The ZEM is a completely different approach to hearing protection which still enables you to hear conversations or range commands. The ZEM units use “sound chamber” technology to achieve a NRR26 noise reduction rating in a very small, lightweight and foldable unit. The really great feature is your ability to hear speech frequencies with great clarity while wearing the ZEM. No muffs, batteries, electronics or control knobs. The SG31 has the same features as the ZEM, but with larger sound chambers that provide an astounding NRR31 noise reduction for even more hearing protection.
Sinclair, Issue:4A, Page:231