Does "speak in a low voice" refer to volume/loudness or to pitch?

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?

Solution 1:

low

(Of a sound or voice) not loud or high:
keep the volume very low
his low, husky voice

It has, therefore, both the meanings.
The same dictionary defines husky thus:

1 (Of a voice or utterance) sounding low-pitched and slightly hoarse:
his voice became a husky, erotic whisper

A low voice can be loud! (click 'More example sentences' in the low entry to open)

His voice was low and loud, like a volcanic eruption sounds from inside the volcano.

[EDIT]
By the same token, lower your voice would also be ambiguous. However, there would hardly be an occasion to ask someone to reduce the frequency rather than the amplitude, other than maybe at an audition/ recording session.

Solution 2:

The most common sense would be for "low voice" to mean low levels of loudness.

The most common way to ask for a low pitch would be to ask for a "deep voice"

Solution 3:

It depends entirely on context. "Darth Vader has a low voice." or "Why are you speaking in such a low voice?" "Because my throat is sore."