14.2 (L) The Physics of Music

14.2 The Physics of Music

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Main Idea

Music consists of complex sound waves produced by vibrating objects.

Essential Questions

• What is the origin of sound?

• What are the characteristics of resonance on strings?

• Why are there variations in sound quality among instruments?

• How are beats produced?

Sources of Sound

• Recall that sound is produced by a vibrating object.

• The vibrations of the object create particle motions that cause pressure oscillations in the air.

• Gongs, cymbals, and drums produce sound after being struck.

Sources of Sound

In brass instruments (trumpet, tuba, trombone, etc.), the performer's lips vibrate.

Sources of Sound

In woodwind instruments (clarinet, saxophone, oboe, etc.), a thin wooden strip called a reed vibrates as the result of air blown across it.

Source of Sounds

• Stringed instruments (piano, guitar, violin, etc.) work by setting wires or strings into vibration.

• In the piano, the wires are struck.

• In the guitar, strings are plucked or strummed.

• In the violin, the friction of the bow causes the strings to vibrate.

Resonance in Air Columns

• Resonance increase the amplitude of a vibration.

• The length of an air column determines the frequencies of the vibrating air that will resonate.

• Changing the length of the column of vibrating air varies the pitch.

Resonance in Air Columns

• A resonating tube with one end closed to air is called a closed-pipe resonator.

• A wave of alternate high- and low-pressure variations moves through a column of air, bounces off a surface and is reflected back through additional waves.

• When emitted and returning waves "collide" and reinforce each other, the waves create a standing wave, and resonance occurs.

Resonance in Air Columns

• An open-pipe resonator is. a resonating tube with both ends open.

• High-pressure parts of the wave "strikes the open end", and the rebounding wave will be low-pressure.

Resonance on Strings

• Plucking, bowing, or striking strings produces variation in waveforms.

• A string on an instrument is clamped at both ends, therefore, the string must have a node at each end.

• Resonance occurs when one wavelength fits on the string.The resonant frequencies are whole-number multiples of the lowest frequency.

Resonance on Strings

• The speed of a wave depends on the medium.

• The tension of a string determines the speed.

• The tighter the string, the faster the wave moves.

• Tuning a stringed instrument involves tightening or loosening the strings.

Sound Quality

• Complex sound waves are blends of several different frequencies.

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Beats

• Two frequencies that are nearly identical interfere to produce oscillating high and low sound levels called a beat.