Airplanes, helicopters, and high-speed road vehicles all produce annoying sounds or noise. Industrial processes and machinery produce noise, as do home appliances such as vacuum cleaners and fans. Some sounds such as music are pleasing to the ear, but most unwanted sounds cause annoyance to humans; the level of annoyance depends on the loudness, the duration, and the frequency content of the sound. Some sounds have dominant tones, such as the “singing” of high-voltage power lines in strong winds, and others are broadband, such as the noise of a jet-engine take-off.