Chirper
Send secret messages in Morse code at pitches that some people can hear and others can't. High-frequency hearing tends to decline with age, so a younger listener may hear a message that an older listener cannot.
Works best with speakers. Start at a low volume.
Often audible to adults
Hz
Hearing range presets Tap to set frequency
Morse code reference
A dash lasts three dots. Pause three dots between letters, seven between words.