MUSIC 150 MUSICAL ACOUSTICS

BRASS INSTRUMENTS

REFERENCE: CHAPTER 11 IN SCIENCE OF SOUND CHAPTER 14 IN THE PHYSICS OF MUSICAL INSTRUMENTS

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

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 OF FREQUENCY

ACOUSTIC IMPEDANCE IS SOUND PRESSURE DIVIDED BY VOLUME VELOCITY Z = p/U

APPARATUS FOR GRAPHING THE ACOUSTIC IMPEDANCE OF WIND INSTRUMENTS

FEEDBACK

INPUT VALVES

REED-GENERATOR CONFIGURATIONS

(a) Corresponds to a woodwind reed

(b) Corresponds to brass player’s lips

(c) 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 FREQUENCY INCREASES MODE FREQUENCIES ARE NEARLY IN THE RATIOS 0.8 : 2 : 3 : 4

BERNOULLI EFFECT

REDUCED PRESSURE IN THE CENTER SECTION CAUSES THE LIQUID IN THE U-TUBE TO STAND AT A HIGHER LEVEL

REDUCED PRESSURE IN AIR JET FROM A VACUUM CLEANER HOSE GIVES RISE TO A NET UPWARD FORCE FB

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 AS HIGH AS 175 dB INSIDE A TRUMPET

NONLINEAR EFFECTS CREATE SHOCK WAVES

TRANSIENTS

TRANSIENTS ARE IMPORTANT IN IDENTIFYING MUSICAL 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