Sound‐transmission‐loss tests were conducted using steel‐stud drywall partitions with a single layer of12‐in.wallboard on each side to study the effect on acoustical performance due to the physical properties of cavity fill. These tests also compared the results using mineral wool and glass fiber and using studs of different widths. All tests were conducted in accordance with ASTM E90‐66T. The transmission loss and sound‐transmission class (STC) were found to be related directly to the thickness of fibrous absorbing material when used in the full cross area of the stud cavities. The increase in STC due to increasing thickness of the cavity fill is not a linear relationship, since the first inch has the greatest effect. When the absorbing material is placed in less than the full cross area of the stud cavities, the gain in acoustical performance is governed by the volume of the absorption material. The low‐density glass fiber performed as well as high‐density mineral wool except in the middle portion of the curve, where the transmission loss averaged 1 dB less. Increasing the stud size increases the transmission loss. This gain is limited to the lower frequencies when cavity absorption is present.