most of the good bass horns start around the 600litre volume (>20cubic feet). if you go for sub-bass then expect at least 4times that volume. The Labhorns need 12 units stacked for free space loading. This results in 3stacked units for combined floor and wall loading, that is 1800litres ~=60cub ft
Go and read the tapped horn thread at DIYaudio.com for a smaller volume horn that has narrowed bandwidth as a consequence.
