The Speaker Key and the Bb Clarinet

 

The B-flat clarinet is an orchestral instrument going back to the time of Mozart and beyond.  It is a transposing instrument, which means that the note heard is a major second below the note written on the page. 

The B-flat clarinet has a small key on the back, just above the thumbhole.   Unfortunately, you cannot see it in this picture.   It starts where the black line hits the instrument.   This key is generally referred to as the speaker key or octave key.   It was first added by J.C. Denner (1655-1707).   It allows the production of an extended scale. [1]   It also changes the timbre of the instrument substantially. 

The hypothesis was that the speaker key shortens the column of air substantially, thus selecting a different mode.  To determine whether this was the case, 4 notes were recorded.   These were the lowest note available on the instrument (E below middle C), the first note to use the speaker key (B above middle C), the lowest note available without pressing any keys (G below middle C), and the note that uses the same fingering as the G but adds the speaker key (D an octave above middle C).  Then the instrument was measured.

The clarinet is 0.67 meters long from the tip of the mouthpiece to its’ end.  In making the calculations, the instrument was assumed to be a square pipe with perpendicular flat ends, in order to simplify things.  

Sound is produced in the instrument by means of a single reed, which vibrates as the player blows into the mouthpiece.   The reed is set against the mouthpiece and the player blows into the mouthpiece.  Then keys are opened and or holes are closed to change the length of the pipe and thus change the pitch.  The fundamental of the instrument is the low E at 245 Hz.  Each note has other pitches that are produced, but not heard on the instrument and these can be seen very clearly in the graph of the intensity of the low E.   As can be plainly seen, the fundamental is not the only note produced, but it is the only one generally perceived by the listener. 

When the speaker key is opened, the same fingering (with of course, the addition of the speaker key) creates a much higher pitch with different harmonics.  Here you can see that the harmonics are in fact rather different for the higher note, instead of lining up with the harmonics for the low E, as was expected.