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.

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Presentation transcript:

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.

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

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 )

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

INTENSITY LEVEL Intensity expressed in terms of power with a reference power of W/m 2 or W/cm 2 is called INTENSITY LEVEL (dB IL) Intensity expressed in terms of power with a reference power of W/m 2 or W/cm 2 is called INTENSITY LEVEL (dB IL)

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

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: dynes/cm 2, or 20µPa (20 x Pa) Sounds that are damaging to the ear: Sounds that are damaging to the ear: 2 x 10 8 µPa

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

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

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

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?