertainties and their effect on this equation, I've concluded that there's not enough uncertainty to explain such a large difference.
In addition, my data is consistently showing 1.2d, which suggests that my methodology is correct.
My question is, what could possibly be causing this error? I've determined it's not pipe length, pipe diameter, speed of sound, the frequency emitted by the speaker, and the sound wave recorded by the microphone.