Test 2 Review Outline
Basic Instrument Functions:
Oscillators
Resonators
High Q => select pitch; low Q => respond to a range of pitches
Radiators
Efficiency depends on size wrt wavelength of sound in air
[examples of various combinations of functions]
Control of pitch, loudness, timbre, envelope, etc., by performer?
Independence of such controls?
Pitch selection
Choose among registers
Normal modes
Basic shapes
Intervals betw modes indep of length => Bessel horns
Useful (harmonic) intervals => e = 0, 2
Fund wavlngth = 2Leff; all harmonics available
Cylinder open at both ends
Cone (same mode fs, difft standwave patterns)
Fund wavlngth = 4Leff; only odd harmonics available
Cylinder closed at one end
Boundary conditions
Impedance changes at boundaries
Degree of
Large => slow decay, soft sound
Smaller => louder, quicker decay
Nature of
Open/Closed, Fixed/Free, PhaseRev/Not
Pressure controlled valves
Vib lips, reeds, vocal folds
Effectively close end
Mode frequencies at impedance
maxima
Flow controlled valves
Fipple, embouchure hole, open end
Effectively leave end open
Mode frequencies at impedance minima
Change effective length
Freq dep if bell, mouthpiece backbore
Tone hole lattice
Tradeoffs between tone hole sizes, positions
Effects of covered tone holes
Valves, slides
Frets, fingers on fingerboard or at string (Chinese instr.)
Cutoff frequency for reflection from open end/tone hole
Brass mouthpiece
Woodwind tone hole lattice
Sympathetic vibrations
In piano for strings with dampers raised
Sitar, etc
Marimba, vibes resonators
Drones
Hurdy-Gurdy, strings
Bagpipes, winds
Mutes
Change (effective) impedance match at bridge
Violin mute
Piano una corda pedal effect
Filter spectrum
Brass mute
Transposing/NonTransposing
Hand in horn bell
Cloth on percussion heads
Characteristics of Specific Instrument Types:
String instruments
Bowed string
Stick/slip
String forms two straight segments
Junction traces lens-shaped envelope
Timing of stick/slip transitions (det. by standing wave fundamental freq)
Loudness control
Bowing speed
Bowing location
Vibrato
Plucked string
Plucking location
Wrt displacement nodes and antinodes
Plectrum width
For electric instrument also location, nature, and size of pickups
Violin mechanisms
Helmholtz resonator
Air in body, f holes
Breathing mode
Bridge feet, Sound post as piston, Bass bar as brace
Rocking modes
Bridge feet, Sound post as fulcrum, Bass bar minimizes rot inertia
Keyboard instruments
Clavichord
Metal tangent both excites string and sets effective length, damping to left
Soft upper limit to dynamic range
Vibrato possible
Harpsichord
Plectrum excites string, jack includes damper
Relatively loud, brash; no dynamic range for given set(s) of strings
Use w voice, small ensembles
Variation in pluck position (lute stop)
Muting (buff stop)
In box
Fortepiano
Hammers excite strings, more complex action required
Much louder upper limit to dynamic range
Evolution from harpsichord-like construction
Large venues
Concerti w virtually full orchestra
Modern Piano
Multiple strings
Loudness
Control over decay [through fine tuning]
Stiffness
Stretch tuning
Maximize length, tension to reduce effect
Wrapped strings to reduce effect
Multiple bridges
Transition [also for no. of strings, wrapped/not]
Bottom of soundboard always open to room
Hammer sizes/softness: impulse change with hammer speed
"Tone Regulation" [needles, sandpaper on hammer felts]
Pipe Organ
Mechanical action
Windchests associated with manual and pedal claviers (keyboards)
Trackers (pulling), stickers (pushing) connect to pallet valves
Pallet valves supply wind to each key’s groove under all its pipes
Roller bars to connect trackers above keys to those beneath valves
Rockers used to separate rows of trackers to various windchests
Sliders (across grooves) act as valves (stops) for each rank of pipes
Pneumatic action
Barker lever = pneumatic amplifier of key force
More flexibility in location of windchests
Key no longer directly connected to pallet valves
Electro-pneumatic action
Multiple electrical and pneumatic relays between key and pipe
Complete freedom to locate pipes anywhere, control many pipes
Enabled fundamental change in concept/design of pipe organs
Most general categories of pipes:
Flue (flow-controlled valve)
Cylindrical metal or square-section wood
Open or “Stopped” [far end closed]
Reed (pressure-controlled valve and resonator)
Cylindrical or conical
Percussion instruments
Pitched
Tympani, tabla
Tuning via head (membrane) tension
Marimba, vibes etc.
Sympathetic tube resonators to increase loudness
Vibrato via modulation of resonating tubes
Chimes, orchestra bells, etc.
Unpitched, or indefinite pitch
Snare drum, tom tom, bass drum, cymbals, blocks etc.
Size and hardness of "beaters" and striking location
analogous to piano hammers, string plectra
Wind instruments
Examples of all possibilities of basic shapes and mouthpiece types
PRESS
FLOW
CYLIND
cl, tpt
fl, organ principal
CONIC
hn, ob
recorder
Brass
Valves, slides (tone holes on some early instruments)
Effects of adding bell, mouthpiece to bore
Effective length freq. dependent
Lowest normal mode out of tune => not used
Attack ease vs. intonation, etc.
Reflections from bends, disturbances to smooth bore
Woodwinds
Register keys / overblowing
Tone holes
Human voice
Pitch determined by vocal folds fundamental
Spectral manipulation of tongue, jaw, etc. to control formant peaks
Techniques
Formant tuning by sopranos
Lowering larynx (covered sound, singer’s formant)
Chordal chanting
Instrument Evolution / development:
Instrument design, composition and performance styles, techniques, venues
Examples (violin, tympani, horn, kbds, ww, etc.)
Room Acoustics Approaches:
Ray analysis
Paths, early images within association time
Distinct echoes?
Focusing at certain locations?
Appropriateness for Sabine analysis?
Modal analysis
Lowest mode frequency
Spacing of available normal modes (equalization)
degeneracy (many modes at same frequencies) bad
reverberant sound must be stored in available modes
Steady-state (Sabine) analysis
Reverberation time, Absorption, Room volume, (max Intensity)
[Max Intensity -> Total Absorption -> Room Volume -> Number of Seats]
in your dreams!
Recent integration of a variety of acoustic analysis programs with CAD software
Ray tracing, materials properties, loudspeaker specs
Plots of levels, speech intelligibility measures, reflection timings, ratios, etc.
Calculations:
Wind, string effective length, normal modes, frequencies
Basics [boundary conditions and mouthpiece valve types]
Symmetries
Rooms
Lowest mode (wavelength = longest dimension * 2)
Sabine equation
Detailed method for all surfaces and their absorption coefficients
Frequency dependence of coeff, total absorp, Treverb
Predicting effect of changes: same equation in two different ways
Calc of existing Atot from Treverb and Volume
Calc of anticipated Treverb in terms of changes to Atot(f)