Some people who attend the World Cup games enjoy blowing plastic horns called vuvuzelas, which are loud and make an ugly sound. Trevor Cox, president of the UK Institute of Acoustics explained to New Scientist why they sound so awful.
The vuvuzela is like a straightened trumpet and is played by blowing a raspberry into the mouthpiece. The player's lips open and close about 235 times a second, sending puffs of air down the tube, which excite resonance of the air in the conical bore. A single vuvuzela played by a decent trumpeter is reminiscent of a hunting horn – but the sound is less pleasing when played by the average football fan, as the note is imperfect and fluctuates in frequency. It sounds more like an elephant trumpeting. This happens because the player does not keep the airflow and motion of the lips consistent.
UPDATE: How to filter vuvuzela out of World Cup audio
What makes the sound of vuvuzelas so annoying? (Via Kristie Lu Stout)
Photo by Dundas Football Club | Attribution 2.0 Generic