1. MUSIC 150 MUSICAL ACOUSTICS
BRASS
INSTRUMENTS
REFERENCE: CHAPTER 11 IN SCIENCE OF SOUND CHAPTER 14 IN THE PHYSICS OF MUSICAL INSTRUMENTS

2. 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

3. SOME BRASS INSTRUMENTS
PLAYING RANGES

4. COMPARISON OF BRASS INSTRUMENTS

5. RESONANCES OF OPEN AND CLOSED PIPES

6. 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, Hz
But these are quite different from the resonances of a trumpet.

7. 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

8. 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)

9. 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.

10. 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)

11. 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

12. 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

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

14. MOUTHPIECE

18. 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)

19. IMPEDANCES COMPARED TO A MUSICAL SCALE

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

21. 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

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

23. IMPEDANCE CURVES FOR TWO TROMBONES

24. HAND IN A FRENCH HORN BELL

25. PRESSURE INSIDE AND OUTSIDE A TRUMPET

26. 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.”

27. 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).

28. MUTES
MUTES

29. VIBRATIONS OF A TROMBONE BELL

30. PERFORMANCE

31. OBSERVING THE LIPS THROUGH A GLASS MOUTHPIECE WINDOW

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