Free The Sound Book: The Science of the Sonic Wonders of the World by Trevor Cox Page B

wineglasses ring when gently tapped. But rest a finger on the edge of the glass, and the sound disappears almost immediately. The friction between the glass and the finger dampens the vibrations and prevents the ringing. For rocks, the damping comes from the internal structure of the stone rather than your finger.
    In 2010, I interviewed violin manufacturer George Stoppani for a BBC radio program about how to choose the right wood for the best-sounding violin. He went around his dusty workshop tapping pieces of wood to allow me to hear the different sound qualities. Only wood with the right grain density and microscopic structure produces a clear tone, which rings on for a few seconds—evidence that it can be used to make a world-class violin. It is similar with rock. 31 Within the stone, vibrations are being passed from molecule to molecule. If there are any cracks or hairline fractures, then it is more difficult for the vibration to travel within the rock and the stone will ring less well. In the age of steam, wheel tappers working on the railways exploited the same principle, checking mechanical defects invisible to the naked eye by tapping the wheels of the trains with a small hammer. Lack of a satisfying ring indicated cracks, which could lead to a catastrophic failure of the wheel. But there is more to this than just cracks. Hit a piece of sandstone and it will not ring, whereas a piece of slate, like those I played at the Keswick museum, can impersonate a gong. Both stones originated from layers of sediments, but slate has been transformed by hundreds of millions of years of pressure into a denser material with a more ordered molecular structure. Vibrations can pass more easily between the neatly arranged molecules in slate than between the loosely packed grains of sand in sandstone.
    M y wife likes to have long phone calls while wandering around the house. As she walks between rooms, her voice changes in fascinating ways, both for her family in the house and for people at the other end of the phone line. Her voice is stronger and harsher in the kitchen because of the hard, reflective tiles and flooring, and clearer and warmer in the living room with soft furnishings, which deaden the sound. The microphone in the handset is picking up a mixture of the sound that travels straight from her mouth and the reflections bouncing off the walls, floor, ceiling, and objects in the room. She cannot sneak into the bathroom during a phone call with me because the bright reverberation is a dead giveaway. Size also matters: larger rooms tend to create a livelier, booming sound.
    Now imagine you are prehistoric person wandering around a dimly lit cave system. Your voice will alter as you move from cavern to cavern, through narrow entrances and down tortuous tunnels. The sound quality varies because of the changing patterns of reflections from the rocks. In large caverns a booming reverberance might be heard, in extreme cases mimicking the sound of a church. But in smaller caverns and tight squeezes, the key acoustic effect is coloration .
    An old staff room at my university had an amazing ability to color sound. It was a plain, narrow, rectangular room with chairs lined up on either side; it was like a waiting room at a train station. The first few times I went into the room I noticed a strange distortion as other people spoke. Moving my head back and forth dramatically changed the timbre of my colleagues’ voices. With my head in one position, their speech sounded very bassy and powerful, but elsewhere their voices went all tinny, distorted, and horrible. Colleagues probably wondered if I had been drinking, as I gently swayed back and forth listening to our lunchtime conversations, scientific curiosity trumping self-consciousness.
    As I moved my head from side to side, voices in the room changed as if someone was rapidly altering the settings on a hi-fi’s graphic equalizer. This coloration was caused by a change in the