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PHYSICS 1L03 (115)
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Lecture

# Set9-Ave.pdf

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School
McMaster University
Department
Physics
Course
PHYSICS 1L03
Professor
Reza Nejat
Semester
Fall

Description
Phys 1L03 Tutorial session Set 9 Ninth Set - Loudness of Sound Sound Intensity level • The base-10 logarithm is used often in Physics, such as to define the sound intensity level. • The logarithm unit of sound intensity level is called the decibel (dB). • The base-10 logarithm is used to determine the sound intensity level in decibels (dB), which is a kind of measure of the loudness of the sound. • The sound intensity level, β, in dB (decibel) is defined as: β = 10 log I (dB) I 0 Where: I is the intensity of sound inWatt /square meter (W /m ) 2 and −12 2 I 0 10 W /m , which it is the minimum sound intensity (called the threshold intensity). Examples: 1. Determine the sound intensity level at a point where the sound intensity is −4 2 I = 10 W /m I −4 2 −12 2 β = 10 log I = 10 W /m , I 0 = 10 W /m , β = ? I0 −4 2 β = 10 log I = 10 log 10 W m/ = 10log(10 )8 I 10 −2 W m 2 0 β = 10(8 dB) = 80 dB 2. The sound intensity level at a point is 91.0 dB. What is the sound intensity at that point? −12 2 β = 91.0 dB I0 = 10 W /m I = ? β = 10 log I I I ⇒ β /10 = log 0 I0 I ⇒ = inv log (β /10) I0 I ⇒ = inv log (β /10) = inv log(91/10) = inv log(9.1) I0 ⇒ I = inv log(9.1) = 10 9.1= 1.26 × 10 W /m 2 I 0 −12 2 9 2 −3 2 I = (10 W /m ) ×1.26 ×10 W /m =11.3×10 W /m −3 2 I = 11.3×10 W /m Phys 1L03 Tutorial session Set 9 Ninth Set- Sound intensity level 1. What are the int– 12ty le2els of sounds with intensities of a. 1.0 ×10 W/m (1 W = 1 J/s) – 6 2 b. 5.0 ×10 W/m – 12 2 I0 = 1.0 ×10 W/m IL (dB) = 10 log (I / I 0 2. At a busy street corner, the sound intensity level is 78 dB. What is the intensity of sound there? I = 1.0 ×10 – 12W/m , IL (dB) = 10 log (I / I ), 0 0 Δ(IL) = IL 2– IL 1 10 log (I / 2 ) 1 3. How does the intensity level of a sound change if the intensity of the sound is doubled? – 12 2 I0= 1.0
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