The Science of Silence
How Sound Moves Underwater & Why It Matters
🔇The Silence
Three weeks ago, I was diving 30 meters below the surface of the Philippines’ Tañon Strait, feeling quasi-weightless and fully immersed in the sights from my vantage of the slight current. A green turtle lazily paddling by over here, little clownfishes guarding their anemone nests over there. Then—a distant roar. Low, steady, and growing louder. I glanced over each shoulder in turn and straight overhead. Nothing.
My dive buddy did the same. Still nothing.
A boat was undeniably cruising somewhere above, but down here, sound was moving far faster than in air. The minuscule time delay between our ears that helps us determine direction on land is almost nonexistent underwater, rendering it virtually impossible to identify exactly where a sound originates. For a diver, this isn’t just a weird phenomenon—it can be a genuine safety concern, particularly if you’re relatively close to the surface.
Because imagine, all of a sudden, a boat running over you. I picture it almost any time I hear a boat overhead, regardless of my depth.
Now, imagine you’re an orca.
🧬 The Science Behind Underwater Sound
As I recently worked through PADI’s Dive Theory course, I found myself drawn most to the physics, physiology, and environment sections. Learning how the aquatic realm affects the human body is intriguing on its own, but even more so by illustrating how much we influence it in return.
Sound behaves differently in mediums of varying densities. As a general pattern, sound waves travel through solids faster than through liquids, and through liquids faster than through gases. Because seawater is much denser than air, accordingly, sound waves travel in the ocean nearly four and a half times faster than at the surface, reaching distances far greater than we can comprehend on land.
For marine life, this means sound is a vital tool of communication, navigation, and survival. Sperm whales use powerful clicks to echolocate over massive distances. Fish like the Atlantic croaker produce drumming sounds to warn each other of predators. Pistol shrimp, for their part, create a loud snap! (218 decibels loud—twice as much as a jackhammer!) with their specialized claw, producing a shockwave strong enough to kill small fish.
But these sounds are all short and infrequent. Contrary to what Jacques Cousteau described in Le Monde du Silence, human activities like shipping, deep sea drilling, and, yes, diving-related boating create a relentless hum that disrupts natural soundscapes. Unlike physical pollution that we can see, we often find noise pollution easy to ignore… unless we’re the ones submerged.
This, of course, becomes immediately apparent for divers. When a boat passes overhead, its rumble doesn’t just come from above—it seems like it’s everything everywhere all at once. You see fellow divers with their heads on a perpetual swivel, scanning for the source of the noise until an elliptical shadow approaches and eventually, gratefully, retreats to the other side. It’s not that we’re panicked. We simply care about our own self-preservation in an environment where sound clues don’t behave the way we ordinarily expect.
🐳 The Orca Connection: A Crisis Close to Home
I felt an immediate connection while reading this article from SeaLegacy about the threat of underwater noise pollution to orcas in the Pacific Northwest. I call Portland, Oregon home, so it’s happening in nearby waters. Plus, it has been a common topic of conversation with one of my students whose passion for cetaceans has piqued my own interest about their challenges.
Aligned with characteristics of all orca groups, the endangered Southern Resident Killer Whale (SKRW) population in the Salish Sea rely on echolocation to hunt and communicate. But unlike other orca populations that hunt marine mammals, the Southern Residents rely almost exclusively on Chinook salmon, itself a species that’s facing the dual threat of overfishing and habitat destruction.
Now, add noise pollution to the situation.
The waterways between British Columbia and Washington are crisscrossed with busy shipping lanes, ferry routes, and recreational boating activity. Low-frequency vessel noise drowns out the orcas’ clicks, making it harder for them to find already scarce prey. If an orca can’t hear well enough to hunt, it can’t eat. If it can’t communicate with its pod, it loses its remarkable, collaborative hunting advantage.
Scientists have recorded drastic changes in orca vocal behavior in response to incessant noise. According to the article, the SKRWs are increasing the volume of their calls, essentially shouting, while others are reducing communication entirely—perhaps because it’s too difficult to make themselves heard. imagine attempting to have a conversation in a room where that jackhammer from earlier was constantly running. I’d give up immediately.
Shipping is one of the primary contributors to undersea noise pollution. Photo by author.
🔮 What This Means for the Future
Experiencing omnipresent sound as a diver can be unsettling for those otherwise blissful 50-60 minutes of bottom time. But for marine life, it’s persistent and life-threatening. Fortunately, science, conservation, policy, and other fields are working on ways to mitigate noise pollution and restore some balance to the marine world.
Some solutions include:
Quieter ship designs: Engineers are developing modified propeller and hull designs that cut through water more silently.
Speed limits in marine mammal areas: Reducing vessel speeds can significantly lower noise pollution and decrease the risk of boat strikes.
Designated marine protected areas: Certain areas legislatively limit industrial activity and excessive boat traffic.
Public awareness campaigns: Divers, boaters, and coastal communities are beginning to take simple steps, like avoiding known marine mammal habitats and using quieter electric boat motors.
As divers and ocean lovers, we have a role to play, too. We can support policies that protect marine soundscapes, choose eco-conscious diving and tour operators while traveling, and spread awareness of the issue—much like this LinkedIn article.
📻 Tuning In to the Ocean
Diving has given me such an acute appreciation for the aquatic realm and continuously makes me more aware of the ways in which human activity shapes it, often in ways we don’t hear.
The next time you find yourself near (or in!) the ocean, I invite you to pause for a moment and listen. Really listen. And think about the blue 71% of the planet. What might it be like to navigate it by primarily using your sense of sound? How can we listen better—to each other and to the ocean?
A tube sponge enjoying sporadic sunlight. Photo by author.