Reverberation time

In this session we will learn the basics of reverberation time and the associated calculations

1. Reverberation time Mr. Mota Manoj H. (Assistant professor, SITCOE) Mr.Kate Ganesh (Assistant professor, ADCET)

2. Reverberation happens when the sound produced in an enclosure hits a hard reflective surface.  The sound reflects back in the room and ads to the original source. The strength of the reverberation decreases with the distance from the source to the reverberating surface. In some cases, the reverberated sound may dominate the original sound. In such cases, padding the reflective surfaces with sound absorbing materials will reduce the sound level. Reverberation time

3. The  reverberation time  of a room or space is defined as the  time  it takes for sound to decay by 60dB. For example, if the sound in a room took 10 seconds to decay from 100dB to 40dB, the  reverberation time  would be 10 seconds. This can also be written as the T 60   time .

4. How to Calculate Reverberation Time Measuring the reverberation time of a space is a good way to identify a noise control problem. If your large open space is plagued by echo and difficulty understanding speech, you may have a reverberation problem. Here we will discuss how to calculate the reverberation time for your multi-purpose use space. Acoustic Calculation The first step to calculate the Sabins (a) with the below equation. Formula for Sabins : a = Σ S α Where: Σ = sabins (total room absorption at given frequency) S = surface area of material (feet squared) α = sound absorption coefficient at given frequency or the NRC

5. After we calculate a, we can then use the Sabine Formula to calculate the reverberation time. Sabine’s Formula: RT60 = 0.049 V/a Where: RT60 = Reverberation Time V = volume of the space (feet cubed) a = sabins (total room absorption at given frequency) Remember: Units used in the formula id feet. For unit of meter the formula is modified as follows….. RT60 = 0.161 V/a

6.   Location Material Absorption Coefficient (a) or Sabins Octave Band Center Frequencies 125 Hz 250 Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz All Walls Gypsum Board 0.29 0.10 0.06 0.05 0.04 0.04 Ceiling Metal Deck 0.05 0.04 0.03 0.03 0.02 0.01 Floors Concrete Painted 0.01 0.01 0.02 0.02 0.02 0.02 Ceiling Acoustic Baffles 0.27 0.65 1.06 1.16 1.14 1.12

7. Sr. No. Room type RT60 (sec) 1. Class rooms having volume @ 200m 3 0.6 2. Class rooms having volume @ 300m 3 0.7 3. Rooms like audio-visual room (Shorter reverberation time is suitable) 0.4 4. Rooms like music practice room (Longer reverberation time is suitable) 0.8 5. Spaces like gymnasiums having volumes @ 5000m 3 1.5

8. Example: Calculate the reverberation time for a classroom having following details. Dimensions…. Length=10m, Width=6m and height =4m Specifications…. Flooring… Terrazo , walls…. Brickwork and painted, cealing … slab Wooden Doors…. 2 of size 1.2m* 1.8m, Glass windows….4 of size1.2m*1.8m No. of benches in class…70, surface area of each bench….1m 2 Calculate the reverberation time and comment .

9. Solution: Floor area = 10m X 6m = 60 m 2 Ceiling area = 10m X 6m = 60 m 2 Door area = 2 X1.2mX1.8m=4.32 m 2 Window area=4X1.8mX1.2m= 8.64 m 2 Wall area = (32mX4m)- Door area -Window area = 115.04 m 2 Bench area = 70 X 1m2 = 70 m 2 Volume of the room= 10mX6mX4m= 240 m 3

10. Sr. No Item Area (m 2 ) S α a 1 Floor area 60 m 2 0.02 1.20 2 Ceiling area 60 m 2 0.02 1.20 3 Door area 4.32 m 2 0.3 1.30 4 Window area 8.64 m 2 0.2 1.73 5 Wall area 115.04 m 2 0.04 4.60 6 Bench area 70 m 2 0.3 21.0 ∑….. 31.03

11. RT60 = 0.161 V/a = 0.161*240/ 31.03 = 1.24 sec. Comment….. Expected reverberation time is around 0.7 sec. So the efforts are to be taken to reduce reverberation time. Possible measures may include use of curtains, use of carpets, Open-cell foams, Acoustic Partitions.