Calculate sound reduction index R, field R′, or required R from source and receiving SPL, specimen area, absorption area, volume, and T60.

Use energy-averaged room SPL and absorption data for the same frequency band. Lab R assumes diffuse fields and no flanking transmission; field R′ includes other paths. This arithmetic does not establish STC, Rw or compliant testing.

Acoustic absorption area, not room floor area.

Sound Reduction Index Formula

The following equation calculates the sound reduction index from energy-averaged source and receiving room SPL in the same frequency band. Lab use assumes diffuse fields and negligible flanking transmission; field R′ includes other transmission paths. It does not produce STC or Rw ratings or establish compliant testing.

R = L1 - L2 + 10 log10(S / A)
  • Where R is the Sound Reduction Index
  • L1 is the average sound pressure level in the source room
  • L2 is the average sound pressure level in the receiving room
  • S is the area of the test specimen (m^2)
  • A is the equivalent sound absorption area of the receiving room (m^2)

To calculate a sound reduction index, subtract the average sound pressure level in the receiving room from that in the source room, then add 10 times the base-10 logarithm of specimen area divided by equivalent receiving-room absorption area, using matching area units. This absorption area is an acoustic equivalent, not the room floor area.

What is a Sound Reduction Index?

Definition:

A sound reduction index is a metric used to measure the level of sound insulation a structure provides.

How to Calculate Sound Reduction Index?

Example Problem:

The following example outlines the steps and information needed to calculate Sound Reduction Index.

First, determine the sound pressure level of the source room. In this case, the sound pressure level is 40dB.

Next, determine the sound pressure level of the receiving room. In this example, this is measured to be 10dB.

Next, determine the area of the test specimen. This is calculated to be 1 m^2.

Next, determine the equivalent sound absorption area in the receiving room. This is measured to be 3m^2.

Finally, calculate the sound reduction index using the formula above:

R = L1-L2 + 10 log10(S/A)

R = 40-10 + 10 log10(1/3)

R ≈25.23 dB