[Arve]

Someone pointed out low-pass filters to you, but that's not the entire story. Built-in speakers in phones and laptops generally don't have a naturally extended frequency response.

A while ago, I made audio measurements of the built-in speakers on the iPhone 7 Plus - https://imgur.com/gallery/DRbu5

The iPhone's extension is not entirely atypical of such devices, and leaves you with a reliable passband of 500 Hz to 10 kHz - above and below those frequencies, you're getting too much noise.

If you're airgapping with audio cables, you'll have a reliable passband of 20 - 20 000 Hz or thereabouts, with some caveats for built-in audio on devices such as a Raspberry Pi that fizzle out at 15 kHz.

As noted by brian-armstrong's comment, you'll only get reliable near-ultrasonic airgapping over a wired connection.

[brian-armstrong]

This isn't correct, by the way. Quiet's near-ultrasonic mode (~19kHz) works fine from several feet away using phone hardware. As long as you're using relatively narrowband communication, it works fine.