Rms value of random vibration

We saw that [2.26]: G(f)=limΔf→0T→∞1TΔf∫0TℓT2(t,Δf)dthttps://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780367812096/f2fb0756-bff5-4de5-ae2c-be0eacfb3062/content/eq299.tif"/>