Acoustic short circuit (loudspeakers): If the wavelength of a frequency is greater than the scope of the oscillating Membran, the forward and backward radiated sound cancel each other out. The pressure in front of the membrane (see membrane) then opposes a negative pressure behind the membrane. That's why almost all loudspeakers are closed at the back.
Open constructions such as electrostatics (see Electrostats) or magnetostats (see Magnetostats) usually have a very weak bass.