Last month’s challenge was not necessarily to name a particular device, but rather to name the type of device that generates a class of audible interference. You can listen to the sound again if you need your memory jogged!

While many immediately recognized the sound as interference caused by a GSM or EDGE phone, Jered wins the prize for his very precise analysis of the root cause of the noise:

The reason for the buzz is the nature of time-division mulitple access (TDMA). In the US, we operate mobile phones at 850 Mhz and 1900 Mhz; in Europe, 900 Mhz and 1800 Mhz. Good so far; that’s not going to make noise that we can hear. TDMA fits more subscribers into the same bandwidth by assigning different terminals different timeslots (vs. CDMA, which uses black magic). These timeslots happen to be spaced 4.615 ms apart, yielding a signal envelope which looks a lot like a dirty 217 Hz square wave.

All sorts of things (like “wires”) are good at picking up a 217 Hz square wave at 0.5 W, and 217 Hz is conveniently smack dab in the middle of our auditory capabilities.

Congratulations Jered! Email me for your prize.

I thought this noise was noteworthy because a surprising number of people do not realize where it is coming from. I’ve often heard this noise on conference calls, and its fairly obvious that some participants don’t understand that their cell phone is causing this interference. The thing that befuddles most is the range at which this interference can occur: their phone could be well across the table, yet with the proper antenna orientation, the noise is loud and clear. Often times, the problem can be ameliorated simply by rotating the phone by about ninety degrees.

What disturbs me about this noise is that it’s a prominent reminder of exactly how powerful this RF transmitter is that I happily stick next to my cerebral cortex and my gonads on a daily basis. 0.5 watts is not a trivial amount of power! And of course, Bluetooth hands-free sets are not much better. Granted the power is lower, but Bluetooth operates at 2.5 GHz — and it’s no mistake that microwave ovens also run at that frequency, as it is absorbed particularly well by the water that makes up 60% of our mass.

While there is no conclusive evidence that cell phones cause any sort of biological harm, there is precedent for entire societies that have fallen victim to the myopic use of technology to better life. For example, even a child can tell you today that lead causes poisoning and brain damage…and so we remark at the Roman’s folly: “Gosh, what idiots! They sweetened their wine with lead and used lead pipe to deliver drinking water. Duh, of course the Roman empire collapsed.”

I often wonder if a millennium from now, people will read about us as we do about the Romans. “Gosh, what idiots. They stuck half a watt of radiation on their heads every day for decades at a time. No wonder they all died of debilitating brain disease.” Or, my other favorite is, “Gosh, what idiots! The made their clothes, cars, and even utensils out of plastics. Everyone knows that plastics outgas damaging free radicals. No wonder they all died of cancer”…and in the end, the meek did inherit the Earth.

Then again…there is no conclusive evidence that anything we do really causes that much damage. We’ve learned from the Romans and gotten more clever, and we use “model” organisms and sophisticated extrapolation mechanisms. But then again, those are just models, and there’s no such thing as accelerated lifetime testing on a real human being…and as any engineer knows who has done a lot of reliability testing, there’s always that one corner case that gets through (e.g., the Xbox360 Red Ring of Death). So with enough new technology entering our lives, the chance that we’ll encounter unforeseen consequences goes up and up. You and me — we’re the ultimate guinea pigs in this grand experiment with technology!