Please help!!!

68.

68. I| You are standing 2.5 m Walk directly in front of one of the two loudspeakers shown in FIGURE P17.68. They are 3.0 2.5 m m apart and both are playing a 3.0 m 686 Hz tone in phase. As you begin to walk directly away from the speaker, at what dis- tances from the speaker do you hear a minimum sound intensity? FIGURE P17.68 The room temperature is 20"C.71. I| The three identical loudspeakers in FIGURE P17.71 play a 170 Hz tone in a room where the speed of sound is 340 m/s. You are standing 4.0 m 3.0 m in front of the middle speaker. At this point, the amplitude of the wave from each speaker is a. a. What is the amplitude at this point? 3.0 m b. How far must speaker 2 be moved to the left to produce a maximum amplitude at the point where you are standing? 4.0 m c. When the amplitude is maximum, FIGURE P17.71 by what factor is the sound inten- sity greater than the sound intensity from a single speaker?73. II A flutist assembles her flute in a room where the speed of sound is 342 m/s. When she plays the note A, it is in perfect tune with a 440 Hz tuning fork. After a few minutes, the air inside her flute has warmed to where the speed of sound is 346 m/s. a. How many beats per second will she hear if she now plays the note A as the tuning fork is sounded? b. How far does she need to extend the "tuning joint" of her flute to be in tune with the tuning fork?75. II Two loudspeakers emit 400 Hz notes. One speaker sits on the ground. The other speaker is in the back of a pickup truck. You hear eight beats per second as the truck drives away from you. What is the truck's speed?79. III When mass M is tied to the bottom of a long, thin wire sus- pended from the ceiling, the wire's second-harmonic frequency is 200 Hz. Adding an additional 1.0 kg to the hanging mass increases the second-harmonic frequency to 245 Hz. What is M