14.2 (L) The Physics of Music

Music consists of complex sound waves produced by vibrating objects.

• 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?

• 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.

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

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

• 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 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.

• 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.

• 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.

• 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.

• 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.

• Complex sound waves are blends of several different frequencies.

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