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How to measure reverberation time?



Reverberation time is defined as the length of time required for sound to decay 60 decibels from its initial level. Classrooms should have reverberation times in the range of 0.4-0.6 seconds, but many existing classrooms have reverberation times of 1 second or more. In such cases, the teacher is competing against the lingering reflections of his or her own voice for the student's attention. The result is a chaotic jumble of sounds.

There are two ways to determine reverberation time: 1) to measure it with a meter or 2) to estimate it. If you want to actually measure reverberation time, you'll need to hire an acoustical consultant. But, you can make a fairly simple estimate for the classrooms you suspect are problems. The following section from the booklet Classroom Acoustics describes the procedure.

Don't be intimidated by the math. If you don't understand the procedure, someone at your school or high school math department should be able to do the calculations for you if you take the room measurements described below (the following is from Classroom Acoustics I):

REVERBERATION TIME

Over 100 years ago, a Harvard physics professor named Wallace Clement Sabine developed the first equation for reverberation time, which has since been named after him and is still used today. Reverberation time is defined as the length of time required for sound to decay 60 dB from its initial level. Sabine’s simple formula is:

where:

RT(60) = reverberation time (sec)
V = room volume (ft3 )
S = surface area (ft 2 )
a = absorption coefficient of material(s) at given frequency
S indicates the summation of S times ? for all room surfaces

To use this formula, the volume of the room, surface area of each material in the room, and absorption coefficients for those materials must be known. Absorption coefficients are measured in specialized laboratories, and represent the fraction of sound energy (not sound level-dB) the material will absorb as a decimal from 0 to 1. Figure 15 gives absorption coefficients for common classroom materials.

A commonly used one-number rating called NRC, Noise Reduction Coefficient, is simply the average of the absorption coefficients at 250, 500, 1000, and 2000 Hz. This simple, one-number rating can be useful for comparing the relative absorption of two materials; however, examining absorption coefficients in each octave band gives a better idea of the performance of a material at various frequencies.

Reverberation time is often calculated with the room unoccupied. Since people and their clothing provide additional sound absorption, an unoccupied room is the worst-case scenario, though not an unreasonable one, since occupancy of most classrooms varies. In a complete analysis, this calculation should be performed for each octave band, as the RT can vary widely at different frequencies. However, for a quick estimate, the RT of a classroom can be calculated for just one octave band representative of speech frequencies, such as 1000 Hz. If this RT is acceptable, then the RT throughout the speech range will likely be acceptable.

To demonstrate the use of the Sabine equation, Figure 16 provides an example calculation of the RT at 500 Hz for the acoustically poor classroom example given in Figure 10a. Try calculating the RT at 500 Hz of the acoustically satisfactory classroom in Figure 10b with only a sound-absorbing ceiling added. Note that the ceiling is lower in that example, so the volume and surface areas will change. The RT of the satisfactory classroom is approximately 0.4 seconds.