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PHY 100—Physics of Music Presentation • Nature of sound waves: vibrations that transfer energy o Sound is longitudinal, or the motion of the wave is in the same direction as the transportation of energy o Speed of sound: ~343 m/s • Sound waves are also pressure waves o Compressions are high pressure areas, rarefactions are low pressure areas o Difference between successive areas corresponds to wavelength o Changes in pressure are measured in amplitude • Human range of hearing: 20 Hz to 20,000 Hz (measure of frequency) o Below 20 is infrasound and above 20,000 is ultrasound o Humans can detect differences usually starting at around 7 Hz in sounds o Pitch refers to the different notes in music o High frequencyhigh pitch, low frequencylow pitch • Consonance: When notes are played simultaneously, their sound waves interfere o Sound waves with frequencies of integer ratios are consonant (they sound good together and form intervals) • Harmonics: o Harmonics are key to not only understanding what harmonics are but the functioning of musical instruments. o A sound that comes out of an instrument is a mixture of many frequencies which are called harmonics. A higher note has a different set of harmonics than a lower note. o Harmonics are well mixed so that when one plays a note one does not hear the separate harmonics, but one sound. o In effect, harmonics add to sound quality. • Where do harmonics come from? o Playing a G on the saxophone using vibrato (a musical technique), creates a vibrating air column. Once I have my vibrating air column, harmonics are produced in halves, thirds, and fourths. o Remember all these harmonics occur at the same time to create a rich, complex sound! o Due to resonance, or the ability of a sound to vibrate at maximum amplitude at a particular frequency, overtones can be heard. Overtones are heard in the higher harmonics such as for the thirds and fourths. • Timbre: The color or “tone” of an instrument o Timbre is not determined by pitch, loudness, or length of the note played, but rather the materials and design of the instrument itself. This is what allows us to tell the difference between instruments, even when they’re playing the same note. o Timbre has many descriptions including: Clear, Brassy, Reedy, Harsh, etc. o Unique timbres are created by the unique harmonic frequencies that occur when a note is played on an instrument. These harmonic frequencies (overtones) happen at multiples of the note’s main frequency due to resonance in the instrument. • Resonance: In physics, resonance is the tendency of a system to oscillate at maximum amplitude at certain frequencies. o There is mechanical resonance, acoustic resonance, electromagnetic resonance, and resonance of quantum wave functions. o Resonance occurs widely in nature, and is exploited in many man-made devices. It is the mechanism by which virtually all sinusoidal waves and vibrations are generated. Light and other short wavelength electromagnetic radiation is produced by resonance on an atomic scale, such as electrons in atoms. • Acoustics o Acoustics is the science of sound. The acoustics of a room are those qualities that together determine its character with respect to the perception of sound. o Decibels provide a relative measure of sound intensity and quantify sounds levels relative to another decibel reference o Using decibels you can help to quantify the acoustics of a room. Decibels basically help to take sound quantify and make it understandable to humans with regards to the human ear. • Constructive and destructive interference: o When the two waves are in-phase they interfere constructively and the result has twice the amplitude of the individual waves (the sound is louder) and when the two waves have opposite-phase they interfere destructively and cancel each other out (silence). • Doppler Effect: o Change in the observed frequency (or wavelength) of waves due to relative motion between the wave source and the observer. o In terms of acoustics it helps to explain why things get louder and quieter as you move and as the other objects move.