Which component is the microphone for the speakerphone?

Hello all,

When I'm using the speakerphone on my iPhone 6, the mic does not work well at all - I'm told it breaks up badly or I can barely be heard. Siri seems to catch only part of what I say.

I suspect this is a result of the screen replacement I did a few months ago. The problem is, I don't know where the speakerphone mic is, so I'm not sure if I didn't connect it right, or didn't place it correctly when re-assembling the phone. Can someone point out where the hardware is, and if applicable, what I may have done wrong?

For that matter, if there is a known issue/common problem/common pitfall with the mic that I should be aware of, I'm all ears.

Thank you much,

Jason