Magic phone ringtone, inaudible to adults

"There is a kind of mobile phone ringtone that students can hear but teachers cannot. Have you downloaded it?" Recently, a so-called "magic mobile phone ringtone" has spread among students in Guangzhou. It was said that adults over 25 years old often could not hear the sound. For this reason, many students have downloaded the ringtone so that they can bring their mobile phones into the classroom without worry.