An apparatus, method and computer program is described comprising: receiving a near-field audio source signal from a near-field microphone (22); receiving a far-field audio signal from an array comprising one or more far-field microphones (23); determining a filter length of a first portion of a room impulse response filter for the near-field microphone, wherein said filter length of said first portion is the same at each of a plurality of frequency bands of the filter and wherein said filter length of said first portion includes a direct acoustic propagation delay; and determining a filter length of a second portion of the room impulse response filter at each of the plurality of frequency bands, wherein the filter length of said second portion is frequency-dependent.