When it comes to clapping don't knock it. TB Eruvin 104

Yesterday we learned that fixing a musical instrument outside the Temple's precincts on Shabbat is forbidden. Obviously if one can’t fix a musical instrument, he certainly may not play it on Shabbat. Today's daf TB Eruvin 104 discusses whether clapping or knocking falls under the rubric of a musical instrument.

“Ulla happened to come to the house of Rav Menashe when a certain man came and knocked at the door. Ulla said: Who is that? May his body be desecrated, as he desecrates Shabbat by producing a sound. Rabba said to him: The Sages prohibited only a pleasant musical sound on Shabbat, not the rasping sound of knocking on a door...

“Abaye raised another objection to Rabba from a baraita: One who is guarding his produce from birds or his gourds from beasts may guard them, in the manner that he typically does so, on Shabbat, as his guarding does not entail a prohibited labor, provided that he neither claps, nor slaps his hands against his body, nor dances and produces noise with his feet, in the manner that is performed on weekdays to chase away birds and animals.

“The Gemara asks: What is the reason that these activities are prohibited? Is it not because he is producing a sound on Shabbat, and any production of a sound is prohibited? Rav Aḥa bar Ya’akov said: This is not the reason. Rather, it is a decree issued by the Sages, lest while acting in his usual weekday fashion he mistakenly picks up a pebble to throw at the birds, thereby handling an object that is set-aside.” (Sefaia.org translation)

The Gemara brings example after example making the case that knocking or clapping is forbidden, but like Abaye's objection is rejected. Nevertheless according to our classical sources, one is forbidden to use objects as musical instruments on Shabbat and knocking on a door rhythmically with the intention to make music. A person is permitted to knock on the door to announce his presence.(Shulkhan Arukh, Orekh Hayyim, 338:1) I have seen Orthodox Jews who clap their hands in a different manner like using the back hand of one of the two hands as a way not to violate the letter of the law.

I have to admit that I ignored this ruling in my synagogue. All my favorite cantors with whom I worked have kept the beat by knocking wherever they are leading services. These are the people in my mind who have not only gotten into the prayer, but also have encouraged the congregation to join them in song. The same is true when it comes to clapping along. I see only benefits as I watch my congregants swing, sway, and clap along with the cantor. So when it comes to clapping don't knock it.