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.

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 T60 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 @ 200m3 0.6
2. Class rooms having volume @ 300m3 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 @ 5000m3 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….1m2
Calculate the reverberation time and comment .

9. Solution:
 Floor area = 10m X 6m = 60 m2
 Ceiling area = 10m X 6m = 60 m2
 Door area = 2 X1.2mX1.8m=4.32 m2
 Window area=4X1.8mX1.2m= 8.64 m2
 Wall area = (32mX4m)- Door area -Window area = 115.04 m2
 Bench area = 70 X 1m2 = 70 m2
 Volume of the room= 10mX6mX4m= 240 m3

10. Sr. No Item Area (m2)
S
α a
1 Floor area 60 m2 0.02 1.20
2 Ceiling area 60 m2 0.02 1.20
3 Door area 4.32 m2 0.3 1.30
4 Window area 8.64 m2 0.2 1.73
5 Wall area 115.04 m2 0.04 4.60
6 Bench area 70 m2 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.