[MikeL (OP)]

We are taught about bats sending out a sound whose echo it reads to inform it about the environment. Do we know if a bat is able to pick up secondary and tertiary echoes as well? If we can detect the bat squeak from inside our house, then surely the bat can read that echo and see us too? Does anybody know if it has layered vision like this?

[chris]

I think that they do compensate / take these extra signals into account. Whales adjust the "beam angle" of the sounds that they emit in order to alter the resolution of their sonar, so I suspect that, faced with a similar problem, bats will also adjust their echolocation approach.

In fact, this paper in PNAS explored this phenomenon, and provides a nice review in the introduction: http://www.pnas.org/content/112/26/8118.full

Some references to other bat coverage on the Naked Scientists that you might find informative:

Non-echoloating bats, such as fruit bats, do actually echolocate!

Bats echolocate to within a wingbeat of each other.

Bats jam each other's sonar to compete for food.

General information about echolocating bats - https://www.thenakedscientists.com/articles/interviews/brilliant-bats

[evan_au]

The amount of energy that is returned from a wave is determined by the Fresnel equations.

If there is a big change in impedance or refractive index, then most of the incident energy will be reflected back.
There is a huge change in refractive index between air and wall material - and equally large between the wall material and the other side. It must bounce of someone inside the room and traverse the equally lossy path to reach the bat outside.
Add in some insulation which would attenuate high frequencies severely (and must be traversed twice).

So a useful amount of ultrasonic energy could not make its way back to the bat from someone inside the house.
See: https://en.wikipedia.org/wiki/Fresnel_equations

There is, however, a more severe problem for the bat - glass windows.
- bats interpret a loss of reflected sound from below as a smooth pool of water, and will often swoop down to brush against the surface of the pond, licking the water off their chest.The smooth surface of the pond reflects the sonar away from them, so they can infer its presence.
- bats interpret a loss of reflected sound from the side as open space, and will often swoop through this area. Unfortunately, glass windows also reflect the sonar away from them, so they can't detect it, with lethal consequences due to their fragile bones.

So the real problem is that bats can't detect smooth vertical walls.

[chris]

Evan, I don't understand - why can't the bats "see" the windows with their sound?

[Bored chemist]

If the bat isn't travelling exactly "square on" to the window, the sound will bounce off the window at an angle- away from the bat. The bat won't hear an echo so it will assume there's a gap.
Occasionally people do the same thing and mistake a large mirror for a doorway because they see a reflection of a doorway.
The bat "sees" a reflection of the space  behind it (and to one side) and thinks it's flying into empty space.

[chris]

If the bat isn't travelling exactly "square on" to the window, the sound will bounce off the window at an angle- away from the bat. The bat won't hear an echo so it will assume there's a gap.

Wouldn't this potentially be true for a lot of objects, or is it the hard, flat surface presented by a window pane that makes this particularly problematic?

[Bored chemist]

Very few things are as flat as a window pane. In much the same way that you can see your face reflected in a window, but not a wall.

[evan_au]

There is a short video of the smooth wall problem here (1.5 minutes):
https://www.nature.com/news/bats-slam-into-buildings-because-they-can-t-see-them-1.22583

[MikeL (OP)]

Thanks guys,
I heard when they were designing the stealth plane there were a lot of dead bats all over the floor because they couldn't see the wing.
You would have to think there would be a massive selective advantage to picking up a secondary echo, such as: tracking a rodent through its burrow, spotting a snake in long grass, seeing an insect hiding on the underside of a leaf etc.
Even though there would be massive energy loss in the initial reflection, say as a function of density, I think it would make a lot of sense if we found they could. I will have to have a close read of the papers you guys have linked.

[Bored chemist]

You would have to think there would be a massive selective advantage to picking up a secondary echo, such as: tracking a rodent through its burrow, spotting a snake in long grass, seeing an insect hiding on the underside of a leaf etc.

Lots of things that are impossible would offer an evolutionary advantage, but evolution has to follow the limits of physics.

[MikeL (OP)]

When it comes to life, I subscribe to the idea that if you can conceive of it and it makes logical sense, it happens.

[Colin2B]

When it comes to life, I subscribe to the idea that if you can conceive of it and it makes logical sense, it happens.

But as @Bored chemist points out, that isn’t the case with physics, you can’t change the way it works just by conceiving of it.
Oddly, it doesn’t happen with life either.

[evan_au]

I heard when they were designing the stealth plane there were a lot of dead bats all over the floor because they couldn't see the wing.

I don't see why bats would collide with a stealth wing much more than they would with a normal metal wing.

While there are similarities between radar and sonar, they are fundamentally different physical processes.

A carbon-fiber wing would absorb microwaves better than a metal wing - but it wouldn't absorb ultrasound any more than a metal wing.

Bats would have trouble detecting a smooth metal wing with sonar. But conventional planes have curves, which would reflect some sound back to the bat from some part of the curve.

A carbon-fiber wing (for example) would also have to be very smooth, for aerodynamic reasons, so bats would have trouble detecting it too. But stealth planes tend to have a have a more boxy shape, with more flat/angular surfaces, so I guess more of the sound would be reflected away from the bat, just like more of the radar signal is bounced away from the transmitting radar.

See: https://en.wikipedia.org/wiki/Stealth_aircraft

[MikeL (OP)]

I've just come across this article on seeing through walls from a distance using radar which you might find interesting. It  addresses a lot of what we talked about. If we can do it with radar, surely bats must come close with their sonar.
http://news.mit.edu/2011/ll-seeing-through-walls-1018 [nofollow]

[Colin2B]

If we can do it with radar, surely bats must come close with their sonar.

No, there are big differences between bat calls and the radar in the article.
Frequency is one aspect, those used by bats are great for spotting small flying insects but the attenuation in air is very high which limits their range. The radar frequencies are hardly attenuated by air so most of the transmitted energy reaches the wall. Obviously the bat could try more power, but it is already at the limit of what a small animal is capable of - bats emit exceptionally loud sounds exceeding 140 dB SPL (at 10 cm from the bat's mouth), which is the highest level reported for any animal in air.
Also, look at the radar system. It uses a complex phased array of 13 transmitters and the power output will be well beyond the capabilities of a bat.
Just because we can do it with sophisticated technology doesn’t mean a bat can come close, if it did we should see supersonic, stealth bats.

[Bored chemist]

When it comes to life, I subscribe to the idea that if you can conceive of it and it makes logical sense, it happens.

So, you bump into Kylie Minogue on the bus and she gives you her telephone number. That must be interesting for you.
For what it's worth, quite a lot of what really happens- like quantum tunnelling- doesn't make sense.

[MikeL (OP)]

For what it's worth, quite a lot of what really happens- like quantum tunnelling- doesn't make sense

Not yet it doesn't... although I have a theory.
PS How do you know about me and Kylie?