GRADES 5-8
SOUND

BACKGROUND INFORMATION: Sound The science of sound is called acoustics from the Greek word for hearing. Sound and light are both forms of waves. To control sound, light or water waves, engineers need to understand how waves are created, behave and move. Not all waves behave the same way. For example, light can travel through the void of outer space but sound must travel through air or some other type of material. What Are Waves? Toss a small stone into a pool of water. Waves are formed from the point the stone hit the water. The waves form a circular pattern around this entry point and spread farther and farther apart. When you look at these waves from the side you can see the crests (high points) and troughs (low points). These Water Waves Form A Circular Pattern Click On The Picture To See Movie Sound Waves Sound waves also are known as compression waves. Compression means to press or squeeze together. (The opposite of compression is expansion, to increase in size or volume.) Sound waves are produced by a series of compressions and expansions within the material it moves. An analogy (a similar situation) is the inflation and deflation of a balloon with a hand pump. When you pump air into the balloon, it quickly expands. However, the air right outside of the balloon is compressed (squeezed together) to make room for the larger balloon. That layer of air compresses the air next to it and the compression wave travels through the air away from the balloon in all directions. When you reverse the process and pump the air out of the balloon, the balloon contracts (gets smaller) and the air outside the balloon expands to fill in the space the balloon occupied. The same situation occurs when a firecracker explodes or any large "bang" occurs. The explosion compresses the air closest to it and that compression wave moves outward in all directions, just like the waves in the pond pictured. Nerves inside our ears are able to sense compression waves allowing us to hear the sound. Reflection When a wave strikes a surface or moves through an area where the material's properties change, the wave will either change direction or reflect (cast) back. In a swimming pool, waves which strike the sides of the pool, reflect back. The reflected waves and original set of waves can pass through one another. In the movie below, water waves which strike the stick are reflected. The reflected waves pass through the original set of waves moving towards the shore. When a wave is moving through air and then travels into water, the wave changes direction. Wave Reflection Click On The Picture To See Movie Sound waves can be reflected by hills, mountains, walls, the ground, structures on the ground and low clouds. A "roll of thunder" is generally caused by the reflection of sound waves between clouds and ground surfaces. In the picture and movie above, the waves reflected off of the stick pass right through the waves coming ashore. The two sets of waves can move right through each other without any interference. Sound waves operate in a similar manner. This is why separate sounds can be distinguished. You can hear someone speaking and music playing at the same time because the sound waves can pass through each other. Intensity Intensity refers to the loudness of sound. Examine the pictures or movies of the water waves. The waves are marked by crests and troughs. The distance between the crests and troughs is called the wave's amplitude. In the case of a sound wave, the greater the amplitude (the greater the distance between the crest and trough), the greater the intensity (louder) the sound will be. The smaller the amplitude of the wave the softer the sound will be. In addition to amplitude, distance affects intensity. If the sound wave is not disturbed and allowed to travel freely (there are no disturbances such as reflections) the intensity will diminish (taper off or dwindle). The farther from the source of the sound, the softer the sound will be. In the activity, the farther the watch was held from your ear, the softer the sound was until you could no longer hear the sound. Placing the watch over the cardboard tube prevents the waves from dispersing into the air and diminishing its intensity. The waves are reflected within the tube. REFERENCES: Freeman, Ira M. and Durden, William J., Physics Made Simple, Doubleday, New York, 1990.