Today I want to address a question that has been on the minds of concertgoers for, perhaps centuries.
Why do French horn players put their hand in the bell?
So I did a little research and I found out many things, but the first thing I found out is this:
French horns are complicated.
Playing them is complicated, their history and evolution is complicated, assembling them can even be complicated.
But back to the original question. A long time ago, horns did not have valves, the buttons you press to alter the pitch. It turns out you can produce several tones blowing through a valve-less horn, the natural tone of the horn and the overtones of that pitch (here’s an article on overtones). But you can’t produce the whole scale.
When horn players put their hand in the bell of the horn, using the technique used as “hand stopping,” depending on how they shape their hand, they can alter the pitch of the tone, and get a full scale. When the hand blocks the bell, it can also alter the timbre of the instrument.
Now that horns have valves, horn stopping is not strictly necessary, but is still used to alter timbre.
The scientifically-inclined among you may enjoy this article, “Spectral Analysis of the French Horn and the Hand-in-Bell Effect,” which also contains references to articles on the acoustics of other brass instruments.
Here is a portion of an interview with the first female French horn player (and first female in the horn section) of the Berlin Philharmonic, Sarah Willis, demonstrating horn stopping.
By the way, the instrument is called a French horn only in English. Everywhere else it is called a horn. Whether the instrument originated in France is a matter of some debate.
In the References section you will find a number of great resources to find out more details about this fascinating instrument.
But I can’t leave you without some actual horn music. There is, of course, Mozart’s Horn Concerto No 2, among other great horn works, but I wanted you to hear this lovely horn and organ duo performance of Pie Jesu by Gabriel Fauré.
“Ten Facts You Should Know About the French Horn,” https://blog.sheetmusicplus.com/2014/04/18/ten-facts-you-should-know-about-the-french-horn/
The Arizona State University Horn Studio (a fine collection of articles and music) http://www.public.asu.edu/~jqerics/articles_online.htm
Horn WikiBook with information on technique, repertoire, and more https://en.wikibooks.org/wiki/Horn/Introduction
Watts, Adam, “Spectral Analysis of the French Horn and the Hand-in-Bell Effect”, http://physics.illinois.edu/undergrad/reu/2009/Watts_Adam.pdf
WIkipedia-French horn https://en.wikipedia.org/wiki/French_horn
Image attributions: Hand from http://www.clipartpanda.com/clipart_images/outline-of-an-hand-clip-art-21795208; French horn from Dover Ready-to-Use Old-Fashioned Music Illustrations.