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What is power?
- the rate at which energy is transferred; energy transferred per unit time
- energy per second
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What is energy?
the capacity to do work, whereas power is the rate at which energy (the work) is expended (done)
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What is a Watt?
- energy transferred per unit time
- 1 watt = 1 joule/s
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What is a relative measure?
a comparative measure
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What is an absolute measure?
an exact number or amount
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What unit of measure is power expressed in?
- watts
- sound = 2 x 10^-8 watts (.00000002)
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What is relative measure of power?
- absolute power in one sound wave is compared with absolute power in another (reference) wave
- A = 2B or B = A/2
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What is intensity?
energy per second per square meter
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What is the unit of measure of intensity?
watt/m2
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What is Ix?
absolute intensity
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What is Ir?
reference intensity
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What is the ratio for the level of intensity?
Ix/Ir
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What is a Bel?
- the log of the ratio Ix/IrBels = log10(Ix/Ir)
- if the base is not give, assume it is 10
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What do positive bels mean?
the absolute measure is bigger than the reference
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What do negative bels mean?
the absolute measure is smaller than the reference
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Ix = 10-6 watt/m2
Ir = 10^-10 watt/m^2
What is the absolute intensity?
What is the level of intensity in bels?
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How do you go from bels to decibels?
- bels = log10(2 x 10-8/ 10-12)
- bels = 4.3 = 43 decibels (dB)
- to convert from bels to decibels, multiply by 10
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What is the equation for dB?
- 10log10(Ix/Ir)
- a decibel is 10 times the log of an intensity ratio or of a power ratio
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Sample dB problems page 53 slide 26
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For every 10-fold change in Ix, dB changes by? Why?
- 10 dB
- log1010 = 1; 1 x 10 = 10
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For every 2-fold change in Ix, dB changes by?
3 dB
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What do 0 decibels mean?
Ix = Ir
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What is dB IL?
- intensity level
- the reference intensity must always be specified!
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What is the standard reference intensity for dB IL?
Ir = 10-12 watt/m2
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dB IL problems page 54 slide 31
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Sample problems page 54 and 55
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What are the four types of intensity problems?
- given an intensity ratio, calculate decibels
- given decibels, calculate intensity ratio
- given Ix, calculate dB IL
- give dB IL, calculate Ix
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What is sound pressure?
force/unit area
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What is the unit of measure for pressure?
- N/m2 = 1 Pa
- microN/m2 = 1 microPa
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What is impedance?
an opposition to motion
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What is the relationship between intensity and pressure?
- I is directly proportional to p2
- p is directly proportional to the square root of I
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How much does p increase if I increases by some factor?
p increases by the square root of that factor
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What is the equation for sound pressure?
20log10(Px/Pr)
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Pressure sample problems page 58 and 59
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What are the four types of pressure problems?
- given a pressure ratio, calculate decibels
- given decibels, calculate pressure ratio
- given px, calculate dB SPL
- given dB SPL, calculate px
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What is the relationship between dB IL and dB SPL?
they are equal
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If two sounds have an intensity level of 60 dB IL each thatn what is the total intensity level of them?
63 dB IL
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When you double intensities how much does it increase by?
3 dB
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Combining intensities page 63
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*If intensity doubles, dB increases by 3, which means pressure also increases by 3. If we are ONLY talking about pressure, not intensity, dB increases by 6
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What is the equation for equal source intensities?
- dBN = dBi + 10log10N
- i = dB SPL (or dB IL) from one source (only add intensities so use 10logX)
- N = # of sources combined
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Two sources each produce 100 dB SPL. What is the total SPL?
- dB = 100 + 10log2
- = 103 dB SPL
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Three sources each produce 100 dB SPL. What is the total SPL?
- dB = 100 + 10log3
- = 104.8 dB SPL
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Eight sources each produce 91.2 dB SPL. By how many dB is SPL increased?
- dB = 10log8
- = 9 (100.2 - 91.2)
- *not asking for total
- *talking about ratios
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