I did a simple experiment as illustrated below
Basically, I used an app to let my phone produce a 393Hz sine wave sound. This is roughly the resonant frequency of a metal can. When the phone is outside the can, the sound is not large. While when I put the phone into the can, the sound becomes noticeably louder. I used a second phone to measure the spectrum of the sound, and the recording indicates a 15dB increase in magnitude. At the link below you may find a video recording of the experiment (though the quality of the video is not satisfactory).
Here is the question: the phone has a constant power, which means the sound source are producing a fixed amount of energy per unit time. Then, why do we hear louder sound when the phone is inside the can? From the conservation of energy, shouldn't the total sound power remain constant? Obviously the can cannot produce energy by itself, so where does the extra power come from?
Also, the louder sound can be heard from almost any direction, from top or from side, which means that the metal can did not attempt to focus the sound toward specific directions.
Thanks for any comment.