20 Megapascals of negative pressure... What? Please explain

[DaveE]

This article (sorry, there's a paywall) describes ultrasound waves creating 20 Megapascals of negative pressure. What does that mean?

 

[Baluncore]

I can see how, following cavity collapse, there can be a negative pressure transient of 20MPa, but that would be measured relative to the positive impulse of 20MPa, delivered by the final closure of a cavity.

 

[DaveE]

So I guess pressure is just a measure of net force over a defined surface. The acceleration of particles in some local area?

I'm confused about the sign. Is the bubble created with positive pressure on the inside, or negative pressure from the outside? I guess that pressure is a scalar and the direction would be defined by the geometry of the surface used. So we could chose any polarity we like. Is there an obvious, or standard, polarity definition?

PS: Wikipedia says negative pressure is tension, positive is compression. That seems to make sense.