![]() ![]() Alternatively (and usually more accurately), one can excite resonant frequencies of the tube (much like those of a flute) by inducing a vibration at one end with a loudspeaker, tuning fork, or other type of transducer. One can then derive the speed of sound from a measurement of the time that an impulse of sound takes to traverse the tube. These experiments often use tubes of gas or liquid (or bars of solid material) with precisely calibrated lengths. These measurements all suffered from variations in the media themselves over long distances, so most subsequent determinations have been performed in the laboratory, where environmental parameters could be better controlled, and a larger variety of gases and liquids could be investigated. They found a value only 0.2 percent below the currently accepted value of ~1,440 m/s at 8 degrees C. ![]() Daniel Colladon and Charles-Francois Sturm first performed similar measurements in water in Lake Geneva in 1826. Early experimental values were based on measurements of the time it took the sound of cannon blasts to cover a given distance and were good to better than 1 percent of the currently accepted value of 331.5 m/s at 0 degrees Celsius. The first known theoretical treatise on sound was provided by Sir Isaac Newton in his Principia, which predicted a value for the speed of sound in air that differs by about 16 percent from the currently accepted value. In fact, one can turn such measurements around and actually use them to determine thermodynamic properties of the medium (the ratio of specific heats, for example). ![]() These parameters thus need to be included in any reported measurements. Because sound requires a medium through which to propagate, the speed of a sound wave is determined by the properties of the medium itself (such as density, stiffness, and temperature). ![]() In its simplest form, sound can be thought of as a longitudinal wave consisting of compressions and extensions of a medium along the direction of propagation. Although the two phenomena share these measurement approaches, the fundamental differences between light and sound have led to very different experimental implementations, as well as different historical developments, in the determination of their speeds. (The frequency of a wave is the number of crests that pass per second, whereas the wavelength is the distance between crests). The second method makes use of the wave nature common to these phenomena: by measuring both the frequency (f) and the wavelength () of the propagating wave, one can derive the speed of the wave from the simple wave relation, speed = f×. The first method is based on simply measuring the time it takes a pulse of light or sound to traverse a known distance dividing the distance by the transit time then gives the speed. Chris Oates, a physicist in the Time and Frequency Division of the National Institute of Standards and Technology (NIST), explains.ĭespite the differences between light and sound, the same two basic methods have been used in most measurements of their respective speeds. ![]()
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