Acoustics, meaning the way sound is transmitted or behaves within a space, is affected by a space’s size, shape, and the materials from which it is constructed. High ceilings, flat and parallel surfaces, and hard materials make a space more reverberant while things like uneven surfaces and soft materials reduce reverberation. Having soft furniture and rugs or carpet can reduce reflection considerably, as do acoustic panels designed specifically for the purposes of absorbing sound.
Listen to the different spaces below to get a sense of how these aspects of a space affects the acoustics:
Reflection is not a bad thing. A little bit of it is pleasant and not having any can feel unnatural. A lot of reflection or an echo, however, can make it hard to discern sounds, in particular, speech and can simply sounds unprofessional and unappealing.
Besides getting a sense of the acoustical nature of the spaces above, you also can hear the ambient noise. Like reflection, ambient noise is not a bad thing, and, like reflection, not having any feels unnatural. The key is to not have it be distracting or overpowering.
When recording, it is best to "capture" at least ten seconds of just the ambient noise, or room tone, as it is called when recording. Doing so will be helpful if you need to fill in empty spaces while editing. If these spaces are left empty, there will be complete silence, which is jarring to a listener. In this Bapst Library example, you can hear the difference that filling in and not filling in the room tone makes.