from Longman Contemporary:
a) in order to make something happen, make something possible etc He lowered his voice so Doris couldn’t hear.
Why don’t you start out early so that you don’t have to hurry?
b) used to say that something happens or is true as a result of the situation you have just stated
There are no buses, so you’ll have to walk.
The gravestones were covered with moss, so that it was impossible to read the names on them.
Q: a) He lowered his voice so Doris couldn’t hear.
I think 'so Doris couldn't hear' sounds more like a result than a purpose.
Am I correct? Is there any way to properly distinquish between them when you use 'so that'?
If you are a native English speaker,
When you read, what's the first feelling of it? result or purpose
When you hear, what's the first feelling of it? result or purpose