A horn loudspeaker comprises: a horn (22) having a throat (26) and a mouth (30); a primary electro-acoustic driver (24) mounted at or adjacent the throat of the horn and directed generally along the horn; and at least one secondary electro-acoustic driver (32T, 32B, 32L, 32R) mounted part-way along the horn and directed generally across the horn. The secondary driver(s) can be used to change the local impedance conditions in the horn and therefore to change the polar response of the horn loudspeaker. At least one filter (12A, 12E) is provided for filtering an input signal (34) for the primary driver to produce a filtered signal for the or each secondary driver. Such a filter may be chosen or designed so as to optimise some aspect of the polar response of the horn loudspeaker, for example to increase directivity, or flatten the polar response within a specified included radiation angle, or to increase omnidirectionality.