‘…here’s a sound that I can’t hear. It’s a high-frequency, around 17.4 kHz. I can’t hear it because that frequency of sound (and anything above it) is typically only audible to people under 25. It’s the result of a phenomenon called presbycusis, a term used for this progressive, age-related form of hearing loss.

In 2005, Howard Stapleton adopted this frequency for an invention he called The Mosquito, an ultrasonic alarm designed to “disperse unwanted youth gatherings” and “combat vandalism,” as the company puts it. Because the sound is only audible to those under 25, UK business owners — including even McDonald’s — can flick on The Mosquito alarm to repel potential vandals while retaining the patronage of the 25+ crowd who can’t hear it. Adults are sneaky fucks.’