Are you talking about bass on headphones or earbuds? The human-perceived bass levels (relative to real levels) on those tend to decrease as the surrounding noise level increases, which is partially why you may find yourself cranking the volume up in noisy places, and outdoor settings tend to have more ambient noise than indoor ones.
If you’re talking about loudspeakers or live instruments, closed rooms have resonances, known as room modes, at certain frequencies, which tend to boost overall bass levels. Basically, the room itself wants to vibrate kind of like a giant wind instrument, and at which frequencies will depend on its length, width, and height. Especially in smaller rooms, this can provide a considerable low-end boost that makes bass sound deeper and rumblier.
e: Audio engineers will use fiberglass or foam panelling to absorb those indoor bass waves instead of letting them bounce off and mess with the sound of the mix.
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