When thinking about sound in a room, we are often concerned with improving the acoustics—how to make sure people can clearly hear the soundtrack of a movie, the music of performers, or a speaker giving a presentation. We don’t usually consider how to keep sound from escaping a space. But the people watching a movie on the other side of the wall at the cinema don’t want to hear your movie, and vice versa. And the people living near a music venue don’t want to hear the band at midnight.

Actually, there are many situations in which the job of engineers is to make an enclosed space as soundproof as possible. Manufacturers and industries with noisy equipment need to keep the noise contained; hospital rooms and apartment buildings are also spaces where the objective is to keep sound from seeping out and disturbing people in adjoining rooms. And there are certain situations where privacy is paramount and nobody outside a room should be able to hear what’s being said inside it.

The engineers who optimize the acoustics in a space, as well as work on projects that make loud noise tolerable or safe for human ears, are acoustical engineers. They are experts at reducing noise from various sources, and they work on keeping sound from escaping spaces: they can make a room behave like a soundproof box.

We measure sound in units called decibels, abbreviated dB. The smallest sound we can hear—almost silence—is 0 dB; every 10 dB corresponds to a 10 times increase in sound. A whisper is 15 dB, a
normal conversation is 60 dB, and a rock concert may reach over 110 dB. Sounds over 85 dB can cause hearing loss. A decibel meter measures sounds in decibels.

Many materials absorb sound. Specifically, they absorb echo, which muffles sound and is useful for improving the acoustics in a room. Other materials block sound by preventing it from entering or leaving a space. Engineers often combine these types of materials to reduce dB as much as possible.

In this activity, students design a box that prevents as much sound as possible from escaping the inside of the box.

1. Explain that acoustical engineers use decibel meters to measure and manage the sound level inside a space or coming out from it. Today we are considering how to keep sound from traveling outside an enclosed space.
2. Ask students to think about situations where keeping noise from leaving a space would be important.
3. Give students the specs:
   • The box must be the same dimensions for each group.
   • The box has to rest on the same surface for all groups.
   • Enough space has to be left inside the box to hold the
     Bluetooth speaker. (Students may decide to create an enclosed box with insulated flaps/lid that completely holds the speaker or a box with one open side that gets flipped over the top of the speaker.)
4. Divide students into teams of 3 to 4 and distribute boxes, paper, pencils, scissors, tape, glue, and rulers. Show students the range of insulating materials.
5. Instruct teams to investigate the available materials and choose the ones they’ll use. Tell them to sketch their ideas.
6. Instruct teams to experiment with their materials and refine their sketches to include the measurements and locations of the materials they have decided to test. If students are unsure of how to experiment, suggest having one student hold materials up against his or her ear while another person whispers into the material. Students can also press materials against one side of the box and say something while another student has his or her ear against the other side of the box. Students can press materials against their ears and snap their fingers to see how muffled they can make the sound. (Be sure to caution students that it can be seriously damaging to make any loud noises into another person’s ear, and not to joke around with that.)
7. Tell students to build their sound-reducing boxes. Assure them that after their first designs have been tested, they can improve their results by testing different materials and altering their subsequent designs.
8. Place the speaker in the uninsulated control box. Play the song at the decibel level you have chosen for the test. Place the decibel meter on top of the box for 30 seconds and record the peak reading. Write it on the board and tell students to note it down. This reading is the baseline against which their soundproofing will be evaluated.
9. Test the teams’ designs one at a time, placing the speaker in each box. To standardize the test, turn on the same song at the same exact volume level. Place the decibel meter on top of the box for 30 seconds and record the peak reading.
10. Ask teams to look at each other’s designs and materials. Discuss why some boxes were better at preventing sound from escaping than others.
11. Send teams back to the drawing board to tweak designs as well as materials.
12. Conduct new tests.

• How much sound is escaping at box seams? Is it better to flip your box over the speaker on the table or sit the speaker on the box with closed flaps/lid?
• What would happen if you filled more of the space inside of the box with soundproofing material?
• What might the effect be if you left air pockets between layers of soundproofing material?
• What other materials might be good for blocking sound waves?

• Sound waves cause vibrations in the air that travel into the ear and are converted into fluid vibrations inside the cochlea, a fluid-filled structure of the inner ear. The cochlea is divided by a membrane that has sensory or hair cells sitting on top of it; these hair cells ride the wave created by the sound vibrations. As the hair cells move, they bump against structures that release chemicals, which in turn create an electrical signal. It is this signal that travels to the brain and translates the vibration into a sound that we recognize. Sounds above 85 dB may damage and eventually kill these hair cells, which cannot grow back.
• How do decibel meters work? They calculate the pressure of the sound waves traveling through the air from a source of sound. Unlike a ruler, which is a linear scale, decibel scales are logarithmic, which means they increase by powers of ten. This is why a sound of 20 dB is 10 times more intense than a sound of 10 dB. The decibel scale is logarithmic because our ears respond to sound similarly: a 10-fold increase in sound intensity feels as if the loudness has doubled.
• Acoustical engineers are hired to reduce unwanted sound in buildings. They use sound-insulating materials to block sound waves from traveling between rooms (or from the outdoors) and they use sound-absorbing materials to reduce the echoes that bounce inside each room. Heavy, dense, solid materials—such as gypsum board or concrete block—are best for blocking (insulating) sound because it’s harder for sound waves to vibrate through them. Soft, fluffy, porous materials—like carpeting or cork—work best as sound absorbers, because sound waves don’t reflect off of their little air pockets as easily as they do off smooth surfaces.
• Engineers need to figure out how to reduce outdoor noise coming through windows in busy cities. It can be very loud on the streets, and simple windows do not reduce sound waves much; sound waves travel right through them. One approach is to use very thick glass that doesn’t vibrate easily. Another is to use sandwiched panes of glass, with a vacuum between them, so that sound can’t make air molecules vibrate between the panes.