Unfortunately i was away, so my apologies for the delay in response. Lets deal with your dilemma though.
Intensity levels of 85dB for 8 hours a day can be harmful to your hearing. The best way to find out if there has been any damage to your hearing is to have your hearing evaluated by an Audiologist. If you don't know of someone in your area, please let me know and i will gladly refer you (email@example.com).
In the meantime - the group can get together and discuss their concerns and frustrations with management. Maybe they can change the type of tones. It is always valuable to hear about true concerns from your employees.
Please let me know if you need any further or more specific information
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