|MadSci Network: Physics|
Hi! It seems I'm getting all the "speed of sound" questions. Below you find two links to the MadSci archive describing one particular method of measuring speed of sound in air, the "Kundt tube": http://www.madsci.org/posts/983199157.Ph.r.html http://www.madsci.org/posts/archives/jul2000/964817012.Ph.r.html For science classes this experiment is the easiest to perform and the most instructive. You will find that the Kundt tube experiment has two disadvantages namely that it is only practical in air and that it measures the speed of sound inside a tube, which is not necessarily the same as the speed of sound in free air (though the difference is admittedly small). The latter is actually a basic problem in determining the speed of sound in general. It is not only dependent upon the medium (and the state it is in) but also on the shape of the medium. This is particularly tricky when the wavelength is of the same order as the smallest dimension of the medium. Conversely, the speed of sound is wavelength dependent. Something like the kundt tube experiment can be used to determine the speed of transversal waves in solids. Such waves are found on metal plates which are made to resonate. For example, ever tried playing a cymbal using a violin bow? If you put some dry sand on the cymbal the standing waves generating the shrieking sounds will be visualised by the sand settling down on the nodes. For longitudinal waves in liquids and solids it is not easy to visualise standing wave patterns. Some indirect way of detection must be used instead. A bar of iron can be struck with a hammer on one end and the ringing requency measured. For liquids I can't really think of an easy way. Probably the most generally applicable method of measuring the speed of sound is the direct method: apply a pulse (a very short burst of energy) and measure at some distance of the source how much time it took the impulse to make its way there. To eliminate measurement errors caused by the unknown delay in the source you might use two identical sound transducers (for gases: microphones, for liquids: hydrophones, for solids: accelerometers) spaced a known distance apart and measure the time difference between those two. It be said that such a setup is quite specialised and expensive. Conclusion: Practical experiments not involving precision measurement equipment rely on creating and detecting standing waves in the medium to be measured. From there, the speed of sound can be calculated. Regards, Bruno
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