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# The Physics of Brass Musical Instruments PowerPoint PPT Presentation

The Physics of Brass Musical Instruments. Or, what do horn players do with their right hands, anyway? Brian Holmes SJSU Dept. Physics, horncabbage@aol.com. Intended structure of this talk. Standing waves Waves in tubes How to build a trumpet

The Physics of Brass Musical Instruments

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## The Physics of Brass Musical Instruments

Or, what do horn players do

with their right hands, anyway?

Brian Holmes

SJSU Dept. Physics, horncabbage@aol.com

### Intended structure of this talk

• Standing waves

• Waves in tubes

• How to build a trumpet

• What horn players do with thei right hands (anyway).

### Actual structure of this talk

• One skit

• A skit

• Another digression (if time allows)

• Much rushing to leave out material I nearly included

Playing frequencies of straight tube

Length: 1.41m

f

cyl

79

181

304

428

545

670

Playing frequencies of straight tube

Length: 1.41m

f/odd

f

cyl

79

181

304

428

545

670

79

60.3

60.8

61.1

60.6

60.9

v/4L = 60.4 Hz

What happens when you add the bell

f

f

bell

cyl

79

181

304

428

545

670

93

221

334

449

574

691

What happens when you add the bell

f

f

f

bell

cyl

79

181

304

428

545

670

93

221

334

449

571

691

14

40

30

21

26

21

What happens when you add the bell

f

f

% change

f

bell

cyl

79

181

304

428

545

670

93

221

334

449

571

691

14

40

30

21

26

21

--

22

9.9

4.9

4.8

3.1

The bell raises all the frequencies; but it raises

the low frequencies more than low frequencies.

• Interpretation: the effective length of the    instrument is different from the actual    length.

• High frequencies reflect closer to the open    end than low frequencies do.

• The bell acts as a high-pass filter.

• Above a cut-off frequency, no sound is    reflected back to the lips.

• The cutoff-frequency is higher for a more    rapidly flaring bell.

### Why make the bell an ineffective radiator of sound?

• Because that makes the bell effective at reflecting sound back to the lips.

• The sound returning to the lips gives feedback to them, controlling their vibrations and feeding more energy into the standing wave.

• This control makes the instrument easier to play.

The mouthpiece has a

bowl-shaped cup that connects

to the conical backbore.

The backbore connects to

instrument.

The column of the cup determines a popping frequency. Any sounds near this frequency will be amplified. The popping frequency of the trumpet mouthpiece is near 800 Hz.      A deeper cup will result in a lower popping frequency, yielding a less strident tone quality.

What happens when you add the

f

f

f

cyl

bell

trumpet

79

181

304

428

545

670

93

221

334

449

571

691

93

232

348

465

578

696

Again the frequencies rise; and again, the

low frequencies rise more than the high ones.

What happens when you add the

f

f

f

f /integer

cyl

bell

trumpet

tr

79

181

304

428

545

670

93

221

334

449

571

691

93

232

348

465

578

696

93

116

116

116

116

116

The playing frequencies are a set of multiples

of 116 Hz; except for the first multiple.

What if you put holes in the side of a trumpet?

The result, a keyed trumpet, would have uneven

tone quality.

Keyed bugles had a more even tone quality,

but didn’t sound like trumpets.

Other keyed instruments met with even less

success. For example, the Serpent.

And the Ophicleide.

Why do horn players keep

their hands in the bell, anyway?

• To restore resonances above the cutoff    frequency.

• To improve the tone quality of notes below    the cutoff frequency.

• To adjust the intonation of the instrument.

• To allow for special muting effects.

open

stopped

A good on-line reference

http://www.phys.unsw.edu.au/jw/brassacoustics.html