Speed of sound in water

Feb 2018
For a science project, I need to measure the speed of sound in water. Obviously, I don't have any fancy equipment to do this so I came up with a set up I'm not sure will work:

I have a PVC cut to 29.8 cm long. I attached two thin pieces of glass (a screen cover for a phone) on each end to seal the tube, and filled it up with water. There's a wire leading to a speaker inside the tube, and the speaker's in a plastic bag to keep it from getting wet. I also have a frequency generator attached to the speaker.

The idea was to increase the frequency the speaker produces until a standing wave is created. Ideally, the sound wave inside the tube will reflect off the air outside the ends of the tube. At the right frequency (and wavelength), the reflected waves will line up with the incoming waves from the speaker, producing a standing wave and a spike in amplitude. I have a decibel meter to measure the amplitude of the waves being created. Once I have the standing wave, I'll know the wavelength of the wave (the fundamental will be 0.596 m), the frequency, and thus the speed of sound. (Ideally)

Things aren't going as planned, and I wanted to know if it was because of accuracy of the frequency generator and decibel meter I'm using, or because the whole idea is just incorrect. Help?