This FAA site
Aircraft Noise Levels Appendix 10 and 11 provides the noise ratings of most helicopters. Nonetheless, it would be wise to get a db meter and check it out for yourself. In the past we built and tested rocket engines and as such measured the db levels and then had head sets made specifically for the noise environment. The cost was about $300 per set but none of us have hearing issues, so well wort it.