The external ears of a horse or mule — the pinnae (singular: pinna) — are much larger than ours. And unlike ours, they can be turned to focus on the source of a sound. When your horse spooks on the trail and freezes, he will turn his ears toward the source of the alarming sound he just heard. Watch those ears, and you will know where the sound came from, even if you didn’t hear it yourself.
Horses differ in what they then do with the information in what they hear. Some “bombproof” horses may not change their behavior at all. Other more reactive horses may stop in the tracks, raise their heads, freeze their body position, and wait. On receipt of further alarming information, a reactive horse may turn and move in the opposite direction from the sound.