How does fitting a magnetic pickup to your guitar affect the acoustic tone?

pdg

If you partly cover the soundhole, it lowers the "breathing mode" of the guitar (the note you get if you thump the top -- usually G on 6th string for oval holes, maybe F#). That mode supports the notes close to it. Also, the oval hole is already small, so you make it harder for the air to come out, probably reducing the overall volume.