Note: problem difficulty is ranked using a star system.

• (*) One-star problems are fundamental to the unit, and can be done relatively quickly. Use these problems to introduce yourself to the material.

• (**) Two-star problems are more difficult, and require an understanding of one or two key concepts. Use these problems to test your understanding of the material.

• (***) Three-star problems are the most difficult, and require some creative thinking in addition to a deep familiarity with multiple key concepts. Use these problems to challenge yourself; if you can complete one of these, you’re on your way to mastering the material.

*Q16.1) A dog barks with a sound intensity of 0.0035W/m². What’s the intensity level (in dB) of the bark?

*Q16.2) A speaker radiates 200W of acoustic power in a spherical wave around it. If you stand 10m away from the speaker,

a) What sound intensity do you hear?

b) What sound intensity level (in dB) do you hear?

**Q16.3) You’re walking through the African Lion Safari when you suddenly encounter a lion. It stands 18m in front of you and let’s out a loud roar. Taking advantage of a rare opportunity, you take out your decibel meter and walk 10m closer to the lion. The lion then lets out another roar which you measure to be 81dB. What intensity level did you perceive the first roar to be? Note: assume the lion roared with the same power output both times.

**Q16.4) An average conversation has an intensity level of about 60dB, and a shotgun blast at close range has an intensity level of about 155dB. How many times louder (or more precisely: how many times more intense) is a shotgun blast than a conversation?

**Q16.5) What’s the farthest away you could stand from a person while still being able to (barely) hear them speak? Assume that human speech radiates 2.5 x 10⁻⁵ W of acoustic power in a spherical wave, and that no power is lost as the sound transmits through the air, and that there are no other sounds present that might overwhelm the sound of the voice.

**Q16.6) What’s the change in intensity level if the intensity of a sound is tripled?

**Q16.7) How much energy is transferred to the eardrum over the course of a 2-hour movie with an average intensity level of 92dB? Note: the eardrum has a surface area of 65mm².

**Q16.8) You place four identical speakers in a room, all equidistant from you. If each speaker individually produces a 83dB sound, what is the intensity level that you hear when all four speakers are on?

***Q16.9) You’re getting ready to give a talk at a physics conference when you decide you’d like to estimate the number of audience members in attendance using your decibel meter. After your talk, you measure the audience’s applause to have an intensity level of 80dB. You later measure the sound of one person clapping to be 67dB. Use this information to estimate the number of audience members in attendance. Note: assume that everyone liked your talk and clapped.

***Q16.10) You decide to try a trendy new exercise that involves screaming at the top of your lungs all day, without stopping, to burn calories. Using your trusty decibel-meter, you measure your scream to have an intensity level of 95dB at a distance of 1 meter. Assuming that you start screaming the moment you wake up and stop screaming the moment you fall asleep for a total of 16 hours of screaming, how many Calories would you burn in a day using this technique? Also assume that the human body is 20% efficient at converting food energy into sound. Note: 1 Calorie = 1 kcal = 4184 J.