Sound localization: Where did that noise come from? -- ScienceDaily
"We humans find it difficult to assess, either visually or acoustically, how far away an object is from us," Wiegrebe says. "Our visual system makes use, among other things, of the phenomenon of parallax. When we move, the apparent position of an object that is closer to us moves more within our visual field than an object located further away. This relative motion provides information about the relative distance of the two objects. Localization of sounds is particularly challenging when the nature of the sound source is not clearly defined. It is not that difficult for us to estimate our distance from a speeding ambulance when we hear its siren. But when the sound is unknown, we cannot tell whether we are hearing a faint sound close by or a louder sound further away.