Responses showed a high degree of agreement: knocking is characteristic of Germans and Austrians. One source added German-speaking Swiss. It is done in academic milieux; some respondents added meetings and pubs. Opinions differed more on when "knockers" would applaud: "Applause is restricted to non- academic performances, like the theater." Someone replied that "applauding could even be misunderstood as an attempt to ridicule the lecturer."
"If an audience is quite carried away, it tends to clap in a situation where it usually would knock." (Stefan Goes) "The occasions where we would rather applaud than knock are for example: If there is a guest lecturer (not necessarily from a foreign country) we would rather clap our hands, to show a kind of special appreciation that he came over here."
Speaking as a "native knocker" from Germany: I think it's only common in an academic context, at university or at a conference. Here applauding could even be misunderstood as an attempt to ridicule the lecturer. But nobody tells you to knock at your first day at university, it's rather some kind of common sense, handed over from generation to generation, without knowing why.
When I was doing a PhD in England in the early 1980s I shared an office with a student from Germany and we attended a seminar together (a maths/statistics seminar). He couldn't believe it when the audience at the end of the seminar gave a round of applause for the speaker at the end. He said it made the thing look like a comedy show rather than a learned seminar.
It seems to me that knocking is restricted to certain domains: academia, political meetings, club meetings (less so in business meetings, I think). In meetings it is not only used in the function of applause (after a presentation, for insce), but also as sign of consent.
Knocking on a table top is one very common form of greeting used in German pubs, when someone joins a group of friends or colleagues already seated at the table. In this case there are only one or two raps on the table, rather than the dozen or so (or more) used as a form of applauding. So perhaps in both contexts rapping on wood symbolises some sort of bond between a group and an individual?
I can't tell you where it originated, but here are some observations I
1. this custom not only is followed in academic circles, but also in cultural societies like literature clubs etc. and the typical german "Verein", that is club with activities ranging from gardening to mountaineering etc.. The knocking takes place usually on dignified occasions such as the annual report of the chairman of the tulip lovers club. One can notice that in societies with predominantly working and lower middle class this knocking has the aspect of a ritual which only lends the occasion dignity and portent. Also, there often seems to be some disagreement on whether to clap or to knock. In such cases members of the handclapping fraction, if outnumbered, often turn with an abashed look to knocking. The group pressure sometimes is very high to join in the knocking business, but don't ask me why.
2. If an audience is quite carried away, it tends to clap in a situation where it usually would knock.
3. In Germany and Austria it is customary in, again, mostly working class and lower middel class "Kneipen" (i.e. the traditional bars) to knock on the table when joining or leaving a group of acquainces at a table. Maybe there's a relationship.
4. The younger academic generation tends to dislike this ritual, but nonetheless bows to conformity, sometimes with a bit of irony. There's a difference, though, between the knocking after lectures, which is unquestioned, and knocking at the above mentioned portentous meetings.
As you generalized correctly, knocking is typically german (and austrian maybe), and is practiced especially on universities by students, but also in other public places after longer speeches.
On universities students knock after the lectures or after a presentation by other students. If a lecture is well done, but really really well done, they applaud to show their appreciation; something that happens rarely nowadays.
In Germany ...knocking on the table is restricted to the university, particularly to conferences. We always knock on the tables after a conference, and it's the intensity that reveals the degree of enthusiasm and appreciation. We don't do so after tutorials. Generally, other students' oral presentations are granted with knocking, out of a feeling of loyalty, I guess. Out of university, Germans applaud like other civilized people :-) (perhaps there's no table in a theatre ???:-) )
The habit of knocking on a table instead of applauding also occurs in the German-speaking part of Switzerland. I heard it in Basel and in Zurich, not in Geneva. In the Netherlands, where I come from, the habit is not known. I noticed that in Switzerland on certain occasions applauding occurs when no tables are around or for a foreign speaker. In the latter case an audience seems to hesitate between applauding and knocking.
...From a sociological point of view, I believe that the audience of a lecture knock, since first, the hearers intend to document solidarity with the speaker, that is because he is a colleague (and of course because of the fact you could be at his place). Second, they reveal allegiance to an elite. In my opinion, "knocking" means "collegial appreciation in the consciousness of membership in a select group" Interestingly, at first I applauded instead of knocking, perhaps because I didn't feel integrated. Consistent with this view, (besides the fact that I surely would applaud, if there's no table...), I applaud on all other occasions (a concert, a recitation, in the stadion), but I knock after lectures at a linguistic workshop. Another account would be, that 'knocking' is a kind of 'reserved applause' used by "German intellectuals", and the reason German scholars knock could be explained in terms of "habituation" and "group constraint".
Here in Brazil, as far as I know, at all Conferences of linguistics, everyone applauds, no one knocks. That confirms what you have written about the non-usage of this method in Americas. On the other hand, in a film whose name is simply "Freud", black-and- white, sorry for not remembering the name of the director and the actors, everyone knocks. And they are in Austria. I thought that it would be a practice at Medicine Conferences (you can test this possibility), but later, when I saw the fim again, I noticed that they knocked only with one hand, having the other holding a paper, or writing something. Would be this the reason ? Well, we can calculate that knocking on the table, comes from the last century, at least.
An English film attestation: I happened to see the c.1934 British movie Goodby Mr. Chips the other night on TV. It's set at a boys' school in England, and at a farewell dinner for Mr. Chips everybody knocked on the table where probably here nowadays people would applaud.
One respondent mentioned knocking in parliament: Knocking is in fact the normal way of applauding after the presentation of a paper. I have also seen it in parliament, as well (ok, on TV, off course)
This appears in at least one other parliament in the world: I was surprised to see that knocking is used elsewhere in the world! In the Canadian House of Commons, where (unlike in the British House) Members of Parliament are seated at desks, it used to be the case that approval of a member's interventions was shown by MPs thumping their desktops with the palms of their hands. However, sometime over the past decade this practice has died out and hand clapping is now the way of showing approval. Just a few weeks ago, though, I saw a news report on a debate in the British Columbia legislature (I believe that was the one), which showed the members thumping their desktops in approval of someone's speech, so the practice appears not to have completely died out in the Canadian parliamentary tradition. As to how this practice originated, I have no idea. Since the British Commons has no desks, it could not have come from there (although we have imported the terms "front benches", "back benches" and "backbenchers" from their Parliament, even though unlike them, neither our Commons and Senate nor any of the provincial legislatures sits their members on benches).
..to show appreciation, a Taiwan audience might invite the lecturer to a banquet! But there are some special customs at a performance of Peking opera. Whenever an actor does an especially impressive job on an aria, scattered members of the audience will call out "hao3!" in a loud voice, one here and one there, in rapid and random succession.
I can confirm that it is practiced in Oberfranken (upper Franconia) because that is where our partner school is. We are in Gefrees, just about 20 KM east of Bayreuth. Where every they go, they rap their knuckles on the table as a greeting. I never saw it in Niedersachsen and I lived there a year.