# Acoustics ch. 4

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1. What is power?
• the rate at which energy is transferred; energy transferred per unit time
• energy per second
2. What is energy?
the capacity to do work, whereas power is the rate at which energy (the work) is expended (done)
3. What is a Watt?
• energy transferred per unit time
• 1 watt = 1 joule/s
4. What is a relative measure?
a comparative measure
5. What is an absolute measure?
an exact number or amount
6. What unit of measure is power expressed in?
• watts
• sound = 2 x 10^-8 watts (.00000002)
7. 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
8. What is intensity?
energy per second per square meter
9. What is the unit of measure of intensity?
watt/m2
10. What is Ix?
absolute intensity
11. What is Ir?
reference intensity
12. What is the ratio for the level of intensity?
Ix/Ir
13. What is a Bel?
• the log of the ratio Ix/Ir
• Bels = log10(Ix/Ir)
• if the base is not give, assume it is 10
14. What do positive bels mean?
the absolute measure is bigger than the reference
15. What do negative bels mean?
the absolute measure is smaller than the reference
16. Ratios page 52 slide 20
17. Ix = 10-6 watt/m2
Ir = 10^-10 watt/m^2
What is the absolute intensity?
What is the level of intensity in bels?
• 10-6 watt/m2
• 4 bels
18. 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
19. 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
20. Sample dB problems page 53 slide 26
21. For every 10-fold change in Ix, dB changes by? Why?
• 10 dB
• log1010 = 1; 1 x 10 = 10
22. For every 2-fold change in Ix, dB changes by?
3 dB
23. What do 0 decibels mean?
Ix = Ir
24. What is dB IL?
• intensity level
• the reference intensity must always be specified!
25. What is the standard reference intensity for dB IL?
Ir = 10-12 watt/m2
26. dB IL problems page 54 slide 31
27. Sample problems page 54 and 55
28. 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
29. What is sound pressure?
force/unit area
30. What is the unit of measure for pressure?
• N/m2 = 1 Pa
• microN/m2 = 1 microPa
31. What is impedance?
an opposition to motion
32. What is the relationship between intensity and pressure?
• I is directly proportional to p2
• p is directly proportional to the square root of I
33. How much does p increase if I increases by some factor?
p increases by the square root of that factor
34. What is the equation for sound pressure?
20log10(Px/Pr)
35. Pressure sample problems page 58 and 59
36. 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
37. What is the relationship between dB IL and dB SPL?
they are equal
38. If two sounds have an intensity level of 60 dB IL each thatn what is the total intensity level of them?
63 dB IL
39. When you double intensities how much does it increase by?
3 dB
40. Combining intensities page 63
41. *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
42. 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
43. Two sources each produce 100 dB SPL. What is the total SPL?
• dB = 100 + 10log2
• = 103 dB SPL
44. Three sources each produce 100 dB SPL. What is the total SPL?
• dB = 100 + 10log3
• = 104.8 dB SPL
45. Eight sources each produce 91.2 dB SPL. By how many dB is SPL increased?
• dB = 10log8
• = 9 (100.2 - 91.2)