1. Bel One Bel = log (I 1 /I 0 ), where I 1 is the absolute measured power, and I 0 is a reference power. Reference used is usually the least sound level that can be detected.

2. Normal human hearing in terms of bels Least amount of sound power that can be detected: Least amount of sound power that can be detected: 0 bel Sounds that are damaging to the ear: Sounds that are damaging to the ear: 14 bels

3. Decibel (dB) One Decibel = one-tenth of a Bel, i.e. One Decibel = one-tenth of a Bel, i.e. 1 Bel = 10 Decibels Therefore dB = 10 log (I 1 /I 0 )

4. Normal human hearing in terms of decibels Least amount of sound power that can be detected: Least amount of sound power that can be detected: 0 dB Sounds that are damaging to the ear: Sounds that are damaging to the ear: 140 dB

5. INTENSITY LEVEL Intensity expressed in terms of power with a reference power of 10 -12 W/m 2 or 10 -16 W/cm 2 is called INTENSITY LEVEL (dB IL) Intensity expressed in terms of power with a reference power of 10 -12 W/m 2 or 10 -16 W/cm 2 is called INTENSITY LEVEL (dB IL)

6. Pressure and Intensity Pressure  Square root of Intensity Pressure  Square root of Intensity P   I OR P 2  I

7. Normal human hearing in terms of pressure Least amount of sound pressure that can be detected: Least amount of sound pressure that can be detected: 0.0002 dynes/cm 2, or 20µPa (20 x 10 -6 Pa) Sounds that are damaging to the ear: Sounds that are damaging to the ear: 2 x 10 8 µPa

8. Decibel: Expressed in terms of pressure Decibel = 20 log (p 1 /p 0 ) where p 1 is the absolute pressure of sound p 0 is the reference pressure

9. SOUND PRESSURE LEVEL 20 micropascals (20 µPa) or 0.0002 dynes/cm 2 Intensity expressed in terms of pressure with a reference pressure of 20 micropascals (20 µPa) or 0.0002 dynes/cm 2 is called SOUND PRESSURE LEVEL (dB SPL) One µPa = 10 -6 pascals

10. Absolute Power Absolute Pressure BelDecibel Minimum detectable sound levels 10 -12 w/m 2 20µPa00 Whisper at 1 meter 10 -10 w/m 2 2 x 10 2 µPa 220 Normal conversation at 1 meter 10 -6 w/m 2 2 x 10 4 µPa 660 Heavy traffic 10 -4 w/m 2 2 x 10 5 µPa 880 Damaging sound levels 10 2 w/m 2 2 x 10 8 µPa 14140

11. Decibel questions Two types of questions: Two types of questions: 1. What is the intensity in dB IL or dB SPL of a sound with an absolute power of X w/m 2 or absolute pressure of X µPa? 2. What is the absolute power or pressure of a sound with an intensity of X dB IL or X dB SPL?