Complex strangers speak their own languages, which vary in complexity and similarity to human languages, but nonetheless resemble human speech.
Imitative strangers repeat speech and sounds in their own voices.
Inarticulate strangers possess human-like voices, and whisper, laugh, or emit other vocal sounds, but do not create verbal statements.
Inert strangers speak in tones, inorganic clicks and taps, or other non-voices.
Inhibited strangers speak in a voice too distorted for any words to be individually discernible.
Mute strangers emit no audio, aside from sounds produced by physical processes (such as footsteps).
Predicative strangers speak using human language, although they do not respond to human speech. Although statements may appear to have semantic meaning, conversation is impossible.
Reactive strangers echo audio in their surroundings. Unlike imitative strangers, which repeat statements in their own voices, a reactive stranger's echoes are repeated as though recorded, with various levels of distortion.
Resonant strangers emit broadcast from non-physical sources, such as radio and telephone signals.