There is also the problem that that impedance mismatch between the gas in the tube and the glass surrounding it is huge so very little power is coupled. Interesting things would be for the tube length/resonance to be closer to the sound frequency of interest (eg 200-2000Hz) and close to critically damped. I have no idea if that's the case.
The second question is whether one can simply modify the tube/ballast in a visually imperceptible way. For example, one could insert an IC in the ballast that modulates the power draw of the lamp dependent on the ambient coupled acoustic/vibration power (eg possibly using the lamp as a vibrational microphone). Then pulse modulating the power draw of the lamp at a relatively low noise part of the power spectrum and within the band of the main power transformer filters. The lamps easily draw 40-80W so there's lots of room to work, and there are already Power Factor Correction implementations that use this simple technique to make lamp's inductive loads "look resistive".
So what's notable is that either the ballast of the lamp could be tampered with or one of the bulbs. Either one could allow transmission out along power lines, if standard transformers and capacitors are used for power delivery. This is how home powerline ethernet works and it transmits Gb/s rather than the few kb/s needed to get voice out over the power lines.
Case in point. I saw a single Korean Samjung lamp interfere with AM radio stations (<1MHz) from 100 meters, by just clipping the power draw of a small fluorescent with a thyristor and oscillator at the ballast of a desk lamp to provide PFC so that thousands of lamps could be placed in huge open plan offices.
Fluorescent lamps are already RF noise sources. Modifying a tube to modulate the noise would be a useful eavesdropping device. If you used a digital modulation mode with a scrambler (the pseudorandom device that makes the number of 1 and 0 bits roughly equal) the emissions would look just like standard white noise to receivers that didn't know the decode pattern.
Now all you have to do is to get your modified lamps into the supply chain for lamp replacement in the secure area.
One way I could see this working is with old bulbs - when you’re near the threshold for it flickering, perhaps sound can modulate the light.
Or perhaps the contacts need to be realistically cruddy and intermittent, and the long glass tube acts as a sounding body to trigger an intermittent contact, encoding sound into light.
There is also the problem that that impedance mismatch between the gas in the tube and the glass surrounding it is huge so very little power is coupled. Interesting things would be for the tube length/resonance to be closer to the sound frequency of interest (eg 200-2000Hz) and close to critically damped. I have no idea if that's the case.
The second question is whether one can simply modify the tube/ballast in a visually imperceptible way. For example, one could insert an IC in the ballast that modulates the power draw of the lamp dependent on the ambient coupled acoustic/vibration power (eg possibly using the lamp as a vibrational microphone). Then pulse modulating the power draw of the lamp at a relatively low noise part of the power spectrum and within the band of the main power transformer filters. The lamps easily draw 40-80W so there's lots of room to work, and there are already Power Factor Correction implementations that use this simple technique to make lamp's inductive loads "look resistive".
So what's notable is that either the ballast of the lamp could be tampered with or one of the bulbs. Either one could allow transmission out along power lines, if standard transformers and capacitors are used for power delivery. This is how home powerline ethernet works and it transmits Gb/s rather than the few kb/s needed to get voice out over the power lines.
Case in point. I saw a single Korean Samjung lamp interfere with AM radio stations (<1MHz) from 100 meters, by just clipping the power draw of a small fluorescent with a thyristor and oscillator at the ballast of a desk lamp to provide PFC so that thousands of lamps could be placed in huge open plan offices.
Fluorescent lamps are already RF noise sources. Modifying a tube to modulate the noise would be a useful eavesdropping device. If you used a digital modulation mode with a scrambler (the pseudorandom device that makes the number of 1 and 0 bits roughly equal) the emissions would look just like standard white noise to receivers that didn't know the decode pattern.
Now all you have to do is to get your modified lamps into the supply chain for lamp replacement in the secure area.
One way I could see this working is with old bulbs - when you’re near the threshold for it flickering, perhaps sound can modulate the light.
Or perhaps the contacts need to be realistically cruddy and intermittent, and the long glass tube acts as a sounding body to trigger an intermittent contact, encoding sound into light.