Ceiling joists consist of two narrow low grade timbers often bolted together sistered above a supporting wall.
Attic conversion ceiling joist loft framing.
Lining walls and ceilings with a double layer of acoustic plasterboard can also be very effective as can laying a carpet over thick rubber underlay.
Joists insulation and boarding.
You can sister the joists by cutting 2 x8 boards the same length as the joists applying construction adhesive and affixing each to a joist with nails.
There are many ways of altering roof structures for loft conversions but they all have one common element the ceiling joists will almost certainly be inadequate as floor joists.
You might open up your attic and find bare joists with fiberglass or cellulose insulation between the joists.
Existing loft joists are designed only to support ceiling boards so a structurally sound floor is needed.
Fortunately this is a fairly easy fix.
Loft floors loft floor construction.
This means that new floor joists are fitted alongside them slightly raised above the ceiling plasterboard to avoid contact with it.
At least 50 of the usable area calculated above must have ceilings of at least 7 feet.