A question regarding the radio broadcast

[izzyalienqueen.livejournal.com]

I understand everything in public is fair game in so far as it is seen to happen. But how do we denote that a conversation isn't heard.

For example, I need to do a scene on the beach. It's fine if it's reported she was there. It's fine if they say she was seen doing this or that. But if I were to say "The breaking of the waves was loud enough you couldn't hear what she was saying unless you were standing right next to her," keep the conversation itself from being reported?

Thanks in advance!