Soffits with MDF?

I just used coarse thread drywall screws. I don't recall drilling first (don't think I did).

They actually make screws specifically for MDF, but can use drywall screws if you wish. Pre drilling will make it easier, but probably isnt necessary except for any places you are very near the edge...you might want to pre drill in those spots.

The screws wont sink into the MDF very well, so if your fabric is going to lay flat against the MDF you may consider using a counter sink so the screws will be more flush.

Excellent advice

I always use a pilot counter sink bit with mdf it makes the job much cleaner.

Or liquid nails and a 16 ga nail gun ...

If its going to be covered with fabric, why not fill with some fiberglass and cover with fabric (no MDF), to get some bass trapping out of it?

Check the last page of my build thread, I'm doing something like that - the bottom is open and filled with insulation (added a layer of rigid insulation since those pics), and I have a rope light tray and recessed lights. Also check Moggie's thread, and Sandman's.

Adding the MDF will act just fine as a bass trap. The MDF won't add up to a hill of beans compared to most low frequency wave lengths, and the addition of the MDF will actually enhance the bass trapping characteristics. A wavelength that passes through the MDF will be attenuated, and it will be attenuated even further when a portion of it is reflected back through the MDF making for reduced SPL. Keep in mind as well, your whole room is one big bass trap. Brad is correct in that you should put some insulation in there. Hope this helps! Best wishes!

You can counter sink the drywall screws on MDF without going through the effort of pre-drilling. Just make sure that you are not screwing it in close to the edge or it will crumble it. People have been doing this in the car audio business for years and it just takes practice.

The typical method is to install 2x2's on the wall and ceiling using screws and a construction adhesive. The screws through the 2x2's should anchor into your HAT channel (if that's what you are using). The soffits (preconstructed) are then raised into place and screwed into the 2x2 runners without construction adhesive. This provides a more secure anchorage and allows for easy future removal of the soffit if required.