PH 105 Dr. Cecilia Vogel Lecture 19

OUTLINE  Woodwinds  single reed, double reed, air reed  bores: cylinder, cone  resonances, harmonics  register holes, tone holes

Woodwind Classification  Woodwinds can be classified by the type of reed that disturbs the air.  single and double reeds: thin piece of wood  air reed: disturbed stream of air  Woodwinds can be classified by the type of bore (shape of the air column inside)  cylindrical bore  conical bore  Any combination of reed and bore.

Woodwind Pitch  String, and vocal instruments  get their pitch from the source of disturbance (string or vocal folds),  not from resonators.  Woodwind instruments  get their pitch from the resonances in air column,  not from vibrating reed or air stream.  The reed (or air reed) is very light, and is driven by the mass of air in column.

Bernoulli Principle  When relaxed, the reed(s) of woodwind are open.  When you blow past the reed(s)  why doesn’t the air just go through?  Why do the reed(s) close?  Moving air sucks,  that’s the Bernoulli principle.  Reed is sucked closed, but when closed no Bernoulli, opens up to get sucked closed again…

Reeds are Pressure Driven  The vibration of a reed is driven by pressure in the air column:  When a low pressure reaches the reed,  reed sucked closed &  cannot equilibrate to atmospheric pressure.  When a high pressure reaches the reed,  reed pushed open &  puff of high pressure air is blown in.  Again cannot equilibrate to atmospheric pressure.

Resonances of Tube  Because the air column cannot come to equilibrium at the reed end,  that end acts somewhat like a closed end.  The clarinet is a cylindrical-bore reed instrument.  We know the resonant frequencies of a tube closed at one end:  where  v = speed of sound in air  L = length of tube  n = only odd integers

Clarinet Resonances  Resonances of cylinder-bore reed instrument, such as clarinet:  At low freq  acts like tube with one end closed  only odd harmonics: 1, 3, 5  At higher freq  affected by bell  all harmonics: 6, 7, 8, 9, 10…  Missing 2 nd and 4 th harmonics make a tone sound “woody”

Conical Bore  Saxophone, oboe, bassoon  have a cone-shaped bore  Resonances of a conical column of air  are the same as a tube open at both ends  where  v = speed of sound in air  L = length of tube  n = all integers  compare fig and fig  timbre of sax is not woody

Air Reed  Flutes, recorders, flue pipes, whistles  blow air stream past obstruction.  Air flow is disturbed,  starting a sound wave.

 The vibration of air reed is driven by flow of air in the column:  When high pressure reaches end of air column,  stream of air is blown away from the opening,  air flows out of column.  When low pressure reaches end of air column,  stream of air is pulled into the opening,  air flows into the air column, Air Reeds are Flow Driven

Resonances of Tube  Because the air can flow in and out of air column at the embouchure end,  that end acts like a open end.  Flute is a cylindrical air-reed instrument.  We know the resonant frequencies of an open tube:  where  v = speed of sound in air  L = length of tube  n = all integers  Flute also doesn’t have woody quality.

Recall  To play notes in higher register on brass instrument  increase the freq of lip vibration.  To play notes in higher register on string instrument  suppress the lower resonance  by placing your finger on string  you force a node  where the lower mode has antinode.

Register Holes  To play notes in higher register on c larinet  Suppress the lower resonance.  By opening a register hole  allowing air in and out,  you force a node  where the lower mode has antinode.

Clarinet  Fig  shows position of register holes for clarinet-like instrument.  note odd harmonics  3f 1 is (2)(3/2)  is an octave plus a fifth  5f 1 is (4)(5/4)  is 2 octaves plus a major third

Overblowing  To play notes in higher register on flute  Suppress the lower resonance.  If the air stream crosses the embouchure hole too quickly  low freq resonances not excited.  Changing embouchure and blowing speed to bring out overtones  is called overblowing.

Flute  Fig  shows resonances that can be overblown on flute.  note all harmonics  2f 1  is an octave  3f 1 is (2)(3/2)  is an octave plus a fifth

Tone Holes  How to play the notes in between on woodwind:  An open tone hole along the air column  is almost like an open end.  The open hole effectively shortens the air column.  To raise the pitch one semitone, how far from end should you open a tone hole?  approx 1/18 the (remaining) length  How many notes between on flute?  On clarinet? 11 18

Woodwind Demo  Listen to timbre. Observe spectrum.  Play chromatic scale using tone holes.  Which holes are closer together? Why?  How many tone holes?  Play in different registers.  what technique is used?  Where are nodes?  What intervals can be played with same fingering?  Change of timbre with dynamics

Summary  Pitch of woodwind,  determined by tube resonances.  Cylindrical reed instruments  missing 2 nd and 4 th harmonic  Conical reed instruments, and cylindrical air-reed instruments  all harmonics  Play in upper registers  by using register holes or overblowing  Play chromatic scale using tones holes