(LSES) Propagation of Sound Lesson

Propagation of Sound

We hear sounds every day. Rarely do we think about how sound actually makes it to our ear. Sound is simply a compression wave that travels through a medium.

A medium can be anything that contains molecules able to vibrate and propagate a compression wave. The speed at which a sound wave can travel through a medium is determined by the medium's density. In general, the denser an object, the faster sound can travel. This is due to the arrangement of the molecules in a substance. If we look at a solid object, the molecules are very close together allowing the sound wave to travel from one molecule to another. This means the wave moves fairly quickly. If we look at a liquid or a gas, the molecules are spread further apart. This means it takes longer for energy to move from one molecule to another; therefore, a sound wave travels more slowly. As you can see, the denser the material, the faster sound can travel. You may have noticed this while swimming in a pool. Sounds generated under water travel to your ear much faster than sounds generated in the air. Sound actually travels about 4.3 times faster in water than in air, so it reaches your ear 4.3 times as fast!

So far we have learned sound is transmitted through solids, liquids, and gases strictly as a compression wave. In a solid, sound can also be transmitted as a transverse wave. This makes sound traveling through solids behave a little differently. Compression sound waves simply move molecules closer together as a wave passes. In solids, transverse waves are also present. In solids transverse waves move in 90° angles to the flow of energy, and disperse the sound in many directions. This is because the molecules are so close together that they literally "touch one another" like a continuous string in a rope. This vibration moves through the whole solid instead of just a narrow band in front of the energy source. These factors allow sound waves to travel very quickly in a solid. In fact, sound travels 15 times as fast in iron as it does in the air. You may have observed a person in the movies placing their ear to a railroad track to see if a train was coming. They were using this principle to hear the train so that they could predict when it would arrive at their location. From the sound vibrations delivered through the iron tracks they could predict how far away the train was.

Seismic Waves

A compression wave and a transverse wave travel at different speeds in solid materials. We see this during a seismic event such as an earthquake. One wave created during the event is called a P wave. It moves through the Earth at a fast rate because it is a compression wave. A second wave called an S wave arrives later because it is a transverse wave. These waves are recorded on a seismograph and the differences in arrival time are used to locate the source of the earthquake called the epicenter. As you can see compression waves are the fastest of the sound waves and they can travel through any material, while only solid materials can transmit transverse sound waves.

The propagation of sound is also affected by the motion of the medium, the source of the sound or the observer. For example, if the wind is blowing, the air is moving independent of the motion of sound. If the air is moving the sound is transported faster than the movement of the sound itself. This can cause the sound to take on a different frequency than would occur if the air were still. Another effect of motion occurs when a moving object makes a sound that is moving relative to the observer. This phenomenon is called the Doppler Effect. The Doppler Effect accounts for the difference in sound that we experience when the source of a sound is moving toward or away from us. If you have ever heard a siren emitted from an emergency vehicle, you have experienced this effect. The frequency is higher than the stationary frequency during the approach, it is identical at the instant of passing by, and it is lower as it moves away. This makes the sound much different upon the approach than when an object is moving away. The Doppler Effect is now used to show approaching storms on weather radar. By using the difference in frequencies of Doppler radar, waves that are reflected from the storm return to the station at different speeds. This allows a meteorologist to determine how fast a storm is moving and the type of precipitation that it is producing.

Sound

Most sounds that we hear travel through the air. These sounds travel at a fairly standard rate called the speed of sound. The speed of sound is measured at 343.2 m/s (1,126 ft/s). This is equivalent to 1,236 km/hr (768 mph). This is a standard unit based on specific calculations, and it varies depending on the type and density of the material it is traveling through. We can estimate the value at Standard Temperature and Pressure (STP) as we did above, but it definitely varies based on many factors. According to standard calculations, sound can travel one kilometer in three seconds, or approximately one mile in five seconds. This can be used to judge how far an object is relative to the observer. For instance, if a thunderstorm is producing lightning you will see the lightning before you hear the thunder produced. If you count five seconds between the lightning and thunder, you can determine that the storm is one mile away.

When an object travels at the speed of sound, it is said to be traveling at MACH I. Many military aircraft can travel much faster than the speed of sound and break the "sound barrier." Some new experimental military planes are capable of reaching Mach XX, or 20 times the speed of sound! An aircraft that can break the sound barrier is called supersonic, and they have the ability to create a phenomenon called a sonic boom. A sonic boom occurs when an object travels faster than the speed of sound, causing compression waves to form at the front and the rear of an object. These waves travel at the speed of sound, and as the speed of the object increases, the waves are forced together. Because the waves cannot get out of the way of each other, they eventually merge into a single shock wave at the speed of sound. We hear this as an explosive sound that moves along the ground under the object that is creating it. In nature, we hear a sonic boom every time it thunders during a storm. As the lightning discharges, it creates superheated air that expands faster than the speed of sound, creating a sonic boom. A sonic boom also occurs when a bullwhip is cracked, producing a sharp sound that we hear as the tip of the whip breaks the sound barrier.

Conduct an Internet search to hear a Sonic Boom or find a video of a Sonic Boom.

As we have learned, sound travels through different mediums at different speeds due to the arrangement of molecules in the substance. If the molecules are close together, as in a solid, then the sound wave can travel very quickly between the molecules. If the molecules are further apart, as in liquids and gases, then the sound wave takes longer to travel from one molecule to another, making the sound wave travel slower. We also learned that sound travels as a compression wave in gases and liquids, but can also travel as a transverse wave in solids. With this knowledge we can now look at how sound is propagated through these different mediums. When we hear sound, it had to travel as a wave through a medium to our ears. When we hear a sound it is transmitted to our brain where we detect the sound wave and it is converted into what we perceive as sound. In the next lesson we will look into how we hear and how we are able to perceive the array of sounds around us.

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