Note: As any audio engineer can tell you, shower acoustics are a result of resonant frequencies within the range of the fundamentals being sung. If you start with a one- by one- by two-point-five-meter shower stall, and cover it with tile (which reflects roughly 98% of the acoustic energy striking it) the whole thing can be considered at first as an organ pipe two-point-five meters long. The lowest fundamental frequency can be computed by taking the speed of sound (call it 340 meters/sec) and dividing it by twice the pipe length. In this example, 340/2.5 gives us 136 cycles per second. In equal-tempered piano tuning that is the C# below middle C (C2#), or close enough to it for an oboist, at any rate.
But it doesn't stop there -- the harmonic series continues at two, three, four, and five times the fundamental (and beyond, into infinite inaudibility) - a C4#, a really flat G4#, a sickly C5#, and a blue note between E5 and F5 that will make the dogs howl. A shower baritone who hits that first C2# just right (or rather who MISSES it just right) will get all the rest of those notes as harmonic accompaniment, although at much lower amplitudes.
And we're STILL not done. The transverse vibrational modes are functions of those one meter dimensions, providing us with fundamental resonances on a blue F4, F5, an almost C6, and so on. Again, these are frequencies at which, when the singer in question sings them, the sound waves will reflect in phase such that they amplify each other rather than cancelling each other out.
The flip-side to all this is that there are out-of-phase, non-resonant frequencies as well -- notes which, when sung, seem to disappear completely.
All of this means that as much as you may like the sound you get in the shower, anyone with a half-trained ear can tell that you sound terrible, and the shower is rewarding you for missing notes and singing the worst ones as loud as you can.