Wind instruments
(technical name: aerophones)
• The sound vibration usually begins at the mouthpiece.
• The rest of the instrument magnifies, and turns this
vibration into a pretty sound
• Wind instruments are classified by mouthpiece types:
A. Brass instruments
The players lips vibrate against each other and against the rim of a
cup mouthpiece. Lips act as a valve, introducing puffs of air at just
the right time to maintain oscillations of the air column
• Note that an instrument is classified as brass not because it is
made of metal, but because it has this type of mouthpiece, which
relies on vibrating lips.
B. Woodwind instruments
The player blows air against a sharp edge or through a reed
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A. Brass Instruments
Construction:
•Mouthpiece
•Mouthpipe
•Cylindrical section (with valves)
•Bell
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Oscillations in a pipe (review)
v
fn  n
4L
n  1,3,5,7...
Trumpet:
L  140cm  1.4m
v
343m / s
fn  n
n
 61.3Hzn  61, 184, 306, 429 Hz
4L
4 1.4m
.
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Acoustic impedance
Definitions:
Specific acoustic impedance:
Acoustic impedance:
p
z
u
p
p
z
Z 

U uS S
p2
I
z
 2
I
u
Volume velocity:
U  uS
p – sound pressure
u – speed of particles of media (air)
S – cross-sectional area of the tube
I – intensity
For plane wave:
z  v
ρ – density
v – speed of sound
For air:
v  400kg /(m 2 s)
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Acoustic Impedance curve
•Acoustic impedance is a function
of frequency
•Acoustic impedance curves have
peaks at resonance frequencies
Role of acoustic impedance in
brass instruments
At the frequencies of impedance
peaks the excess pressure helps
to open the lips (pressure
cooperates with the lips) –
regenerative or positive feedback
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( 1   2   )
Bernoulli’s equation
y
A2
v2
W  K  U
d2
W  F1d1  F2 d 2  P1 A1d1  P2 A2 d 2  P1  P2 V
A1
v1
d1
K  12 mv22  12 mv12  12 V (v22  v12 )
U  mg( y2  y1 )  Vg ( y2  y1 )
P1  P2  12  (v22  v12 )  g ( y2  y1 )
P1  gh1  12 v12  P2  gh2  12 v22
P  gh  12 v2  const
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The bell
All brass instrument have bells
• Bell changes spectrum of the radiated sound
• Bell changes acoustic impedance curve (both the frequencies and
heights of the resonance peaks)
• Bell changes the radiation pattern, making it more directional at high
frequencies (Why at high frequencies?)
• It allows more efficient radiation of sound by matching the high pressure
inside the horn with the lower pressure outside
The mouthpiece
It shifts the frequencies of the impedance peaks, but has more dramatic
effect on peak heights, especially enhancing low resonances.
Popping frequency – the lowest resonance of a brass instrument mouthpiece
(750-850 Hz)
Question: Estimate the length of the mouthpiece.
f1  v 1  v 4L 
v
340m / s
L

 10cm
4 f1 4  800Hz
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Valves
Valves are more commonly found on brass instruments. Pressing a valve
makes the air flow through an extra section of tube, temporarily making the
instrument longer in between the mouthpiece and the bell. The slightly
longer instrument gets a slightly lower fundamental harmonic, and a lower
harmonic series. (A few valves are ascending valves, which cut off a
section of tubing and so raise the pitch.)
animation
http://cnx.org/content/m12364/latest/trumpetvalve.swf
Slides (trombone)
Seven positions of the slide are needed to play all desired notes of the scale
These positions are not equally spaced (for a semitone down ~6% elongation)
Problems with valves position and slides length
L1 2  21 12  1  0.0595
L1  21 6  1  0.1225
L3 2  21 4  1  0.1892
L1 2  L1  0.1819
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Woodwind instruments
The player blows air against a sharp edge or through a reed
Reed
• A thin strip which vibrates to produce a sound
• Usually made from “Giant cane” (bamboo), sometimes from synthetic
materials
• Single reeds instruments: clarinet, saxophone
• Double reed instruments: oboe, english horn, bassoon, contrabassoon
• Free reeds: a) framed free reads - accordion, harmonica, bayan, ancient
instruments; b) unframed free reads – some primitive instruments
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