The best replacement for knocker is doorbell.

The best alternatives to knocker are rated as following:
★★★★★  doorbell

Your alternative for knocker is not listed? Suggest!
Speaking about knocker, one probably shouldn't forget about door viewer.

Other search terms related to knocker and their ratings are:
★★★★★ door viewer
★★★ peek hole
★★★ viewer
★★ eye viewer
★★ peep hole
peephole
You have in mind a companion for knocker and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of knocker in all years. Click on graph for more.
knocker
doorbell