As long as the frequency is between 1 and 10kHZ then well fitting earplugs should reduce the pressure level by 20 to 30dB. If you combine with earmuffs or noise cancelling headphones then that could approach 40dB [1]. Indeed the manufacturer of one system recommends that operators use hearing protection such as earplugs [2].

[1] http://scienceandglobalsecurity.org/archive/sgs09altmann.pdf

[2] http://www.adsx.co.za/information/faq-2/