An office wall has an STC of 65.
Which of the following statements is correct?

A.
The wall will cause sound to be diminished by 65 percent.

B.
The wall will cause sound to be diminished by 65 dB.

C.
The wall needs to be 65 mm thick in order to provide inaudibility.

D.
The wall will not provide any sound absorption.

Explanation:
The wall will cause sound to be diminished by 65 decibels (dB). Cisco recommends walls with a sound
transmission class (STC) of 65 if inaudibility is desired for a room. STC indicates a material’s ability to block the
transmission of sound waves. Normal human speech is typically registered at 60 to 65 dB, so a wall with an
STC rating of 65 should completely block normal human speech.
The wall might not need to be 65 millimeters (mm) thick in order to provide inaudibility. Generally, you must
double the existing material to increase the STC rating by 5. For example, if one inch of paneling provides an
STC rating of 40, it would take two inches of paneling to provide an STC rating of 45, four inches for an STC
rating of 50, eight inches for an STC rating of 55, 16 inches for STC 60, and 32 inches for STC 65. Rather than
increasing wall thickness, it is much more practical to install acoustic paneling and sound barriers.
The wall will not cause sound to be diminished by 65 percent. Noise-reduction coefficient (NRC) ratings are
used to indicate the relative amount of sound energy that can be absorbed by acoustic paneling, carpeting, tile,
and other sound absorbing materials. An NRC rating of 0 indicates no sound absorption, and an NRC rating of
1 indicates a complete absorption of sound. The higher the NRC rating, the greater the level of sound
absorption. Cisco recommends using acoustic panels with an NRC rating of 0.75 or greater.

Cisco: Cisco TelePresence Room Design Guide: Building for Acoustic Isolation (PDF)