Ocean’s Unseen Symphony: SOFAR Channel, Hippo Echolocation & Sound

The ocean, far from being a "silent world," is a vibrant soundscape, a fact often overlooked due to the unique physics of sound in water. While light is an excellent long-distance messenger in air, sound plays that role in the ocean. This difference has profound implications for marine life and scientific exploration.

The Misconception of a Silent Ocean

Jacques Cousteau's influential 1956 film, "The Silent World," despite its groundbreaking visuals, perpetuated a significant misconception. The film, which won the Palm d'Or at the Cannes Film Festival, introduced the underwater world to a global audience. However, its title was a misnomer. The ocean is anything but silent.

The physics of sound in water differs significantly from air due to water's higher density and other physical parameters. Light, which travels far in air, is quickly absorbed in water, limiting visibility. Conversely, sound travels much further and faster in water—approximately four times faster than in air. This makes sound the primary means of understanding the ocean.

The film's soundtrack, largely overlaid music, further obscured the natural sounds of the ocean. It wasn't until about 20 years later that the public became aware of underwater sounds, notably through the popularization of whale song. These haunting sounds, now familiar, were astonishing to audiences 50 years ago, revealing the intricate communication of marine animals.

The Physics of Sound Travel in the Ocean

The speed of sound in the ocean varies with temperature and pressure, ranging from about 1,450 m/s to 1,600 m/s. This variability means sound does not travel in a straight line but can be steered. Higher temperatures and pressures both increase sound speed. This phenomenon is analogous to a line of people walking: if one side walks faster, the line bends.

This steering effect creates a unique feature in the ocean called the SOFAR (Sound Fixing and Ranging) channel. This channel, typically found around one kilometer deep, is a region where sound speed is at its minimum. Above this depth, sound speed increases due to higher temperature (from sunlight). Below it, sound speed increases due to higher pressure.

When sound is generated within the SOFAR channel, it is continuously refracted back into the channel, preventing it from escaping upwards or downwards. This effectively traps sound, allowing it to travel horizontally for immense distances with minimal energy loss. This is why whale songs can travel thousands of kilometers, not just because whales are loud, but because they utilize this natural acoustic waveguide.

Walter Munk's Global Ocean Thermometer Experiment

In the 1990s, oceanographer Walter Munk proposed using this principle to measure the average temperature of the entire ocean. His idea was that if sound speed is affected by temperature, then measuring the travel time of sound across vast oceanic distances could provide an average temperature reading. This would address the challenge of monitoring global ocean warming.

The Herd Island Feasibility Test in 1991 aimed to prove this concept. Herd Island, a remote location, was chosen for its isolation and for a pun Munk appreciated. Researchers deployed powerful, low-frequency transducers in the SOFAR channel and set up listening stations worldwide. The experiment confirmed that sound could indeed travel across entire oceans. An accidental early test even resulted in a phone call from Bermuda, confirming the sound's arrival hours before the official start.

However, the experiment faced challenges. The transducers, despite being designed for loud, low-frequency sound, frequently broke down due to the immense power required. A daily bulletin, typed and faxed via satellite phone, documented the progress and the dwindling number of working transducers. While the experiment established the principle, it was deemed inefficient and raised concerns about its potential impact on marine mammals. Consequently, this large-scale acoustic thermometry has not been repeated, though natural sounds like glacier movements are being explored for similar purposes.

Hippos and Underwater Echolocation

Beyond global ocean monitoring, underwater sound offers insights into animal behavior. Recent research on the Chobe River in Botswana, a challenging environment with opaque water and abundant wildlife, highlights the importance of sound for understanding aquatic ecosystems. Scientists studied the river's soundscape, including boat noise and wildlife interactions, to aid local communities in managing their environment.

One particularly intriguing area of study involves hippos. These semi-aquatic animals, known for their aggression, communicate both in air and underwater. A key question is how hippos navigate and interact in the murky water where visibility is near zero. There's a hypothesis that they use a form of echolocation.

A scientific paper described an experiment involving captive hippos trained to search for carrots underwater. The study found that hippos produced "clunky clicks" only when actively searching for carrots, not during social interactions or when no carrots were present. This suggests a rudimentary form of echolocation, similar to a human using sound to gauge the shape of a dark room. While not as sophisticated as whale or dolphin sonar, it could help hippos avoid bumping into each other in opaque water.

This research, though challenging to conduct in the wild due to the hippos' aggressive nature, points to the ongoing mysteries of underwater sound. Hippos are the closest living relatives to whales, sharing a common aquatic ancestor and a reliance on sound for navigating their environment. The "devil in the details" of hippo echolocation remains an active area of research.

The Physics of Music: Scales, Tunings, and Harmony

The science of music delves into how sound waves are organized into meaningful patterns that evoke emotion. While often associated with mathematics, the true essence of music lies in its ability to move us.

Notes, Frequencies, and the Octave

Most music is built upon discrete notes, or pitches, arranged in scales. The pitch of a note corresponds to the frequency of its sound wave. For example, concert A vibrates at 440 Hertz (Hz). Higher notes have higher frequencies. While the audible range contains a continuous spectrum of frequencies, music selects only a few discrete steps. This is partly because we need to distinguish notes, but more importantly, it's a cognitive choice: too many notes make music difficult to process.

A fundamental concept in music is the octave. Two notes an octave apart sound like the "same" note, just higher or lower. This is a direct consequence of physics: the higher note in an octave has exactly twice the frequency of the lower note. Our auditory system is wired to perceive this simple 2:1 frequency ratio as a special relationship.

The Harmonic Series and Overtones

The complexity of natural sounds, including musical notes, arises from the presence of overtones. When a musical instrument, like a guitar string, vibrates, it doesn't just vibrate as a whole. It also vibrates in halves, thirds, quarters, and so on. These shorter, higher-frequency vibrations are called overtones and form the harmonic series.

The lowest frequency, the one we perceive as the note's pitch, is called the fundamental or first harmonic. The second harmonic is an octave above the fundamental, the third is a perfect fifth above that, and so on. Our auditory system blends these harmonics together, allowing us to perceive a single, rich note rather than a cacophony of individual frequencies. The prominence of octaves in the lower harmonics explains why our ears readily blend them.

Scales and Tuning Systems

The diatonic scale (major and minor scales) is a common structure in Western music. There's a popular, though often oversimplified, idea that the major scale is "natural" due to simple mathematical ratios between note frequencies. The ancient Greek philosopher Pythagoras is often credited with discovering these harmonic relationships, supposedly by observing blacksmiths' anvils. While the story is likely apocryphal, it highlights the historical link between music and mathematics.

The Pythagorean scale, based on simple ratios like 3:2 for a perfect fifth, works well for certain keys but can sound "out of tune" in others. During the Renaissance, theorists like Gioseffo Zarlino proposed "just intonation," which aimed for tidier mathematical relationships. However, just intonation also suffers from the problem of not translating well across different keys. Playing a piece in a key other than the one it was tuned for can sound jarring.

Attempts to solve this led to instruments with more keys, like Marin Mersenne's 16th-century keyboard with 31 notes per octave, but these were impractical. The ultimate solution for playing in any key without sounding "weird" is the equal-tempered scale. In this system, each step in the chromatic scale (all 12 notes within an octave) is precisely the same, achieved by making the frequency ratio between adjacent notes the 12th root of two – an irrational number.

This means that in equal temperament, many of the "nice" simple frequency ratios of Pythagorean or just intonation are slightly altered. While this introduces a subtle "beating" effect (a rising and falling in volume due to interference between harmonics), our ears have become accustomed to it. J.S. Bach's "Well-Tempered Clavier" famously showcased the possibilities of composing in any key using these new tuning systems.

Consonance, Dissonance, and Cultural Perception

The concepts of consonance (sounding harmonious) and dissonance (sounding clashing) are often linked to the simplicity of frequency ratios. A perfect fifth, with its simple 3:2 ratio, is generally considered consonant. However, the "diminished fifth" (or tritone), like C to F-sharp, is often described as highly dissonant, even earning the moniker "diabolus in musica" (the devil in music).

Yet, the perception of consonance and dissonance is also heavily influenced by culture and familiarity. For instance, the major third, now a staple of Western harmony, was considered dissonant in the Middle Ages and avoided in sacred music. Our modern ears find it perfectly acceptable.

The German physicist Hermann von Helmholtz, in the 19th century, provided an objective way to calculate acoustic roughness, a measure of sensory dissonance. His work showed that while octaves and fifths indeed stand out as particularly consonant, the "terrible" tritone is not significantly more dissonant than many other intervals. Its demonization was more due to theoretical issues in Pythagorean tuning than its inherent sound.

Ultimately, much of what we experience as "good" or "bad" in music is shaped by our cultural exposure. While some fundamental acoustic principles, like the octave, are universal, the specific scales, tunings, and harmonic preferences vary widely across cultures. Indonesian Gamelan music, for example, uses scales like Pelog and Slendro, which divide the octave differently from Western scales, demonstrating that there is no single "natural" way to organize musical pitches. This cultural variability encourages us to explore and appreciate the diverse musical traditions of the world.