Vary the relative weightings of the join sub-cost component (F0, power, spectrum).
The default weights are all 1.0 (the features have already been normalised to unit variance)
F0 weight:
festival> (du_voice.set_jc_f0_weight currentMultiSynVoice 0.5)
Power weight:
festival> (du_voice.set_jc_power_weight currentMultiSynVoice 0.5)
Spectral weight:
festival> (du_voice.set_jc_spectral_weight currentMultiSynVoice 0.5)
Setting a weight to 0.0 means that the join cost effectively ignores that acoustic feature.