I have this Casio alarm clock for well over 25 years now. This alarm clock followed me from home to my new country in Perth Australia, and is a reliable workhouse - never fails to wake me up for all my exams and so on. Once I entered the workforce I’ve established a good biological clock routine and do not need for the use of a alarm clock again - but this clock stayed faithfully by my beside table till now.
The one feature this clock has is the EL backlight - high tech in it’s day, it’s something of a dated relic now with today’s OLED and backlit displays.

I love this clock, but there is something that annoys me always. There is this high pitched noise whenever the EL back light is lit. My curiosity is piqued when my wife commented she can only hear this noise when the clock is right by her ears, about 10 centimeters or more away she could not hear this noise any more. Definitely a no no when it’s more than 30 cm away. On the other hand, I still manage to hear this hum with the alarm clock an arms length away, possibly further.
Via my phone app, the the tones of the hums - 2.8 kHz, 4.3 kHz and 7.1 kHz.

All three bands and its amplitude are well within the human auditory spectrum. And yet my wife cannot hear it while I can. Why? Do I have super ears? The answer sadly is no.