resonance = sound more louder?

When a vibrating tuning fork is held near the mouth of a glass tube (with one closed end), the observed loundness is greater than just with the fork alone.
One may account this by the resonance of air molecules in the glass tube, and I personally think that they are just resonant inside the tube, the sound wave reaches outside should not be standing wave and sould be equivalent to just with the fork alone, but why it sounds more louder? I think it sounds louder only when we are hearing the sound inside the tube, which has double amplitute and four times the intensity.