Absorbing sound and increasing the attenuation between source and receiver (or sound isolation) are two different issues. Think about it, right now they have the best absorption possible in there being no reflective surfaces around them at all, the perfect absorber. So it is adding attenuation rather than absorption that seems to be needed.
Putting flats with absorption toward the performers may help slightly reduce the
orchestra levels compared to flats without any absorption but adding absorption to a lightweight panel will do nothing to increase the minimal attenuation the panel provides. Carpet as absorption may help the piccolos but won't do much for the bassoons, double basses, tympanis or tubas. A barrier wall could help reduce the levels in the front rows but a wall that lets you see the sound sources will provide little, if any, attenuation of those sources since it does not
block the direct path. Incorporating a barrier that at a minimum breaks
line of sight may not be
practical, although a plexi barrier or plexi used to extend a barrier may be possible.
So, add a pit or don't use the first six rows of seating or get the
orchestra to
play softer. The latter may be most
practical. As noted, some things like enclosures around certain instruments or diffusion instead of absorption toward the performers, creating better ensemble among the performers so they can hear each other and themselves, may permit some members of the
orchestra to
play softer.