What the @#$% is an Anechoic Chamber…

Contrary to popular belief, the quietest place on earth is not a Buddhist temple or the room after you tell a corny joke. It’s the anechoic chamber at Orfield Laboratories in South Minneapolis; trust me, it’s Guinness Book of World Records-certified (learn about Orfield’s backstory and its pioneering work here). In more precise terms, the chamber is attuned to -9.4 dBA (the human ear can detect sounds above 0 dBA). In fact, the double insulated, concrete reinforced chamber is rumored to be so quiet that spending a mere 30 minutes alone in the room will lead to insanity (according to the Discovery Channel). This is because the only sound you can hear is that of your insides beating and gurgling and expanding and settling, which your brain tries to counter by creating imaginary noises.

Let’s start with an analogy. Have you ever tried to construct the “perfect” sandwich? You have to make sure you have premium base ingredients, the optimal ratio of bread to spread to condiments (determined by careful testing), and geometrically exacting slicing techniques. There’s a lot of room to be picky. Designing a “perfect” anything requires rigorous standards, constant critiquing, and a healthy budget. So when engineers in the 1970’s were tasked with building a chamber with no internal reflection (no waves bouncing around) to test wave-emitting devices, they were really being asked to build a “perfect” wave-cancelling apparatus.

One type of wave we might want to work with in this way is a sound wave (thus the name “an-echoic”, meaning non-echoing). The most familiar example of an acoustic anechoic chamber is a recording studio, which ensures the singer’s voice is not affected by any outside interference. But an anechoic chamber can also be used to test loudspeakers and microphones, as well as very quiet devices and even the accuracy of a person’s hearing. The University of Salford’s website on anechoic chambers mentions an additional unorthodox example application in which a remarkably resourceful (and probably a little OCD) student made use of an anechoic chamber to isolate and record the sound of a raindrop landing on a roof tile for science class.

Noise cancelling rooms are all well and good, but we live in a world where some of the most important waves are imperceptible electromagnetic waves. Antennas, televisions, and computers all employ waves that are much harder to notice without the aid of a instrument. Nevertheless, these products require the same “ideal” testing conditions as their acoustic counterparts. In fact, there is a whole branch of science dedicated to measuring the unwanted effects of electromagnetic energy called “electromagnetic compatibility” (EMC).

Now, if you’re wondering what this has to do with our work here at Intellectual Ventures Lab… Yup, you guessed it: we built our very own anechoic chamber for RF testing! As part of our research into MSA-T, our team needed the ideal testing conditions provided by the chamber in order to conduct their experiments. As you can see from the launch of Kymeta, this tool has definitely paid off.

Fifty years ago, the most accurate EMC tests were conducted in Open Area Test Sites (OATS), essentially large spaces where there was minimal electromagnetic wave activity. However, this method was relatively imprecise and impractical. The weather and the amount of sunlight dictated when the OATS could be used, and there were few measures in place to restrict the number of wave reflective objects at the sites (i.e. wires, lights, and walls).

An Open Area Test Site; far from ideal (ETC, Taiwan)

One way to negate some of the problems with the OATS was to conduct tests in a radio-frequency (RF) shielded chamber (this concept is based on the Faraday cage principle; read more on How Stuff Works and watch Walter Lewin, MIT physics professor explain it here). However, this enclosed space model now opened up issues with cavity resonances and internal surface reflections.

To further complicate matters, regulatory agencies in the 1970’s and 80’s began to implement new testing standards for products that emitted or were susceptible to RF waves. The economic threat of electronic devices being taken off the shelf for not meeting international standards spurred large multi-national corporations, such as IBM and Hewlett Packard, to look for better and more reliable alternatives for vetting their products.

The solution they settled on was to invest in absorber materials (already used for radar and microwave tests) to attenuate (weaken) the waves and inhibit internal reflection in anechoic chambers. In order for this radiation absorbent material (RAM) to be lossy*, it could neither be a conductor nor an insulator as neither actually absorbs energy. RAM is essentially a foamed plastic/rubber, such as polyurethane or polystyrene that is impregnated with carbon and iron (the carbon promotes destructive interference with incoming waves). In addition, the RAM had to be a certain shape and height depending on the frequencies and wavelengths being tested; a pyramid shape cut at precise angles relative to the wave source is typical. Balancing these different factors allowed RAM to efficiently absorb RF frequencies from microwave to below 500 MHz.

* This is a fun word for you non-engineers meaning a material that is designed to have a high attenuation**

**Attenuation is the weakening in force or intensity of, in this case, electromagnetic waves

IBM built the first full size EMC Anechoic Chamber in Boca Raton, Florida. It had a $2 million price tag and a testing range of 3 meters. However, it turns out high-end computing devices require chambers with a 10-meter testing range (due to the far-field criteria). This meant more space and more expense, so IBM had to figure out how to determine the effectiveness of the absorber materials in order to optimize the design of the chambers. IBM enlisted the University of Colorado at Boulder to develop a testing model for RAM that would broaden its effective frequency range. The biggest breakthrough came in the application of a new material called “ferrite”. A great absorber, ferrite could also be made into tiles only a few millimeters thick. Taking advantage of this super-RAM, IBM successfully built the first high performance, 10-meter range anechoic chamber in 1986.

Today, the hybridized anechoic chamber is the standard for EMC testing. They are versatile enough to emulate a wide variety of test conditions and cheap enough to be a reasonable investment for companies, labs, and universities.  However, the US government and NASA operate the largest anechoic chambers, some capable of holding an entire fleet of fighters! Check out NASA’s explanatory video here.

Benefield Anechoic Facility at Edwards Air Force base (USAF Stock)

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One Comment

  1. Cory
    Posted February 26, 2013 at 1:50 am | Permalink

    I’d be interested to hear more about the anechoic chamber you made.

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