2x 90 minute hoods?

Or a 90 with a bigger/smaller 30 minute hood?

Or a multi-layered plasterboard box, might help boost the bass frequencies

Interesting thought - do the rating just add up though, or do you run into issues with how you can install them and getting the right seal from hood to hood/ceiling?

Boxing may be possible - but the ceiling is of course finished and building a bigger box through a loudspeaker cutout might be challenging!!

Jason.

You must be in a tall building or one with substantial number of floors below ground? Anyway, 120mins fire resistance is hard to achieve but like all fire rated ceilings, it comes with very tight specification. Banging holes in them, fire hoods or not, could torpedo that specification. 
Any one objecting, perhaps via some future fire safety inspection, could have you on the rack! If I were you, I would definitely not puncture a 120min fire rated ceiling without written approval from a fire engineer or the building’s fire risk assessor.

how deep are the speakers from front to back - could the speaker be entirely below the firebreak level and be mounted in a suspended tile or something hanging trendily  a few inches below?

Mike

Indeed so - the architect and fire people are involved as it's a new build.  We merely installed the consultant-specified loudspeaker in the defined location on the approved drawing, with the hole cut by the main contractor for us - only then to be told that there is a problem!

The specified units are quite deep at 240mm, which would put us in pendant loudspeaker territory as an alternative, but the ceiling is a bit low for that.  We're exploring wall loudspeakers as an alternative, or lower profile units in an enclosure, or surface mounted units.  It is all a bit tricky though.

Building a 240mm high box through a 200mm square hole does sound challenging.

But you can build it on the ground and cut a larger hole and then put it up screwed into battens in the ceiling. Thinking a cross-section would look like a top hat. Would need a bit of decorating around it.

Also the layers of plasterboard would need to overlap at any joints, so stepped.