MUSIC 150 MUSICAL ACOUSTICS BRASS INSTRUMENTS REFERENCE: CHAPTER 11 IN SCIENCE OF SOUND CHAPTER 14 IN THE PHYSICS OF MUSICAL INSTRUMENTS http://newt.phys.unsw.edu.au/jw/brassacoustics.html

FAMILIES OF BRASS INSTRUMENTS VALVED BRASS INSTRUMENTS SLIDE BRASS INSTRUMENTS NATURAL BRASS INSTRUMENTS KEYED OR FINGERED BRASS INSTRUMENTS BRASS INSTRUMENTS MAY BE CLASSIFIED ACCORDING TO MAIN BORE: CYLINDRICAL BORE (TRUMPET, TROMBONE, etc. CONICAL BORE (CORNET, TENOR HORN, FRENCH HORN EUPHONIUM, TUBA, etc. SOME BRASS INSTRUMENTS COMBINE CYLINDRICAL AND CONICAL

SOME BRASS INSTRUMENTS PLAYING RANGES

COMPARISON OF BRASS INSTRUMENTS

RESONANCES OF OPEN AND CLOSED PIPES

OSCILLATIONS IN A PIPE The air column of a Bb trumpet has a length of about 140 cm. The resonances of a closed pipe of this length should occur at: fn = nc/4L = 343 n/4(1.4) = 61.3 n (n = 1 , 3, 5, 7, . . .) = 61, 184, 306, 429 . . . Hz But these are quite different from the resonances of a trumpet.

OSCILLATIONS IN A PIPE One way to study the resonances of a pipe (or a wind instrument) is to make a graph of its ACOUSTIC IMPEDANCE as a function o frequency. ACOUSTIC IMPEDANCE js sound pressure divided by volume Z = p/U

APPARATUS FOR GRAPHING THE ACOUSTIC IMPEDANCE OF WIND INSTRUMENTS (The tube filled with fiber-glass provides a constant volume flow from the driver, so that the pressure recorded by the microphone is proportional to acoustic impedance)

FEEDBACK Feedback—The addition of part of the output of a system to the input. Negative feedback---Used to reduce distortion in amplifiers, etc. Positive feedback---Used to sustain oscillations in musical instruments, etc.

Pressure-controlled valves---Oscillation is INPUT VALVES Pressure-controlled valves---Oscillation is possible at frequencies of pressure (impedance) peaks (brasses, reed woodwinds) Flow-controlled valves---Oscillation is (impedance) minima (flutes, flue organ pipes)

Pressure-controlled valves---Oscillation is INPUT VALVES Pressure-controlled valves---Oscillation is possible at frequencies of pressure (impedance) peaks (brasses, reed woodwinds) Flow-controlled valves---Oscillation is (impedance) minima (flutes, flue organ pipes) VALVES: Outward-striking, inward-striking, sideways-striking

REED-GENERATOR CONFIGURATIONS Inward striking Corresponds to a woodwind reed (b) Outward striking Corresponds to brass player’s lips (c) Sideways striking Alternative model of brass player’s lips

BENADE’S “WATER TRUMPET” (INPUT VALVE IS CONTROLLED BY PRESSURE FEEDBACK)

MOUTHPIECE

THE BELL: CHANGES THE FREQUENCIES AND HEIGHTS OF THE IMPEDANCE PEAKS CHANGES THE RADIATION PATTERN (More directional) CHANGES THE SPECTRUM OF THE RADIATED SOUND ALLOWS MORE EFFICIENT SOUND RADIATION (MATCHES IMPEDANCES)

IMPEDANCES COMPARED TO A MUSICAL SCALE

REFLECTION POINT IN THE BELL The turning point moves outward in the bell as the frequenncy increases Mode frequencies are nearly in the ratios 0.8 : 2 : 3 : 4

BERNOULLI EFFECT REDUCED PRESSURE IN AIR JET FROM A VACUUM CLEANER HOSE GIVES RISE TO A NET UPWARD FORCE FB REDUCED PRESSURE IN THE CENTER SECTION CAUSES THE LIQUID IN THE U-TUBE TO STAND AT A HIGHER LEVEL BLOWING DOWNWARD THROUGH THE SPOOL CAUSES AIR TO FLOW OUTWARD, SUPPORTING A CARD AND PIN BY MEANS OF THE BERNOULLI FORCE

SLIDES AND VALVES PLAYING POSITIONS OF A TROMBONE SLIDE PRESSING DOWN ON TRUMPET VALVE LENGTHENS THE THE TRUMPET TUBING

IMPEDANCE CURVES FOR TWO TROMBONES

HAND IN A FRENCH HORN BELL

PRESSURE INSIDE AND OUTSIDE A TRUMPET

SHOCK WAVES IN BRASS INSTRUMENTS Only a small part of the sound energy that reaches the bell is radiated. The rest is reflected. Sound levels become as high as 175 dB inside a trumpet. This is well into the nonlinear range for acoustic waves. Nonlinear effects create shock waves, which result in the sound of “brassiness.”

TRANSIENTS Transients are important in identifying music instruments (see Section 7.11) BELOW CUTOFF: feedback stabilizes oscillation ABOVE CUTOFF: partials build up slowly without strong feedback Lips are more or less “on their own” until the first reflected wave comes back’ To attack a note correctly requires trained lips (especially on a French horn).

MUTES MUTES

VIBRATIONS OF A TROMBONE BELL

PERFORMANCE

OBSERVING THE LIPS THROUGH A GLASS MOUTHPIECE WINDOW

Assignment for Monday: Exercises 1-5, p.244 Read Chapter 12 Woodwind Instruments