The external Research Program of Canada Mortgage and Housing Corporation accepted the proposal by MJM Acoustical Consultants INC. to conduct a study on the noise isolation provided by exterior walls in wood construction. The report is addressed to acousticians, builders and construction professionals. A total of nine Sound Transmission Loss measurements were conducted on four exterior walls, which have a thermal insulation factor of RSI 3.5 (R20): two walls with 38 mm x 140 mm (2" x 6") studs and two with 38 mm x 89 mm (2" x 4") studs. Five tests were carried on walls with no exterior finishes, and four on walls with PVC cladding. In order to establish the effect of varying the stud spacing, one sound transmission loss test was performed on a wall whose studs were spaced 600 mm (24") apart; the rest of the specimens were constructed with studs spaced at 400 MM (16") o.c. which corresponds to the stud spacing most often used for exterior walls in Canadian construction. The interior finish was the same for all the walls tested: 13 mm (1/2") drywall located on the receiving room side; the exterior side of the wall was located on the source room side.