![]() ![]() By design, in case of a tuning fork, eventually the body is left vibrating in just one dominant mode while the energy in most other modes has decayed away faster. In this transient phase, the exact motion is, at least in theory, just the evolution of certain superpositioned normal modes. This sound reflects off of the object's boundary and traverses across it again and again and so on. This displacement wave which is set off in the material is the sound that travels through it. The solid itself, the air around it too.So the vibrations from one tong of the struck tuning fork travel to the other part via its bulk and set it in motion. This sets of a chain reaction throughout anything in contact with the point of impact: The displaced atoms being linked to other atoms via inter atomic forces, push on them in turn. When an impulse is delivered to any point on such an object, the atoms at the place of impact get displaced from their equilibrium positions. At low temps**, the atoms can be approximated as staying in place. The constituents are held in equilibrium positions by inter atomic forces. For a metallic tuning fork, its a metal lattice. This arrangement is responsible for stability of Most solid materials we encounter in daily life have some sort of equilibrium arrangement of atoms. This question is in general about how motion spreads to other parts of a body when impulse is applied to only a particular part.Įlastic solids like a tuning fork, comprise of atoms (or molecules) in a certain structure. The short answer is that its connected to the first. On how the second prong is set into motion after the first prong is struck When struck, objects vibrate in a superposition of these normal modes so the motion can be quite complicated and the sound produced multi-tonal.įortunately for high Q systems like tuning forks, when struck, the fork settles into one particular frequency-the normal mode its designed for. Each normal mode has a characteristic frequency and motion. 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). I'm currently writing a book on music theory and I'd like to include some background information on the physics of sound waves. ![]()
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