The optimal volume of 15 liters is worked out on the basis of a Qtc of 0.7.
A smaller box than 15 liters will raise the Qtc above 0.7 and you'll get a peak in the upper bass with a steeper roll off in the deep bass.
In effect the sound will be boomy. This type of enclosure will compensate for a system with poor bass by making it sound fuller. But if you have an efficient midbass, then the overall output in the sub frequencies will be boomy. The small box will also need more power to reach the same level of excursion (since the air from the rear cone is more compressed in a smaller enclosure and offers a higher resistance to the motor)
A larger box will reduce the Qtc, the trade off here is that the roll off will start at a little higher frequency but will go lower.
Taking cabin gain into consideration, I would suggest a lightly filled enclosure of Qtc between 0.6 (25 liters/0.88 cubic feet) to 0.65 (20 liters/0.7 cubic feet) or a heavily filled optimal enclosure.
you reinforced what I was thinking......
will now definietly go with a box with gross volume of 0.88 cubic feet.
thanks a lot for your help