Ceilings have their own testing system (CAC) that is equivalent to wall ratings and more applicable to ceilings per the ASTM.
Walls are rated using single pass tests to determine their STC rating because only 1 wall separates two rooms. This test isn’t really applicable to ceilings because sound has to travel through 2 ceilings to move from 1 space to another. (Imagine sound passing through the ceiling of an office, bouncing off the structural deck and then having to pass back through the adjacent office’s ceiling) This is why the ASTM established tests specifically for ceilings with the ASTM E1414 and gave them their own rating (CAC rating) by using a double pass test.
Test results which can be obtained from Armstrong (a leading ceiling tile manuf.) are conducted strictly according to ASTM E1414 and show that it is possible to meet a 42db reduction in sound between spaces using insulation adjacent to the wall ceiling connection of ACT to a STC 42 wall. This detail shows a STC 42 wall, A ceiling tile system and 4’ of insulation on either side of the wall.
(From the Armstrong website)
What is the difference between STC, CSTC and CAC?
STC – Sound Transmission Class – sound reduction from one side of barrier to the other – walls; single pass rating.
CSTC – Ceiling STC – AMA 1-II-1967 Test Method – Two room test, measure of sound that passes through a ceiling across a common plenum and down through ceiling in adjacent (receiving) room; double pass.
CAC – Ceiling STC – ASTM E1414 – ASTM test method introduced in 1995; essentially replaced the AMA 1-II test.