Reed Instruments 2

Air Pressure Patterns w/ Reeds

• At the mouthpiece of a flute it is easy to make the air move and so we say that the flute has a low input impedance.
• At the mouthpiece of a reed instrument it is hard to make the air move, it takes large pressure changes to force air into the instrument, and so we say that the reed has a high input impedance.
• In order to get a good coupling between the high impedance of the reed and the tube, there must be a high impedance at the mouth of the tube.
• This means that there must be an anti-node of pressure at the mouth of the tube.

Beating Reed Instruments

• Beating reeds completely close the opening as they vibrate. The pressure changes at a beating reed are large but the amount of air that flows through is small. Thus reeds approximate closed ends of a tube.
• If we couple a beating reed to a cylindrical tube then it will sound as a half-closed tube.
• Lowest frequency will be 1 octave lower than a completely open tube of the same length.
• Sound will be lacking in even harmonics. This accounts for the characteristic hollow sound of the low range (the chalumeau range) of the clarinet family.

Tone-Hole Cut-Off

• By itself, a reed will excite an extremely wide range of harmonics in the tube.
• The diameter of the tube opening will determine the maximum frequency at which the tube will resonate well. There is a (rather soft) cut-off once the wavelength of the harmonic approaches the diameter of the tube.
• For Tone the lowest note of the instrument only the foot opening plays a role in setting the cut-off.
• Once there are open tone holes, they too play a role in setting the cut-off frequency.
• If the cut-off frequency of the row of open tone holes is too different from the cut-off frequency of the open end of the tube then the very lowest notes will sound quite different from the notes only a few semitones higher.
• If the cut-off frequency changes abruptly then there can be significant changes in the timbre from note to note.
  • Forked fingerings on the recorder show this effect to a slight degree.
  • The bagpipe shows this effect quite strongly and some notes sound stifled compared to others.
• One way to compensate for this effect is to modify the diameter of the last few inches of the instrument. This is why the clarinet has a slight bell at the end.