FM Synthesis and Bat Echolocation
Humans rely on vision to navigate through the physical world. The eyes take in light information and the brain interprets the stimuli to construct an image. Bats craft an image without using their eyes, in nighttime conditions where vision is rendered useless. The force of natural selection favored the use of sound in place of vision, so that bats can form pictures using vocal calls. Richard Dawkins’ The Blind Watchmaker states that physical stimuli are arbitrary, but nerve impulses are the same. Bats may in fact see in ‘color’ through the stimulus of sound. This process is known as echolocation.
During echolocation, bats compile discrete pieces of aural information to produce depth perception. The vocal chords fire chirps at varying rates depending on what type of information the bat is looking for. The baseline rate is about 10 chirps per second, but when the bat nears an insect it wants to eat, the rate increases to up to 200 chirps per second so that it can gather more detail.
How does the bat discern between the sounds sent out and the echoes coming back in? Like FM modulation on a synthesizer, the animals modulate the shape of their call. A higher-pitched sound indicates that the object is farther away. The motor action of the larynx, used to produce the shriek, is preceded by contraction of the middle ear muscle. This motion separates the inner ear bones, reducing hearing sensitivity so that bats don’t get deafened by their own cry.
Bats carved out their own niche and hunt at night, because the daytime economy is heavily exploited by other mammals. Different groups of bats have evolved echolocation separately. Smaller bats with tiny eyes have trouble supplementing echolocation with their eyesight, so they emit ultrasound waves to discern a higher level of detail when flying. Bats also use the Doppler effect and rotate their outer ear flaps to modulate sound dynamically.
Active sonar is used in naval warfare to communicate a ship’s position and determine the location of enemy vessels. Military researchers spent much of the early 20th century developing the technology, but the bat had evolved it millions of years earlier.