To apply step-by-step measurement for acoustic holography, transfer function between signals of different position must be time-invariant. However, when the transfer function is time-variant, there are errors when we construct hologram.
When position or direction of excitation to the source is different, depending on each measurement step, spatial distribution of sound radiation from source will be different with respect to the excitation. Therefore, the transfer function varies with regard to the measurement step. On the other hand, if the relative positions between sound source and microphones varies with time, the transfer function also varies with time.
Errors due to the varying transfer function are discussed, when they are seen in time domain. It is proposed to interpolate the transfer function to gain time domain signals. To reduce the effect of time-dependent transfer function, it is proposed to arrange reference microphone near center of the aperture.