Conceptual Physics

at which each one can talk without exceeding the limit?

dB

5.4 A cafe is known for its tasty blended juice drinks. The owner worries that if too many blenders are running in the preparation
area at once, the noise will drive customers away. She determines that the maximum allowable sound level at the cash
register should be 84 dB. If each blender in the cafe emits sound with a level of 79 dB at the register, how many blenders can
be running at once before the maximum is exceeded?

blenders

5.5 The human eardrum has an area of 5.0×10–5 m^2. (a) What is the sound power received when the incident intensity is the

minimum perceivable, 1.0×10–12 W/m^2? (b) What is the sound power at the eardrum for conversational speech, which has a

sound level of 65 dB? (c) What is the sound power at the threshold of pain, 140 dB?

(a) W

(b) W

(c) W

5.6 Pete's family is watching a movie at home, with the TV emitting sound at a level of 71 dB as heard by the family. His dad
turns on the popcorn popper, which emits sound that has a level of 68 dB as heard by the family. (a) What is the total sound
intensity perceived by Pete's family? (b) What is the total sound level?
(a) W/m2
(b) dB

5.7 Suzy revs her monster truck, producing a sound with a sound level of 84 dB as heard by Mrs. DiNapoli across the street.
Next door to Suzy, Mike guns his station wagon which produces a sound with a sound level of 71 dB at Mrs. DiNapoli's. At
this poor neighbor's house across the street, what is the unbearable ratio of the intensity of the truck's sound to the intensity of
the wagon's sound?

5.8 Ju Yeon sits in a park where people come to fly small radio-controlled airplanes. She can hear down to a sound level of
20 dB. If a plane emits sound with a power of 1.6eí6 W, how far away will the plane be when she is just able to hear it?
m

Section 7 - Doppler effect: moving sound source

7.1 Passengers on a train hear its whistle at a frequency of 735 Hz. What frequency does someone standing by the train tracks

hear as it moves directly toward her at a speed of 22.5 m/s?

Hz

7.2 A particular ambulance siren alternates between two frequencies. The higher frequency is 617 Hz as heard by the ambulance
driver. If the ambulance drives directly away from you at a speed of 16.8 m/s, what frequency do you hear as the higher
frequency?
Hz

7.3 A small plane is taxiing directly away from you down a runway. The noise of the engine, as the pilot hears it, has a frequency

1.13 times the frequency that you hear. What is the speed of the plane?

m/s

7.4 A train moves away from you at 29.0 m/s, sounding its horn. You perceive the frequency of the horn's sound as being

415 Hz. What is the frequency heard by the train's passengers?

Hz

7.5 A stuntwoman is preparing to take a punch, crash through a "candy glass" window, and fall a long distance. The script calls

for her to emit a piercing scream just before she hits the "ground." In reality, she will land on a waiting airbag. Lights! Camera!

Action! The primary camera crew, filming from her starting height, hears her last-instant scream at a frequency 3.77 kHz. Her

scream has a frequency of 4.05 kHz when she is at rest. How far did she fall? Report this as a positive distance.

m

Section 9 - Supersonic speed and shock waves

9.1 The Mach angle of a jet traveling at supersonic speed is 19.4°. What is the Mach number?

9.2 What is the Mach angle of a jet moving at Mach 2.2?

°

Copyright 2007 Kinetic Books Co. Chapter 16 Problems^321