Maple, welcome; I know your question doesn't require this, but we have a board policy that generally helps us minimize the number of posts required to help people; it's here
http://www.johnlsayers.com/phpBB2/viewtopic.php?t=4062
I know you've missed it, or your location would be posted along with your user name. I'd appreciate it if you take care of that part at this time -
As to high frequency absorption, that won't change with or without spacing from the ceiling; the spacing causes absorption at LOWER frequencies to improve. While this is generally a GOOD thing (makes absorption more uniform) if you still have too much high frequency you will probably need more surface covered with absorption.
Way it works is this - lowest peak absorption occurs at 1/4 wavelength. Wavelength is = to speed of sound divided by frequency (If I knew your location, I'd know whether to use metric or english units - just one reason for that "read first" announcement) - since I don't know where you live, I'll use english units. Wavelength = 1130/frequency, so for 10 kHz tone the wavelength would be 1130/10,000 or .113 feet, or 1.356 inches. Divide this by 4 to get 1/4 wavelength, and you have maximum absorption of 10 kHz with only 1/3 inch of depth.
This is linear, so 1 inch would get you 3 times as low frequency, or 3.3 kHz, and 2 inches would get you down to about 1.6 kHz - this is just 1/4 wave length, actual absorption continues to be useful to at least an octave below that, or about 800 hZ for 2" absorption mounted against the wall or ceiling. Spacing the material off the wall does even more to lower this bottom end, but the only thing that will increase UPPER frequency absorption is MORE SURFACE AREA.
Only exception to this I can think of is if you've covered your absorbers with some sort of non-breathing material, such as plastic... Steve