Chp 13

Sound and Music

What is sound?

• A sound wave is a wave of alternating high pressure and low pressure regions.

• Anything that vibrates in air creates a sound wave.

• It is a longitudinal wave because the air compresses in the direction of travel

Loudness• The loudness of a

sound is measured in decibels (dB)

• The louder the sound the higher the amplitude of the wave

Decibel Scale

• The decibel scale is a logarithmic measure of sound.– An increase of 20 dB

means the wave has a 10 times greater amplitude

– But, our ears perceives a 20 dB increase to be about twice as loud

Frequency of Sound

• We hear the different frequencies of sound has having a different pitch.– Low frequency has a low pitch

– High frequency has a high pitch

• Humans can hear from about 20 Hz to 20,000 Hz• Human ears are most sensitive for sounds between

100 and 2,000 Hz (range of speech)

Wavelength • The frequency and wavelength

of sound are inversely related.

• When frequency goes up, wavelength goes down.

Speed of Sound

• The speed of sound depends upon the material the sound is traveling.

• Sound travels faster in a solid than in air.

• Sound requires a medium to travel. As a result, sound DOES NOT travel in a vacuum.

Beats• Beats – when two frequencies are close, but not

exactly the same, the resulting sound seems to oscillate

• When the two waves are in phase, the sound gets louder. When the two waves are out of phase, the sound gets softer

Music

• The same note sounds different when played on different instruments.

• Instruments produce a mixture of different frequencies.