Engineering Sound Barriers

In this lesson, students bring together what they have learned about sound energy and how it interacts with different materials to design a sound-absorbing wall that reduces noise transmitted from one room to another.

• Engineers can use scientific knowledge about sound waves and energy transfer to design technologies that control sound. 
• There are different ways that sound walls work. Some sound walls reflect sound waves off of them so they aren’t transmitted through to the other side. Other sound walls absorb sound energy. The goal is that the sound energy isn’t transmitted to the other side. 

• Why would a person need to wear ear protection?
• Why do recording studios care about sound quality?
• Why do people use sound walls along freeways?
• How do you think sound walls work to reduce noise?
• How can you use what you have learned about sound energy and materials to help you design a technology that absorbs sound?

Misconception: Sound energy moves in the empty space between particles of matter.

Fact: Sound energy moves by vibrating molecules that collide with one another, passing on the energy from one molecule to another.

Fact: Sound moves faster in air than in liquids or solids because air forms less of a barrier.

Fact: Sound moves fastest in solids and slowest in air because the molecules of solids are more closely packed together, so the sound can transfer from one molecule to the next more quickly.

Absorb : to take in

Acoustics :  the properties of a space that determine how sound waves travel

Reflect :  to bounce off of

Sound :  energy that is carried in waves by vibrating molecules

Transmit :  to pass onward

Vibrate :  to move back and forth quickly

How Sound Travels

When you slam a door, the sound travels outward from the source. It is carried in sound waves, which are patterns of vibrating molecules caused by the movement of sound through a medium. A medium is the matter a wave travels through. It can be solid, liquid, or gas.

Sound travels through the molecules of air that fill the room. If you are within the range of these vibrating molecules, your ears pick up the vibrations and hear them as the sound of the slamming door. The farther away you are from the source of the sound, the quieter the sound seems.

Because sound energy is transferred from molecule to molecule, sound waves cannot travel in a vacuum, where there is no matter. It is important to note that waves don’t carry matter. Molecules stop vibrating and return to their original position once they have passed on the energy.

Movement of a Sound Wave

Sound waves are called longitudinal waves. They are caused by the back-and-forth vibration of the molecules of the medium. If a sound wave is moving from left to right through the air, air molecules will vibrate to the right and left as the energy of the sound wave passes through it. A transverse wave is the other kind of wave. In a transverse wave, if the wave moves from left to right, the disturbance moves up and down. When a crowd of people in a stadium do “the wave,” they are modeling a transverse wave. The individual people move up and down, while the wave moves from left to right (or right to left). After the disturbance passes through, the people go back to where they were before the wave moved through.

How Sound Interacts with Matter

If you are in one room and you hear a door slam, it means the sound has been transmitted through the air. To transmit means to pass from one place to another.

In order for there to be an echo, the sound waves have to come into contact with a reflective material. To reflect means to bounce off of something. Sound waves are reflected when they collide with matter that acts as a barrier.

Echoes can only happen in large spaces. Sound moves very quickly, but it still takes time for the energy to move from one place to another. Echoes happen when there is a break between when you hear the original sound and when you hear the reflected sound.

Finally, the sound needs to be loud enough so that it has enough energy to move through the large space and back. In the mausoleum, the sound waves produced by the slamming door were transmitted through the air. They traveled until they reached the hard, reflective walls and high domed ceiling. They were then reflected off of the surfaces and produced the 15-second echo.

For the hands-on activity of this lesson, students apply what they know about sound energy and matter to figure out how to engineer a sound-absorbing wall for a classroom. Students use what they know about the criteria and constraints of the problem to decide on a possible solution for a sound-absorbing wall, and then test their prototypes. Students use the data they gather from each prototype test to improve it and support a claim about how well it solves (or does not solve) the problem.