If you happen to shower outdoors for whatever reason, or in a room without a roof, or in a room not made of porcelain tiles, you might not sound much better when you sing. Or, if your shower room happens to be a perfect cube and you’re standing in the exact center of it, which would be weird. In any of these cases, there might not be much reverb, i.e., reverberation. As Premium Beat explains, reverb happens if two basic conditions are met: 1) Sound travels out in all directions, and, 2) That sound bounces back at different times. On the second point, showers are usually shaped like rectangular prisms, so one’s voice bounces off of certain surfaces sooner than other surfaces. This is what creates reverb’s layered echo, which Mic says makes your voice “hang” in the air longer.
Reverb becomes extra potent if sound bounces off of a hard surface, and continues to bounce multiple times. This is exactly what happens in a shower because of two additional reasons: 1) Showers are usually made hard surfaces like tiles, which reflect sound better than sound-absorbing soft surfaces, and, 2) Showers have a “cavity structure,” as Mic puts it, meaning that they’re not only fully contained on all sides, but small enough for sound to bounce around quickly and overlap again and again. This is why showers amplify certain frequencies and make them both more resonant — which can make them sound deeper — and louder.