into the next air molecules, and the next, and so on. At the end of the Slinky, the waves are bounced back. This back and forth movement continues but it will slow down over time until stopping, due to a loss of energy. The coils compress and stretch apart as the wave passes through the coils. At the secured end, the wave bounces back, sending the wave in the opposite direction. The person holding the end of the Slinky can feel the energy of the modeled sound wave because the last coil pushes against their hand. This back and forth movement occurs several times. Likewise, sound waves also bounce off of objects. Sound waves are absorbed by soft surfaces and reflected by hard surfaces. Figure 3 compares compression and rarefaction in the Slinky to a sound wave in air.
5 Pressure Waves
Sound waves are also called pressure waves because they increase and decrease in pressure as they move through a medium. An area where the pressure is greatest and the medium is squeezed together is called compression. An area of decreased pressure, where the medium is stretched out, is called rarefaction. Sound waves are initiated when a force (a push or pulling motion) is applied to a medium that can vibrate, meaning to move back and forth. Vibrating objects give surrounding air molecules a push, which starts a pressure wave that moves out in all directions like the ripples in a pond.
Blowing across the open mouth of an empty bottle starts a pressure wave in the bottle causing the air inside to vibrate. The air in the bottle will vibrate at a specific frequency, called its natural frequency, and the sound produced will be specific to that frequency. The height of the air column in an empty bottle is measured from the top of the bottle to its bottom. Adding water to the bottle will shorten the height of this air column. The sound waves bounce off the surface of the water in much the same way as they bounce off the bottom of the bottle. Thus, the sound heard depends on the height of the air column. As the height of the column decreases, the frequency of the sound produced increases. The highness or lowness of the sound, called the pitch, is directly related to the frequency of the sound.
FIG 1
See for Yourself
Materials
6 empty water bottles, same size and shape
water
What to Do
1 Support each empty bottle with your hand, and then blow across the top of each one.
2 Compare the sounds made by each bottle.
3 Change the height of the air column inside each bottle by adding varying amounts of water and compare the sounds that are produced.
What Happened?
The results of this experiment confirmed that as the height of a vibrating air column, which is closed at one end, decreases, the frequency of the sound produced increases. It also confirmed that the pitch of sound is directly related to the frequency of the sound. The lower the frequency, the lower the pitch, and vice versa. The empty bottle produces the lowest pitched sound because it has the air column with the greatest height; thus, the sound produced has the lowest frequency. By increasing the quantity of water added to the bottles, the height of the air column in each bottle was shortened, increasing the frequency of the sound waves, and so higher pitched sounds were produced.
6 Pitch
Pitch is a property of sound. Pitch describes the highness or lowness of a sound. The frequency of sound waves determines the pitch heard. Remember, sound is how pressure waves entering your ears are perceived by your brain. Not everyone perceives a specific sound the same. But all can agree that the lower the frequency, the lower the pitch, and the higher the frequency, the higher the pitch. Pitch has nothing to do with the loudness or softness of a sound.
A flute made from a drinking straw is a simple homemade instrument that, like all wind instruments, vibrates at a specific frequency. The source of the pressure wave that starts the air vibrating is the reed at one open end. The reed is made to vibrate by blowing on it. A straw flute is made with a double reed, consisting of two parts that vibrate, which sends a pressure wave into the straw causing an air column in the open tube to vibrate. The longer the straw, the longer the air column inside and the lower the frequency. This means a longer straw will produce a lower pitch. Conversely, decrease the length of the straw and there is an increase in both the frequency and pitch.
See for Yourself
Materials
bendy drinking straw
scissors
ruler
marking pen
helper
What to Do
1 With your fingers, flatten the short end above the accordion folds in the straw.
2 Figure 1 has marks indicating where to cut the straw to form the reed part of the straw flute. Once the two cuts have been made, cut the sharp point off to blunt the end.FIG 1
3 Moisten your lips, then place the reed in your mouth past your teeth and blow. If the flute is not producing sound, try drawing in the lips around the teeth rather than puckering them. Slightly lifting or lowering the straw flute can also help. Don't be too quick to give up. It might take a few tries to reach the goal of making a sound.FIG 2
4 Once you have mastered playing the straw flute, ask your helper to cut small sections off the end of the straw as you continue testing the straw flute.
What Happened?
The length of the straw component of the flute affects the sound frequency. A shorter straw will have a higher frequency. The higher the frequency, the higher the pitch of the sound produced. Thus, as the straw was shortened, it produced a sound with a higher pitch.
7 Sound Transmission
Sound transmission is the movement of sound energy via longitudinal waves. In most
