Sound is created through vibrating objects going through a medium (air) in the forms of sound waves. These sound waves reach the eardrum causing them to vibrate. Then the brain perceives these vibrating waves as sound. The phenomenon of hearing sounds like a complicated process, believe me it is! However, the curriculum unit, "The Science of Sound" will attempt to simplify the process so that it can be utilized in the third to fifth grade classroom.
When an object such as a drum vibrates, it produces sound. The sound causes the molecules in the air to move back and forth. This motion called sound waves vibrates in all direction within the atmosphere.
Sound waves within the atmosphere are invisible, but there are several ways in which you can demonstrate movement of sound within your classroom. (See Lessons I on sound waves.) This simple experiment uses water as the medium in which the waves travel through. This is a simulation of vibrations traveling in the atmosphere as sound waves. The sound waves would then reach the eardrum.
Sound waves travel at different rates of speed and have different intensities and frequencies. The frequency of a sound wave is determined by the number of times an object, or sound waves it produces, vibrates in one second. Scientist measures frequency in units called Hertz. Hertz are the number of times a sound wave cycle passes a given point each second.
The frequency that sound vibrates determines the pitch of that sound. If a sound wave vibrates rapidly, it will produce a higher pitch. If a sound wave vibrates slowly, it will produce a lower pitch. The experiment in the second lesson demonstrates this phenomenon.
Blowing across the top of the bottles containing different amounts of water causes a column of air to vibrate inside the bottle to vibrate and the vibration produces sound waves. The strong flow of air travels down into the bottle and expands. When it reaches the surface of the water it is reflected back towards the opening. The back and forth movement of these air molecules across the opening of the bottle reaches the atmosphere and produce sound.
The flow of air in the bottles with the less water has to travel farther inside the bottle producing a larger air column, which causes the air to vibrate slower. The slow vibration produces a lower pitch. Likewise, the flow of air in the bottles with more water has to travel a shorter distance inside the bottle producing a smaller air column, which causes the air to vibrate faster. The faster vibration produces a higher pitch. High pitches have high frequencies and short wavelengths. Low pitches have low frequencies and longer wavelengths.
All objects have a natural frequency or period of vibration. If we consider the strings on the inside of the piano, we will find that it is composed of string or wires with its different lengths and thickness. Each of the wires has been tightened so that they will vibrate at a definite frequency. But the string can also vibrate without being struck because anything vibrating near it at the same rate, as its own natural vibration rate will set a string into motion. This action is called sympathetic vibration.
In order to demonstrate sympathetic vibration of an object in the classroom, a simple pendulum can be used. Once the pendulum is set into motion, it has a natural frequency. You can set up additional pendulums at the same length and it will have the same frequency as the first pendulum. But if you place two or more pendulums, hang them on the same source and just swing one of the pendulums the other ones will begin to swing. This happens because the other pendulums feel the vibration on the source that the first pendulum is hanging because they have the same natural frequency as the first pendulum. This is an excellent example of sympathetic vibration. See experiment in Lesson IV if you would like to do this experiment in your classroom.
In the atmosphere vibrations of one object at its natural frequency produce vibrations in another object with the same natural frequency. This phenomenon is called sympathetic resonance. The best known example of resonance occurring in nature is the opera singer hitting a note that shatters the glass. Resonance occurs when small vibrations of one object at its natural frequency produces large vibrations in another object with the same natural frequency. A better understanding of resonance and sympathetic resonance can be achieved by performing the experiment in Lesson IV at the end of this unit.