... when you check out a book on an rfid self-checkout machine in a library. I assume it has something to do with an electromagnetic field, but does anyone know more?

Some systems will trigger sounds so e.g. you know the item was scanned. Other sounds could be the electronics that are generating the EM field. The EM field does not have in inherent sound to it, and at low power, I can't think of any sound it might induce in the surroundings.

1 hour ago, Prometheus said:

... when you check out a book on an rfid self-checkout machine in a library.

..you should record it using smartphone and attach .wav or .mp3 or .mp4 so we could hear it..

 

Apart from what Swansont mentioned, some types of relays are quite loud.

https://en.wikipedia.org/wiki/Relay

 

This one, that I have, is clicking every time, my Arduino circuit is sending signal to turn it on or off. You can hear trigger sound around 4m 10s in this video:

Older/cheaper relays can have moving parts and electromagnets inside.

SSR (Solid State Relays) have no moving parts, therefore they are soundless.

 

Edited July 17, 20196 yr by Sensei

There are plenty of beeps and sounds coming from the monitor, but this isn't one of them. It's more of a thunk, coming from the belly of the machine, so i'm thinking the electronics that create the EM field as swansont said. I tried to find a sound of it online, but came across a load of ASMR stuff instead - that's some weird stuff. I'll record next time i take out a book.

I developed one such machine, but it was a quarter century ago, so they can have evolved in a completely different direction meanwhile.

This automatic cashier could read a barcode, let the customer pay with a bank card, and deactivate the anti-theft stripe, optionally reactivate it.

The anti-theft stripe is covered with permanent magnets which, after they're made, deactivate the stripe by saturating it, so the detectors at the store's exit don't notice the stripe. The magnets are chosen weak, but it still takes a good magnetic field to make them, and at some distance through the air. This demands a strong current in the deactivating coils, which I obtained over a short time by capacitor discharge.

The combination of significant field, strong current and short time creates a mechanical shock that is noisy. It could be reduced by making the coils stiffer and suspending them separately from all surfaces. I didn't care at that time, possibly nobody did meanwhile.

In a different experiment, I had 350V and 20kA in several turns that created 7T, and this one was much more noisy. Even the feed cables had to be hold on the table or they would jump away from the other. The friend how did that regularly told me that once a connection screw wasn't tied, it just vaporized when the current passed, and he stays deaf for a day.

Edited August 14, 20196 yr by Enthalpy

That's exactly what i was looking for, thanks.