This book is a good example of “think ink”: it makes you think about familiar things in a new way. Sound is familiar to almost everyone, but it’s a much more complex phenomenon than most people realize. Only part of it takes place in the air; there’s a lot going on in the brain too. Trevor Cox looks at both sides of sound. And more sides than that, because it’s a three-dimensional phenomenon. And four-dimensional: time is involved.
That’s most obvious with echoes, which Cox explores in chapter 1, “The Most Reverberant Place in the World”:
I like to play a game with my fellow acousticians: guess the reverberation time. They usually pick an acoustically outrageous number, maybe 10 or 20 seconds. Even so, they always guess far too low. At 125 hertz, the reverberation time was 112 seconds, almost 2 minutes. Even at mid-frequency the reverberation time was 30 seconds. The broadband reverberation time, which considers all frequencies simultaneously, was 75 seconds. I called Allan over to give him the good news. We had discovered the world’s most reverberant space. (ch. 1, pg. 57)
And where is that? It’s the “oil storage complex at Inchindown, near Invergordon”, in Scotland. Huge tanks, in other words, that were built in the 1930s to store oil for a nearby naval anchorage. They’re “dug deep” into a hillside and now are decommissioned, so they’re empty, dark and rarely visited. But, as Cox discovered, they’re also acoustically fascinating: “Never before had I heard such a rush of echoes and reverberation” (pg. 55).
As professor of acoustics at Salford University, Cox is well-equipped to investigate and analyse places like that, but sound is too big a subject for any individual to understand everything. This book discusses a startling range of animals, objects and acoustic phenomena: bats, moths, barking fish, claps, bells, whispering galleries, echoes that conduct conversations, musical roads, icosidodecahedral loudspeakers, wave organs, abandoned radomes, waterfalls, singing sands and ringing rocks. Last and least, it discusses silence:
The ear is exquisitely sensitive. When perceiving the softest murmur, the eardrum barely moves. For the quietest sound that a young adult can hear, the eardrum vibrates by less than one-tenth of the diameter of a hydrogen atom. Even in silence, tiny vibrations of molecules move different parts of the auditory apparatus. These constant movements have nothing to do with sound; they stem from random molecular movements. If the human ear were any more sensitive, it would not hear more sounds from outside; instead, it would just hear the hiss generated by thermal agitation of the eardrum, the stapes bone of the middle ear, and the hair cells in the cochlea. (ch. 7, “The Quietest Places in the World”, pg. 209)
I’m not sure about “one-tenth of the diameter of a hydrogen atom”, which seems very small, but measurement is certainly essential to acoustics. Modern instruments can turn sound into shape, which is appropriate, because sounds are essentially shapes. You might call them sculpted air, just as waves are sculpted water. And light is sculpted energy. The parallels between light and sound weren’t obvious to the ancients, but they explain lots of things, such as why some frequencies of sound, like some frequencies of light, can travel further than others through air or water or stone.
And the visual phenomenon of iridescence has its acoustic equivalent: sonic crystals, which “reflect some frequencies intensely, mimicking the iridescence of butterfly wings” (ch. 8, “Placing Sound”, pg. 256). Unfortunately, Cox says that the sound is “unpleasant”. But this is a new field and perhaps beauty will be born in time. Ugly or exquisite, thunderous or threnodic, sound is a fascinating subject and this book will open both your ears and your mind to its wonders and weirdness.