The not-so-silent seas

 kw: book reviews, nonfiction, science, oceanography, acoustics, bioacoustics, sound, noise pollution

A number of years ago at the beach at Cape Henlopen, Delaware, a pod of porpoises came by, a hundred yards or so offshore, as they usually do in the afternoon. This particular day a couple of busloads of Amish people from the Lancaster, PA area had been brought for a day at the beach, and I was standing about chest-deep in the water among a dozen or so Amish teenagers. As I like to do when the porpoises swim by, I ducked my head under water to listen to them whistle and chirp. It is a sound I love. I liken it to the social honking of a V of geese flying overhead.

I said to the nearest Amish boy, "Try putting your head under water. You'll hear the porpoises." He tried it, and then shouted something in German to the other kids. Soon they all were bobbing up and down and chattering among themselves about this novel experience.

As a teen I often watched the Flipper TV show, centered on a bottlenose dolphin (the "poster child" of porpoises). I was familiar with the chatty noises the animal could make. A few years later I was enthralled to listen to the album Songs of the Humpback Whale. As an avid reader of science articles and books, I've kept up in a general way with the findings of bioacoustics about the whole range of animals that make sounds under water, from snapping shrimp and mantis shrimp to the fish called Grunts—and many others that do more than grunt—to the mammals, particularly whales (I count porpoises as small whales).

The title Sing Like a Fish: How Sound Rules Life Under Water was a slam-dunk for me. The reading was as enjoyable as I'd anticipated. The author, Amorina Kingdon, muses in the Epilog whether it was worthwhile to devote two years of her life to the book and its research. She concludes a resounding Yes, and I agree. Who else could do this? Another might write on the same subject, but Ms Kingdon's voice is unique: heartfelt, lyrical and eloquent.

Early in the book we learn why we've ignored the underwater soundscape for so long. Our ears don't hear that well under water. In the age of sail, many sounds were heard by sailors trying to sleep below decks. When there is little wind, a sailing ship is so silent, the sounds of the sea that make their way through the wooden hull are quite evident. These days, professional sailors would be hard pressed to hear anything coming through the hull of a noisy, engine-driven ship. Those who never go to sea, who experience it only from above, have no idea how sound-filled it is. Early SCUBA divers, with Jacques Cousteau in the lead, hampered by air-adapted ears and the noisy SCUBA apparatus, heard so little that they produced a documentary film about the ocean titled The Silent World, about ten years before the first episode of Flipper aired. As I recall, the main sound heard under water in that film is the noise of the SCUBA. I wonder if Cousteau ever learned that fish have songs.

Although I wanted to learn SCUBA diving, I wasn't willing to pony up for expensive diving lessons and equipment. I snorkeled instead; I could surface dive to 40 feet without much trouble, and once went 60 feet. Snorkeling can also be noisy if you never leave the surface; we breathe rather loudly. But if you pop a surface dive down even 10 feet and just hang there, gradually the sounds will become evident. In my case, though, I dove offshore of tide pools, to see what animals were on the rocks. Most of the sound was wave noise, but I did hear a little of the fishy chorus around me: clicks, squeaks, and mournful groans. Better equipment has helped us realize that fish sing! Not just whales.

Once the hydrophone was invented, the soundscapes of the seas came into sharper focus (Hmm, bit of a mixed metaphor there; I guess you can figure it out anyway). Much of the book is devoted to surveying the breadth of underwater animal species that detect sound, hear sound (different senses), and communicate using sound. In one place it is stated that a catalog of fish sounds contained around 900 species. Compare that to the 94 species of cetaceans (whales of all varieties), all of which can vocalize. Though there are about 34,000 species of fish, the 900 "cataloged" to be vocal simply reflects the very early stage of "auditioning" the world's census of fish. I don't recall reading about any fish that were tested, that they were unconditionally silent, so perhaps I can fairly conclude that most, or nearly all, fish will be found to not just receive sound but to use it.

Certain invertebrates also make sounds intentionally. Snapping shrimp and mantis shrimp make noises loud enough to be weapons, though a mantis shrimp mainly relies on the fastest punch in the animal kingdom as its primary weapon. Considering the wide range of insects that make noises to communicate and court, it's reasonable to assume that many of the active invertebrates also use sound. I immediately thought of scallops, which make a clapping sound when fleeing danger, but in their case I think the sound is incidental to the "jet propulsion" their clapping produces.

Sadly, there is a new player on the soundscape. Us. Ship engines are noisy. The propellers ("screws") are noisier. Sonar is noisy. Seismic surveying is incredibly noisy. The racket of pile driving to install offshore wind turbines drives whales and fish crazy and can kill them. The low rumble of the turbines once they are operating is a chronic noise that drives migrating animals to change migration routes, and hinders the feeding activities of many animals that need to hear their prey. So does the persistent growl of ships' propellers and the hammering of depth-finding Sonar. A couple of chapters are devoted to describing all the problems that sea animals are having with the sounds we make while we use the oceans for commerce and entertainment.

The book ends on a hopeful note regarding the confluence of regulation and public attitudes that can result from wise use of scientific data being collected right now. We know we can't protect everything, but if we know certain areas that are most sensitive—such as a special bay where certain whales raise their calves—we can focus our efforts where they are more effective.

Just in case you're wondering: This sonogram illustrates why we call the sounds made by humpback whales "songs":

For reference, middle C on a piano has a frequency of 261 Hz. 200 on these diagrams would then be near the G below middle C, and it happens to be the natural frequency of my speaking voice. To sing along with this whale, just follow the lowest, darkest line of each group. The highest blip on the second track (~700 Hz) is almost an octave higher than the highest note I can sing, which is an A with a frequency of 440 Hz. Most women can just reach the next A at 880 Hz, so a woman can sing with this whale, but not even a low-voiced man can reach the low tones near 50 Hz seen in much of the second track. This whale has a very wide vocal range!

But why are these a song rather than tuneless noises such as some people make when concentrating on whittling or something similar? In a word, structure. Phrases and shorter elements (words?) repeat and are sometimes repeated with variations. There is something intentional going on. One verse of a typical popular (human) song lasts from half a minute to a minute. Most song tracks on your playlist are 3-4 minutes in length. A typical section (verse?) of a humpback song lasts a couple of minutes, and such verses are grouped into soliloquies lasting on average 8 minutes, but the whale may sing the song, over and over again, with variations, for an hour, or even a day. Maybe a whale is a philosopher, and this is his way (only males sing) of thinking out loud. Or maybe these are courting songs, which the whale will repeat until a female responds, perhaps from miles away, and joins him.

A book like this reminds us of what we don't know, and hints at the level of effort we'll need to exert to find out what the natural world contains, while it is still there. That's the author's aim. The fish are singing, for their own reasons. But now we are able to hear. Will we value their songs enough to preserve them, not just as recordings, but by preserving the singers?

Trajectory of the soundscape

 kw: book reviews, nonfiction, science, acoustics, sound, noise pollution, silencing

Summer nights we hear mostly katydids, my wife and I. In late spring, there will be a few. By late July there are myriads, and they out-shout the annual cicadas ("locusts" to some). Just by listening through a slightly opened window we can tell how warm it is outside: at a comfortable temperature, they sing their usual 3-part song, "katy-did"; when it is uncomfortably warm for me, we hear "katy-didn't"; cooler temperatures elicit only a 2-part song ("katy") and the coolest temperatures at which they will sing (around 60°F) they just burp "kate!".

The earliest noisy ancestors of katydids lived about 350 million years ago, about twice as long ago as the earliest dinosaurs. Animals probably made inadvertent sounds as far back as a billion years or more, but we don't know of specific hearing structures prior to about half that.

We learn the history of sound and sound-making by living beings in the early chapters of Sounds Wild and Broken: Sonic Marvels, Evolution's Creativity, and the Crisis of Sensory Extinction, by David George Haskell. This book is lyrically written, celebrating the sounds of living things of every kind. The author also laments the gradual silencing of the natural world, and then goes on to point out the ways dominant human cultures silence others.

Many of the sounds animals make go unheard by humans. Firstly, the sonic realms we call "ultrasonic" (frequencies above about 20,000 Hz, such as the laughter of mice or the echolocation beeps of bats) and "infrasonic" (below about 15 Hz, where most elephant-speak occurs) cannot be perceived with our natural auditory systems. Secondly, we are very badly equipped to hear with our head under water, although we can hear some louder sounds such as the whistles and creaky-door sounds of nearby dolphins. But when out of water, we hear nothing: sounds made underwater stay there (kinda like "staying in Vegas").

The ears of land animals are elaborate transducers between airborne sound and the liquid environment in our inner ears. Most hearing is mediated by ciliated cells, which first evolved 3+ billion years ago, in the oceans. They are most sensitive to waterborne vibrations.

We humans go to great lengths to have quiet living spaces. I live in a suburb that is noted for being very quiet, at least from human noises. We do hear, blocks away, the occasional siren, and a bit of road noise. Otherwise, we hear a small amount of birdsong and insect noises. Bird song here is less by far than what I heard as a child in the 1950's and 60's. As I wrote above, summer nights can be noisy with insect sounds. Not just katydids but several kinds of crickets. Once I put a small microphone at the focus of a curved reflector taken from a desk lamp, and went about the yard pointing it here and there. I was amazed at the multitude of very high-pitched sounds, like crickets but 2-3 octaves higher. Being on the edge of hearing anyway, these sounds are usually too faint to hear.

But not that many of us have the luxury of a quiet home. Cities are noisy. VERY noisy. As the author brings out, the historical development of cities has resulted in poorer areas being the noisiest. It's almost an exact relationship. By that measure, the oceans are becoming impoverished. Human noise fills the seas. If the designers of ships' propulsion systems were required to spend a cruise wearing earphones attached to hydrophones placed on the ship's keel, they'd probably be permanently deafened within a day by the noise from the screws (propellers); I hope then they'd me more inclined to design quieter systems in the future! A single oil tanker or container ship, powered by multiple motors the size of houses, makes as much noise as a city. Multiply that by about 6,000 mega-ships, plus another 60,000 "smaller" commercial vessels afloat at any one time, and you have a sonic apocalypse. It is growing steadily. No wonder whales aren't rebounding that much. They did in the past—just for the past few decades—, but that has leveled off as our commerce fills their world with literally deafening racket.

This big book (400+ pages) is filled with wonderful information I've skimmed past or skipped here. We need sound, and we need a balance of louder and quieter places in our life; and so it is for all creatures. The sounds of animals are so many and varied and amazing, it is good to be able to hear them. That includes, not just taking an evening stroll amidst the thrums of katydid and cicada and cricket; not only going outside at dawn for the dawn chorus of bird song; even going to Cape Henlopen or Cape May when dolphins are passing and going under to listen to them. It includes being attentive to what is around us, not just sights but sounds and even smells.

I don't know what else to say about this amazing book. It's big, and I could have done with an even bigger book.

It can drive you nuts, and it can help you hear

kw: book reviews, nonfiction, acoustics, signal processing

NOISE by Bart Kosko is the most intensely mathematical book I've yet seen at the local library, outside the Reference section. The author is a professor of Electrical Engineering, a popular author, and judging from the diversity of his formal education, a polymath. Judging from the text, he is also an autodidact in a number of areas.

I'll begin with a quibble: Dr. Kosko has the teacher's habit of repetition—we all know our students don't get it the first time—, but takes it to an extent I found tiresome. There is a little English bird named the great tit (Parus major) that is an easy subject, so it is studied extensively. A number of studies have shown that it sings on a higher pitch in the presence of industrial noise. According to the index to the book, this bird appears three times. The indexer missed the other four times the story is repeated. Dr K? You had me by the second time. Your meme has now successfully reproduced!

Noise has a Jekyll-and-Hyde quality. It helps and it hurts. I have tintinnitus (I think tinnitus is the more modern term) in both ears, probably due to mowing too many lawns without hearing protection. I've never used firearms or listened to loud music; my brother, once a rock drummer, got his hearing problems the musical way. I found out something about tinnitus, when I had physical therapy for a sore neck. Certain head positions caused the ringing to get much louder. I went to an audiologist. He said the cause was that, moving the head that way triggered the "too loud" muscle that closes or narrows the ear canal in response to loud sounds (drummer's earache, my brother called it, when it went on too long and got sore). Tightening that muscle increases the resonance inside the ear, making the tinnitus louder. Tinnitus due to hair cell damage is an actual sound produced in the cochlea; a sensitive microphone can record it.

When I nap, as distinct from sleeping at night, I usually find that turning on the radio so I can barely hear it helps me nod off. It isn't quite white noise, but it masks a lot of other sounds, and its familiarity seems to help. Some noise is helpful. Indeed, noise of my choosing may be sufficient to let me rest when a neighbor is using a lawn mower or stereo up too loud. The author goes into legal implications of noise and noise ordinances, showing his familiarity with law (as it turns out, that's one of his degrees).

Low levels of noise also help detecting certain signals. This works only in nonlinear systems, but I have yet to find a truly linear system in nature. I've also occasionally heard a third tone when a friend and I whistle two loud, different tones. The third tone is always lower, the subtraction of the two higher frequencies. Theoretically, a fourth tone, the sum frequency, is also present but I haven't heard it. This phenomenon arises from nonlinearity, which is slightly present in the air transmitting the sounds, but is much greater at the air-eardrum interface, and perhaps equally so in the drum-stirrup-anvil-cochlea chain.

Anyway, this nonlinearity means a faint sound could be more discernible in the presence of an even fainter hissing noise ("white" noise). I suspect the tinnitus I already have would overcome both signals, so someone else will have to do the experiment.

In the realm of signal processing, myriads of experiments have been performed, as Dr. Kosko writes. He goes quite deeply (for me...and I am a mathematician by trade!) into the math of noise statistics and signal-noise convolutions. One aspect from which I learned much was the diversity of "bell curves". Like most classically-trained statisticians, the only bell curve I knew was the Gaussian normal. Of course, I know quite a variety of bell-like distributions (Weibull, Lognormal, Logistic, and a number of others). However, only the Normal curve has a related central limit theorem. Thanks to this book, I learned that there are a number of central limit theorems, leading to a family of symmetric bell curves, of which the Gaussian is on end-member. There were hints of other families thereof, not as relevant to signal processing.

All of these bell curves apply to the analysis of white noise, which is noise with a flat spectrum. Of course, an entity with a truly flat spectrum from f=0 to infinity is impossible, because the energy required is infinite for any nonzero signal level. In real systems, a signal can have a flat spectrum over a range of interest, but fall off at higher frequencies. Any "near-white" signal will be very jittery, but the shape of the jitteriness can vary. Thus, "well-behaved white noise" with a uniform range of excursions in sound pressure will sound like a steady hiss, "gaussian" noise with a larger number of small excursions and fewer large ones will sound crackly, and more leptocurtic distributions of sound pressure such as "cauchy" noise sound like popcorn over a fainter hiss.

Much more common are pink noise, more rumbly because the sound power at higher frequencies falls off steadily as 1/f. I suspect real "pink" noise is more like lognormal noise, with the lowest frequencies sharply attenuated, but a 1/f response above a low-frequency mode. I learned of brown noise, whose frequency spectrum is essentially the square of pink noise, and black noise, which is cubic or higher. I suspect black noise sounds a lot like an earthquake rumble, or is perhaps felt but barely heard. (NOTE to self: transduce pink, brown, and black signals into WAV files to see how they sound).

Stephen Hawking was once told that each equation in a book cuts the potential audience in half. If this were true, NOISE would have an audience of one or fewer. But the text is actually quite readable, and I can attest that, if one simply skims over the equations and scans the accompanying explanations, sufficient understanding results.

A large part of the book illustrates and explains stochastic resonance, the enhancement of detection that faint noise can confer on small signals. I can add an example. Astronomers have been taking advantage of this for decades. When recording a negative, an astronomer will balance the exposure time so that sky brightness (which is never zero) produces a density of 0.5. This means there is a background speckly gray caused by several percent of the silver grains.

It is known that for the best astro emulsions, it takes ten or more photons to make a grain "convert". If there is a faint nebula in view, so faint that only one or two photons will strike the average grain during the exposure, then statistically a larger proportion of grains will "convert" because of the added contribution of the sky brightness, which is noise to the uninitiated. The nebula would be undetectable in a much darker sky without a much longer exposure (which would then of course be possible).

The author shows how stochastic resonance works for "linear" signals and for images, and how it seems to help our neurons do their job better. He speculates that, though noise probably isn't the cause of life, it probably makes life possible.