14 Sound Waves character of sound waves speed of sound sound intensity resonance Homework: 2, 3, 4,...
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Transcript of 14 Sound Waves character of sound waves speed of sound sound intensity resonance Homework: 2, 3, 4,...
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
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.
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.
15
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
16
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.