This horn is part of the European Space Agency's Large European Acoustic Facility in Noordwijk, the Netherlands. It purpose is to blast noise at spacecraft components to measure whether they can withstand the vibrations.
When turned up fully, according to the European Space Agency, the noise emitted by the horn would be loud enough to kill any human in the vicinity.
However, as Popular Science point out, it would probably take quite a while. The horn puts out 154 decibels, which is only marginally louder than a gunshot. The eardrums rupture at around 150 decibels. The lungs rupture at around 200 decibels.