Your ears; your hand or a book; a blindfold if possible; if a friend is available that will make the experiment more objective.
Famously, bats and dolphins can “see” by detecting echoes. These are the stars of echolocation (and I’ve always found it amazing that dolphins also have acute eyesight, so they are doubly gifted).
But other species can navigate by echolocation. This includes humans. Some blind people have learned this skill to a high degree, and can walk or bike around freely, as though they were sighted, by making clicking sounds and hearing the echoes.
But we all possess echolocation and use it instinctively. Imagine you dropped your bunch of keys on the floor, what would be the first sense to detect their landing? Hearing, which would know where they fell. Vision doesn’t know, it has to hunt around. If you shut your eyes and focused on the sound you might retrieve the keys as quickly as you could have by looking.
But the idea of this experiment is to show that you can do more than locate an object from the noise it emits. You can generate a noise (as bats do) and get information about what is in front of you from the echo (as bats do).
Close your eyes very tightly or put you blindfold on.
Now you will need to try different sounds: experiments with human echolocation seem to show that people do best if they pick what their instinct suggests is a good sound to “see by”. But for this experiment you need so emit a steady high-pitched sound. The one that works for me is if I begin to say “Sssssssnake” but get stuck on the “Sssss…”.
While you are Ssss-ing get your friend (or do it yourself but it’s not s good) to hold a book up and move it in front of you and away again. Can you hear the difference?
I can, from up to about a foot away. If the book is moved fairly rapidly fr0m one side to the other I can also pick up a Doppler-like effect, so I’m able to detect motion as well.
Reflection (or echo)
It’s been known for over a century that humans can echolocate. A good survey of the experimental evidence is published here and there is lots of more recent investigation.
The next post in the series, 7/101, will be ‘Footsights’.
Looking two steps ahead, 8/101 will be ‘Seeing the Future’.