Does the low in
Speak in a low voice!
He said in a low voice.
refer to the volume/loudness or to the pitch? Does it mean quiet, or low-frequency?
EDIT: After understanding from your answers that the meaning is ambiguous (it may apparently mean either the first or the second or both), and after reading some answers to Please "Mute your voice"! suggesting "lower your voice" as a request to speak more quietly, I am adding a related question:
Is the meaning of
Lower your voice!
also ambiguous, or does this particular phrase refer unambiguously to loudness?