1. Unit 3 Day 8 – Common Logs

2. Warm Up #1

3. Page 378 in textbook Take 3 minutes to answer the “Think about the situation”

4. Decibels and Sound Intensity
Use the table to answer the following questions.
I agree, whispers should have a low decibel rating and a jet engine should have a high decibel rating.
For every increase of 1 in the exponent of the sound intensity, there is a corresponding increase of 10 in the relative intensity of the sound.

5. What if you wanted to find….
Two options to find:
Guess and Check to find an x-value that works.
Put the left side into y1 then right side into y2 and find the intersection point. The x-value of the intersection point is your answer.

6. 1) Express each of the numbers below as accurately as possible as a power of 10.
a. 100 b. 10,000 c. 1,000,000 d e f g h. 345 i

7. Check your work!

8. 2) Suppose that the sound intensity of a screaming baby was measured as 9.5 watts/cm2. To calculate the equivalent intensity in decibels, 9.5 must be written as 10x for some value of x.
a. Between which two integers does it make sense to look for values of the required exponent? How do you know? b. Which of the two integer values in part a is probably closer to the required power of 10? c. Estimate the required exponent to the nearest hundredth. Then use your estimate to calculate a decibel rating for the loudness of the baby’s scream. d. Estimate the decibel rating for loudness of sound from a television set that registers intensity of 6.2 watts/cm2.

9. Check your work!

10. Remember inverses from yesterday
Remember inverses from yesterday? When you think inverse, think ________________!
Opposites or Switch the x and y

11. A logarithm base b of a positive number y is defined as follows:
What is the inverse of ?
A logarithm base b of a positive number y is defined as follows:
If y = bx, then logb y = x.

12. The definition of common logarithms is expressed as:
If y = 10x, then log10 y = x.

13. 3) Use your calculator to find the following logarithms
3) Use your calculator to find the following logarithms. Then, compare the results with your work on Problem 1.
log 100 b. log 10,000 c. log 1,000,000
log e. log (-0.001) f. log 3.45
log (-3.45) h. log 345 i. log

14. Check your work!

15. 4. A logarithm can only be taken for a positive number since will always be positive for any value of x.

16. 5) Logarithms can be used to calculate the decibel rating of sounds, when the intensity is measured in watts/cm2.
a. Use the logarithm feature of your calculator to rewrite 9.5 as a power of 10. That is, find x so that 9.5 = 10x.
b. Recall that if the intensity of a sound is 10x watts/cm2, then the expression 10x can be used to convert the sound’s intensity to decibels. Use your results from Part a to find the decibel rating of the crying baby in Problem 2.

17. Check your work!

18. 6) Assume the intensity of a sound I = 10x watts/cm2.
a. Explain why x = log I.
(Think about the definition of a log) 
Rewrite the expression for converting sound intensity reading to decibel numbers using log I.
Remember: Converting to decibels: 10x + 120

19. Check your work!

20. 7) Use your conversion expression from Problem 6 to find the decibel rating of the television set in Problem 2 Part d.

22. Summarize the Mathematics
Log y = x implies
“The Logarithm of y is the exponent required to

23. Use your understanding of the relationship between logarithms and exponents to help complete these tasks.
Find these common (base 10) logarithms without using a calculator.
log 1,000 ii. log iii. log 103.2
b. Use the function y = 10x, but not the logarithm key of your calculator to estimate each of these logarithms to the nearest tenth. Explain how you arrived at your answers.
log 75 ii. log iii. log 7.5
If the intensity of sound from a drag race car is 125 watts/cm2, what is the decibel rating of the loudness for that sound?

24. Check your work!

25. Homework
Independent Practice on Common Logarithms