Flat voice - Tradução em português

É uma voz chata, sem graça, robótica, tediante, ... As traduções do Google me deixaram confuso.
Gostaria que me ajudassem a entendê-la.

Segue abaixo, um contexto:

Van Helsing opened her mouth and showed Seward her teeth.
'See - they are like knife points now. How many times does she have to use them on a child's neck before you believe me?'
Seward could not take his eyes off the evil but beautiful face of the woman that he loved. 'What do we have to do?' he asked in a low, flat voice.