Three things could put a voice where no speaker is, and they differ in a way that can be checked rather than argued. Two of them make real sound in air: a transducer bonded to a window or wall drives the surface as a loudspeaker, and an ultrasonic beam demodulates into audible sound along its path. Both are commercial products. Anyone standing in the room hears them, and any recorder captures them. The third — the claimed delivery of speech by pulsed radio-frequency energy — produces its pressure wave inside the skull, at a tenth of a pascal to three pascals, conducted through bone to the inner ear. If that is what is happening, there is no sound in the room at all: a person beside you hears nothing, and a microphone records nothing, not because the equipment failed but because there is nothing in the air to capture. So the question is not whether you believe any of this. It is whether a second person and a cheap recorder find something, and that is a question with an answer.
- Structure-borne audio — a surface exciter bonded to glass or plasterboard makes the panel itself radiate. Real airborne sound: a bystander hears it, a microphone records it. Sold with adhesive pads from about $33.
- Parametric array — an ultrasonic carrier around 40 kHz self-demodulates in air. Real airborne sound inside the beam: a bystander standing in the beam hears it, a microphone in the beam records it; off-axis level falls below a tenth.
- Microwave auditory effect — thermoelastic expansion produces 0.1 to 3 pascals of pressure INSIDE the head. No airborne sound exists. Not audible to anyone else and not recordable, by physics rather than by equipment failure.
- Foster, Garrett and Ziskin (2021) add that equipment capable of the third at any useful range would be, in their words, large and very obvious.
Open questions
- Does a second person in the room hear it, and does an ordinary recorder capture it?
- If a surface seems to be the source, does a contact microphone on that surface register more than an air microphone beside it?
- If nothing is captured, was the recording uncompressed — and was the equipment capable of the frequency in question?
- Does any published work demonstrate intelligible speech, rather than clicks or tones, delivered by radio-frequency means at a distance?
References
- Structure-borne audio
- Parametric array
- Microwave auditory effect
- Contact microphone
- Sampling limit
- What is the neurotech bullhorn?
Read the sampling limit before trusting a negative result. A recording that captured nothing may only mean the recorder was not built to hear it.
This is a way to tell mechanisms apart, not a way to establish what happened. A negative result rules out the two airborne mechanisms; it does not establish the third, which remains unverified.
Topics: Proposed solutions · Technology