While playing the guitar, I remembered a photo I had seen which was taken from inside of the guitar showing the vibrating strings. So I decided to emulate it.
Well, I couldn't get my phone inside the guitar and take a photo so I just decided to take it looking down, while focusing on the vibrating strings part.
I played around with the shutter speed settings, as well as the ISO and the flash intensities to try to capture that vibrating effect. It seemed the optimal shutter speed range was about 1/2000 seconds. I even went as fast as 1/6000 seconds but I got more movement than the standing wave effect.
My preliminary conclusion is that the shutter speed needs to match the frequency of the vibrating strings to get the "standing wave" effect. But I need to explore this further, with a more precise setup and a more scientific approach.