Abstract

Objective

The goal of this project is to learn about sound and hearing using a decibel meter to compare noise levels in different settings and locations.

Introduction

Click here to watch a video of this investigation, produced by DragonflyTV and presented by pbskidsgo.org

There's an old adage that claims if you really want someone to pay attention, then whisper. But how softly can someone speak and still produce a sound that can be heard? On the other hand, how loud is too loud for our ears? At what level does sound become harmful to our hearing, and is the damage temporary or does it have long lasting effects? In this project, you'll discover the answers to these questions and more in your experiments and research. You will use a sensitive sound meter to measure the range of noise and sound in your neighborhood, home, school, and surroundings. You'll then compare the results from the various sites to see if those noisy spots are really as loud as they seem and if the supposedly quiet zones are truly an acoustical haven for the ears. You might be surprised at what you discover.

Check out the project video to see how two curious students, Tarissa and Sabrina, were surprised at what they found when they measured noise levels in one of the world's busiest cities, New York City.

The student scientists in the video took sound level readings from places they thought were quiet, like the New York City Library, and compared them to sites they knew were loud, like the subway, airport, and the Empire State Building. They also tested various sounds and locations they thought would rank in the medium range of noise level. In the process, they discovered a couple of unexpected results and learned some things about sound and hearing as well. Read on to see how you can repeat a similar experiment in your city or town and find out if, you too, have some acoustical surprises in your environment.

Before you begin your project, you will want to do some background research on sound and hearing. You will learn, for example, that sound as we hear it is described in relative units called decibels (abbreviated dB), named after the American inventor of the phonograph and telephone Alexander Graham Bell. The smallest audible sound (near total silence) for humans usually correlates to 0 dB, and soft whispers are rated at a relative increase of about 15 dBs. At the other end of the scale, the roar of jet engines can register up to 120 dB while firing machine guns top out at 140 dB. That's about the loudest sound level our ears can take at close range, although anything over 120 dB can start to cause pain and potential damage. Scientists say, in fact, that sounds above 85 dB may lead to at least temporary hearing loss. Whether the damage is permanent or not depends on the decibel level, the length of time, and how often the ears are subjected to the injurious sound.

The numbering system for decibels is a little complicated. Even if decibel numbers look fairly close, the actual sound difference associated with the two numbers is really much greater (see Table 1). That's because decibel numbers represent changes over a logarithmic scale rather than a linear scale, so decibels refer to changes represented by exponents. Focusing on the exponential changes, auditory techies describe a 100 (10^2) fold increase in sound as a change in 2 Bels, or 20 decibels (deci= one tenth). Likewise, a 1,000 (10^3) fold increase is represented by a change in 3 Bels, or 30 decibels, and a 10,000 (10^4) fold increase is represented by a change in 4 Bels, or 40 decibels. Scientists decided to use decibels instead of the larger Bel unit because our ears are capable of differentiating between impressively small changes in sound. In fact, one decibel equals about the "just noticeable difference" (or JND) in sound intensity we can pick up under the most quiet circumstances. So the original Bel unit was broken down into the smaller, more convenient decibel units, to better describe the relationship between sound intensity and our hearing.

When the intensity of a sound is constant, distance has a lot to do with how loud it seems to our ears. A boom box set outside the on highest volume 10, for example, may sound painfully loud when played right next to your ears, but it becomes much more tolerable even without adjusting the volume when played from across the street. The intensity of the music didn't change, but your perception of its loudness did because of the increased distance.

Scientists have shown that, in most cases, sound decreases with an inverse relationship to the distance squared. This is technically known as the inverse square law. It basically means that if the distance is doubled, the intensity of the sound will drop by a factor of 4 (2^2=4); if the distance is tripled, the intensity of the sound will drop by a factor of 9 (3^2=9); if the distance is quadrupled, the intensity of the sound will drop by a factor of 16 (4^2=16), and so on. You'll have a chance to experiment with the effect of distance on sound in one of the experiments in this project. Then you can see if your results support the findings of the auditory experts.

In addition to understanding the decibel scale system and the importance of distance to sound, you might want to investigate the physics of sound waves and hearing as part of your background preparation for this project. We've included some useful websites and suggested search terms to get you started. It would be good to know, for instance, what exactly happens when sound waves hit your ear and how that information is translated into a interpretable signal to your brain. You also might be interested in figuring out what causes "ringing in the ears" and if that is an indication of permanent hearing loss or not. Then you'll know whether those repeated parental warnings about too high volumes on iPods, computers, or TVs is actually good advice for you that shouldn't fall on deaf ears.

So pump up the excitement, but please not the volume, and get started on this auditory adventure!

Terms, Concepts, and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• Sound and sound waves
• Hearing
• Decibel
• Sound intensity
• Threshold of hearing
• External, middle, and inner ear
• Noise-induced hearing loss
• Tinnitus

More advanced students should also study:

• the inverse square law equation (for sound applications).
• sound power, measured in microwatts per meter².

Questions

• What is sound and how do we hear?
• What is a Bel, a decibel and what are the decibel levels of some common sounds and sites?
• What is the relationship between decibels and intensity of sound?
• How does distance affect sound levels?
• What levels of noise are linked to pain or hearing loss? Why do some sounds lead to permanent damage and others do not?

Bibliography

• Easy to read information on how we hear:
  Harris, T., 2007. "How Hearing Works," How Stuff Works website [accessed May 24, 2007] http://www.howstuffworks.com/hearing.htm.
• Brief explanation of decibels:
  Staff, 2007. "What is a decibel, and what is the loudest sound I can listen to before it hurts my ears?" How Stuff Works website [accessed May 24, 2007] http://www.howstuffworks.com/question124.htm.
• Mathematical explanation of decibels and decibel calculations:
  Nave, C. R., 2005. "Decibels," Hyperphysics website, hosted by the Department of Physics and Astronomy, Georgia State University [accessed May 27, 2007] http://hyperphysics.phy-astr.gsu.edu/hbase/sound/db.html#c1.
• Introductory information on noise-induced hearing loss:
  DRF, 2007. "Toxic Noise," Deafness Research Foundation [accessed May 27, 2007] http://www.drf.org/hh_dictionary/noise_hearing.htm.
• Another good website for information on noise-induced hearing loss:
  NIDCD, 2007. "Noise-Induced Hearing Loss," National Institute on Deafness and Other Communication Disorders" [accessed May 27, 2007] http://www.nidcd.nih.gov/health/hearing/noise.asp.
• The idea for this project came from this DragonflyTV podcast:
  TPT, 2007. "Extreme Sounds by Tarissa and Sabrina," DragonflyTV, Twin Cities Public Television [accessed May 24, 2007] http://pbskids.org/dragonflytv/show/extremesounds.html.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• Sound meter with a range from at least 50 dB to 125 dB (you can find a suitable meter at RadioShack for under $50: "7 Range Analog Display Sound Level Meter," part number #33-4050)
• Boom box, radio, CD player, or iPod with speaker
• Metric measuring tape (or use a standard tape and convert the feet to meters)
• Note pad
• Pen or pencil
• List of 10 or more locations to test for sound level
• Quiet area outside or a large, quiet room to test the effects of distance on sound

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Experimental Procedure

Experiment 1: Environmental Testing

1. Make a list of at least ten different places to measure sound level. Try to include places that are very quiet, very noisy, and somewhere in between. When you are doing your experiment, you might find a few additional sounds to measure, so these can be added to your list at that time.
2. Make a prediction of the decibel level or range you think each place you have selected will register on the sound meter (refer to decibel levels for common sounds obtained from your background research). Rank your locations on a list from highest predicted decibel level to lowest predicted decibel level.
3. Practice taking decibel readings with your sound meter to be sure you know how to use it. You might take several readings from the same spot to be sure that your meter is consistently measuring the same or nearly the same decibel level.
4. Now, the fun begins. Go out and make those observations and readings. Take three readings at each location.
5. In your notebook, write down the date, time, location, decibel levels, and a brief description of each noise or sound. Also, measure or estimate how close in meters you were to the sound source each time you take a reading.

Experiment 2: Effect of Distance on Sound

1. Use your boom box, CD player, or iPod with an attached speaker as your source of sound in this experiment.
2. Do this experiment in a quiet spot outside or in large, quiet room.
3. Turn on the piece of equipment and select a medium-high level volume for your testing. Play music that is fairly consistent in its range of sound level (not a lot of loud or soft sections), or use a talk show station on a radio station.
4. Measure and record three decibel readings with the meter right next to the speaker.
5. Move the sound meter away from the speaker 1 meter (39.4 inches) and take another three decibel readings.
6. Repeat the measurements at 2, 4, 8, and 10 meters away from the speaker.

Analyze Your Data

1. Calculate the average decibel level from the three sound meter readings recorded for each of your locations in Experiment 1 and Experiment 2.
2. Using your data from Experiment 1, make a table with four columns listing:
   1. location
   2. predicted decibel level
   3. average decibels recorded
   4. distance from source
3. How close were your predictions to the actual decibel readings in Experiment 1?
4. Which locations registered levels that could be associated with temporary hearing loss?
5. Were there any locations or sounds that were too soft to detect with your meter?
6. Could altering the distance from the sound source change the results for the locations that were very loud or too soft to register?
7. In Experiment 2, what effect did increasing the distance have on your decibel readings?
8. Calculate the change in decibels from 1 meter to 2 meters; from 1 meter to 4 meters; from 1 meter to 8 meters; and from 1 meter to 10 meters. Do this by subtracting the average decibel reading found at 1 meter from the average decibel reading found at 2 meters, at 4 meters, at 8 meters and at 10 meters.
9. How did the decibel changes in experiment 2 compare with decibel changes you might expect according to Table 1? (Hint: If you had used 5  meters in your experiment, moving the sound meter from 1 meter away from the speaker to 5 meters is a relative change in distance from the speaker of 5x. According to the inverse square law, under ideal conditions that would equal a 25-fold (5^2) drop in signal. Looking at Table 1, a 25-fold drop in signal should mean losing about 14 decibels from the 1 meter sound reading to a 5 meter sound reading).
10. For help with data analysis, see Data Analysis & Graphs.
11. For a guide on how to summarize your results and write conclusions based on your data, see Conclusions.

Variations

• Changes over time: Take readings of the same location at different hours and/or over several days. Do you see any changes in the decibel readings? Are they what you expected?
• Analyze your electronic sources of sound: Measure the full range of sound that is produced from your TV, computer, iPod, or CD player. Take sound meter readings starting at the lowest volume level and continue to take readings at each interval indicated on the equipment by numbers, bars, etc. Be sure that you keep the sound meter the same distance from the sound source throughout your testing. Repeat the experiment with head phones if you have them. Is there any difference between the sound levels at each interval from the speaker versus from the head phones?
• You do the math: For those of you familiar with logarithmic calculations, use the inverse square law to calculate how a large change in distance would affect the decibel readings of a location in your first experiment. Select the data from one of the noisier locations in your original experiment above.
  • Determine what the decibel level would be if the same sound were heard from an additional 20 meters away; from an additional 50 meters away.
  • How far away would you have to move from the sound to get the decibel level under 85 dB (less than the level scientists believe can cause hearing damage)?
  • Investigate and explain the relationship between decibel numbers and the power of sound as measured in microwatt/m^2. Convert the decibel readings in your experiment to changes in microwatts/m^2.
• For an experiment that shows you how to measure how good your hearing is, see the Science Buddies project Measuring Your Threshold of Hearing for Sounds of Different Pitches.

• For more science project ideas in this area of science, see Physics Project Ideas.

Credits

Darlene Jenkins, Ph.D.

Sources

The idea for this project came from this DragonflyTV podcast:

• TPT, 2007. "Extreme Sounds by Tarissa and Sabrina," DragonflyTV, Twin Cities Public Television http://pbskids.org/dragonflytv/show/extremesounds.html.

Last edit date: 2008-06-17 00:00:00