Acoustics:Acoustics:The Properties of SoundThe Properties of Sound

Acoustics:Acoustics:The Properties of SoundThe Properties of Sound

• air pressure changes when objects vibrate, air pressure changes when objects vibrate, producing sound in the earproducing sound in the ear• pound a tablepound a table• pluck a stringpluck a string

Sound WavesSound WavesSound WavesSound Waves• vibrations are transmitted to our ears by a medium vibrations are transmitted to our ears by a medium

— usually air— usually air

• sound waves radiate through air outward from the sound waves radiate through air outward from the source sound like source sound like <W1:28><W1:28> waves in water that waves in water that radiate in circles from a tossed pebbleradiate in circles from a tossed pebble

Sound WavesSound WavesSound WavesSound Waves• sound waves also radiate through water and sound waves also radiate through water and

solid materials, such as doorssolid materials, such as doors

• listen to underwater listen to underwater sounds in the swimming sounds in the swimming pool!pool!

• get a friend to hum get a friend to hum underwaterunderwater

• how far away can you how far away can you hear the sound?hear the sound?

Vibrating StringVibrating StringVibrating StringVibrating String

String VibrationsString VibrationsString VibrationsString Vibrations• rubber band experimentrubber band experiment

• hold both ends of a rubber band hold both ends of a rubber band • have a partner pluck the rubber band have a partner pluck the rubber band • notice the movement of the rubber band notice the movement of the rubber band

• the whole length is vibrating

String VibrationsString VibrationsString VibrationsString Vibrations• strings and pipes strings and pipes

vibrate in segments of vibrate in segments of their length, as well as their length, as well as in their entirety — at in their entirety — at the same timethe same time

• fundamentalfundamental• frequency produced by frequency produced by

vibration of total length vibration of total length • usually loudest usually loudest

frequency producedfrequency produced

String VibrationsString VibrationsString VibrationsString Vibrations• harmonicsharmonics

• exact whole-number multiples of fundamentalexact whole-number multiples of fundamental• proportion of string is reciprocal of frequencyproportion of string is reciprocal of frequency

string length = f = fundamental frequency

1/2 string length = 2f

1/3 string length = 3f

1/4 string length = 4f

• [i:30][i:30] harmonics harmonics

on sanxianon sanxian

Harmonic SeriesHarmonic SeriesHarmonic SeriesHarmonic Series• harmonic seriesharmonic series

• a series of harmonics or a series of harmonics or partialspartials above a given above a given pitchpitch

• harmonic harmonic partialspartials• occur at integer multiples of the fundamental• wind and string instruments

• example: if the frequency of C= 64 Hertz, the frequency of the 2nd partial is 2 * 64 = 128 Hertz

HarmonicsHarmonicsHarmonicsHarmonics• <w1:29><w1:29> pipes like the flute have pipes like the flute have

harmonicsharmonics

Harmonic SeriesHarmonic SeriesHarmonic SeriesHarmonic Series• The French horn also has The French horn also has

harmonic partials – live demoharmonic partials – live demo

How Does Sound Work?How Does Sound Work?How Does Sound Work?How Does Sound Work?

• Many musical sounds are made of Many musical sounds are made of special combinations of harmonicsspecial combinations of harmonics

How Loud Are the Harmonics?How Loud Are the Harmonics?How Loud Are the Harmonics?How Loud Are the Harmonics?

• Here is a graph of the first 15 Here is a graph of the first 15 harmonics of a harmonics of a [i:31][i:31] vocal tone vocal tone

• Here is the average loudness of the Here is the average loudness of the same same [i:31][i:31] vocal tone vocal tone

How Loud Are the Harmonics?How Loud Are the Harmonics?How Loud Are the Harmonics?How Loud Are the Harmonics?

• The vocal tone's first harmonic The vocal tone's first harmonic fades in and out like this:fades in and out like this:

How Loud Are the Harmonics?How Loud Are the Harmonics?How Loud Are the Harmonics?How Loud Are the Harmonics?

• The vocal tone's The vocal tone's frequencyfrequency moves moves up and down this:up and down this:

How Steady is the Sound?How Steady is the Sound?How Steady is the Sound?How Steady is the Sound?

Harmonic SeriesHarmonic SeriesHarmonic SeriesHarmonic Series• Using its harmonic pattern , we can Using its harmonic pattern , we can

recreate a recreate a [i:32][i:32] vocal tone vocal tone