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STC: Deciphering ICD 705 Acoustic Requirements and when “testing” is required (Part I)

Updated: Jan 28

STC stands for “Sound Transmission Class” and is the primary terminology used for determining acoustic requirements for SCIFs and SAPF’s in ICD 705 policy.  It is a sound rating derived

primarily in a laboratory setting where wall partitions utilize a standardized rating methodology (STC 45, STC 50) showing how well they reduce sound transmission.  In the Accreditation world, this is commonly referred to as the “Can you hear me now” standard, because STC and Sound Groups (3,4) typically aren’t using instrumentation to determine the acoustic integrity of the space.  Instrumentation testing typically falls under a “NIC” rating (Noise Isolation Class) and is a very specific type of testing based upon ASTM standards.  One thing to keep in mind is the importance of conducting an architectural review of the design of your space not only to ensure your wall system can meet the required STC rating, but also to keep in mind potential threats and vulnerabilities comparative to the Security In-Depth (SID).  The exact policy language for Sound Group Ratings from ICD 705 is as follows:

B. Sound Group Ratings

The ability of a SCIF structure to retain sound within the perimeter is rated using a descriptive value, the Sound Transmission Class (STC). To satisfy the normal security standards of SCIFs, the following transmission attenuation groups have been established:

  • Sound Group 3 - STC 45 or better. Loud speech from within the SCIF can be faintly heard but not understood outside of the SCIF. Normal speech is unintelligible with the unaided human ear.


  • Sound Group 4 - STC 50 or better. Very loud sounds within the SCIF, such as loud singing, brass music, or a radio at full volume, can be heard with the human ear faintly or not at all outside of the SCIF.



Stakeholders are often focused on the acoustic integrity of their future space and want to ensure technical testing is done as confirmation that the wall system was assembled correctly.  While this concern is understood, technical testing is usually not a requirement where money should be spent.  Focusing on a good design, implementation of that design and understanding the Security In depth (SID) around your space are far more important.   NIC technical instrumentation is the first thing people jump to, but as the policy explains below, SCIFs aren’t designed to protect against a technical acoustic audio attack:


Chapter 9. Acoustic Protection

A. Overview


1. This establishes DNI guidelines to protect classified conversations from being inadvertently overheard outside a SCIF.


2. This is not intended to protect against deliberate technical interception of audio emanations.


In the next segment, we will be going beyond the common vocal testing that’s typically conducted and will provide guidance in further detail on the three testing types.  Feel free to Contact Us should you have a unique problem or issue, but more importantly want to be assured of the acoustic integrity of facility the first time.  Tearing out walls to fix a sound issue is typically a costly retrofit you should never have on any project.  SPG’s Subject Matter Expertise will allow you to “Measure twice and cut once” to meet Schedule and Cost requirements.


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