Whistle Hook Up

My father is back to playing with his trains (N scale I believe)! He bought the attached Big Sound round speaker - says it is an engine whistle. However, he doesn't know how to hook it up (do you attach 2 wires, 4 wires). I tried searching internet but can't find anything. I noticed there is one small hole on the side of the speaker, but he thinks you attach the wires to the nubs that sit on the top of the speaker.

Can anyone help?

Thanks.