Update: Thanks! It's called a voice amplifier.
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Does this thing exist, and what is it called?
A microphone and a small speaker that you can attach on your body with a magnet or something, for speaking to crowds outdoors? Or just the speaker that you can connect any BT microphone to?
I've been on a few surveys and guided tours outdoors, and it's often so hard to hear the person speaking – even for me with good hearing. Surely there must be a tool for fixing this issue?