Biosonar: Delay and Range

Differences between vision and biosonar

(3) Resolution - animals using biosonar cannot focus sound waves like light
The resolution of an image roughly means its sharpness. Most animals can form high-resolution images with vision. Light waves are focussed through a narrow aperture (the pupil of the eye) by the lens, and cast a pattern of light on the retina, the light-receptive tissue at the back of the eye. However, when animals use biosonar, they have none of the same mechanisms for producing high-resolution images. They have no acoustic lens, nor an acoustic retina. The mechanisms by which they achieve high resolution are completely different and produce images that we can scarcely imagine.

Delay and range resolution - the crux of the matter?
Have a look at the interactive delay demonstration

Many scientists think that bats' sonar image resolution is based on the perception of range to sonar targets (reflecting surfaces). Bats (and other animals that use biosonar) have an advantage over humans in judging distances using sound. We judge the distances to sound sources by a rough impression of the intensity and atmospheric distortion of the sound. However, bats can calculate the distance of the source of echoes because they can measure the time it takes for sound to travel from their mouths to the target and back to their ears. A bat knows exactly when it emits a sonar pulse and can time the delay from pulse to echo very precisely. Some scientists claim that this precise echo delay measurement is at the root of biosonar images and is so accurate that it can result in high resolution in range.

If bats can judge echo delay and range to targets very accurately, they can separate echoes that they receive at their ears very closely spaced in time. They have resolution (sharpness) in their sonar images in the range direction (scientists sometimes say "along the range axis").

Our visual images are arranged in different directions. The resolution of our images is in the left-to-right and up-and-down directions. A bat's images are arranged primarily in the backwards-and-forwards direction (the range axis).