For each body, there exist some natural ways in which it can vibrate. The exact vibration state is determined by how much of each normal mode of the body the initial impulse "awakens". Macroscopically, we see this as propagation of sound through the material. When struck, the atoms/molecules are set abuzz gyrating slightly about their equilibrium positions. Things which are nearly but not perfectly rigid are capable of vibrating. This is characterized by the moduli of elasticity of these bodies. A very nearly rigid body or elastic is the one which can be slightly deformed under applied stress(~force).
An absolutely rigid body comprises of constituent atoms/molecules that absolutely don't budge from their initial positions. Here are some generic images illustrate the point: Its the pressure variation which travels away from the fork towards the listener. These generates local pressure variations which are what we call rarefactions and compressions. So the rapid movement compresses and "stretches" the nearby air volume. Once the prong starts vibrating at a fixed frequency, it moves rapidly towards and away from its nearby air molecules. In fact the prong would vibrate in pretty much the same way in vacuum too. You are right in thinking that the second prong, the one not struck, is not set in motion by the interveining air. Infact you don't even need the second prong(buzz of fly wings for example). Your emphasis is on explaining compression and rarefaction-for this how the tuning fork reaches its equilibrium vibrational motion isn't important. The exact mechanics of how the tuning fork vibrates is complicated*-however once set vibrating, the equilibrium motion is easy to understand-the to and fro movement of tuning fork prongs. I would really appreciate any diagrams to help my understanding too if possible however my ability in physics runs out after this thought! My second thought is that perhaps the vibrations are induced by the wave moving up and down the stem and not the air particles. So how do the tines end up moving together and apart (out of phase)? My assumption is that this would increase the pressure in-between the tines and move the second tine outwards but this would mean the tines are moving in phase. When the first tine is struck against an object (table) it would move towards the second tine. I understand the vibrational motion of the tines once the tuning fork has been struck however I'm confused with how it starts. My example of choice is to explain compression and rarefaction using a tuning fork example (seeing as this is a simple device that musicians will be familiar with). that might be a way.I'm currently writing a book on music theory and I'd like to include some background information on the physics of sound waves. So they wonder how they cut those stones, etc etc. So they had that thing that when the tuning fork end was put to resonate, the hole thing would resonate like pushing a child on a swing. on the other end of the rod they had other shape that it would probably vibrate at the same frequency. The old egyptians had some staffs called "was scepter" that is a long metal rod (which if made in steal would have a longitudinal natural vibration of about 4~5 KHZ) and in one end of the rod they had a tuning fork, with the prongs short enough to vibrate at about 4~5 KHZ. The most wonderful about the tuning fork is that dampening the longitudinal wave in the handle does not stop the tuning fork oscillation. That is why you put the handle over a surface and can hear the sound much better, because the tuning fork sends the longitudinal wave through the handle. So in the middle of the "U" shape of the tuning fork the two waves collide and make a standing wave which travels down through the handle.
But the thing is that when something bends it is doing it like a wave. The 2 prongs are in a transversal movement (bending towards each other). Well, as Kyle said the sound produced by a tuning fork (or any other source) is longitudinal.īut, the tuning fork itself does both, transversal and longitudinal.
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