The lower the tone frequency the less audible it tends to be.

当然,这些音频信号的频率越低,越难被听到.

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The types of - voice signals considered include voiceband data , network signalling, circuit continuity tones, and dual - tone - frequency signalling.

考虑的 非话音 信号的类型包括:话带数据 、 网络信令 、 电路连接单音以及多音多频信令.

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