Physics 1 Notes: Sound

Chapter 16
• Sound
o Longitudinal traveling wave that propagates away from the source using a
medium
o Wave speed
§ Depends on the elastic modulus and density in solids
§ Depends on Bulk modulus and density in gases/fluids
§ In air, at 0°C and 1atm pressure, sound travels at 331m/s
• Over limited range of temperature, speed varies as v=(331+0.6T)
m/s
• T is in °C
• At room temp (20°C), v=343m/s speed of sound in air




§
§ Ex: For a distance of 1 mile, how long do we hear thunder after a
lightning strike?
• 𝑑 = 𝑣𝑡
!
• 𝑡="
# %& #+,-%
• 𝑡 = '('%/* ∗ # %&
• 4.69 sec
o Mathematic Representation of Sound
§ Sound is a pressure wave
§ Amplitude is pressure
§ 𝐷 = 𝐴𝑠𝑖𝑛(𝑘𝑥 − 𝑤𝑡)
• D=displacement of wave
• A=amplitude
./
§ Wave number: 𝑘 = 0
• 𝜆=wavelength
§ Angular frequency: 𝑤 = 2𝜋𝑓
• f=frequency

, • Human ears sensitive to frequency/pitch of 20Hz-20000Hz
o As we age, we lose ability to detect high pitch by a factor
of ~2
• Ultrasonic: sound waves beyond 20kHz (20000Hz)
• Infrasonic: sound waves below 20Hz
o Intensity of sound
§ Loudness
§ Power per unit area
§ Human ear sensitive to 10-12 Watts/m2 – 1 Watt/m2
§ Not linearly proportional to intensity, but logarithmically
• Sound in Decibels (dB/b)
1
• Difference in intensities: 𝛽 = 10 log ;1 <
!
o In dB
o I, = 102#. W/m2
§ Base level of human hearing
o I = measured intensity
• Ex: What is the difference in intensity, given I = 102#, W/m2?
1
o 𝛽 = 10 log ;1 <
!
#,"#!
o 𝛽 = 10 log ;#,"#$<
o 𝛽 = 10 log(100)
o 𝛽 = 10(2)
o 20dB
• Intensities of different sources




o