  Creating sound zones has been an active research field since the idea was
first proposed. So far, most sound zone control methods rely on either an
optimization of physical metrics such as acoustic contrast and signal
distortion or a mode decomposition of the desired sound field. By using these
types of methods, approximately 15 dB of acoustic contrast between the
reproduced sound field in the target zone and its leakage to other zone(s) has
been reported in practical set-ups, but this is typically not high enough to
satisfy the people inside the zones. In this paper, we propose a sound zone
control method shaping the leakage errors so that they are as inaudible as
possible for a given acoustic contrast. The shaping of the leakage errors is
performed by taking the time-varying input signal characteristics and the human
auditory system into account when the loudspeaker control filters are
calculated. We show how this shaping can be performed using variable span
trade-off filters, and we show theoretically how these filters can be used for
trading signal distortion in the target zone for acoustic contrast. The
proposed method is evaluated based on physical metrics such as acoustic
contrast and perceptual metrics such as STOI. The computational complexity and
processing time of the proposed method for different system set-ups are also
investigated. Lastly, the results of a MUSHRA listening test are reported. The
test results show that the proposed method provides more than 20% perceptual
improvement compared to existing sound zone control methods.
