When in discussion regarding sound, the frequencies involved are grouped into what are known as the audible range and the inaudible range, however, such ranges are standards set to the auditory capabilities of the homo sapiens species by which research is observed. In other zoological subjects, the audible range varies from that of a human, possibly with longer or shorter wavelengths which may influence a seemingly simple task to become complex as observed from a human point of observation. One such example is what is known as infrasound, where sound wave frequencies are found to be below those of the audible range, in opposition to frequencies found above the range, or ultrasound, both of which may be utilized in survival by several living organisms.

In essence, sound is a vibration produced by oscillating objects which may be perceived by the auditory system and described as a number of vibrations per second, also known as the frequency (f, measured in Hertz, symbolized as Hz) of the sound waves which may be expressed thus as wavelengths, or the distance a wave travels before repeating its motion. A medium- such as air- is required to transmit sound waves in which particles may vibrate perpendicular to transmission direction (transverse), or particles may vibrate parallel to transmission direction (longitudinal), or, particles may even spiral in transmission direction as a torsional wave, the most dangerous of all sound waves (SPH1).

Infrasound, therefore, is a vibration with frequencies composed of short wavelengths, which travel for long distances through air and particularly through earth, where deeper infrasound travels further. Due to the way in which sound travels through the atmosphere, infrasound possesses the ability to travel further than ultrasound which is composed of longer wavelengths. The study of infrasound waves may be known as "infrasonics" or "infra acoustics", a contemporary study in both biological and geological sciences (11). 

Infrasonic wavelengths possess the ability to travel through both water and air mediums, being measured in decibels (dB), however the measurement scale for decibels in air differs from the scale used for those in water (4). Such distinction may also be recognized in the measurement of temperature, which may be measured in Celsius ( C), Fahrenheit ( F), or kelvin (K)(4). Present in the measurements of infrasound, there is a 63.0 dB difference between the two scales, where 26.0 dB of the difference is attributed to the conventional choice of sound reference level: in air, the reference level is 20.0 microPascals where in water, the level is 1.0 microPascal. The remaining difference found in the 63.0 dB is due to the differences in density and compressibility of water and air (4). 

Due to the differences between the mediums and the different scales, there are drastic differences in the perception of the level of sound when wavelengths in air are compared to those in water. For instance, 160.0 dB in air waves are liable to produce tissue damage in the auditory systems of many mammals, including humans, where in water, 160.0 dB in water is equivalent to 100.0 dB in air, which is the sound level perceived when standing approximately three feet from a loud radio. Or 125.0 dB in water is equal to approximately 65.0 dB, which constitutes the level of a normal conversation (4).

Although there would seem to be no apparent connection relating infrasonic sound waves and seismic "ground" waves, there does exist a relationship between the two: seismic waves are infrasonic, however they are a specialized type of infrasound, or acoustic, wave which is capable of traveling great distances and as frequencies as long as several months a cycle within the earth (11). Seismic waves are energy waves generated by the sudden movement of earth, breaking of rock, or an explosion- they are a means by which energy may be transferred between two points within the earth. Although there are several forms of seismic waves, the focus is upon two main forms: body waves, which travel through the inner earth, and surface waves, which move solely along the surface of the planet such as a water ripple in a pond (36).

Body waves are called so as they travel through body of material in all directions and not merely at the surface; they may be classified as either a primary wave (P wave) or a secondary wave (S wave). P waves are known to be the fastest seismic waves, longitudinal waves which may travel through both solid and liquid material, including the molten core of the earth, with short wavelengths and high frequency (36). They are also known as compression waves, as they rely on the compressional strength and elasticity of substances to propagate, traveling generally at approximately 6.0 km/s (approximately 3.7 miles/s) in the earth’s crust, which is more than seven times the speed of sound (21). P waves provide pushes and pulls on rock as they move through the earth, just as infrasound waves in the air push and pull on the air. An analogy is thus: when thunder sounds, only to rattle a glass window, it is due to the sound waves applying pressures and releasing the window, very much like P waves on the earth. In accordance with P waves, there are S waves, which are known to move slower than the P waves and only through solid material, however with short wavelengths and high frequency, similar to the P waves, traveling the same at approximately 3.5 km/s (approximately 2.2 miles/s)(21). They are transverse waves, which move up and down, or side to side in forward travel in the solid earth, dependant on the strength of the material. An analogy of such waves may be found in observing a slinky: if the slinky is shaken side to side, a longitudinal wave is produced, however if given an abrupt sideways deflection (by snapping one’s wrist), a transverse wave results, which travels at both ends (21). 

However, body waves are only one form of seismic wave where surface waves are another, occurring only at the surface of the earth without penetrating the earth’s interior as deeply as the body waves with long wavelengths and low frequency, but at a slower velocity than the body waves. It is this form of seismic wave which produces the most damage, as found in earthquakes, which may be classified as either Love waves (L waves or Q waves) or Rayleigh waves (R waves)(36). L waves, named after A.E.H. Love, an English mathematician credited with success in producing the mathematical model for a wave in 1911, are the fastest of surface waves, which cause the earth to move from side to side in a strong, horizontal transverse waves (36). However, Rayleigh waves are the worst of the two forms: named after John William Strutt, Lord Rayleigh, the man credited for mathematically predicting the existence of such waves in 1885, R waves are vertical elliptical, or torsional waves, which roll along the surface such as waves roll across water (36). Such waves move the earth up and down and sideways in the same direction as transmission, which provides for most of the shaking as found in earthquakes and adds to the possible immense destruction which occurs. ⁶

Although seismic waves are usually only felt by homo sapiens, it is possible for other living organisms to perceive the waves beforehand, either as seismic waves or as sound. Some organisms are capable of employing infrasound and seismic waves as communication, which may influence them to be sensitive to changes within the earth, under the surface, as earthquakes occur. In the case of elephants, they may be able to hear the grinding of rock below the earth’s crust as well as receiving tremors from a far distance away, as they are known to employ both infrasound and seismic communication (V1). Zoological forms have often been observed behaving strangely before an earthquake: in 1975, prior to the Haicheng Earthquake in China, it was noted that rats, snakes, birds, cows, and horses behaved oddly, perhaps being able to feel seismic pre-shocks which remain undetected by even sensitive equipment (J). Some fish, including the catfish particularly, have become agitated before an earthquake, sometimes leaping out of water onto dry land. Snakes have been found frozen in snow after leaving the safety of underground hibernation dens in the middle of winter; mice have appeared in a daze, allowing for easy capture; homing pigeons take a longer time to navigate; hens may lay fewer eggs, if any at all; pigs have become aggressive towards one another; and bees have been known to evacuate their hives minutes before the quake occurs, not returning until approximately fifteen minutes after the end of the quake, just as millipedes, ants, squid, and leeches have also been observed exerting odd behaviour (J).

Much biological and zoological research has been focused on the auditory and optical systems of organisms as they are senses possessed by homo sapiens, where no specially designed organs to detect terrestrial vibrations are existent. However, research is now being pursued in biology and perhaps even increased in zoology as to such organs and how they are employed. For instance, there is at present an interest in whether lions possess the ability to utilize infrasound and vibration detectors in their paws (J). 

Such seismic sensitivity may be found in species of mammalia, insects, amphibia, and reptilia as means of communication, defense, location of prey, social interactions, and mating, signals which may be discovered by use of computers and geophones, as used in the Vietcong jungles of Vietnam to observe foot steps (M). Prairie mole cricket males sing from burrows to females, sending ground vibrations to other males in warning to maintain reasonable distances; some rodents use seismic communication by drumming their feet; Leafcutter ants use vibration to signal for aid when buried alive or to recruit foragers; snakes and other reptiles which lack external ear openings detect ground vibrations, allowing for the assumption that a rattlesnake never hears its own rattle; and vibration is also employed in spiders, frogs, bison, the rhinoceros, hippopotami, alligators, and kangaroo rats (M). However, one of the most prominent examples of seismic communication and infrasound utilization may be observed in elephants: when frightened, elephants employ seismic waves to send messages of warning to other herds by stamping their foot to the ground, from where shock waves are sent out, traveling up to 48.2 km (30.0 miles) away (V1). 

Research pursued by homo sapiens is more than often based on the limitations of the human senses, however it must be understood that abilities become more complex or simple in other organisms, which may be able to sense that which humans cannot. One such example of this is the concept of infrasound and seismic communication, which several organisms employ for survival, however confused it may make humans, who are incapable of sensing through natural means the same which is perceived by other species.