We are used to the idea that there are parts of the electromagnetic spectrum that our eyes can not see; and in our everyday lives we meet equipment that works in these parts of the spectrum; our radio sets, microwave ovens, x-ray machines etc.
It's not so obvious that there are parts of the pressure wave (sound) spectrum we can not hear.
Human hearing has a very wide frequency range; 20Hz - 20kHz. Thats a range of 1000:1 - MUCH wider than the range of frequencies we can see! It allowed us to hear predators, and to communicate.
Bats have evolved to use the pressure wave spectrum between about 20KHz and 200kHz to allow them to find prey and avoid obstacles in total darkness by echolocation - sonar. We can not normally hear these sounds - and its not because they are quiet. Actually, they are so loud - as loud as a pneumatic drill - that the bats have evolved a special mechanism to turn off their hearing as they call. The calls are just too high pitched for us to hear. If we want to pick up these sounds we need special equipment - a "bat detector".
Why do bats use such high frequencies?
Well, there are many good reasons. Firstly, its easier to make high pitched sounds than low pitched ones.
Look at a pipe organ. The small pipes take little air to blow, and make high pitched sounds. The long pipes take much more air, and make low notes.
Its the same for your Hi-Fi system. Low = big, high = small.
To the left you can see a typical Hi-Fi low frequency speaker (Woofer). This reproduces the bass notes. It is 12 inches in diameter.
Right is a Hi-Fi "Tweeter" unit. It reproduces the high-pitched notes. The working part - the dome in the center is about 1 inch diameter.
Bats have very small mouths and could not produce low notes. Lions roar, birds tweet. Bats chirp - at very high pitch.
Bats use their calls to find prey. The wavelength of their call needs to be small compared to the size of the object to be detected. This is called interference limiting. A call at 50kHz has a wavelength of 0.25 inch and will allow detection of small insects. If bats called at 5kHz (2.5 inches) they could detect the trees - but not leaves and not moths.
Frequency Modulation (FM) and Constant Frequency (CF)
Some bats calls use a single frequency - for example lesser horseshoe bats call at around 100kHz. This "Constant Frequency" (CF) call enables them to use less energy whe calling. Others such as Brandts bats use a "chirp", (FM) while the common pipistrelle uses both - a chirp ending in a constant frequency. This is called a "hockey stick" (HS).
CF calls enable detection at greater distances, (useful in open environments) while FM calls give more precise localization of the prey, especially in cluttered suroundings. Example spectrograms of their calls are shown below.