[chris (OP)]

When walking into an empty room or beneath a bridge voices begin to echo. What's causing this?

Chris

[lightarrow]

When walking into an empty room or beneath a bridge voices begin to echo. What's causing this?

Chris

Sounds bouncing off the walls. In an empty room sounds are not damped by the various objects which absorbs them.

[RD]

In a room without carpets curtains or upholstered furniture to absorb the sound there are reflections of reflections of reflections etc, creating a reverberation effect.