Sound and Intensity

• Transverse vs. Longitudinal Waves• Sound Frequency• Sound Loudness• Loudness and the Decibel Scale• db Examples• Other db scales

Transverse vs. Longitudinal Waves• String (transverse) wave animation

https://sites.google.com/site/physicsflash/home/transverse

• Air (longitudinal) wave animation

https://sites.google.com/site/physicsflash/home/sound

• Wavelength – distance between peaks at fixed time• Frequency – time between repetitions at fixed position• Velocity from wavelength and frequency

sound.swf

transverse.swf

Transverse vs. Longitudinal Slinky

• Transverse vs. longitudinal waves

http://www.animations.physics.unsw.edu.au/jw/sound-pressure-density.htm

• Transverse

• Longitudinal

sound_transverse.swf

sound_longitudinal.swf

Transverse vs. Longitudinal Comparison

• Transverse vs. Longitudinal Waveshttp://faraday.physics.utoronto.ca/IYearLab/Intros/StandingWaves/Flash/long_wave.html

long_wave.swf

Comparison of waves on string and air

• Both have– Wavelength – distance between peaks at fixed time– Frequency – rate of repetitions at fixed position (like your ear)– Wave velocity

• Differences– String wave velocity varies with tension and mass/length – Air wave velocity set at 343 m/s (at 20° C) *

*at any temperature )

Frequency (pitch) of sound waves

• Frequency (pitch) of sound waves

http://www.animations.physics.unsw.edu.au/jw/sound-pitch-loudness-timbre.htm

http://www.animations.physics.unsw.edu.au/jw/frequency-pitch-sound.htm

• Human ear ~ 20 Hz to 20,000 Hz (dogs higher)

quantifying_doremi.swfquantifying_500_1000_2000.swf

sound_frequency.swf

Loudness (volume) of Sound Waves• Loudness (volume) of sound waves

http://www.animations.physics.unsw.edu.au/jw/sound-pitch-loudness-timbre.htm

• Human ear can hear from about 10-12 W/m2 to 100 W/m2 - about 14 orders of magnitude!

sound_volume.swf

Intensity and decibel scale• Range of human ear Intensity 10-12 – 100 watts/m2

• Make scale more convenient - “compress” this • Try Logarithms

• Method 3 gives most convenient scale

• Definition decibel (sound)

Method Intensity Log

Log(I)

Log(I/10-12)

10 Log(I/10-12)

Example - Intensity and db scale

• Auto interior sound intensity 3 x 10-5 W/m2. What is decibel level?

(use calculator)

(fractional logarithms OK)

• The sound level for a jet plane at takeoff is 140 db. What is the intensity?

()

Logarithm rules on textbook inside back cover

Example 12-4 – Loudspeaker volume

• 3 db Intensity difference

(difference/quotient rule)

Example 12-5 – Airplane roar

• Translate 140 db at 30 m to intensity

• Scale from 30 m to 300 m using inverse square law

• Translate at 300 m back to db

FYI - Other “Decibel” scales• Sound db - referenced to 10-12 W.

• Electrical dbm - referenced to 10-3 W.

• 2G/3G/4G/WIFI signal strengths(WIFI > -20 dbm near router.)

• Comcast checks cable modem this way.

• db always log of power ratio to some reference power.

Cellphone signal strengths