Transcript from ANIMUSIC

Chordophones
from
ANIMUSIC
by Wayne
Lytle
Physics of
Music
PHY103
Tradeoffs in chordophones
Strings only
• High tension means a lot of stress on the
instrument.
• More massive strings have lower pitches
under lower tension
• However thick strings don’t bend very
easily
– Loss of high frequencies leading to dull or soft timbre.
– Shift of overtones sharpwards
Piano spectrum
347Hz 697Hz 1094Hz
1396Hz
347*2=694
347*3=1041
347*4=1399
Higher harmonics are higher than multiples of
fundamental
Why? Wave equation requires more energy for
shorter waves – non-linear wave equation on string
Are these frequency shifts
important?
Butler(example 2.4).
a) Piano playing C4
b) Piano playing C4 but the partials have been
lowered by digital processing so that their
frequencies are exact integer multiples of
the fundamental.
Pair of tones repeated 3 times.
Bass strings
• If you lower the tension too much the tension changes during plucking
or hammering. The pitch changes after the pluck. The strings flop
around leading to buzzing and fast damping.
•  Low notes then requires thick and heavy strings (metal ones) to
prevent these problems.
• Wound strings help reduce problem of loss of high frequencies.
Problems with wound strings: damage to fret-board and fingers
particularly for fretless basses.
• To keep corrosion from reducing high frequency response  plastic
covering. However stretching of plastic may damp string motion
faster.
• If the tension is too low then the string will hit the finger board. This
is less of a problem for a harp but is a big problem for a guitar or lute.
Soprano strings
• Require light strings and high tension (for a
given tension).
• Metallic strings are tiny and kill your
fingers. Many steel string lutes/guitars are
not plucked by hand.
• Gut or nylon strings are softer but damp
faster and are less bright.
Tradeoffs in the strings
• Length/tension/density – ease of play, position of
plucking, having strings of different notes on the
same keyboard or fret-board, strength of
instrument.
• String composition
– metallic – less damping but heavier, harsher and
more damaging to fingers and fret-boards
– Gut or nylon – softer/duller but lighter and
damping faster
Amplification:
• A string by itself is not a very good radiator – it
has a small surface area.
• To increase volume the vibration must be coupled
to something with a larger surface area.
• Box: guitar/zither - vibration passes through
bridge– faces of the box vibrate.
• The surfaces of the box vibrate in modes as does
the air inside.
Amplification (continued)
• Box with holes – air moves in and out
(violin, guitar)
• Box with a membrane (African lutes)
vibrations excited in the membrane too.
The string excites harmonics, some are
amplified more than others depending on
the coupling of the string to box and the
way the box resonantes.
Examples of Chordophones
Box Zither- Santoor India
• Box zither –
trapezoid box with
many parallel
strings, strings are
struck
• Santoor (Indian)
Music taken from CD
Musical instruments of
the World 1990 CNRS
Vietnamese Board Zither
• 16 steel strings
above an oblong
convex sound
box.
• Strings are
plucked and
pressed to
change the pitch
African
Lute
Chad
• Skin below the strings on the gourd.
• Gourd resonator
Tar: Lute from
Azerbaijan
• Belly is covered
with ox pericardium
membrane
• 24 movable frets of
gut?
Harp –ngombi Central Africa
• plucked
KoraGuinea
• ox tendon
strings slid
up and down
for tuning
• notched
bridge
Mbela- Musical Bow
Central Africa
• What is the
resonant cavity?
Sounding the string
• Plucking finger/plucker
Sound is influenced by position of plucker
• Hammering
Sound is influenced by weight of hammer,
material of hammer and leverage of
hammer.
• Bowed –stick/slip continuous excitation
Hammered/Plucked/Bowed
Which one is which and
how might you expect
the sound would be
different?
The importance of attack
transients in sound identification
• Listening example 4.8 (Butler) Three sustained tones
are presented, each with the attack transient (initial
60 millseconds) removed. Identify the instruments
• Listening example 4.9. First the sound of a violin
(C4). Next the attack transient of that tone elongated
to a hundred times its normal length through digital
time expansion.
• Note this scratchiness makes it difficult for computer
programs to transcribe music played by violins.
Mixed timbres
• Listening example 8.3. Starting with a
French horn sound and ending with a
clarinet sound. 11 short tones each
shifted 10% further toward the clarinet
tone.
• Listening example 8.4. Intermediate
between piano and violin timbre. Both
spectral mix, and attack envelope is
intermediate.
Timbre and sound identification
• The tone color of an instrument
• Often used to describe spectral overtone
mix.
• However attack transient, decay, vibrato all
contribute to our identification of the
instrument played. Auditory cues such as
given in temporal changes are important.