07 Jan 2013

Listen Up – How Sound Transmission Class Ratings Relates to SCIF Security

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When it comes to breaching a facility’s security, sometimes it doesn’t take high tech electronics, all it takes is the lowest tech of all: ears. That’s right, all the high tech security in the world will do little good if a classified conversation can be heard by a covert operative standing nearby. This is why the DOD has created a series of sound transmission class ratings to standardize the science of audio security.

What Do STC Ratings Cover?

The established DNI guidelines are designed to protect classified conversations from being inadvertently overheard outside a SCIF. This is not intended to protect deliberate technical interception of audio communications.

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 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 ouside of the SCIF.

How is Compliance Assured?

To begin with, it is vital that audio tests be conducted to verify standards are met. Tests may be instrumented or non-instrumented as approved by the AO. Test methods used shall be detailed in the CSP. Other requirements are as follows:

    • If performing instrumented acoustic tests, it is vital that only those with training on audio testing techniques be permitted to conduct the tests.
    • With all SCIF doors closed, all perimeter walls and openings (e.g., air returns, doors, windows, etc.) shall be tested at multiple points to ensure that either Sound Group 3 or 4 is met.
    • Other testing requirements include audio test sources shall have a variable sound level output.
    • The output frequency range shall include normal speech.
    • Test speakers shall be placed six feet from the test wall and four feet off the floor.

Audio gain of the test source shall produce “loud or very loud speech” as defined by Sound Group 3 or 4 levels respectively.

  • As an alternative, instrumented testing may be performed to Noise Isolation Class (NIC) standards. Results shall comply with NIC 40 for Sound Group 3 and NIC 45 for Sound Group 4.
  • All non-instrumented tests shall be approved by the AO.


Audio integrity of a SCIF is paramount to its ability to keep conversations from being overheard. When it comes to securing a SCIF, no detail is too inconsequential to consider. The nation’s security could depend on it.

When it comes to securing the nation’s secrets, Jaye Andone is an expert. Her company, SCIF Global Technologies, is in the business of designing, manufacturing and transporting Sensitive Compartmented Information Facilities worldwide. To learn more, visit http://scifspace.com/

About the Author

Jaye Andone is president and CEO of SCIF Global Technologies. Jaye will most likely be your first point of contact to lead you through the process from concept to procurement of your facility. Request a Quote or give her a call: 904-524-0911

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