Sound travels through vibrations, with frequency measured in Hertz (Hz). Humans perceive sounds from 20 Hz to 20,000 Hz, or 20 kilohertz (kHz). This range allows us to experience speech, music, and environmental cues. Sounds exceeding 20 kHz are categorized as ultrasound, existing beyond human hearing. This vast world of high-frequency sounds is imperceptible to most people, yet routinely used by many animals.
Animals with Ultrasonic Hearing
Many animal species possess the ability to detect and utilize ultrasonic frequencies, far surpassing human hearing. Bats, renowned for their nocturnal activities, can hear frequencies up to 160 kHz, with some species reaching 212 kHz. Similarly, marine mammals like dolphins and porpoises employ high-frequency sounds, with porpoises having an upper hearing limit around 160 kHz.
Beyond these well-known examples, other animals also perceive ultrasound. Moths, for instance, have evolved to hear frequencies up to 300 kHz, primarily as a defense against predatory bats. Rodents like mice and rats communicate using ultrasonic vocalizations. Mice often use sounds during mating, and rats for social cues or danger signals. Even common domestic animals like dogs and cats exhibit ultrasonic hearing. Dogs can hear up to 65 kHz, and cats up to 64 kHz, aiding them in detecting small prey.
The Function of Ultrasonic Hearing
Ultrasonic hearing provides significant evolutionary advantages, primarily through echolocation, a biological sonar system. Animals like bats and dolphins emit high-frequency sound pulses and interpret the returning echoes from objects. This process allows them to create detailed mental maps, enabling navigation and hunting in limited or absent visibility.
Bats use echolocation to pinpoint flying insects in darkness, emitting chirps as loud as 120 decibels. Echoes provide information on distance, size, shape, and texture of objects, allowing bats to differentiate prey from other elements. Dolphins and other toothed whales use similar high-frequency clicks and whistles for underwater echolocation, where sound travels faster than in air. This enables them to locate prey like squid in murky waters or deep ocean depths, detecting objects the size of a golf ball from significant distances.
Other animals use ultrasonic hearing for defensive purposes. Many moth species detect the ultrasonic calls of approaching bats, initiating evasive maneuvers like loops, spirals, or dives to avoid capture. Some moths produce their own ultrasonic clicks, which can startle bats or jam their sonar systems, allowing escape.
Biological Mechanisms of High-Frequency Sound Perception
Perceiving high-frequency sounds relies on specialized anatomical and physiological adaptations within the inner ear. The cochlea, a fluid-filled, spiral-shaped structure, is central to this process. Sound vibrations from the middle ear transmit into this fluid, causing the basilar membrane within the cochlea to move.
The basilar membrane varies in stiffness and width along its length. At the base of the cochlea, the membrane is narrower and stiffer, making it most responsive to high-frequency sounds. As sound waves travel further into the cochlea towards its apex, the membrane becomes wider and more flexible, resonating with lower frequencies. This differential responsiveness means high-frequency sounds stimulate specific hair cells near the cochlear base. These hair cells convert mechanical vibrations into electrical signals, transmitted along the auditory nerve to the brain.
Human Interaction with Ultrasonic Frequencies
Humans do not naturally hear ultrasonic frequencies. However, humans have developed technologies that harness ultrasound for various practical applications.
Medical sonograms, or ultrasound imaging, are a common example, using high-frequency sound waves to create images of internal body structures without invasive procedures. This technology is used to detect abnormalities like tumors or fluid pockets, and to monitor pregnancies.
Another application is SONAR (Sound Navigation and Ranging), which uses sound waves to detect underwater objects and measure ocean depths. This technology is employed by boats, ships, and submarines for navigation and exploration. Ultrasonic frequencies are also used in devices like dog whistles, which produce sounds above 20 kHz, audible to dogs for training but imperceptible to humans. Ultrasonic pest repellers are also marketed, though their effectiveness lacks consistent scientific validation.