Few adults can hear sounds above 18kHz. I remember making tone generators in Pascal as a kid, beeping out audible 22kHz notes that my older brother and my dad couldn't hear. Later on, I'd try similar programs on my mobile phone, but the damn device couldn't generate tones above 16.5kHz. Poor hardware, I thought, until I tried it on my 16 years old nephew.
Grasshoppers aren't nearly that high pitched.