By Jake Weiner

Many species in the forest interact with their environment through the use of sound waves. Whether it be the trumpet of an elephant or the ultrasonic call of a bat, many organisms use sound to interact. Sound waves are very responsive to and highly impacted by the surfaces that they come in contact with. While some surfaces may enhance the range and tonality of a sound, others may partially or totally impede the movement of those sound waves through space. It is no coincidence then that different species produce calls at varying frequencies, as these deliberate frequencies optimize range given the surfaces present in the forest. 

The basics of acoustics dictate that harder surfaces reflect sound more intensely, and softer surfaces absorb sound more fully. When sound waves travel through space they ricochet off of every surface within range and are subsequently altered by these interfaces. Forests contain a wide variety of surfaces from which sound may strike and reverberate from. A tree, for example, embodies the spectrum of hard to soft surfaces that exist within the forest. The wood of the trunk is a dense, hard surface that causes tremendous reverberation. Conversely, the fan-like, broad leaves in the canopy of the Cecropia tree in the Amazon Rainforest serve the opposite role and greatly absorb sound waves. It is because of this acoustic gradient that different animals intentionally produce specific frequencies.

The Western Small Footed Bat produces a supersonic pitch that is approximately four-thousand hertz above the human auditory range, which tops out at around twenty-thousand hertz. It is not surprising then, that this bat also lives in the top canopy of Pacific Northwestern forests. The reason that these bats, among other upper canopy dwellers, produce such high frequencies is because of the physical nature of the sound waves produced at such high pitches. Low pitch sound waves are produced in a sine wave, which is categorized by smooth, equally amplitudinal waves that travel at much slower frequencies and are easily absorbed. However, high frequencies, typically any above ten-thousand hertz, travel at much faster rates and are able to penetrate absorptive surfaces such as the thick forest canopy. By producing calls in the supersonic frequency range these bats are able to prosper in the top canopy which also allows them to live at a safe distance from their forest floor dwelling predators, predominantly snakes. This behavioral adaption increases their fitness greatly by allowing them to utilize the acoustics of their habitat, and interact with the surfaces of their environment at a frequency that the vast majority of their cohabitants cannot detect. 

Similar to bats, elephants’ survival depends considerably on the interaction between their use of sound waves and the natural environment. The forests in Kenya receive heavy rainfall which consequently causes the soil to be especially moist and thick, a combination that equals substantial sound absorption. The soil found here can be described as Nitisols, being made up by over thirty-percent clay. Despite these sound exterminating ecological conditions, the largest inhabitant of these wet forests have become masters at traversing this acoustic ecosystem. African Elephants produce extremely low frequencies that allow them to utilize their otherwise unfavorable sonic habitat to their advantage. These elephants purposefully create sound waves that are so incredibly low, and so amazingly forceful, that they result in a seismic reading. While an immediate assumption may be that this spectacle is only a result of their sheer size and weight, that would be categorically inaccurate. African elephants generate sound waves below twenty hertz, which is considered to be in the sub-frequency spectrum and is too low for humans to hear. The reason that they consciously do this is because these exceptionally low frequencies travel at a long enough rate to penetrate the opposing soil beneath. The force required to generate these sub frequencies is enough to provide the energy for the sound waves to travel over three kilometers underground. This is an ingenious tactic that herds use to communicate with each other because their trumpeting would be quickly absorbed by the various soft surfaces found within the forest. An African Elephant’s trumpet is typically located within the mid-range frequency spectrum which spans from about five hundred to two thousand hertz, leaving these sound waves at an optimal amplitude and frequency rate to be absorbed before ever reaching the other herd.           

The ecological conditions of a given forest also determine its overall acoustics, and essentially regulate how loud or quiet that environment is. For example, the Hoh forest of Washington state is an extremely dense forest, and subsequently known as one of the quietest in the world. These two features are directly correlated and function jointly to create the jungle-like feel felt in Hoh. The Hoh forest receives between twelve to fourteen feet of precipitation per year, and has a temperature range of thirty to eighty degrees Fahrenheit during the winter and summer months respectively. These conditions promote the extensive growth of various epiphytes such as moss, spike moss, lichens, and ferns which effectively encase the entirety of the forest. This thick coat of epiphytes fundamentally changes the acoustic make-up of Hoh because it covers all hard surfaces such as tree trunks, branches, and rocks which eliminates the ability of sound reverberation. These soft surfaces covering the forest create a phenomenon known as “zero noise,” where virtually all sound waves produced are absorbed and there is little to no ricochet. In addition to these epiphytes, the zero noise environment of the Hoh forest is also contributed to by the presence of Western Hemlock trees. These trees produce flat, soft needles and grow very thick, narrow crowns. Their crown shape allows them to grow in close proximity which closes much of the canopy and stops sound waves from reverberating.     

The acoustic environments found in forests around the world greatly impact the interactions and behavior of the wildlife and plant growth present. This study of bioacoustics can lead to entirely new perspectives being taken when examining the ecological interactions within our planet’s forests. While appearing subtle, this has a great impact on the organization of our biosphere. From the world’s largest land animal to the simplest forms of vegetation, all life is truly informed by these invisible interactions.           

About the Author:

I am a student at the University at Buffalo, studying International Trade within the department of geography. From a young age, I have shown interest in ecosystem diversity, ecology, and acoustics. My focus is to find the relationships, co-dependencies, and intersections that these three fields share. As a musician, I have come to recognize the significance that sound waves play in our biosphere through a personal study of bioacoustics. This has led me to profound new perspectives and has altogether reshaped my approach towards the natural world. It is my position that a comprehensive analysis of the interactions between these three fields can break down a substantial barrier in the field of natural science and will lead to revolutionary discoveries that completely reorder our understanding of the environment.