Title: Physics of the Piano Piano Tuners Guild, June 5, 2000
1Physics of the PianoPiano Tuners Guild, June 5,
2000
- Charles E. Hyde-Wright, Ph.D.
- Associate Professor of Physics
- Old Dominion University
- Norfolk VA
- chyde_at_odu.edu
2The Physics of Music and Musical
ReproductionAutumn 2000, Mon.Wed.
420-535pm Call 18539
ODU PHYS 332W
Topics Physical attributes of music and
sound. Acoustics of musical instruments and
concert halls. Electronic generation and
recording of sound. Neuro-physiology of sound
perception.
3Physics of the Piano
- Oscillations Sound
- Vibrations of a String
- Travelling waves Reflections
- Standing Waves
- Harmonics
- Piano acoustics
- Hammer action
- Sound Board
- Multiple Strings
- Chords, Scales Tuning
4Why does a mass on a spring oscillate?
- It is not because I push it
- The mass continues long after I let go.
- The spring is pushing on the mass.
- Why doesnt the mass just come to rest in the
middle? - After all, the spring(s) exert no (net) force on
the mass when it is exactly in the middle. - No force seems like no motion (wrong).
5Force and Motion
- The mass moves even when nothing is pushing
- The mass moves because of inertia
- Forces do not cause motionforces cause motion to
change - Force is proportional (mass) to the time rate of
change of motion (acceleration) - F ma
- A force acting to left either
- Makes the mass go faster to left or
- Makes the mass slow down as it moves to right
6Net Force causes motion to change
- Fma
- The time rate of change of velocity
(acceleration) is proportional (mass) to the net
applied force. - A force acting to left either
- Makes the mass go faster to left or
- Makes the mass slow down as it moves to right
7Fma Mass on a Spring
- For a Spring, F ? -kx
- Force pushes towards middle
- Force grows with distance
- Force -kx and Force ma
- Acceleration a - (k/m)x
- Frequency (f) measures how fast an oscillation is
changing - Acceleration is rate of change of rate of change
of position
- Car on freeway on-ramp
- 60 miles per hour per 10 sec
- Acceleration is equal to a mathematical constant
times frequency squared times position.
- Frequency increases with stiffness, decreases
with mass
8Vibrations of a String
- Each little segment of a string is like a mass on
a spring - The spring force is supplied by the tension in
the string and the curvature of the wave. - A wave (of arbitrary shape) travels on a string
with velocity
9Travelling waves and Reflections
- Each end of the string is held rigidly.
- To the wave, the fixed point acts like a wave of
opposite amplitude travelling in opposite
direction. - Rigid end of string reflects wave with opposite
sign - Loose end of string (or other wave--e.g. organ
pipe) reflects wave with equal sign.
10Standing Waves
- Each point on string experiences waves reflecting
from both ends of string. - For a repeating wave (e.g. sinusoidal)
- Velocity wavelength times frequency v l f
- The superposition of reflecting waves creates a
standing wave pattern, but only for wavelengths - l 2L, L, L/2, 2L/n)
- Only allowed frequencies are f n v/(2L)
- Pitch increases with Tension, decreases with mass
or length
11Harmonics on string
- Plot shows fundamental and next three harmonics.
- Dark purple is a weighted sum of all four curves.
- This is wave created by strumming, bowing,
hitting at position L/4. - Plucking at L/2 would only excite f1, f3, f5, ...
12Pitch, Timbre, Loudness
- Equal musical intervals of pitch correspond to
equal ratios of frequency - Two notes separated by a perfect fifth have a
frequency ratio of 32. - Notice that 2nd and 3rd harmonic on string are
perfect 5th - Timbre is largely determined by content of
harmonics. - Clarinet, guitar, piano, human voice have
different harmonic content for same pitch - Loudness is usually measured on logarithmic
decibel (tenths of bel) scale, relative to some
arbitrary reference intensity. - 10 dB is a change in sound intensity of a factor
of 10 - 20 db is a change in sound intensity of a factor
of 100.
13Frequency analysis of sound
- The human ear and auditory cortex is an extremely
sophisticated system for the analysis of pitch,
timbre, and loudness. - My computer is not too bad either.
- Microphone converts sound pressure wave into an
electrical signal. - Computer samples electrical signal 44,000 times
per sec. - The stream of numbers can be plotted as wave vs.
time. - Any segment of the wave can be analysed to
extract the amplitude for each sinusoidal wave
component.
14Samples of Sound Sampling
- Clarinet
- Guitar
- Piano
- Human Voice
- ...
15Piano keys (Grand Piano)
- Key is pressed down,
- the damper is raised
- The hammer is thrown against string
- The rebounding hammer is caught by the Back Check.
16Hammer action
- Throwing the hammer against the string allows the
hammer to exert a very large force in a short
time. - The force of the hammer blow is very sensitive to
how your finger strikes the key, but the hammer
does not linger on the string (and muffle it). - From pianissimo (pp) to fortissimo (ff) hammer
velocity changes by almost a factor of 100. - Hammer contact time with strings shortens from
4ms at pp to - Note that 2 ms ½ period of 264 Hz oscillation
17From Strings to Sound
- A vibrating string has a very poor coupling to
the air. To move a lot of air, the vibrations of
the string must be transmitted to the sound
board, via the bridge. - The somewhat irregular shape, and the off center
placement of the bridge, help to ensure that the
soundboard will vibrate strongly at all
frequencies - Most of the mystery of violin making lies in the
soundboard.
18Piano frame
- A unique feature of the piano, compared to
violin, harpsichord. is the very high tension in
the strings. - This increases the stored energy of vibration,
and therefore the dynamic power and range of the
piano. - Over 200 strings for 88 notes,each at ? 200 lb
tension - Total tension on frame 20 tons.
- The Piano is a modern instrument (1709, B.
Cristofori) - High grade steel frame.
- Also complicated mechanical action.
19Piano strings
- An ideal string (zero radius) will vibrate at
harmonics - fn n f1
- A real string (finite radius r) will vibrate at
harmonics that are slightly stretched - fn n f11(n2-1)r4k/(TL2)
- Small radius-r, strong wire (k), high tension
(T), and long strings (L) give small
in-harmonicity. - For low pitch, strings are wrapped, to keep r
small
20In-harmonicity tone color
- Perfect harmonics are not achievable--and not
desirable. A little in-harmonicity gives
richness to the tone, and masks slight detunings
of different notes in a chord. - Each octave is tuned to the 2nd harmonic of the
octave below.
21Multiple Strings
- Multiple Strings store more energy--louder sound
- Strings perfectly in tune
- Sound is loud, but decays rapidly
- Strings strongly out of tune
- Ugly beats occur as vibrations from adjacent
strings first add, then cancel, then add again. - If strings are slightly out of tune
- Sound decays slowly
- Beats are slow, add richness to tone.
22Multiple Strings, Power and Decay Time
- Decay time of vibration Energy stored in string
divided by power delivered to sound board. - Power delivered to sound board force of string
velocity of sound board (in response to force) - Three strings store 3 times the kinetic energy of
one string - If three strings are perfectly in tune, Force is
3 times larger, velocity is three times larger,
power is 9 times larger, Decay time is 3/9 1/3
as long as one string alone (Una corda pedal). - If strings are slightly mistuned, motion is
sometimes in phase, sometimes out of phase,
average power of three strings is only 3 times
greater than power of one string. Decay time of
3 strings is SAME as decay time of one string
alonejust louder.
23Beats from mistuned strings
- Two tones are mistuned by 10. One string makes
10 oscillations in the time it takes the other to
make 11 oscillations. - Yellow curve resulting superposition of two
waves - ½ of beat period is shown. Beat period
20period of individual wave. - Acoustic power would be 4x individual wave, if
strings were perfectly in tune. Because of
beats, average acoustic power is 2x individual
contribution
24Chords, Scales, and Tuning
- For the simple chords of perfect fifths and
thirds, the harmonics of each note match the
fundamental of the other notes in the chord. - Equal tempered tuning was designed (by J.S. Bach
others) to give the closest possible match
between pitch and harmonics within chords, for
any possible starting note.
25Conclusions
- The mathematical theory of music dates to
antiquity. - Music illustrates fundamental physical and
mathematical concepts. - Music is influenced by technology
- Modern examples
- Piano, Clarinet
- Musical Reproduction and Processing (Digital
predates Analog!). - Music appreciation has many layers.
- Science can add another layer