Field measurements of airborne sound transmission loss were made on several operable partitions of the horizontal-sliding-panel type between conference rooms. Apparent Sound Transmission Class (ASTC) and Noise Isolation Class (NIC) ratings were computed. Very significant deviation of field-measured sound transmission ratings and manufacturers Sound Transmission Class (STC) ratings were found. Clients were not satisfied by actual sound isolating performance. Transmission of voice was clearly audible. Some deficiencies of field conditions were found. Some deficiencies of partition installation were found. Modifications were made; acoustical performance did not change significantly.