Fashion in Europe and America: Ringtones That Teachers Can’t Hear
The so-called “noiseless ringtones” are actually sound of 17000 Hertz. Setting it up as ringtones, students can easily hear the sound of coming messages, though; teachers are not in the least consciousness.
What the “noiseless ringtones” use is the most basic acoustics principle. The frequency that can be caught by human beings is at the [...]