In STC ratings, what does an STC of 40 indicate?

• A. Nothing heard
• B. Heard as loud speech
• C. Faintly heard
• D. Clearly audible

Answer: (C)

STC is a single-number rating that estimates how well a wall or partition reduces airborne sound between rooms, focusing on the frequencies where speech mainly lies. An STC of 40 means the partition attenuates about 40 decibels of sound in the mid-frequency range. Practically, if someone speaks at a normal level in one room, you would hear something on the other side, but it would be faint rather than clear. You might catch a word or two, but not understand a conversation easily. Higher numbers would make speech even less noticeable, while lower numbers let it come through more loudly. So a value of 40 corresponds to speech being faintly heard through the partition.