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The Effects of Sound Barrier Design on Highway Noise Attenuation

Researched by Steven P. 

1999-2000 

 


 


PURPOSE

The purpose of this experiment was to determine if a sound barrier does reduce noise, and which type of barrier is the most effective in reducing highway traffic noise.

I became interested in this idea because many people are concerned about noise pollution, and there is a great need to reduce this noise.  I wanted to find out which sound barrier is the most effective in reducing highway noise.

The information gained from this experiment will help society because engineers will know what type of sound barrier is the most effective in reducing highway traffic noise.

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HYPOTHESIS

My hypothesis is that a sound barrier will reduce noise and that a concrete or wooden barrier will more effectively reduce highway noise when compared to an earth berm or vegetation.

I base my hypothesis on information about sound barriers and the characteristics of sound available from the Washington State Department of Transportation and from the book, Understanding Sound by Beulah Tannenbaum and Myra Stillman.

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EXPERIMENT DESIGN

The constants in this study were:

a. The calibrated decibel meters used. 
b. The distance from both sides of the sound barrier where the readings were taken. 
c. The intervals of time between each measurement.

The manipulated variable was the type of sound barrier used to reduce traffic.

The responding variable was the level of recorded traffic noise behind the sound barrier. 

To measure the responding variable I used hand-held decibel meters. 

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MATERIALS

Quantity  Item Description
3 Decibel Meters (digital)
3  Decibel Meter Calibrators
Hard Hat
1  Safety Vest
3  Clipboards
1 30m Tape Measure  
(Fiberglass Reinforced)
1  5m Tape Measure (Metal)
3  Watches
1  Camera

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PROCEDURES


Phase I: Determine if a sound barrier is effective in reducing highway noise.

This phase of the experiment will be conducted along a highway with a concrete barrier adjacent to an open field.  This will allow for direct comparison of decibel readings with and without a sound barrier.

1. After arriving at the site, calibrate all three decibel meters using the calibrator made for each meter. 
2. Set up one decibel meter in the open field near the road for the road side reading. 
3. Take a reading every 30 seconds for 10 minutes and record the readings on a data sheet. 
4. Record the Leq (average sound level) and the Max (highest reading) level for this period of testing. 
5. Place a decibel meter 3 meters back from the decibel meter location in step 2. 
6. Place the next decibel meter 17 meters farther back, and the last decibel meter another 17 meters from the second one. 
7. Station a person at each meter to record the decibel readings every 30 seconds for 10 minutes.   Use a watch to keep track of time. 
8. Have a fourth person take a traffic count during this 10 minute period.  Count the number of light, medium, and heavy vehicles passing by the location.  Counting traffic helps determine what is creating the noise during the experiment. 
9. At the end of this 10 minute period, record the Leq and the Max levels from each of the decibel meters on the data recording sheet. 
10. Move the three decibel meters down the length of the sound barrier, far enough so sound coming around the end of the barrier doesnít affect the readings. 
11. Place each meter at the distances described in steps 5 and 6. 
12. Repeat testing procedures 7 and 9 to collect decibel readings behind the sound barrier. 
13. Retest the calibration of each decibel meter to make sure they were accurate throughout the test.  If they are out of calibration by more than one decibel, redo the test.


Phase II: Compare different types of sound barriers to see which is the most effective in reducing highway noise.

1. After arriving at the chosen sound barrier, calibrate the decibel meter. 
2. Measure out 2 meters from the barrier on the road side for the placement of the decibel meter. 
3. Start recording decibel readings every 30 seconds for 10 minutes. 
4. At the end of the 10 minute period, record the Leq and the Max level. 
5. During the same 10 minute period have another person count and record traffic as done in Phase I. 
6. Move to the other side of the barrier and measure out 3 meters behind the barrier to position the decibel meter. 
7. Record decibel readings every 30 seconds for 10 minutes at this new decibel meter position. 
8. Measure off 20 more meters (for a total of 23 meters behind the barrier) for the next decibel meter position, and repeat steps 3 and 4. 
9. Measure off another 20 meters (for a new total of 43 meters behind the barrier) for the final decibel meter position, and repeat steps 3 and 4. 
10. At the end of the testing, check the calibration to see if the decibel meter was accurate throughout the test.  If it is out of calibration by more than one decibel, redo the test.

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RESULTS

 

The original purpose of this experiment was to determine if a sound barrier does reduce highway noise, and which type of sound barrier most effectively reduces highway noise.

A - weighted decibel readings were used to compare noise readings in front of and behind the barrier.  This scale most closely represents human hearing.  Decibel readings were averaged (Leq) to obtain average noise levels for easy comparison.  Lower decibel readings indicated lower noise levels.  A larger difference in readings from the road side of the barrier to behind the barrier meant a greater reduction in noise.

Results for both phases of the experiment are shown on tables and graphs on the following pages. 

Phase I Results: See Graph (long download)

The purpose of Phase I was to determine if a sound barrier did reduce highway noise. 

At three meters behind the wall, highway noise was reduced by 9 decibels when compared to the open field, which was the greatest noise reduction at comparable monitoring positions.  At 20 meters behind the barrier, there was a one decibel reduction compared to the open field readings. There was a 5 decibel reduction difference between open field and barrier readings 37 meters behind the barrier. 

At 37 meters behind the barrier, highway noise was reduced 15 decibels from in front of the barrier.  At 37 meters back in the open field noise was 10 decibels lower than the road side reading. 

At all meter positions behind the wall, decibel readings were lower than in open field.


Phase II Results: See Graph (long download)

The purpose of Phase II was to determine which barrier effective at reducing highway noise.

Concrete Wall: Noise was reduced 17 decibels 3 meters behind the concrete wall, and 20 decibels at 43 meters behind the wall. 

Earth Berm: 3 meters behind the earth berm noise was reduced by 20 decibels, and by 21 decibels at 43 meters behind the berm. 

Wood Fence: The wood fence reduced noise by 10 decibels 3 meters behind the wall, and by 16 decibels at 43 meters behind the wall. 

Vegetation: Noise was only reduced by three decibels 3 meters behind the trees, and by 5 decibels at 43 meters behind the trees. 

The earth berm had the greatest noise reduction at all decibel meter positions, and the concrete wall had the second highest noise reduction at all of the decibel meter positions.  However, the difference in noise reduction between the earth berm and the concrete wall was 3 decibels immediately behind the barrier and only 1 decibel at the farther positions. The vegetation did the worst job at reducing noise at all three of the decibel meter positions. 
 

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CONCLUSIONS

My hypothesis was that a sound barrier would reduce noise, and that a concrete or wooden wall would reduce highway noise better than an earth berm or vegetation.

The results indicate that my hypothesis should be partially accepted.  Data showed the sound barrier did reduce highway noise, so the first part of my hypothesis was correct.  However, the earth berm reduced highway noise better than a concrete barrier by a few decibels, which was different than the second part of my hypothesis.

Because of the results of this experiment, I wonder if a more detailed analysis would show more clearly the shadow zone, since noise may pass over the barrier.  I also wonder what consideration engineers give to cost, aesthetics, and space limitations in the design and building of a sound barrier.  Even though the earth berm was the most effective in reducing highway noise, it took up a lot of space.

My findings should be useful to engineers because they will know that an earth berm is effective in reducing highway noise, and so is a concrete wall.

If I were to conduct this project again, I would try to get four of the same decibel meters for use at the same time.  This could improve data collection and reduce time. I would also conduct the experiment during a time of year when there isnít as great a chance of adverse weather conditions. Weather conditions affected the locations and choice of barriers to study.  I might also include a section for the analyzing of the barriers to see if they were reasonable and feasible to construct. 
 

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RESEARCH REPORT

Introduction

Information that is important to sound barriers includes sound and sound barrier design.  In the first part of this report, I am going to talk about sound, the characteristics of sound, decibels, acoustics, and noise.  In the second part I will explain the types of sound barriers, how they work, and the design and construction of sound barriers.  

SOUND

What is Sound?

Sound comes from the vibrations in molecules, and is always traveling through a substance, whether it is a solid, liquid, or gas.  Sound cannot travel through a vacuum because there are no molecules for the sound to travel through.

When sound waves reach our ears, they vibrate the eardrum.  The vibrations are sent through the three bones in the middle ear.  These impulses are then sent to the brain.  Our ears are never free from sound.  There is always sound, and some of it is pleasant, and some of it isnít.  If your eyes were as sensitive as your ears, you could see a 100-watt light bulb 3218 kilometers (2000 miles) away.   It is hard to find silence, except in soundproof rooms, which arenít always totally soundproof.  There is constant background sound, which we usually donít notice, and an occasional sudden sound.  The background sound can be very helpful to tell us where we are and what time it is.  Sound can also provide other information, and can serve as a warning system.

If a sound is made, and nobody is around to hear it, it is called unheard sound.  If a sound is made and if there is someone around to hear it, it is called heard sound. 
 

Characteristics of Sound

Sound waves are a series of compressions and rarefactions.  Compression occurs when the molecules are compressed. Rarefaction happens when the molecules are thinned out.  Between the compression and rarefaction zones, there is a neutral zone where the density of the molecules is equal to or almost equal to the density before the sound was made.   These three zones of high pressure, low pressure, and neutral pressure form a sphere of waves with the source of the sound at the center. 
  
 
The wavelength of a sound is the distance from a compression or rarefaction zone to the next similar area.    The amplitude is the pressure difference between compression or rarefaction zone and the neutral zone. The term amplitude, intensity, and volume are the same, and can be used to describe loud and quiet sounds.  

Frequency is how many sound waves pass by per second (cycles per second).  The number of cycles per second (or cps) is called hertz.  A higher frequency equals a shorter wavelength, and a lower frequency equals a longer wavelength.  Examples of higher frequency sounds are someone whistling, a flute, and a high voice.  Some low frequency sounds include a foghorn, low voice and the rumble of an earthquake.  Everything has a natural frequency. Humans can hear between 15 Hz and 20,000 Hz. If a sound is above 20,000Hz, it is called an ultrasonic sound. 

Sound travels at different speeds, depending on the substance it is traveling though.  The following chart shows some examples: 
 



SUBSTANCE 
SPEED 

Dry Air/ 0 Degrees C

  330 mps

Dry Air/ 1000 Degrees C
 700 mps
Hydrogen 1315 mps
Carbon Dioxide 182 mps
Water 1400 mps
Copper 3560 mps
Aluminum 5100 mps
Brass 3505 mps
Granite
 3947 mps

Steel
5090 mps

  mps = meters per second

An echo is the reflection of sound off of an object.  Sonar uses echoes to determine depth, and other distances.  A megaphone also uses reflection to direct beam of sound out of its cone.

Free vibration is vibration that dies out in time such as when a tuning fork is struck. Maintained vibration is making the vibration last.  A trumpet holding a note is an example of maintained vibration.  Forced vibration is making something vibrate at an unnatural frequency.  The eardrum has a natural frequency, too, but is made to vibrate at the frequency of sounds it receives.  The stick and slip vibration method is when something is pulled or pushed until it slips such as with a violin.  The strings of the violin are pulled by the bow until they slip, creating a vibration.   Resonance is sound waves making a separate object vibrate.

 

Decibels

Sound is measured in decibels (dB).  This is a unit measuring the relative intensity of sounds.  The decibel is named after inventor Alexander Graham Bell.  Most people speak in a range between 45 and 75 decibels.  A decibel, which is a tenth of a bell, is the smallest change in sound that a human can hear.  The scale starts at 0, which is the quietest sound that can be heard by a human.   The test results taken during my experiment are measured in an A-weighted scale.  This is a scale that is designed to approximate human hearing.
 

Because sound is measured on a logarithmic scale, an increase of 10 dBA sounds twice as loud.  For example, 30 dBA sounds twice as loud as 20 dBA.  A double in the source of a sound equals a 3 dBA increase.  For example, one car may have a 70 dBA sound level.  Two cars at the same speed, distance, and point would then have a 73 dBA sound level, and 4 cars at the same speed, distance, and point would have a 76 dBA sound level. 

The amplitude of a sound increases as you get closer to the source of a sound, and decreases as you move away.   The change in intensity is related to the square of the ratio of the distances.  For example, the intensity of a sound 100 meters away from the source is one-fourth as intense as it is 50 meters away from the source.   The mathematical formula in this example would be (50/100) 2 = _.   Another example, the intensity of a sound 20 meters from the source is 9 times the intensity of a sound 60 meters from the source, or (60/20) 2 = 9.

 

Acoustics

The word acoustic comes from the Greek language and means "to hear".  Acoustics is sometimes referred to as the science of sound.  More often it is referred to as 
architectural acoustics, which is the design of a building's acoustical environment. Recently, this has become a greater area of study. 

Marcus Pollio, a Roman architect who lived in the first century BC made important observations on acoustics, and formed hypotheses on reverberation and interference.  The scientific study of acoustics was not done until 1856 by the American physicist Joseph Henry.  Wallace Sibine, another American physicist, later studied acoustics in more detail in 1900.

Sound reflects off any surface the waves encounter.  Rooms can be designed and built for the best natural reflection with no undesired distortion.  Architects use materials that help enhance reflection and absorption of sound.  Cork and felt are good acoustical dampers while metal and stone reflect sound.  A roomís acoustics are usually good if there is a balance between reflection and absorption materials. 

The science of acoustics is of great concern when designing sound barriers, not only for roads and highways, but in all necessary locations.

 

Noise

Noise has been defined in many ways.  The Council on Environmental Quality defines noise as "sound without value." The World Book Encyclopedia Dictionary entry states that noise is "a sound that is not musical or pleasant".  The American Standards Association calls noise "an undesired sound".  Encartaís article on noise says, "Noise is a complex sound, a mixture of many different frequencies or notes not harmonically related."  AASHTO, the American Association of State Highway and Transportation Officials calls noise "unwanted sound."

Noise can cause physical harm.  If you are exposed to a sound of 120 decibels it is painful.  If you are exposed to a sound of 170 decibels, even for a moment, it will knock you unconscious, and destroy your hearing.

People thought that noise just went along with progress, and that you had to put up with it.  We now know that noise can be reduced with out slowing down progress.  Some people donít mind some noise, while others donít like it at all.  We use sound barriers along highways and other locations to reduce noise to a tolerable level.

There are three major ways of reducing highway noise.  The first way is to improve cars so that the engines and other parts run quieter, which will reduce the noise by 5 to 10 decibels.  The next way to reduce noise is to leave open spaces between highways and houses when either is being designed and built.  You could also put less noise-sensitive buildings or facilities next to the roads.  The last way is to use sound barriers or other noise reducing measures if the noise level is above standards.

Sometimes insulation can be used in buildings to reduce noise.  This is usually not a very good idea because the road contractors donít have the authority to insulate buildings, and if buildings are insulated, then you would have to spend more money on air conditioning since there is no way for air to easily flow in and out.

There are several other ways to solve the noise problem when building new roads.  You can build away from noise-sensitive areas and through non noise-sensitive areas or through undeveloped land.  Constructing partially underground, so walls on side deflect noise from residents is also an easy possibility.  Making the road level the best that is possible so car engines donít work as much on hills can also reduce sound. 


SOUND BARRIERS 
 

Sound barriers are being used more and more as the demand for reduced noise along highways becomes greater and greater.  Sound barriers are built on new as well as existing roads.  The U.S. Congress passed legislation in 1976, which required states to reduce noise along highways beside impacted areas.   The federal government pays for most of the barrier construction, and the state pays for the rest.  Noise barriers reduce the decibel level by 10 to 15 decibels, which is cutting the sound in half.  Usually you get a one decibel reduction for every two vertical feet of barrier added, though the effectiveness and costs evens out at about 13 feet in height. 
 

Types 

There are two main types of noise barriers: earth berm and noise walls (concrete, masonry, wood, etc.).  Construction can include each type separately or in combination.  You use the noise readings to help decide the type, location, length, and height of a noise barrier.  The visual look also is a factor that decides which type to build at the given site.  Vegetation is sometimes used for noise reduction.

Noise walls can be constructed out of concrete, stucco, masonry, wood, and metal.  There are a variety of designs and styles used depending on what is needed at the location.  They are designed to blend into their surrounding but still be effective. 

You should use an earth berm for visual looks, but only if the materials and room allows.   An earth berm blends in with its surroundings very well, and can have plants added to it to virtually cover it up.  An earth berm usually should not have a slope steeper than  about a 22º.

Vegetation can be used if it is tall enough and it is dense enough (you canít see though it).  Vegetation at least 200 ft thick can give a 10 decibel reduction.  It is usually impractical to put in vegetation, because of cost and because there is usually not enough room.  
 

Design and Construction

The engineer designing a sound barrier must consider the most effective way to reduce noise with its location and design.  A sound barrier will only work if the residents behind it canít see the road (meaning that they are in the shadow zone).  The barrier wonít work for residents above wall, such as on a hillside, and homes that are too far apart make it very hard to build a barrier for a reasonable cost. Sections that are taken out (not put in) of sound barriers for roads or other purposes reduces the effectiveness of the barrier, since the sound is able to travel through the opening.

The following are some of the considerations for the construction and design of a sound barrier:

  • The desired noise reduction
  • The right of way needs and maintenance
  • Cost and constructability
  • Maintenance
  • Wind
  • Soil type
  • Earthquakes
  • Vehicle impacts
  • Vehicle access( to and through it)
  • Pedestrian access
  • Matches other structures around it (design, looks)
  • Drainage
  • Water/Electricity
  • The moving of objects or buildings to put in the barrier.

There are several other prospects for an engineer to consider when designing sound barriers.  Here are some factors that help calculate the sound produced by traffic:

  • 2000 cars per hour is twice as loud as 200 cars per hour.
  • Vehicles traveling at 65 miles per hour sound twice as loud as vehicles traveling at 30 miles per hour.
  • 1 truck traveling at 55 miles per hour is the same as 28 cars traveling at 55 miles per hour.  
     

Defective mufflers, other faulty engine parts, hills (making the engine work harder), the engine itself, aerodynamics, and the tires on the road can all contribute to traffic noise.   Traffic noise has a frequency of about 550 hertz, so engineers also have to know how to design a sound barrier with these frequencies. 
  
 
Engineers will only design a sound barrier for an area if it is an impacted location.  An impacted location is an area used by humans where traffic noise (during the design year) equals or exceeds 66 dB.  The level is set at 66 dB because 2 people canít have a normal conversation from 1 meter apart.


  

Design Year Traffic Noise Decibel Level

Allowed Cost Per Household*

 

Equivalent Wall Surface Area Per Household (square meter)

66 dBA $15,000   65 sq. m
67 dBA $17,000  71.5 sq. m
68 dBA $18,500  77.7 sq. m
69 dBA $20,000   84 sq. m
70 dBA $21,500  90.5 sq. m
71 dBA $23,000  96.7 sq. m
72 dBA $24,500  103 sq. m
73 dBA $26,000  109.2 sq. m
74 dBA $27,500  115.5 sq. m
*Reevaluated in January of each year.  Based on $22.10 
per square foot constructed cost.  WSDOT 
 

Sound decreases 3 to 4.5 dBA as the distance from the source is doubled because of  objects that deflect the sound, and because sound also gets weaker as the distance from the source becomes greater.  Generally, a distance farther than 500 ft from heavy traffic on a freeway is not a major concern.  On lightly traveled roads distances of 100 to 200 feet from traffic is not a major concern either.

Another consideration is wind.  You need a taller sound barrier for wind that is traveling with sound just as in the concepts in "Characteristics of Sound."  If you have a wind travelling against the sound, the barrier doesnít need to be as tall.

 

Summary

I hope you enjoyed my report and learned about sound, its characteristics, and sound barriers.  You can continue reading my journal to see how sound barriers reduced highway traffic noise.  
 

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BIBLIOGRAPHY 
 

"Acoustics," Microsoft Encarta, 1998.

American Association of State Highway and Transportation Officials, A Policy on Geometric Design of Highways and Streets, Washington. D.C., 1990.

Cohn, Louis F. Ph.D & Harris, Roswell A. Ph.D. P.E., Special Noise Barrier Applications Phase II, [Online] Availablehttp://www.wsdot.wa.gov/ppsc/research/onepages/WA-RD3781/htm/, November 2, 1999.

Grey, Jerry, Noise, Noise, Noise!, Philadelphia, PA, Westminster Press, 1975.

Knauss, Harold P., "Sound," The New Book of Popular Science, Grolier, 1994.

Rubel, David, Kidís Science, New York, Scholastic Inc., 1995.

"Sound," Microsoft Encarta, 1998.

Tannenbaum, Beulah & Stillman, Myra, Understanding Sound, New York, McGraw Hill Book Company, 1973.

United States Department of Transportation, Acoustics and Your Environment, Cambridge, MA, February 1999.

Washington State Department of Transportation, Design Manual (M 22-01), Olympia WA, December 1998.

Washington State Department of Transportation, "Frequently Asked Questions," Air Quality and Noise Studies, [Online] Availablehttp://www.wsdot.wa.gov/regions/northwest/noise/faq.htm/, November 2, 1999. 

Washington State Department of Transportation, Highway Traffic Noise, [Online] Available http://www.wsdot.wa.gov/regions/northwest/noise/1.html, November 7, 1999.

Washington State Department of Transportation, "Noise Policies," Air Quality and Noise Studies, [Online] Availablehttp://www.wsdot.wa.gov/regions/northwest/noise/policies.html/, November 8, 1999.

"Wave Motion," Microsoft Encarta, 1998. 
 

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