For humans to be able to hear a sound, it must be both loud enough and within the right frequency range – as measured by the number of vibrations per second, or hertz (Hz). The average person is most sensitive to sounds in the 1,000-5,000 Hz range, and most lose the ability to hear very high frequencies (above around 20,000 Hz) with age. Even so a sizeable proportion of the population do seem to remain sensitive to the very low frequency “infrasound”. High-frequency sounds have more than just audible effects as teenagers in Swindon discovered in 2006. Tired of having crowds of youngsters collecting around the town theatre, the owners installed the Mosquito, a device that emits sonic energy at very high frequencies. Only the teenagers could hear it and it forced them to meet elsewhere.