Sound strength, G, represents the logarithmic ratio of this sound energy to that of the response measured in a free field at 10 m distance from the sound source. 
can be calculated according to the following equation, where corresponds to the start of the direct sound, and to a time that is greater than or equal to the point at which the decay curve has decreased by 30 dB.
The terms and can be expressed as:
is the impulse response measured at the measurement point;
is the impulse response measured at a distance of 10 m in a free field,
is 20 µPa;
= 1 s
is the sound pressure exposure level of ;
is the sound pressure exposure level of .
 ISO 3382-1:2009 Acoustics — Measurement of room acoustic parameters — Part 1: Performance spaces.