What effect does a bat's echolocation have on other bats?

Question

Bat echolocation is a perceptual system where ultrasonic sounds are emitted specifically to produce echoes. By comparing the outgoing pulse with the returning echoes, the brain and auditory nervous system can produce detailed images of the bat's surroundings. This allows bats to detect, localize, and even classify their prey in complete darkness. – Echolocation, Wikipedia

Furthermore, we read that:

[ . . . ] these bats contract their middle ear muscles when emitting a call, so they can avoid deafening themselves.

So far, so good. But this gives rise to two questions I've been challenging with. (And unfortunately I seem not to be able to find the correct search terms and I don't get relevant results)

Imagine two bats close together, flying in a distance of, say, 1 meter from each other. As we know, both echolocate in order to perceive the whereabouts of their surroundings. Bat #1 emits ultrasonic waves, but contracts its middle ear muscles so it won't get hurt. I imagine these two bats are close enough to each other for the waves to be harmful to bat #2's hearing as well.

Hence, two problems arise

• Either the waves aren't harmful to the other one (I'd love some calculations on this, or hints on how to do some myself) or something's happening to bat #2 that doesn't let the ultrasonic waves damage its hearing. Certainly bat #2 isn't supposed to know when #1 will emit those waves, so what is happening?

• Won't the waves bat #1 has emitted affect the waves bat #2 will emit? So, won't this affect bat #2's perception? We've seen many bats flying together in real life with [maybe seemingly] no trouble visualizing the surroundings, so this mustn't be the case, but why?

Answer 1

Bat echolocation is an incredible and complex process. Bats can emit two kinds of ultrasonic signals. One is a short pulse (less than 5 ms long) that sweeps a wide range of frequencies (100 Hz to 25 kHz) and is referred to as a FM sweep, or a broadband signal. The second kind in a constant frequency (CF) and is much longer in duration (5-30 ms) and held at a specific frequency. In addition to the fundamental frequency there are first and second harmonics. This important for depth perception, pursuit of prey, and location of other bats!

Specifically, bats measure distance by comparing the time between emitted sound pulses and its returning echo. Rapid FM sweeps are especially well suited for target distance so the bat can adjust easily and locate the target. Bats can determine angle, size, elevation, azimuth, and velocity/movement of object from these time and frequency differences as well. Now, bats calculate their own velocity using the Doppler shift concept of their own echos. Meaning, when their signals comes back to them, they compare the amplitude of what their ultrasonic sound wave would be if they weren't moving to the received sound and calculate actual velocity off that (it is a very complicated process and I'm trying to simplify it to get to your question).

Ok, now that is the basics of echo location, but back to your question: What happens when there are multiple bats? Well, when you have many ultrasonic waves going at once, it might get confusing about which is your signal. At least that what you would think. Since the bats have such a wide range of frequencies they can vocally create (100 Hz to 25 k Hz), bats usually simply change the range they are vocalizing to prevent wave disruption. If you do get an overlap of waves from bats using the same frequency, you get something similar to Doppler effect where amplitudes are not what expected. Since they are pretty drastic changes (sometimes even canceling out the wave entirely), and a bat can sense if another bat is around them by calculating the size of the objects producing noise, a bat will know this distortion is due to another bat and not the prey. As a result one bat will have to change their frequency range. This has become sort of a dominance game for them as a result. The bats heightens (I think, might be lower) the frequency of their signals until one cannot go any farther. The one with the wider range is considered dominant and rules that frequency range. Neat right! Hope that answered your question!

Sources: http://scitation.aip.org/content/asa/journal/jasa/54/1/10.1121/1.1913559

Behavioral Neurobiology by Thomas Crew copyright 2000 (most of info from this source, great read if interested)