Final answer:
Echolocation is the sound-based navigation mechanism that allows bats to fly in the dark without hitting objects by using echoes to determine the distance of objects.
Step-by-step explanation:
The characteristic of sound that makes it possible for bats to fly in the dark without hitting any objects is echolocation. Bats emit ultrasounds that travel through the air and bounce back as echoes when they hit an object. These echoes return to the bat, and based on the time it takes for the echoes to return, which is directly proportional to the distance, bats can determine how far away an object is. This process is similar to the sonar used by ships and is essential for bats to navigate and find prey in the dark. The independence of sound speed from its frequency ensures that all the echolocation sounds travel at the same speed, which is crucial for the bats to accurately judge distances. An interesting application of this is when a bat encounters an obstacle like a door; if the minimum distance that a bat can detect is not less than the width of the door, it may struggle to find the open door based on echolocation alone.