Introduction to Acoustics

2006 Fred Tepfer
1380 Bailey Avenue Eugene, OR 97402
non-commercial use freely granted
 

return to Fred Tepfer's home page
return to School Facilities Reading

Contents

  1. Importance of Acoustics
  2. Notable Failures
  3. The Nature of Sound
  4. Absorption and Reflection of Sound
  5. Transmission of Sound Between Rooms
  6. Amplification of Sound
  7. Myths and Truths
  8. A Pattern for Classroom Acoustics
  9. Glossary
  10. Other Sources

1. Importance of Acoustics

Acoustics are fundamentally important to learning environments. Learning is intrinsically linked with communication, and aural (sound) communication is acoustics. Similarly, learning is about concentration, and external noise is a major distracting factor in education. This article is about typical classroom environments, up to about 1,000 square feet. Large specialized rooms like auditoriums, gyms, and cafeterias needs careful acoustical engineering and should not be designed using the rules of thumb described below.

The importance of acoustics is not limited to classrooms. Noise in corridors and public spaces can soar if they are too reverberant (too much echo), with voices raised louder and louder to overcome the background echo, just like shouting conversations at a noisy cocktail party or restaurant. In addition, sound is an important navigational tool for people who are blind or low vision, and either end of the reverberation scale (too "live" or reverberant, or too "dead" or absorptive) can prevent them from finding their way.

2. Notable Failures

Most of us are familiar with the "schools without walls" of the 1960's. They were based on assumptions about teaching and acoustics that in most cases were not valid. The school without walls is based on the open office environment ["Dilbert" cubicles]. These are moderately successful because of the major acoustical difference between individuals working alone in the offices. However, in the school, groups of people are expected to communicate nearly constantly in the same acoustical environment, which is quite a different acoustical challenge. Solutions appropriate to creating privacy are often inappropriate in learning environments. Beware especially of architects and vendors who think in terms of an office environment while talking about classrooms. The "School Zone" part of the Armstrong Ceilings web site is one example of this. That approach can result in adequate but not excellent learning environments.

[picture of kid moving an accordian-type partition]
What's wrong with this picture? If a wall is light enough to be moved by a child, it is not going to be an effective barrier for sound.

3. The Nature of Sound

Sound is created by vibrations of air or other materials. When someone speaks, their vocal cords vibrate, which creates vibrations in the air that travel to the ears of the listeners much as waves travel across a pond when you throw in a stone. When the sound is higher, the waves are closer together (higher frequency sound) and when the sound is lower, the waves are farther apart. Longer waves (lower sound) pass through thinner materials and curve more easily around barriers. Shorter waves (higher sound) are reflected by relatively thin materials and don't bend much around barriers. Nearly all spoken sound is in the range of 125 Hz (cycles per second) to 4,000 Hz, although people can hear from about 20 Hz to 20,000 Hz. All sound waves carry well through open air, or even through small holes and cracks in walls and ceilings. Because of the logarithmic nature of sound, a small hole will let through a lot of sound.

"Noise" is just unwanted sound, and "signal" is what you are trying to hear. In every sonic envirnment there is background noise, and if the signal isn't much louder than the noise, you will have trouble hearing. In a large room with several groups of students all in conversation, unless enough sound is absorbed instead of reflected off of walls and passed on to the next student, the overall effect is a lot of noise. Typical school cafeterias are built without much (expensive) sound absorbing material, so they are very noisy.

4. Absorption and Reflection of Sound

Sound waves can be reflected or absorbed, and the science of acoustics is largely about what to reflect (send back into the room, what to transmit (sent to the next room), and what to absorb (turn into heat energy). Environments for music want more reverberation, enough to "warm" the sound with reflections. If too much is absorbed, less sound reaches the audience and it sounds "dry" or "dead" , the musicians need to work harder, and the lack of reverberation makes the slightest error more apparent. By contrast, environments for speech want less reverberation, although moderate amounts of reflection are useful to reinforce the sound as long as the overall time that it takes a sound to decay (or die away) isn't too long. A desirable reverberation time for classrooms is about .75 seconds for interactive (discussion-based) spaces and 1.0 seconds for lecture halls. By contrast, a symphony hall might have a two second reverb time. My personal preference for classrooms is toward the reverberant end of what is considered acceptable.

Education is speech-based, whether solely from presenter to listener or a discussion among a whole classroom of students, so the nature of speech informs acoustical design for classrooms. Speech is made up of vowels, which are sounds near the lower end of speech frequencies ("oo", "uh", "ah", etc.), and consonants, most of which are in the upper part of the speech range ("t", "s", "k", etc.).

When we abbreviate written language we remove vowels yet retain meaning, but if we remove only consonants, the sense is usually lost. Take the word baseball, for example, whose consonants are bsbll, still recognizable, and whose vowells are aea, which we don't recognize as being related to the word "baseball". Similarly, if you turn up the treble and turn off the bass in an audio system, speech remains intelligible (try it!). However, if you turn off the treble and turn up the bass, speech becomes a muddy mess.

This suggests that classroom acoustics needs to absorb more in lower ranges of the speech window than in the higher ranges . So how do some common classroom materials perform? NRC, or noise reduction coefficient, is the average the absorption at certain frequencies and is the rating touted by interior materials manufacturers. NRC is a very imperfect indicator acoustical performance. As an average rating, it tells you nothing about which parts of the sound spectrum are being absorbed. Here is a more detailed look broken out by frequency, with higher numbers absorbing more sound, lower numbers absorbing less.

Coefficients of absorption

(table still under construction)        
Material
500 Hz
1000 Hz
2000 Hz
4000 Hz
NRC
carpet (glued down)
.14
.37
.60
.65
concrete block (coarse, bare)
.31
.29
.39
.25
concrete block (painted)
.06
.07
.09
.08
gypsum wall board
.05
.04
.07
.09
wood fiber tile glued to ceiling
5/8" mineral fiber ceiling tile
.55
3/4" mineral fiber ceiling tile
.70
1"  6 lb/cubic ft. fiberglass ceiling tile
.70
.93
.98
1.03
.90

 

Many acoustical materials are optimized for office environments, such as glued-down carpet, acoustical panels, and relatively inexpensive mineral ceiling tile. This environment is designed to provide speech privacy by muddying consonants (high frequency sound) and not absorbing vowes (low frequencies), making speech unintelligible a short distance away. This is the REVERSE of good classroom design where you want to reinforce speech intelligibility. Keep reading for a pattern for excellent classroom acoustics for learning

Open office environment with acoustical partitions, acoustical ceiling, and glue-down carpet

Absorption of sound is particularly difficult in special environments like cafeterias, kitchens, gymnasiums, and swimming pools. Conventional materials may be subject to damage, or absorb odors, or be incompatible in other ways. However, materials do exist that work well. For example, for a gym, walls can be built of a special slotted concrete block. Because the absorption is inside the block's core, no amount of ball impact can compromise its integrity.

5. Transmission of Sound Between Rooms

The control of noise from one room to another is the other major challenge in acoustics. As with absorption, different materials transmit more or less sound at different frequencies. In transmission, blocking the entire speech range is important, and this factor is reflected in the STC rating (Sound Transmission Class) of a wall. Unlike the NRC, the STC takes into account the performance of the wall at its worst frequency. The STC value of a wall is a relatively reliable relative indicator of the number of decibels of attenuation that can be expected from a wall system. STC values are a very useful tool in acoustical design but should be downrated for actual field conditions, as even the best installation will never match the lab rating.

Critical to sound transmission issues is the background noise in the receiving room. If the background noise is higher than the amount of sound passing through (and around) the wall, then users won't hear the sound from the adjacent room. If background is lower than what's transmitted, then room occupants will hear sounds from the adjoining space. This tells us that some acoustical problems can be overcome by increasing background sound, but that can cause problems as people need to raise their voices to be heard.

Transmission tips:

  • In general, materials with more mass block sound more than lighter materials, especially for low frequencies (which are very hard to block)
  • Sound likes to travel around barriers. To be fully effective, walls should go from floor to the structure above, and holes should be carefully caulked or filled. Even small holes make a big difference. Most transmission failures are caused by cracks and holes.
  • Batt insulation is useful, but it has to be installed very carefully or the effectiveness is compromised. It's best to have it inspected by someone who understands this before walls are covered up. Unlike thermal insulation, even minor gaps, cracks, and other installation problems result in a major reduction in effectiveness.
  • Making the wall finishes of different thicknesses/masses helps stop sound waves, perhaps by adding an extra layer of gypsum board to one side of the wall.
  • More effective than batt insulation is decoupling of the two faces of the wall by having two sets of studs, one for each wall surface (staggered studs). Resilient channels can be used with similar effects
wall with staggered studs
Wall with staggered studs

6. Amplification of Sound

Sound amplification goes hand in hand with education in this day and age. Most media now include sound (videos, CD/DVD, Internet content, etc.), so a room needs to provide for and be friendly to a loudspeaker system. On the other hand, except for large rooms not commonly encountered in K-12 education, voice amplification is not normally used, with two significant exceptions:

7. Myths and Truths

Myth: control noise just by installing carpet. Carpet is not an acoustical panacea. It is effective for reducing noise from feet and movement of furniture. However, for absorbing noise, the carpet system commonly used in schools (glued down) absorbs mostly in the high frequencies of speech, i.e. the consanants. Padding the feet of furniture can help with furniture noise, but doesn't help with foot noise.

Myth: Ceiling tile is enough. Acoustical ceiling tile, either glued to the ceiling (usually in 12" squares) or sitting loosely in the suspended steel grid (usually 2 foot by 2 foot or 2 foot by 4 foot), can be a very effective means of absorbing sound. However,there are many types of tile with different absorption characteristics (wood fiber, mineral fiber, glass fiber, etc.). To ensure effectiveness, someone has to match the acoustical problem to the solution. Furthermore, nearly all ceiling tile is ruined by painting unless very special paints are used with great care. Many older schools develop acoustical problems when acoustical tile is painted after it gets dirty or stained or just looks old. "Best Practices" classroom acoustical design for classroom interaction usually makes the center of the ceiling reflective, and provides high-performance acoustical tile near the perimeter. Misplaced or poorly selected acoustical material can require louder voices and ultimately vocal stress.

Truth: Heating and cooling systems can be a major acoustical problem. HVAC systems can contribute to noise three ways.

  1. The heating/ventilating equipment, the fans and terminal units, make noise. This noise can be transferred directly, usually to the spaces under, over, or next to the unit.
  2. HVAC systemscan also created noise through the distribution system because of excess air or water velocity (pipes or ducts too small) or through poor workmanship (such as small holes that create hisses and whistles).
  3. Air ducts can also create a path for sound to travel between rooms, solved only with expensive sound traps (which also increase fan energy consumption). For this reason, ducts should generally not go from room to room but should be branched off from a main in the corridor.

Truth: Outdoor noise can be a significant problem. The location of a school can be a significant determinant of acoustical noise problems, especially if buildling users are expected to open windows. Locations near major roads, airports, or any other source of noise can be a major problem, in some cases even with the windows closed. Measurement of ambient noise should be a part of site selection for a school.

8. Pattern

For a typical classroom, keep the head wall (front) reflective with hard surfaces. Provide absorptive materials around the sides and back of the ceiling and the upper part of the wall, using a high NC material (.8 or higher) such as one inch thick compressed fiberglass. Provide a relatively non-absorbent material in the middle of the ceiling and on the rest of the wall surfaces. Consider using carpet to reduce foot and furniture noise (or use other methods). Also consider a sloping ceiling at the front of the classroom to reflect more sound out into the room.

[diagram and link to nonoise.org article]

Other sources to consult:

The best classroom acoustics source: http://www.nonoise.org/library/classroom/
A good source: http://www.mbiproducts.com/roomacoustics.html
Dictionary of terms: http://www.armstrong.com/commceilingsna/glossary.html

Glossary

Selected glossary items:

Absorption In acoustics, the energy of sound waves being taken in and trapped within a material rather than being bounced off or reflected. Materials are rated in terms of their ability to absorb sounds.

Articulation Class (AC) Rates the listener's ability to understand the spoken work within a space, expressed as a decimal with 1.0 being perfectly understandable. The privacy index is derived from the A1 calculation. Lower A1 ratings (less than 0.2) indicate that adjacent spoken words are less intelligible, therefore less distracting. The sum of the weighted sound attenuations in a series of 15 test bands. Note: AC has replaced Noise Isolation Class (NIC) as the accepted industry standard performance value. NIC is based on hearing sensitivity rather than discernment of actual speech, which is the primary concern in open office layouts prevalent in acoustical design work. Verify the rating methodolgy with manufacturer's published data.

Ceiling Attenuation Class (CAC) Rates a ceiling's efficiency as a barrier to airborne sound transmission between adjacent closed offices. Shown as a minimum value, previously expressed as CSTC (Ceiling Sound Transmission Class). A single-figure rating derived from the normalized ceiling attenuation values in accordance with classification ASTM E 413, except that the resultant rating shall be designated ceiling attenuation class. (Defined in ASTM E 1414.) An acoustical unit with a high CAC may have a low NRC.

DBA (A-weighted decibel) A single-number measurement based on the decibel but weighted to approximate the response of the human ear with respect to frequencies.

Decibel (dB) - A unit to express differences in power. In acoustics, equal to ten times the logarithm of the ratio of one sound and lower-intensity reference sound. One decibel indicates a difference of about 26% and is about the smallest change the ear can detect. The dB level is a logarithm quantity; the maximum normal level is approximately 120dB

Fiberglass Panels - Glass strands laid in mats and formed into a rigid or semi-rigid board, sometimes requiring a separate stable material laminated to the fiberglass.

Mineral Wool - A man-made wool-like material of fine inorganic fibers made from slag, used as loose fill or formed into blanket, batt, clock, board, or slab shapes for thermal and acoustical insulation.

Noise Reduction Coefficient (NRC) - Average sound absorption coefficient measured at four frequencies: 250, 500, 1,000 and 2,000 Hz which rates how well a ceiling or wall panel absorbs sound. NRC is the fraction of sound energy, averaged over all angles of direction and from low to high sound frequencies that is absorbed and not reflected.

Reverberation Time - Time required for a sound to decay to a value one millionth of its original intensity or to drop 60 decibels.

Sound Transmission Class (STC) - A single-number rating of a wall or ceiling's efficiency as a barrier to airborne sound at 16 speech frequencies from 125 to 4000 Hz. STC is a decibel measure of the difference between the sound energy striking the panel or construction on one side and the sound energy transmitted from the other side.

return to reading index

RESPONSE FORM

(you can also respond via e-mail)

Please list the article that you are responding to:
What (if anything) in your current situation or background relates to the information in this article?
How is this applicable to your work?

Do you have a question related to or provoked by the material? How would you suggest that this article be improved?

Your name and e-mail address:
Select or

::::::::::::return to Fred Tepfer's home page:::::::: return to School Facilities Reading:::::::::::::::