|
|
|
 |
 |
Good Vibrations |
Curious investigators long have been fascinated by sound and the way it travels in water. As early as 1490, Leonardo da Vinci observed: "If you cause your ship to stop and place the head of a long tube in the water and place the outer extremity to your ear, you will hear ships at a great distance from you." In 1687, the first mathematical theory of sound propagation was published by Sir Isaac Newton in his Philosophiae Naturalis Principia Mathematica. Beginning in the mid-seventeenth century investigators were measuring the speed of sound in air but it was not until 1826 that Daniel Colladon, a Swiss physicist, and Charles Sturm, a French mathematician, accurately measured its speed in water. Using a long tube to listen underwater (as da Vinci had suggested), they recorded how fast the sound of a submerged bell traveled across Lake Geneva. Their result--1,435 meters (1,569 yards)  per second in water of 1.8 degrees Celsius (35 degrees Fahrenheit)--was only 3 meters per second off from the speed accepted today. What these investigators demonstrated was that water--whether fresh or salt--is an excellent medium for sound, transmitting it almost five times faster than its speed in air!
But how does sound travel? Sound is a physical phenomenon, produced when an object vibrates and generates a series of pressure waves that alternately compress and decompress the molecules of the air, water, or solid that the waves travel through. These cycles of compression and rarefaction, as the decompression is called, can be described in terms of their frequency, the number of wave cycles per second, expressed in Hertz. The human voice, for example, can generate frequencies between 100 and 10,000 Hertz and the human ear can detect frequencies of 20 to 20,000 Hertz. Dogs and bats are examples of many creatures that can hear sounds at much higher frequencies--up to 160,000 Hertz. Whales and elephants, at the other end of the spectrum, generate sounds at frequencies in the range of 15 to 35 Hertz, mostly below human hearing and thus called subsonic, or infrasonic. Sound waves, like light waves, also can be described in terms of their wavelength, the distance between the peaks of two waves; the lower the frequency, the longer the wavelength.
In 1877 and 1878, the British scientist John William Strutt, third Baron Rayleigh, published his two-volume seminal work, The Theory of Sound, often regarded as marking the beginning of the modern study of acoustics. The recipient of the Nobel Prize for Physics in 1904 for his successful isolation of the element argon, Lord Rayleigh made key discoveries in the fields of acoustics and optics that are critical to the theory of wave propagation in fluids. Among other things, Lord Rayleigh was the first to describe a sound wave as a mathematical equation (the basis of all theoretical work on acoustics) and the first to describe how small particles in the atmosphere scatter certain wavelengths of sunlight, a principle that also applies to the behavior of sound waves in water. |
|
|
|
|
|
|
