What’s the area of Hunger of Hadar whispering and slurping sounds?

Hunger of Hadar has this audio effect:

You open a gateway to the dark between the stars, a region infested with unknown horrors. A 20-foot-radius sphere of blackness and bitter cold appears, centered on a point with range and lasting for the duration. This void is filled with a cacophony of soft whispers and slurping noises that can be heard up to 30 feet away.

Can these sounds be heard 30′ from the sphere (50′ radius), or 30′ from the center point (10′ from the sphere)?

Related: I also asked a separate question about the effect of these sounds.