What is Sound?
Sound is a disturbance of the atmosphere that human beings can sense with their hearing systems.
Such disturbances are produced by practically everything that moves, especially if it moves quickly or in a rapid and repetitive manner. The movement could be initiated by: Hammering (rods), plucking (string), Bowing (strings), Forced air flow (vibration of air column – Organ, voice). Vibrations from any of these sources cause a series of pressure fluctuations of the medium surrounding the object to travel outwards through the air from the source.
You should be aware that the air is made up of molecules. All matter is made out of particles and air is no exception. As you sit there in the room, you are surrounded by molecules of oxygen, nitrogen, carbon dioxide and some pollutants like carbon monoxide. Normally these particles are all moving around the room, randomly dispersed and all roughly the same distance from adjacent particles as all of the others. The distance between these particles is determined by the air pressure in the room. This air pressure is mostly a result of the barometric pressure of the particular day. If it’s raining outside, you’re likely to be in a low pressure system, so the air particles are further apart from each other than usual. On sunny days you’re in a high pressure system so the particles are squeezed together. Most of the characteristics we expect of air are a result of the fact that these particular molecules are very light and are in extremely rapid but disorganized motion. This motion spreads the molecules out evenly, so that any part of an enclosed space has just as many molecules as any other. If a little extra volume were to be suddenly added to the enclosed space (say by moving a piston into a box), the molecules nearest the new volume would move into the recently created void, and all the others would move a little farther apart to keep the distribution even or in ‘equilibrium’. Because the motion of the molecules is so disorganized, this filling of the void takes more time than you might think, and the redistribution of the rest of the air molecules in the room takes even longer. If the room were ten feet across, the whole process might take 1/100 of a second or so.
If the piston were to move out suddenly, the volume of the room would be reduced and the reverse process would take place, again taking a hundredth of a second until everything was settled down. No matter how far or how quickly the piston is moved, it always takes the same time for the molecules to even out. In other words, the disturbance caused by the piston moves at a constant rate through the air. If you could make the disturbance visible somehow, you would see it spreading spherically from the piston, like an expanding balloon. Because the process is so similar to what happens when you drop an apple into a bucket, we call the disturbance line the wavefront. It is important to note that the particles of the medium in this case molecules of air, do not travel from the source to the receiver, but vibrate in a direction parallel to the direction of travel of the sound wave. The transmission of sound energy via molecular collision is termed propagation.
If the piston were to move in and out repetitively at a rate between 20 and 20,000 times a second, a series of evenly spaced wave fronts would be produced, and we would hear a steady tone. The distance between wave fronts is called wavelength.