At their core, radio frequencies are simply electrical currents. Like sound waves, electrical currents pulse, or oscillate, at certain speeds; the rate, at which a current pulse, in cycles per second, is called frequency, and is measured in Hertz, abbreviated Hz. The range of frequencies usually being referred to when one speaks of radio frequencies, or RF, is between approximately 3,000 Hz (3 kHz) and 300,000,000,000 Hz (300 GHz). It should be noted that this range includes much more than your standard AM/FM radio, which operates between approximately 300 kHz and 300 MHz. Radio frequencies, and the specific uses allowed at each frequency, are regulated by the government.
The Importance of RF Energy
Energy that falls into the spectrum described above, or RF Energy, has a number of special properties. For our purposes, the most important property is that the energy in an RF current can radiate from a conductor into free space as electromagnetic waves (radio waves). While this concept is the basis for directed wireless communication (your AM\FM radio, Wi-Fi, Cell phone, etc.), RF energy is also created in many other ways. RF energy is generated by most standard equipment used in today’s workplace. Computer chips (microprocessors), monitors, printers, wires: all generate signals that can be monitored, recorded, and reconstructed. Computers are already an indispensable part of the modern world; as they become smaller, lighter, and more capable, the signals that they can generate are also receiving more scrutiny.
Internal and External
The federal government of the United States has recognized the threat posed by RF energy emanating from equipment inside office space and work facilities. Modern eavesdroppers do more than stand around the corner listening to your conversation; sophisticated agents can monitor electronic communications from a great distance. For this reason, government facilities are required to protect themselves through a variety of technical security countermeasures designed to reduce or eliminate the ability of RF energy generated inside the facility to leak out and thus be exploited.
Problems caused by RF energy are not one way concerns
Sensitive electronic equipment can be interfered with by external signals; directed energy can be used to “flood” an area for eavesdropping purposes; and exceptionally high levels or direct contact can induce sickness or burns in humans. For this reason, many companies who operate such equipment, or who electronically store and process sensitive information, often choose to survey the potential location(s) before building a new facility.
The Purpose of RF Testing on Facilities
RF tests are generally performed for two reasons: to assess a barrier’s ability to attenuate RF energy; and to determine the ambient levels that exist in an area. These tests are performed in the field, with specialized equipment used to generate signals at specific frequencies and strengths, as well as equipment to observe and record signals.
Reference Test - Performed in open area with no obstruction between the transmitting and receiving antennae, and is primarily intended to confirm proper equipment functionality and calibration. This is the baseline test to measure the readings at each frequency point in respect to un-obstructed free space measurement to determine the actual attenuation of the Subject Under Test (SUT). A comparison of these measurements to those achieved during the subsequent Data Tests at the same test points will be completed. This test is performed once for each series of tests that will utilize the same antenna separation distance, power, test equipment and frequency spectrum.
Data Test - Performed with both the transmitting and receiving antennae functioning. The transmitting antenna is placed inside the space being tested, and the receiving antenna is placed outside the space at the same distance that the reference test utilized for antenna separation. The test point measurements taken during the Data Test will be subtracted from the baseline test (Reference Test) to determine actual attenuation that the barrier is providing. This test is performed on each wall/barrier that is tested during the evaluation (RF attenuation testing).
Ambient Test - Performed with only the spectrum analyzer and receive antenna, and measures the amplitude and frequency of any detectable signals in the frequency range under test. The transmitting antenna does not operate during this test. This test determines whether any signals generated outside the space interfere with the Reference and Data tests and how these signals can penetrate the facility under test. This test is performed at each Data Test location at the exact point where the receive antenna was positioned for that test.
Walk-Away Test – Performed by measuring the signal at the prescribed distance for the Reference and Data tests, then moving farther away in 10 meter increments to find the “drop off” point for signal reception. This test is typically performed on a smaller spectrum with reduced resolution bandwidth for speed and accuracy. The lower, middle and upper portions of the SUT may be tested separately.
The Sound Transmission Class (STC) Rating
Assigning an STC rating to a barrier is a way to easily determine how much it is expected to dampen, or attenuate, sound waves. When a sound wave comes in contact with a barrier, some of the energy from its vibrations transfers to that barrier. The properties of the barrier will determine how much energy is lost as the sound wave passes through it; this measurement, at a given frequency, is called the barrier’s transmission loss (TL) effectiveness. The higher the TL measurement, the more the barrier will reduce the sound’s power as the wave passes through it.
Since TL measurements are taken across a range of frequencies, it is sometimes difficult to compare two barriers. Sound transmission class (STC) ratings attempt to solve this problem by assigning a single integer value for acoustical performance. ASTM E413 (“Standard Classification for Determination of Sound Insulation”) determines a barrier’s STC value by measuring its TL over a range of 16 different frequencies between 125 Hz and 4,000Hz (roughly the frequency range of human speech). Like TL measurements, higher STC values indicate better attenuation performance on the part of the barrier being tested.
In order to generally predict the ability of a constructed area to contain sound, a wide variety of barriers have been built and tested in laboratory settings. The STC values of these barriers are used by the construction industry to approximate the protection level that a space should achieve if the tested method of construction is implemented. Practically speaking, however, these values are only assumptions; from variances in the material creation process to differences in construction methods, field-built spaces rarely match laboratory test results. The only true way to measure an area’s attenuation properties is to perform field testing.
Digital Audio signal and voice tracks are converted to analog form and injected into loud speakers. The sound is then measured for loss, reproduction and intelligibility. An audible speech test is also performed at 96dB.
Instrumented STC results are usually 7-10 points below the Architectural material listing specifications. No intelligible speech should be heard through these walls. An amplified speech recording is also played in the room at 96dB and should be at a minimum audible but weak, and not intelligible to the naked ear outside the shelter along all four walls. Loud speech is categorized in the 65-75dB range. Shouting or Amplified sound is normally in the 75-85 dB range. The use of amplified sound in these types of rooms can be approved, but must be Operationally controlled at levels not to exceed those of normal to loud speech. Check with your CSA for appropriate guidance and approval for the use of Amplified sound in any facility.