14 Sound Waves

• character of sound waves

• speed of sound

• sound intensity

• resonance

• Homework:

• 2, 3, 4, 10, 17, 41, 45, 81, 91.

Sound Waves

• pressure/density waves

• compressions

• rarefactions

• reflects

• refracts (similar to light)

• diffracts (bends around corners)

Sound Spectrum

• ultrasonic (f > 20kHz)

• (human) audible (20Hz < f < 20kHz)

• infrasonic (f < 20Hz)

• dogs, cats (50Hz < f < 45kHz)

• bats up to 120kHz

• elephants as low as 5Hz

Sound Speed

• in air: v = 331 + 0.6TC m/s

• in solids: ~ 1800 to 5000 m/s

• in liquids: ~ 1100 to 1500 m/s

• in gases: ~ 300 to 1200 m/s

Sound Intensity

• intensity, I: power/area [watt/m2]

• decibel: = 10log(I/Io)

• Io =10-12 watt/m2.

• Example: 10-9 watt/m2 = 30 decibels

Example: Intensity

• point source of sound, 0.010 watts

• I at 10 meters: = power/area

= 0.010watts/(4102m2)= 7.96x10-6 watt/m2.

• = 10log(7.96x10-6/10-12) = 69dB

Resonance in Tubes

• Due to constructive interference of waves within the tube

• open at both ends: all harmonics• closed at one end: odd harmonics only

Doppler Effect

• f’ increase on approach

• F’ decrease on separation

• Eq. 14.14a page 487

demos

• Vernier microphone, logger pro, physics with computers, voice program

• measure voice, chilandi plates, tuning forks, anyone with perfect pitch, meter stick?

• Open/closed tubes

• Take data on chilandi plates, compare to sand patterns with wave driver applied.

End

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Example of wavelength distortion due to source motion:

12

Nature of Sound Waves

• Longitudinal

• Oscillations are:

• Condensations (higher pressure areas) and

• Rarefactions (lower pressure areas)

• Sound travels at about 343m/s at room temperature and normal atmospheric pressure

13

Doppler Effect

• Frequency received is different than the Source frequency due to:

• Source Motion,

• Receiver Motion or,

• a combination of Source and Receiver motions.

14

Height vs. Time

0

0.5

1

1.5

2

2.5

0 5 10 15 20 25

Time (s)

Y (

m)

values of “A” and “f”?

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Decibels

• intensity level

where Io = 1.0x10-12 W/m2.

• Example: Intensity of sound is 4.0x10-5 W/m2. Intensity level is

oI

IdB log10

dBdBdB 76100.4log10100.1

100.4log10 7

12

5

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Sound Intensity (I)

• Intensity = power/area = P/A [watt/meter2]

• Spherical Radiation I = P/4r2.

• Example: Small speaker emits 1.0W of sound in all directions. Intensity 10m from the speaker is 1.0/(4102) W/m2.

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Frequency of Sound

• Audible Range: 20Hz to 20,000Hz

• Infrasonic: f < 20Hz

• Ultrasonic: f > 20,000Hz