Inside the NRC Anechoic Chamber and Control Room: After our stopover to see the electron-scanning microscope, Paul Barton took the group into the anechoic chamber area—an area in which he has been doing research for many years. Indeed, one has almost the sense of the NRC lab being Barton’s “home away from home.”
There is a small control room area featuring the expected banks of signal generators, signal analyzers, audio amplifiers (oldie but goodie Canadian—of course—Bryston 4Bs, to be exact), a smattering of monitor speakers, various test instruments, an oscilloscope, a microphone control bay, and several computer workstations used to run tests, and then gather and analyze data. Off one side of the room, behind thick, double isolation doors, is the anechoic chamber itself, whose walls, floor, and ceiling are entirely covered with a cross-hatch of four-foot deep acoustic damping wedges (the wedges appear to be made of a golden yellow acoustic fiberglass-like material where the wedges are given shape by open weave metal mesh). The entry door to the chamber is itself covered with wedges whose shapes have been partially rounded off to accommodate the arc of the door when it swings open.
The interior of the chamber is a strange and wonderful place to behold. Extending from the entry door is a minimalist, open-weave metal “catwalk” that is suspended well above the floor of the chamber and at the end of which is found a motorized, remote-controlled loudspeaker test fixture. Visitors must be very careful when entering the chamber because, with the aim of minimizing acoustically reflective surfaces, the catwalk has been design with no handrails at all (one wrong move and a guest could conceivably fall down upon the grid of acoustic wedges on the chamber floor below!). Since standing room on the catwalk was at a premium, Paul had to split us into two small groups in order to give each visitor some time in the chamber.
Inside the chamber, illumination is provided by a small set of work lights. Suspended from the chamber ceiling and positioned directly across the chamber from the loudspeaker test fixture is a geometrically precise array of costly, calibrated Brüel & Kjær microphones. The mics are arranged so that one mic is positioned directly on-axis with the loudspeaker under test, while two more mics take measurements from 15 degrees above and below the test speaker, with another pair of mics taking measurements from 15 degrees to the left and right of the test speaker.
Barton explained that this arrangement creates a measurement test “window” that will, when results from the five mics are averaged, tend to minimize if not eliminate any spurious diffraction artifacts that any one mic might happen to pick up. In addition to the mic array, the chamber provides a small closed circuit video camera (to enables testers to verify that loudspeakers are correctly positioned before tests begin), a small laser-type aiming device (which helps testers make sure the speaker is precisely aligned with the microphone array), and a strobe-type emergency warning (so that technicians working in the chamber with the door closed will have a visual means of being alerted if, say, a fire alarm goes off).
Paul invited us to close the chamber door and to observe how extraordinarily quiet the chamber really is. How quiet is it? It’s quiet enough that you might hear blood flowing through vessels in your ears and, if conditions are right, quiet enough that you might just be able make out the (acoustic) sound of your own heartbeat. Trust us on this one: Agent Maxwell Smart’s fictional “cone of silence” has nothing on this place. The chamber just plain swallows up echoes, reverberations, and sounds of all kinds. Paul pointed out that the chamber is certified to be completely anechoic from 20kHz on down to 80Hz, although it does reflect some bass energy below 80Hz. This low-frequency limit is largely a function of how big the chamber is, so that the only way to get a chamber that is anechoic all the way down to 20Hz is to build an exceedingly large (read “expensive”) one.