A segment of the sound wave front surface area increasing with distance. In this angle, the same sound energy is distributed over the spherical surfaces of increasing areas a-s-d is increased. The intensity of the sound is inversely proportional to the square of the distance of the wave front from the signal source. Example: 1d-1, 2d-4, 3d-9, 4d-16
Generally speaking, when we take measurements in a room we do so in the A weighted scale but we might also want to determine if there is a higher low frequency content to the sound spectrum. HVAC noise often has significant low frequency content and if you can hear the HVAC you might want to confirm the nature of the HVAC noise. By switching the weighting button or switch to the C weighted scale you can determine if the low frequency noise is significant. The C weighted scale eliminates the low frequency noise discount that we get in the A weighted scale. If the C weighted noise level is much higher than the same measurement taken in the A weighting, that is a strong indication of a significant low frequency sound content. (Remember the equal loudness contour)
Many classrooms today have computers in the room that are left on most of the time. The CPU's frequently will have fans in them that are quite audible. Note where the equipment is located. Often the CPU is located in a corner or under a desk with adjacent hard surfaces that can reflect and amplify the sound toward the students. Many classrooms also have TV sets, often mounted in a corner close to walls separating classrooms. I have seen classrooms so equipped where the TV set in one classroom was closer to a student in the adjacent classroom than he/was to the teacher in that classroom.
Horror stories abound with respect to background noise levels, such as the Los Angeles Unified School District (LAUSD) who recently equipped hundreds of their classrooms with $400 million dollars worth of window unit air conditioners that had operational noise levels in the plus 50 to plus 60 dBA ranges. Sadly, the LAUSD knew about the ANSI standards but elected to ignore the standards and the scientific rationale that prompted their development.
Identification and quantifying noise levels is an important aspect to classroom evaluation in order to achieve a good signal to noise ratio. It is also important to for evaluation of a room to determine if the room is suitable for the application of Sound Field Amplification equipment.
Reverberation and Reverberation Time
Reverberations along with background noise are the two elements most frequently cited, that can have a negative impact on speech intelligibility. Reverberation is the persistence of sound in a room once the sound signal has abruptly ceased. Reverberation time is the time in seconds that it takes for the sound to die down by 60 decibels or put another way to 1/1,000,000 of its original intensity one the signal has ceased.
Sound propagates spherically in all directions, much like blowing up a very large balloon. Sound waves travel at about 770 miles per hour (1132 feet/second) in air and when the waves contact a hard surface they are reflected, time and time again until the sound energy is absorbed and can no longer be heard. Reverberation is much more audible in a large voluminous space like a gymnasium. It is quite easy to hear the reflected sounds in the form of a hissing or even a clicking sound. Any sound in an untreated gymnasium immediately sounds hollow or it echoes. The same is true even for smaller classrooms but is not quite as easy to detect. In continuous speech the reflections arriving at the student's ear milliseconds after the direct speech sound tend to smear the clarity of the speech signal. You also need to recognize that beyond the critical distance of direct speech, the reflected sounds can sound louder than the direct speech signal.
When reverberation occurs in a hard surfaced room the sounds can actually increase in intensity and is known as a sound build-up caused by the combination of direct and reflected sound energy. If you have ever driven through a tunnel with the windows down you will notice how much louder the sound is. Once again this is an increase due to the combination of direct and reflected sounds. It is not necessary to understand these phenomena as much as it is to recognize that they exist.
Reverberation is dependent on only two things, (a) the volume of the space and, (b) the amount of absorption in the space. As in the case of a large, hard surfaced gymnasium the RT is much longer.
Reverberation Time can be measured with sophisticated sound measuring equipment where the sound sensitive equipment measures the time it takes for a generated signal to decay by 60 decibels upon cessation of the signal. The SLM records the sound signal and decay times at all appropriate frequencies simultaneously. Reverberation Time can also be calculated with a surprising degree of accuracy manually or by a computer aided software program.
The recommended RT for a typical classroom of 10,000 cubic feet or less, contained in the new ANSI standards should not exceed 0.6 seconds at each of the frequencies of 500 Hz, 1000 Hz and 2000 Hz. For classrooms between 10,000 and 20,000 cubic feet in volume, the RT should not exceed 0.7 seconds at the three frequencies.
The Sabine Formula for calculating the RT in a space is a simple mathematical formula presented as T= 0.049V/Sa where T is the reverberation time in seconds, 0.049 is a constant, V is the volume of the space, S is the surface area of all the surfaces and a is the absorption coefficient of the building material at a given frequency.
Calculating the RT manually can be rather tedious and somewhat time consuming but it can be done without too much difficulty. You will have to know what the absorption coefficients are for common building materials, which are readily available from a variety of sources. Practically all-acoustical materials have been tested and their manufacturers do publish the absorption coefficients. For the more common building material absorption coefficients a listing can be found at www.acousticalsurfaces.com under acoustics 101.
The acoustical absorption coefficient of a material is defined, as the incident sound that strikes the material that is not reflected. More simply put it is the percentage of sound of a particular frequency that strikes the material that is absorbed. Absorption coefficients range from 0.0 to 1.0. The higher the absorption coefficient the better the absorption. Frequently you will see acoustical materials that are described as having an NRC of, for example .65. The NRC or Noise Reduction Coefficient is a single number rating that is based on the average of the individual absorption coefficients at 250 Hz, 500 Hz, 1000 Hz and 2000 Hz to the nearest 0.05. The higher the NRC, generally, the higher the absorption characteristics of the material. Many hard surfaces building materials will have an NRC of only 0.01 to 0.10 while highly absorptive materials that are soft and fibrous in nature will have an NRC of 0.90 to 1.00.
A more cost-effective method of calculating the Reverberation Time has been developed by the author, which is now available as a service from Acoustical Surfaces Inc., a Chaska, Minnesota based acoustical materials distributor. ASI will provide you with a questionnaire from which all of the required information is entered into a computer program and the RT is calculated on a worksheet and in report form. Known as RASP (Reverberation Analysis Software Program) the service is available for $200.00 for the first room analysis and $100.00 for each subsequent room on the same project.
In addition to calculating the RT of an existing or a proposed room, a second worksheet is also presented with acoustical recommendations on what options are available to correct the room acoustically to comply with the ANSI standards. Another feature of the program is the ability to enter background noise levels and distance factors in order to approximate the speech intelligibility as a percentage of speech understood correctly in accordance with the default data selected. When comparing an existing space with a treated space the program will also determine the decibel reduction of the reflected sound and further provides a graph from which the percentage of sound reduction as perceived by the human ear, can be charted.
The RASP program currently offered, as an acoustical analysis service may also be available as a user friendly, point, select, and click software program in the not too distant future. The program is designed to run on Windows and is not only a practical tool for calculating RT but is also an educational tool for designers and other persons interested in acoustics and sound. The author compared numerous calculated results with those of field-measured data. Not all classrooms are the same and so it was found that certain correction factors needed to be made with the result that the correlation between the two methods was surprisingly close. The ANSI standards cite the use of the Sabine Formula as a simple means of validating compliance with the standard.
Equally important to the data developed is the manner in which the data is presented so that it can be easily understood by most laypersons. A computer printout commands attention and respect for the data and may be much ore meaningful than a bunch of numbers within a written report. Graphic Typical worksheet of RASP RT program
Understanding the data and evaluating the potential solutions
The only sound that passes through a material is that which can snake through a leak in the material. When sound strikes a wall, the sound energy sets the wall in vibration, much like a radio signal to a speaker. The wall then becomes the transmitter and relays the signal on the opposite side of the wall, albeit at a reduced level of intensity. Some of the intensity of the sound signal will be lost in energy to heat conversion process as the sound waves energize the material.
In the case of acoustically absorptive materials such as acoustical ceiling tile or acoustical wall panels, the sound waves striking the material also cause the fibers to vibrate thereby creating and energy to heat conversion that reduces the sound energy. Acoustical ceiling tiles are the most commonly used acoustical materials because the ceiling area is normally the only available surface area that is not used for other purposes. Classroom walls often have to accommodate blackboards, doors or other wall mounted fixtures. Window walls have glass for light and a view. Floors need to be durable enough to walk on or locate furniture on. Ceiling surfaces are therefore the most logical surface to install acoustics. With sound bouncing around at 770 miles per hour the acoustical ceiling is going to be bombarded with direct and reflected sound waves where some percentage will be absorbed.
All ceiling tile have one characteristic in common and that is that they are fibrous in nature though, some are better than others. The most common ceiling tile is made from a mineral fiber, which is relatively hard. The tiles are frequently perforated with a textured pattern. Other tiles may be manufactured from fiberglass and will be quite distinctive in their appearance either on the back or the edges. Some older schools may have the old 12"x12" standard round hole perforations that are manufactured from wood or mineral fiber.
Most relatively modern ceiling tiles will be 2'x2' or 2'x4' in size and lay in a tee bar grid suspension system. Just because the ceiling is acoustical in nature, do not be deluded in to thinking the acoustics are OK. They may not be, because the tile's acoustical performance is not high enough to meet the recommended ANSI standards.
When evaluating the classroom for background noise, let us suppose that you have identified the problem as one stemming from the transmission of noise from other classrooms. It may be that the partition leaks because they only extend to the underside of the acoustical ceiling. The solution is to block the path of sound through the ceiling over the wall and back through the ceiling next door. Sealing any partition leaks with a gasket or caulking compound can solve this problem. Alternatively, barrier material can be installed in the ceiling space to block off the sound transmitting through the ceiling space. To be effective it has to be handled with detail but rest assured the problem can be resolved with relative ease.
The STC (Sound Transmission Class) is a single number rating that determines a structure's noise blocking performance. The higher the number, the better the construction. The sound levels are measured at each frequency and the resulting noise reduction levels are applied to an STC chart similar to the inverse of the equal loudness contours. In other words, once again the low frequency sound levels are discounted to reflect how we hear.
Another way to evaluate the STC performance of a partition or wall is to set a radio in one room with the volume turned up to about 70 decibels and then go to the adjacent room and see if the sound can be heard easily. You can also measure the sound in the source room and then in the receiving room to determine the difference. This is known as the noise reduction of the wall but in reality it is the total noise reduction from one room to another by all pathways.
Recommended STC performance data for walls and floors between classrooms and other spaces are listed in the new ANSI S-12.60-2002 Classroom Acoustics standard.
As you look at the ceiling in a building that has central heating and air conditioning (HVAC) the problem could be due to noise from the ductwork or the ceiling vents. HVAC ceiling silencers are available to reduce this source of noise by way of sound reducing sound trap ceiling diffusers. The thing you need to know is that there are solutions and that help is available to determine which is the most cost effective solution. In some case HVAC noise may be nothing more than proper maintenance to the equipment or a matter of reducing the fan speed of the unit.
If the sound is coming through the windows, many times the windows simply need to be gasketed with an inexpensive foam seal. If the problem of sound intrusion is through thin glass it may be necessary to install a second layer of glass much like a storm window or screen. If the sound is coming through the door to the classroom, the door frame can be gasketed with a foam sealant and an inexpensive drop seal applied to the bottom of the door that drops down when the door is closed.
Many, many problems are simple problems to identify and fix when you know how. It is not a matter of rocket science but rather, more a matter of practical knowledge. That isn't to say that there are not more serious problems that are more difficult to fix in which case you may need to resort to more extensive expertise.
Reducing reflected noise and Reverberation Time
Replacing old ineffective ceiling tile with a newer higher performing acoustical tile is an option and is easy to accomplish. Be sure to ask what the NRC values of the tile are, the higher the better.
If it is not possible to replace the ceiling tile or only a small amount of additional acoustical treatment is required to correct the room's acoustics, consider acoustical wall panels.
Adding acoustical wall panels will reduce the reflected noise and also reduce the reverberation time. One thing you will not have to worry about too much and that is where to install the wall panels. In the average classroom with chalk and bulletin boards you will be limited thus the solution to where you can place the material is simply, "any where you can". Sound waves generally make no great distinction as to where acoustical treatment should be placed; traveling at 770 miles per hour around the room the sound waves will find the acoustical material sooner rather than later. Generally speaking wall space is available up high on the walls often above the chalkboards; this is OK.