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Comment by stevage | Hacker News Reader

I didn't watch the talk, but wondering if someone can explain this line from the post:

> Since 7.8 kHz is in the audio range

What is "the audio range" in the context of radio frequencies?

As other people have pointed out "audio range" is generally 20Hz-20kHz. Your phone (and other audio equipment) is therefore built to be able to transmit those frequencies. The way a speaker creates sound is by passing electricity through a wire, creating a magnetic field, and pushing against a permanent magnet. Either the magnet or the wire is attached to a membrane that will then get pushed out. Doing this between 20-20k times a second and you make sound. However when charged particles (like the electrons in a wire) accelerate they create radio waves, so the magnetic coil in the speaker will also create a small amount of radio waves in the same frequency as the sound it is producing. This is what's called parasitic EMF, and in this case it turns out that this small amount of radio signal is enough to interact with the radio in the wheels.

> Since 7.8 kHz is in the audio range, you can use the parasitic EMF from your phone's speaker to "transmit" a similar code by playing a crafted audio file

The range of human hearing is about 20 to 20000 Hz. As a by-product of producing physical vibrations at those frequencies (i.e. producing sound) via an electromagnetic coil, a speaker will produce an EMF with the same frequencies.

I believe this is referring to the human ear's frequency range, so 20Hz-20kHz, which is a range that phone speakers can produce pressure waves at. I didn't watch the talk either, but I'm assuming that one of the following cases is true:

1. The phone's speaker generates a small amount of EM intereference at the audio frequency it's playing at 2. The sound waves hitting the locking electronics cause them to vibrate at that frequency and pick up random noise from the environment as a signal.

Either way, by using a frequency between 20Hz and 20kHz, everyone has some kind of "transmitter" that can generate mostly arbitrary waveforms.

Has several definitions, usually 20 Hz — 20 kHz: https://en.m.wikipedia.org/wiki/Audio_frequency, but sometimes may refer to the range of specific codec (telephony might have something like 300—3400 Hz for example, so not this case). TFA means it as “I have a peripherial readily available”.

It's not really because it's "in audio range" but rather "in a range which your phone speaker can transmit" which "happens" to be in audio/hearing range we can mostly hear for obvious reasons.

This is an alternate (probably similar) app to help your atomic clock watches get a signal from your phone rather than waiting for the vagaries of radio signal propagation (WWVB / https://www.nist.gov/pml/time-and-frequency-division/time-di...).

https://apps.apple.com/us/app/radio-wave-sync/id1484233572

...basically: Turn up your phone volume, wiggle the phone speaker (which has magnets), magnets == signals => watch gets the right time from your phone instead of remote radio waves.

I've used the app a few times before and it's generally pretty reliable if you follow the instructions!

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