How effective are sound absorbing panels at reducing office noise levels?
If absorptive panels are placed on the ceiling and walls, the noise levels will most certainly drop, but there could also be a decrease in speech privacy as well. You don't want 60 people to all be able to hear each other talk on the telephone or to each other.
To mitigate this problem, sound masking systems are typically installed in office environments. These are essentially a series of speakers placed above an acoustical ceiling tile grid that generate white noise. The number and placement of speakers are optimized for the particular office environment. The result is a background hiss-like noise that will add a degree of privacy and reduce the perceived impact of sound distractions in an office (e.g. someone coughing or the sound of typing on keyboards).
In short, adding absorption to the ceiling (as previously recommended) and adding sound masking should improve the acoustical environment of the office.