I’ve heard lots of reports of people whose dental fillings picked up radio broadcasts, so I’ll assume the phenomenon is true. My question isn’t how the metal in the fillings acts as a radio receiver (although that would be interesting to learn about), but, rather, how the sound itself gets produced.
There’s no speaker in your mouth, no vibrating membrane or similar, and most confusingly how do radio waves at, say, 1000 kHz get transduced in your mouth into, say, 1 Hz sound waves?
Cecil did a column on it. He was not completely convinced the phenomenon was real. Is it possible to hear radio broadcasts through your teeth? It’s worth noting (unless I have been lied to) that during the Great Depression, there were a number of so-called super-stations that broadcast at greater power than any domestic stations currently broadcast. Most of the radio-in-the-filling stories I have heard date from this era.
Well, I’ll be …
Is there nothing that Cecil hasn’t already addressed? (In fact, Cecil refers to Snopes. And that was one of the places I read about the phenomenon as having been ‘documented’.)
Thanks, bibliophage.
That reminds me of a new device that is going to be on the market soon. It is a lollipop device that recieves and that some how transmits sound to your ear because the lollipop is touching your teeth. When I read about it, the article wasn’t very detailed, but it does sound like it used the same method as people picking up radio through their fillings.
I also have my doubts about this phenomenon. Why buy a radio when you can just get some AM fillings in your mouth!
Is it possible for dental work to act as a radio? Well, it worked on Gilligan’s Island. And the Professor didn’t seem surprised.
To find out, I’ve compile a list of parts we would need to build a simple AM radio. What items do we need?
Item 1: Antenna. You need an antenna to convert the radio station’s electromagnetic signal into electrical current. The resulting current will be AC (alternating current), very low level (amplitude on the order of a millionth of a volt), high frequency (the carrier is around 1 million cycles per second for AM), and amplitude modulated (for AM stations, the amplitude of the high frequency signal exactly follows the music and sound).
Item 2: Amplification. A microvolt signal is very difficult to do anything with, so the best thing to do is amplify the signal as soon as possible, i.e. right after the antenna.
Item 3: Tuner. A band pass filter selects one station frequency while attenuating all the rest.
Item 4: Detector. Since the carrier signal is very high frequency, connecting a speaker directly after the tuner will result in nothing, since a speaker cone can’t vibrate that fast. But even more important, it is a symmetrical AC signal, which means the average voltage will always be around zero. A long time ago someone figured out that if we cutout one polarity of the signal (either the positive or the negative), we’ll be left with a varying DC (direct current) signal. The high frequency carrier is still there, but the average voltage will now be a function of the signal’s amplitude instead of being zero. The detector, which can be as simple as a series diode, allows current to flow in only one direction.
Item 5: Speaker. The speaker (or audible transducer, as engineers like to call it), produces proportional air pressure fluctuations as a function of voltage fluctuations. The air pressure fluctuations is what we call “sound”. Note that the speaker “ignores” the high frequency component since it can’t vibrate that fast anyway (i.e. the speaker resonate characteristics naturally filters out the high frequencies). Also keep in mind that sound also travels efficiently through liquids and solids as well.
The above description would still make an engineer cringe since I left out things like the power supply, feedback, impedance matching, speaker amplification, tonal filtering, carrier filtering, sensitivity & selectability, noise, saturation, frequency response, dynamic range, drift, stability, etc. But they’re geeks. How do think those old crystal sets worked? In fact, if a station’s signal is very strong, we may not need steps 2 & 3. That leaves us with a 3 component, bare-bones radio: antenna, detector, and audible transducer. Don’t believe me? Go ahead and try it: Take a small signal diode and hook it in series with a crystal earphone. Hook the other end of the diode to a long piece of wire, and hook the other end of the earphone to a cold water pipe. Can you now hear Dr. Laura? (“I’ve got a new attitude…”).
Now how can we build a radio in your mouth?
The antenna is easy. Any ol’ conductor that is aligned in such a way that the alternating magnetic field portion of the signal produces a current will suffice. In a head this could be fillings, braces, or even WWII shrapnel.
The audible transducer is almost as simple. There are only two criteria: when you apply a varying voltage, does it physically deform or vibrate in sync? And does it efficiently vibrate at audible frequencies (1000 to 5000 cycles per second) with sufficient amplitude that a person’s ear could hear it? While our bodies are not outfitted with Polk Audio’s Reference Series system, a resonate conductor such as a loose filling may do the trick. Also keep in mind that the amplitude may not be great enough to create air pressure fluctuations for other people to hear it, but it may be strong enough to transfer directly to the person’s ear via vibrations through the body.
The detector is the hard part. No detector, no radio. While engineers like to use silicon or germanium diodes as one-way current valves, rectification may occur at anytime if certain conditions are met. There are a variety of dissimilar materials that, when joined, act as a “poor-man’s” diode. Though I cannot confirm this, it would not surprise me a bit to find that certain combinations of metals, acids, oils, and salts found within the mouth & dental work would act as a rectifier/detector under certain conditions.
So what is the likelihood that:
a) There is a conductor within your dental work that is an efficient antenna at frequencies around 1 MHz, and has enough output to drive an audible transducer.
b) There is a resonate element within your mouth that is excited by voltage, efficiently vibrates at audible frequencies, and has enough amplitude to cause air pressure fluctuations that can be heard by the human ear (or enough amplitude that your own ears hear it through internal vibrations).
c) Dissimilar materials within your mouth have joined to create a rectifier, and thus forming a AM detector.
d) The antenna, detector, and speaker within your mouth are “wired” together in the proper configuration. (It should be noted here that some items may have a dual or even triple role, i.e. a combination antenna/detector.)
e) You’re in the vicinity of a very strong AM signal.
Answer: Extremely unlikely. But possible? Certainly.
Crafter_man.
Good answer.
To add a little to what you said, a detector can be made from a lot of non-linear devices. POW’s in WW2 built detectors from a piece of pencil lead placed across two razor blades.
I know of one case in Texas where a friend and fellow ham was operating a 1000 watt BC610 AM army surplus transmitter and got a call from a next door neighbor complaining that she could hear him talking on her bedsprings. I did not actually go over and lie in the bed, so I can’t say absolutely that she was really hearing his transmissions, but like the filling stories, I suppose it’s possible. The bedsprings and some rust between contact points could conceivably form a non-linear detector and the bedsprings might in some way have acted as a transducer.
Crafter_Man: Many thanks for such an informative answer! I look forward to mulling it over at my leisure after work.
Diver: The story has the ring of truth.
Thanks!
(BTW, in my OP, I left out a ‘k’. I meant to say “transduced in your mouth into, say, 1 kHz sound waves”.
Diver said:
"The bedsprings and some rust between contact points could conceivably form a non-linear detector and the bedsprings might in some way have acted as a transducer."
That happens more often than you might think. Check this out!
http://www.newscientist.com/lastword/answers/lwa1008mysteries.html
matt
Thanks for the URL.
The phenomenon apparently is more common than I would have thought!
May I offer second-hand testimony? My younger brother had braces as a teenager. Once in the middle of the night when he was sleeping in the basement, with the rest of the house dead quiet (on a very quiet residential street) he could hear the “Chariots of Fire” theme. He looked around the room to see if the stereo was on, but it wasn’t. Plugging his ears with his fingers only made the sound clearer. His braces were indeed picking up a radio broadcast.