Lesson: To Absorb or Reflect... That is the Question

Contributed by: Integrated Teaching and Learning Program, College of Engineering, University of Colorado Boulder

An anechoic chamber (room designed to completely absorb reflection from sounds and electromagnetic waves) at the University of Salford.
Students explore sound and acoustics
Copyright © Trevor Cox https://en.wikipedia.org/wiki/Salford_Acoustics#/media/File:Measuring_a_diffuser_in_an_anechoic_chamber.png


This is the last of five sound lessons, and it introduces acoustics as the science of studying and controlling sound. Students learn how different materials reflect and absorb sound.
This engineering curriculum meets Next Generation Science Standards (NGSS).

Engineering Connection

Acoustical engineers help create environments in which we can hear the sounds that we actually want to hear and omit the sounds that we have no desire to hear. For instance, classrooms are designed so that the teachers can hear the students, and students can hear the teachers. Theaters are designed so that the audience can hear the actors. Movie theaters are designed so that you can hear the movie, and the sounds of conversations around you are muffled.

Pre-Req Knowledge

Lessons 1-4 of the Sound and Light unit: Longitudinal and Transverse Waves (Lesson 1), Wavelength and Amplitude (Lesson 2), Frequency (Lesson 3) and Sound Waves (Lesson 4).

Learning Objectives

After this lesson, students should be able to:

  • Define acoustics as the science of studying and controlling sound.
  • Explain that acoustical engineers help design spaces where some sounds are enhanced and others absorbed.

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Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

  • Make observations to provide evidence that energy can be transferred from place to place by sound, light, heat, and electric currents. (Grade 4) Details... View more aligned curriculum... Do you agree with this alignment?
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*Note: You will need a small metal trashcan and a piece of carpet or fabric for the demonstration of sound being absorbed/reflected in this lesson.

Angie, Harmon and Fisherman Frank used the sonar, and after searching for two days, they think they can see the shape of a ship on the sea floor! Is it the galleon? Before they can be sure, they lose the image. The ocean around Fisherman Frank's boat is bubbling. Fisherman Frank tells Angie and Harmon not to worry – it's just natural gas bubbling up from the ocean floor. But they cannot see the galleon anymore through the Sonar equipment. Do you know why?

The reason they cannot see the galleon is because the sound waves from the sonar are being absorbed by the bubbles, so they cannot get a good picture of the ship. Sound is absorbed more quickly in air than it is in water, so the natural gas bubbles absorb the sonar waves, preventing them from hitting the galleon and echoing back. When the natural gas bubbles subside, they should be able to see the galleon again.

The reason that Angie, Harmon and Fisherman Frank could not see the galleon was because the sound was absorbed by the gas bubbles. Today we are going to talk about different ways that sound is absorbed or reflected. We have already talked (see the Pre-Lesson Assessment section) about how some places are really loud, some places are really quiet and some places are in-between. Which type of environment do you think our school is? (Likely answer: In-between — although some rooms are quieter/noisier than others.) Well, did you know that there is a special kind of engineer that design rooms so that they will be quiet or noisy?

The type of engineer that works with sound is called an acoustical engineer. Let's write that up on the board. (Write the word, acoustical, on the chalk/white board.) Can you all say it with me? "Acoustical engineer!" Excellent! Acoustical engineers know all about how sounds can be reflected off of some materials and absorbed by others, and they use this engineering knowledge to help design and create buildings that can be quiet in some parts, and, if necessary, noisy in others. Acoustical engineers design spaces so that we can hear the things we want to hear and block out the noises that we do not want to hear. In our classroom, can you hear cars on the street? If so, are they loud or soft? Is it easy to hear the teacher when he or she speaks? Can you hear the other students? At school assemblies, can you hear the speaker at the front of the room? Do sounds echo in your gym? These are some of the types of things that acoustical engineers think about as they design different types of buildings.

What kinds of issues would an acoustical engineer have to think about if they were building a theatre in which people were going to perform musical concerts? That's right – they would want all the people in the audience to be able to hear all the instruments, and they would also want the sound to travel all the way back to the seats farthest away.

Let's make sure everyone knows the definition of "reflect" and "absorb" — can anyone explain what those words mean? Reflect means that the sounds bounce off of it, like our image is "reflected" by a mirror. And absorb means that the sounds soak into it, like paint soaks into a sponge. Now I'm going to give you a demonstration of how some materials reflect sound and others absorb it.


Speak loudly into a metal garbage can and let the students hear how the sounds are reflected. Next, place a piece of carpet, or some other soft fabric or material into the garbage can, and again speak into it. Discuss with the students how the sounds were much quieter (and echoed less) with the fabric in the can. This is because the empty garbage can reflected the sound. It allowed the sound to bounce back so it was heard easily. When the material was added to the garbage can, the sound got trapped or absorbed into the material so it was harder to hear. Ask the students to discuss their observations with each other. Ask if they can describe why the objects they added lowered the volume (the sound waves were both scattered and absorbed...actually raising the temperature of the fabric a little tiny bit)?

Sound waves are always absorbed — eventually. However, some materials absorb sound waves better than others, causing the sound to get quieter and disappear faster.

We already talked about acoustical engineers, but we did not explain is what the word "acoustics" means. Who would like to try to define that word? That's right! Acoustics has to do with studying sound and controlling how sound is produced, transmitted and received. We say that acoustics is the science of studying and controlling sound. So, it makes sense that an engineer who knows about the science of sound is called an acoustical engineer.

Now you are going to get a chance to become acoustical engineers! Your job is to design and make a pair of headphones that will block out as much sound as possible.

Lesson Background and Concepts for Teachers

Sound travels in waves. Sound waves can travel through almost anything, but they must travel through something. That something is called a medium, or conductor. Sound waves travel faster through denser (thicker) mediums. (Water is denser than air, solids are denser than liquids (of the same substance), etc.) They cannot travel — as light waves can — through completely empty space where there is no air. Light travels faster than sound. Therefore, you can sometimes see something before the sound of it reaches you. Think of thunder as an example: you might see a flash of lightening, but the crash (noise) produced by the lightening bolt (thunder), is not heard for several seconds after the flash of lightning.

A photograph of the KiMo Theatre in Albuquerque, NM. Shown are rows of curved seats, each row rising above the previous row, a stage with a red curtain and corrugated panels on the ceiling.
The KiMo Theatre in Albuquerque, NM.
Copyright © City of Albuquerque, http://www.cabq.gov/kimo/pictures.html

Just as a mirror reflects light, some surfaces reflect sound. A sound reflection that you can hear as a separate repetition is called an echo. We do not hear an echo if we are close to the reflecting surface. When there are many reflecting surfaces, the echo bounces back and forth several times. This is called reverberation. If you have ever visited the Grand Canyon in Arizona, hearing echoes bounce off the canyon walls are very common (and expected!).

Some substances send back few or no sound reflections. They absorb sound. Materials that absorb sound are often used in classrooms, offices, manufacturing facilities and other places to help reduce noise. Some of these materials are carpet, curtains, double-paned glass, plasterboard partitions and acoustic baffles.

Theaters and auditoriums (see Figure 1) are designed and built in ways that cause sound to travel clearly from the stage, preventing echoes. When the sound is well controlled in a room, it is considered to have good acoustics. Architects and acoustical engineers must take many factors — such as reverberation, soundproofing, echoes and sound projection — into consideration when designing auditorium acoustics so that the audience will have a satisfying experience.

Sound is measured in decibels (dBs), from the threshold of human hearing (0 dB) to the threshold of pain (about 120-140 dB). See Figure 2 for average sound levels.

A table showing the average sound levels of: residential home noise (40db), conversation (60db), stereo playing at home (85db), rock band concert (110db) and jet engine (150db).
Average sound levels of common noises.
Copyright © Image created by Janet Yowell, University of Colorado at Boulder, 2007.

Workplace safety regulations may vary for different industries, but all require that sound be kept well within the "threshold of pain" unless ear protection devices are provided. Our ears are very sensitive to loud noises, which can cause permanent damage to our hearing, so it is important to wear good ear protection whenever you are exposed to loud noises. It is even possible to damage your ears from listening iPods, MP3s and other sound equipment too loudly. As fun as it is to rock out, music should kept low to protect our ears!


Absorption: The act of something being consumed by, or absorbed into, something else; a sponge absorbs water.

Acoustical Engineer: An engineer who uses the properties of sound to design environments that allow us to hear the noises we want to hear and block out the noises we do not want to hear.

Acoustics: The science of studying and controlling sound.

Echo: The repetitive noise heard as sound bounces back and forth off of objects.

Associated Activities

Lesson Closure

Today we learned about acoustics and acoustical engineers. Who would like to share what the word acoustics means? You're right! It is the science of studying and controlling sound. Acoustical engineers use their knowledge of sound to design buildings where some sounds are absorbed and others reflected in order to create the desired acoustic environment. Acoustical engineers have a pretty neat job! Maybe one of you will become an acoustical engineer when you grow up. As our story continues, Angie and Harmon and Fisherman Frank wait for the natural gas bubbles to stop bubbling up and absorbing the SONAR sound waves. Once all the gas bubbles are gone, they are able to see the image of the sunken galleon! They are very excited, but wish they could get a closer look. In our next lesson, we are going to start learning about light, and will find out what happens next to Angie and Harmon in their quest for the treasure!


Pre-Lesson Assessment

Quiet, Loud and In-Between: Ask students to think of quiet places, loud places and in-between places. Have several students share their answers and discuss with the class how the sound level of a certain place is related to its use. (For example, a library is quieter than a hockey rink because a library is used for reading and studying, and a hockey rink is used for playing a sport).

Post-Introduction Assessment

What does this word mean?: Ask for a volunteer to define acoustics and give a reason why it is important. Ask for another volunteer to explain what acoustical engineers do, and give an example of a project on which an acoustical engineer would work.

Lesson Summary Assessment

What do we want to hear?: Tell the class that they are now acting as acoustical engineers who have be hired to design an orchestra hall. Make a list on the board of all the noises that the audience and performers will want to hear, and another list of all the noises they will not want to hear. Discuss with the class how acoustical engineers need to know all about the properties of sound in order to create a listening environment that is pleasant for the audience and the people on stage.

(Ideas: Noises we do not want to hear: gum being chewed, people talking on cell phones, babies crying, people talking, people wiggling on their seats, people opening their programs, etc. Sounds we do want to hear: the music, singers, people talking on stage, etc.)

Lesson Extension Activities

Visit various rooms in the school (cafeteria, library, gym, etc.), and have the students shout their names and listen for the echoes. Talk about how and why different rooms absorb or reflect sound differently.

Check the internet for a local acoustical engineer and invite them to come speak to the class.

The next time your class is on a field trip, discuss with the students the acoustics of the building or room you are visiting.

Additional Multimedia Support

These sound files show the differences between someone speaking and clapping in a carpeted vs. uncarpeted room: http://www.acs.psu.edu/drussell/Demos/RT60/RT60.html


City of Albuquerque, "KiMo Theatre," accessed February 22, 2007. http://www.cabq.gov/kimo/pictures.html

Russel, Dan, 1999. KetterlingUniversity Applied Physics, Acoustics Animations, "Reverberation in a Small Room," accessed February 22, 2007. http://www.acs.psu.edu/drussell/Demos/RT60/RT60.html

TechTarget, Whatis?com, Electronics, "decibel," 2007, accessed February 22, 2007. http://searchsmb.techtarget.com/sDefinition/0,,sid44_gci213525,00.html

U.S. Department of Labor, Occupational Safety & Health Administration, Safety and Health Topics, Woodworking, "Production: Noise," accessed February 22, 2007. http://www.osha.gov


Teresa Ellis; Frank Burkholder; Abigail Watrous; Janet Yowell


© 2007 by Regents of the University of Colorado.

Supporting Program

Integrated Teaching and Learning Program, College of Engineering, University of Colorado Boulder


The contents of these digital library curricula were developed by the Integrated Teaching and Learning Program under National Science Foundation GK-12 grant no. 0338326. However, these contents do not necessarily represent the policies of the National Science Foundation, and you should not assume endorsement by the federal government.

Last modified: August 16, 2017