What is the loudest instrument in a standard orchestra?

The trombone, on most measurements - a fortissimo trombone can pass 110 decibels at close range, and brass generally carries over everything else because the sound is directional and comes straight out of the bell at the audience.

This is why orchestral seating looks the way it does. Brass and percussion sit at the back not because they are less important but because they do not need help; strings, which are individually quiet, sit at the front in large numbers. It takes a great many violins to balance three trombones, which is roughly why an orchestra has the proportions it does.

It is also an occupational health problem that took a long time to be taken seriously. Players seated directly in front of the brass are exposed to levels that would require hearing protection in a factory, and orchestral musicians have measurably elevated rates of hearing loss. In 2012 a viola player won a case against the Royal Opera House after a single rehearsal seated in front of the brass section left him with permanent hearing damage - the court found the orchestra liable, and the ruling changed how the industry handles it.