Rw + Ctr are Rw with the addition of a low frequency sound correction factor Ctr. (a negative number). The use of Rw + Ctr has become more relevant due to the increase in low frequency sound sources such as surround sound systems, traffic and aircraft noise, drums and bass guitars and home entertainment equipment. Two walls can have the same Rw rating, but have different resistance to low frequency sound, thus a different Rw + Ctr.
"The whole strata community owes a huge debt of gratitude to you and the OCN executive. Much appreciated."