1. Physics of Sound (continued from Lecture 8) Physics of hearing (I)
PHY238Y Lecture 9
Physics of Sound (continued from Lecture 8)
Physics of hearing (I)
References: Haliday, Resnick, Walker: Fundamentals of Physics, 6th ed., Wiley 2003, Ch. 18 (18.1 – 18.3, 18.5)
Hallett et al.: Physics for the life sciences, 4th ed., Ch.2 (2.1 – 2.5)
Thanks to dr. R. Nave for the permission to use some of the pictures from Hyper Physics:

2. PHY238Y Lecture 9 (slide from lecture 8)
Traveling sound waves:
Displacement and pressure variations

3. PHY238Y Lecture 9 – slide from lecture 8
Sound intensity is
Power/unit area
or: time rate of energy transfer/unit area
Many sound intensity measurements are made relative to a standard threshold of hearing intensity I0,
The most common approach to sound intensity measurement is to measure the sound level on the decibel scale:
The factor of 10 multiplying the logarithm makes it decibels instead of Bels, and is included because ~ 1 decibel is the just noticeable difference (JND) in sound intensity for the normal human ear

4. The threshold of hearing
PHY238Y Lecture 9
The threshold of hearing
The measured threshold of hearing curve shows that the sound intensity required to be heard is quite different for different frequencies.
The standard threshold of hearing at 1000 Hz is nominally taken to be 0 dB (it actually is 4 dB)

5. Threshold of pain is 120 – 130 dB
PHY238Y Lecture 9
Intensity level (dB)
Ear drum rupture
160
Pain threshold
Shotgun blast
100
Car horn (6 m distance) 90
City street 75
Typical room 40
Open country 10
Threshold of hearing
Threshold of pain is – 130 dB
The dynamic range of human hearing extends from zero dB to the threshold of pain.
Example: problem 18-2 p. 404 from Halliday/Resnick/Walker

6. PHY238Y Lecture 9 Pitch (frequency)
The audible sound: a pressure wave with frequency between 20 Hz and 20,000 Hz and with an intensity above the standard threshold of hearing:
Frequency: 20 Hz - 20,000 Hz (corresponds with pitch)
Wavelength (at 20oC in air): m – 17.2 m
Intensity: W/m2 (0 to 130 dB)
Pressure: 2 x N/m2 (2 x atm)
Sounds may be generally characterized by:
Pitch (frequency)
Loudness (is NOT simply the intensity level)
Quality (timbre – determined by the harmonic content of the sound)

7. Variation of intensity with distance: the inverse square law:
PHY238Y Lecture 9
Variation of intensity with distance: the inverse square law:

8. PHY238Y Lecture 9
Inverse square law plot