What STC rating does Cisco recommend for walls if inaudibility is desired for a room?

A.
0

B.
0.75

C.
1

D.
40

E.
65

Explanation:
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. A wall with a rating of 65 will cause a
sound to be diminished by 65 decibels (dB) by the time it has passed into the adjoining room. 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.
An STC rating of 40 is not sufficient to provide inaudibility. Walls in office buildings generally have an STC
rating of 40. Therefore, human speech would decrease to 25 dB by the time it reaches an adjoining room.
Generally, you must double the existing material to increase the STC rating by 5. 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.
STC ratings do not typically go as low as 0 or 1. However, these low values are seen in noise-reduction
coefficient (NRC) ratings. 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)