From The Wall Street Journal Online
"School Builders Design For Better Acoustics"
Amid the largest U.S. school-construction boom in a generation, designers and architects are worrying about more than just how classrooms look. They are also concerned about how the rooms sound.
The usual hard-surfaced finishes in classrooms have always created an echo-chamber quality. But recent changes have made matters worse. Indoor air-quality requirements force schools to install noisy heating and cooling equipment. Suburban sprawl brings roads and schools closer together, making traffic noise a problem. And even modern teaching methods, which emphasize group work over lectures, add more voices and more acoustic clutter to the environment.
More than an annoyance, the increasing classroom clatter can be a detriment to education. According to educational experts, poor acoustics are one of the biggest treatable obstacles to learning. Studies have found that students, regardless of hearing ability, perform worse in noisy classrooms than those in quiet ones, even within the same school. And teachers feel it, too, by having to lecture above the racket. They miss an average of two days per year due to vocal fatigue, according to the National Center for Education Statistics.
The effect of poor acoustics is particularly acute among those without full command of speech, including young children and students whose first language isn't English. People in the early stages of language acquisition, be it as a second language or young children, don't have the ability to "fill in the blanks" when they miss a syllable or hear a word incorrectly, says Donna Ellis, head of Washington, D.C.'s efforts to improve acoustics in its classrooms.
Also, the movement to mainstream hearing-impaired students into regular classrooms means there are more pupils with lower baseline auditory ability. And the increased incidence of middle-ear infections means many more grade schoolers experience temporary hearing loss at some point during the school year. "If in that time they teach something like long division, you might miss something crucial," says David Lubman, an acoustical consultant in Westminster, Calif.
One of the fastest-growing school districts in the country, Clark County, Nev., home to Las Vegas and 13 new schools a year, has had stringent acoustical standards since the mid-1980s. The district uses carpeting, suspended acoustical ceilings and walls that go all the way to the roof deck to prevent sound from oozing from one classroom to the next. But in many parts of the country, the acoustical movement is only now starting to take place, in large part advanced by new standards in construction and design guidelines.
Last year, the American National Standards Institute, a Washington nonprofit that administers thousands of voluntary standards, approved acoustical benchmarks to limit background noise and reverberation in schoolhouses. The states of New York and Washington already have similar sound standards. Los Angeles Unified School District, in the middle of a $3.63 billion construction program, has acoustical guidelines for its designers. And it's not just a U.S. movement. The United Kingdom recently adopted standards on classroom noise, and the World Health Organization devised its own guidelines for nations to adopt.
"Acoustics are a critical factor now whenever you are looking at classroom design," says Tim Dufault, principal at Cuningham Group Architecture, a Minneapolis architecture firm.
The new standards have their critics, who say schools can't afford to make every classroom as quiet as can be. And even the staunchest supporters of creating adequate classroom sound acknowledge it adds 0.5% to 2% to an overall construction budget -- at a time when local governments are gushing red ink. But thanks to an avalanche of funding measures passed by states and school districts when times were good, schools are one of the few strong spots in the construction industry.
"You have a combination of money already voted on, money not yet spent, and new money that the public seems willing to spend -- plus there's simply the pressure of more kids," says Paul Abramson, an educational consultant in Larchmont, N.Y., He predicts school-construction spending will stay near or above $20 billion a year for at least the next few years.
The Burroughs Elementary School in Minneapolis, set to open this fall, will be the first in that city to meet sound-quality targets adopted for new construction in November 2001. Edward Kodet, the architect on the project, says the goal is that the "student who sits in the back can hear as well as the student who sits in the front."
He has done things like add ceilings that slope from front to back so sound "carries, but doesn't echo." The footprint of the rooms, more trapezoid than rectangle, reduces the tendency of sound to reverberate. In terms of materials, the classrooms have double layers of sound-absorbing ceiling tiles, insulated glass windows, and thicker walls where they abut raucous spaces such as stairwells.
Some of the biggest noise culprits in schools are the more-robust heating and air-conditioning systems required in many states for indoor air quality -- exactly what spurred Minneapolis to think about sound.
"We had been doing a lot of HVAC [heating, ventilation, air conditioning] renovations, and getting noisy systems that drove us to where we needed to make acoustics a priority," says Lee Setter, environmental specialist for Minneapolis schools.
Such was the case at the Downtown School, a magnet facility in Minneapolis. It was built in 1999 and immediately drew the ire of teachers and parents because of the noise coming from its climate-control system, combined with its open-classroom design. "Not everybody can filter it out," says Lee Fertig, the school's director.
Modifications including larger walls, additional carpeting and sound-absorbing panels, solved the problem. "The noise-interference level went way down," says Mr. Fertig.
Clanging air conditioners are hardly the only problem. Sound bleeds from one room to the next and from outside sources such as highways and airplanes. One design strategy in the fight against noise is to plug holes, as on a ship.
"Sound is like water," says Matt Ciaglo, an architect with Fletcher Thompson Inc. in Hartford, Conn. "It finds the smallest gap." As a matter of course, his firm uses soundproof caulk along seams between drywall and the floor and adds sound-attenuation blankets in the walls.
Further techniques include using different drywall thicknesses -- one of them five-eighths of an inch, the other three-quarters of an inch -- on either side of a wall. The two widths absorb different sound frequencies and together prevent both low- and high-pitched sounds from getting through. Also, staggering doors in a hallway so classroom entrances aren't directly across from each other and installing carpeting to reduce foot noise can reduce unwanted din.
If acoustic standards for schools were in place nationwide, the additional spending could equal $100 million to $400 million a year, based on current construction spending budgets. As it is, the growing movement has a number of beneficiaries in industry.
Heating and air-conditioning manufacturers, already reaping the benefits of indoor air-quality rules that require their products, now have an additional service to sell -- keeping the equipment muffled. "In all types of buildings, as you move to quieter spaces, typically the cost does go up," says Gary Luepke, applications engineer for Trane, a subsidiary of American Standard Cos. that makes indoor climate-control systems.
Also, manufacturers of materials such as acoustic ceilings, carpets, and sound absorbing insulation will see increased demand for their wares. The Carpet and Rug Institute, an industry trade group, puts acoustics at the top of the list in promoting its products to schools. Armstrong World Industries Inc., Lancaster, Pa., has set up an "acoustic calculator" to assist school designers and architects in selecting its products.
The strongest opposition to the new standards comes from the modular-classroom industry, which call them too onerous. "It's not that we don't want quieter standards," says Susan Stewart, a lobbyist for the modular-classroom industry in Sacramento, Calif. "But the cost in some of these areas would be exorbitant."
Not all sound-abatement tactics are costly. Teachers in Washington are known to put old tennis balls on the end of chair legs to eliminate squeaking on the floor when students fidget at their desks.
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This article describes how RR Audio Laboratory, an acoustics company in California, goes about studying the acoustical character of a space for their clients.
RR Audio Laboratory
636 E. Harvard Rd. unit B
Burbank, CA 91501 USA
Telephone : (818) 843-8212
Facsimile : (818) 563-9372
E-mail : rr@trapagon.com
"Acoustical Engineering"
Most room environments are not designed with the acoustical properties in mind, but rather other aesthetic or practical objectives are of primary importance - until it becomes necessary to either work with sound or enjoy the listening experience without the room's signature detracting from the ability to distinguish detail, proper spectral balance or spatial cues. The answer is to either design the room from the beginning taking into consideration the acoustical properties, or to correct the problems later using any of several techniques.
RR Audio provides acoustical analysis and design services for recording and project studios as well as for home environments.
If you are planning a new room design or want to correct the one you have, give us a call.
Acoustics
Acoustical problems in critical listening environments can be a real nightmare. In the case of a project studio in the home, what normally would make for a comfortable den or bedroom can be a disaster when you are trying to evaluate your latest recording. In the case of a working recording studio, anomalies in the room acoustics can be inadvertently compensated for when mixing the recording, causing the mix to be incorrectly balanced when taken out of the original environment. In the home listening environment, although acoustical problems are not threatening your livelihood, they can be extremely annoying in that they interfere with the ability to "get inside of the music", creating the illusion that you are "there".
In each of the above cases, what is needed is to evaluate the environment to determine where the problems lie in the frequency and time domains, to extract from this information a model of what modifications need to be done to the room to alleviate the problems, and to decide which types of devices and materials should be used to both achieve the desired results and stay within the budget. The same thing holds true when creating a room from scratch. If the room is designed from the ground up to be acoustically balanced, not only is the end result much more satisfactory, but the cost to build the room is generally less than fixing mistakes after the room is done.
The first step in correcting the acoustics of a room is the analysis :
Analysis
Acoustical analysis is accomplished by generating a series of impulses (similar to handclaps) which contain all frequencies up to ½ the sampling rate used and feeding the signal into the room via the playback system. Several different signals with different sampling rates are used to allow for the maximum detail in the data to be acquired (tests are done with data to 23kHz, then another test is done with data to 4kHz, then again with data to 500Hz). The reason for this is it allows for more data points to be available at the lower frequencies, so that when zooming in on the lower frequencies there is enough data (without large gaps between data points) to evaluate the problems without having to interpolate data. Microphone placement in the listening position is tested first, then additional tests are done with the microphone in specific problem areas to confirm that the problem data is correct.
Once data is acquired, the information is displayed in the time domain. This allows the viewing of the reaction of the system to the impulse, and shows when in time any reflections occurred (which allows the calculation of their origination based upon the distance from the microphone) as well as the amplitude of those reflections and any ringing, etc. This data can then be transformed into the frequency domain, where frequency spectrum response curves (amplitude vs. frequency), phase response (phase shift vs. frequency), group delay (time delay, both positive and negative, vs. frequency), spectral decay (amplitude vs. frequency vs. time "waterfall" plots) and other data presentations can be generated. These plots allow for very detailed analysis of the problems, and provide the information required to find the problem areas with certainty and design the correct modification program the first time.
Evaluation
Once the data has been acquired and transformed into the various plot types, the information is evaluated and design work is begun to correct the problems. Many different solutions are available to correct the various kinds of problems which may exist — some methods being more costly than others. After determining which are the primary causes of the problems, several methods of correction are outlined and the required materials and devices for correcting the problems are listed in ways which allow choice as to method vs. budget. The client is then consulted as to which method is preferable, at which point design of the final system is begun.
Design
Computer-aided design of devices, layout of the room, and plans for building the various devices are completed and final approval is gained from the client before construction is begun. Some of the devices used in the correction of typical acoustical problems include:
Quadratic Diffusors — generally n=17 or n=29 types, these are devices which reradiate sound, allowing the frequency response of the reradiated sound to be much smoother than when reflected off of a flat surface. They consist of carefully calculated wells of specific depth and width at staggered frequencies. A complex diffusor would be a composite of an n=29 unit on the left which operates from 240Hz to 1700Hz, a center n=17 unit which operates from 966Hz to 7728Hz, and a duplicate of the left n=29 unit on the right. An n=29 type has 28 wells, and diffuses 14 frequencies and the corresponding harmonics - n=17 types have 16 wells and diffuse 8 frequencies + harmonics.
Parabolic Reflectors — these are curved devices which are generally placed on walls (a different type is used on ceilings) to spray the soundwave on paths which tend to break up standing wave modes. When used on ceilings (generally over a larger area than the wall-types) these devices not only break up the standing wave between parallel ceiling and floor, but reflect the soundwave towards the walls, where absorption by the wall treatment and any acoustical panels can occur.
Wall Treatment — many times the most attractive and least expensive method (depending upon chosen materials) of creating absorption and reflection zones is to treat the wall surfaces permanently using absorptive materials chosen for the proper absorption coefficient for the specific problem area and reflective materials (like sheets of damped wall paneling) interspersed strategically. Absorptive materials (and non-finish-grade reflective materials) are covered with fabric and trimmed with moulding, piping, etc. to maintain the decor of the room. In many cases, the finished result does not look like any acoustical treatment has been done at all (these are affectionately called "camouflage jobs" by our silly designer).
Acoustical Panels — these are permanent or portable devices which either absorb energy, reflect energy, or do a combination of both jobs. Panels when strategically placed can damp problem areas, reflect energy into trap zones, or form the trap zones themselves. These devices can be constructed in such a way as to be very attractive or extremely inexpensive, depending upon requirements and the materials which are used. We have made panels with everything from simple cloth-covered luan exterior frames with inexpensive cloth over the standard inner frame and damping materials to solid-hardwood framed exteriors with raw silk over the interior materials.
Bass Traps and Resonators — many times a single frequency and its harmonics are causing a serious problem. In these cases, it is generally preferable to trap the problem frequency with a tuned device which cancels or reinforces the fundamental frequency rather than use an equalizer, as any signal processor adds noise and other anomalies. These devices work on the same principle as blowing across the mouth of a Coke bottle. If there is liquid in the bottle, depending upon the volume of the bottle and the size of the mouth, a specific note is emitted. Drink some of the liquid and the note changes. Helmholz resonators of this type can greatly reduce problems associated with specific frequencies, and in most cases eliminate them entirely. Other types of Trapping devices can be created by designing a damped hidden chamber which is tuned to one or more frequencies, or using structures (like highly absorptive panels) to create a trap zone in a corner or other strategic area. When properly used, a minimum of devices can correct for a major low frequency problem.
If you would like to ask questions regarding the products and services we offer, or would like us to send you more information, please let us know. We would be happy to provide you with whatever information you require.
RR Audio Laboratory
636 E. Harvard Rd. unit B
Burbank, CA 91501 USA
Telephone : (818) 843-8212
Facsimile : (818) 563-9372
E-mail : rr@trapagon.com