Dissecting reverberation time
Understanding the components of RT60 is important for understanding how absorption is measured in laboratories, specified properly in construction documents, and how absorption coefficients, NRC, sabins, and RT60 all relate.
RT60 = C (V)
A
RT60 Reverberation time (seconds)
C
Constant (0.161 when V is in m3; 0.049 when V is in cf)
V
Volume of the room (average height x average width x average length)
A
Total sound absorption (sabins) provided by a product sample during testing or by the surfaces and objects in a room if calculating RT60
Complying with RT60 requirements in standards
When the acoustic performance metric in a standard is maximum RT60, the value of RT60 is given, C is known, and V is known through simple room take offs and calculation. One solves for A, the amount of absorption needed in the room to achieve the required RT60 in the standard.
RT60 = 0.60
Most standards require RT60 to be 0.60 seconds or less
RT60 = (0.049xV)/A
A = (0.049 x 10,000)/0.60
Assume the volume of the room is 283.17 m2 (10,000 sf) for this example
A = 817 sabins
817 sabins of absorption are needed in the room to comply with the maximum RT60 requirement in the standard
Measuring product absorption
When measuring the absorption performance of a material, product, or object in a reverberation chamber in an acoustics laboratory, RT60 is measured and therefore known, C is known, and V for the test chamber is known based on its dimensions. One solves for A, the total absorption (sabins) provided by the specimen being tested. If the test specimen is representative of a surface finish, such as carpeting on a floor or a contiguous acoustic panel ceiling, then one divides A by the area of the test specimen to get α, the absorption coefficient (refer to ASTM C423, paragraph 12.1.3.).
A = 61 sabins
A came from measuring RT60 in the lab and using the reverberation time equation to solve for A. Sixty-one is used as a hypothetical value for this example.
a = A/area
Divide A by the area of the test specimen to get a
a = 61/72
Insert the numbers for the variables and calculate
a= 0.85
a (alpha, absorption coefficient) is what manufacturers use to calculate NRC
To find α, it is important for one to know the area of the test specimen. For materials or products representing a contiguous room surface, the area of the test specimen is easy to calculate. This is not the case for three-dimensional objects suspended freely inside the test chamber or sitting on the test chamber floor, especially when there are multiple objects arranged in a specific pattern, such as an array. Before discussing this in more detail, a few other fundamental topics need to be covered.
