Direct‐radiator loudspeakers are often mounted with the back of the diaphragm working into a completely enclosed space. Conventional theory states that when the maximum linear dimension of such an enclosure is small compared with the wave‐length, the pressure is uniform throughout, and the acoustical impedance presented to the loudspeaker is ‐j/ω(V/ρc2), whereVis the enclosed volume. Although it has not been clearly established how small an enclosure must be before it is “small compared with the wave‐length,” the foregoing expression is generally used, at low audiofrequencies, to calculate the acoustical impedance of closed loudspeaker housings.It is shown here that while the acoustical impedance of a closed rectangular housing is capacitive at very low frequencies, it passes through zero as the frequency increases and becomes that of an inertance as the frequency of the first normal mode is approached. For a typical housing 11 in. × 22 in. × 22 in., the frequency at which the impedance presented to a very small speaker passes through zero is in the vicinity of 70 c.p.s.; at this frequency, the maximum linear dimension of the enclosure is less than one‐seventh of the wave‐length.These results are obtained by using the methods given by Morse for determining the pressure distribution in a room. A point‐source loudspeaker is assumed and the pressure at the source is calculated as the summation of the pressures due to the normal modes of the enclosure, losses being neglected. Measurements of the pressure within the enclosure support this analysis.From measurements of the pressure distribution over the surface of the loudspeaker diaphragm, it may be deduced that the magnitude of the acoustical impedance which the enclosure presents to the loudspeaker diaphragm and the frequency at which the impedance becomes zero depend upon the dimensions of the loudspeaker diaphragm as well as the dimensions of the enclosure.