1. What is the intensity, in W/m2, of a laser beam used to burn away cancerous tissue that, when 90%
absorbed, puts 500 J of energy into a circular spot that is 2.00mm in diameter in a time of 4.00s?
2. The low-frequency speaker of a stereo set has a surface area of 0.05m2 and projects 1.0W of
acoustical power uniformly in all directions.
a) What is the total sound energy projected by the speaker in 1 hour?
b) What is the sound intensity at the surface of the speaker?
c) What is the sound intensity at a distance 3.0 meters away?
d) What is the difference in decibel level between the sound intensities from (a) and (b)?
3. Ear trumpets were never very common, but they did aid people with hearing losses by gathering
sound over a large area and concentrating it on the smaller area of the eardrum. What decibel increase
does an ear trumpet produce if its sound gathering area is 900 cm2 and the area of the eardrum is 0.500
cm2, but the trumpet only has an efficiency of 5.00% in transmitting the sound to the eardrum?
4. (a) What frequency is received by a person watching an oncoming ambulance moving at 110 km/h
and emitting a steady 800-Hz sound from its siren? The speed of sound on this day is 345 m/s. (b) What
frequency does she receive after the ambulance has passed?
5. Students in a physics lab are asked to find the length of an air column (in a tube closed at one end)
that has a fundamental frequency of 256 Hz. On a day where the speed of sound in air is 345m/s, the
students hold a 1.0 m tube vertically and fill it with water to the top, then lower the water level while a
256-Hz tuning fork is rung and listen for the first resonance. How high is the water in the tube when they
hear the first resonance (at the fundamental frequency)?
6. Two loudspeakers on a concert stage are vibrating in phase. A listener is 50.5m from the left speaker,
and 26.0m from the right one. The listener can respond to all frequencies from 20Hz to 20kHz, and the
speed of sound is the same for all frequencies: 343 m/s. (a) What are the three lowest frequencies that
the listener can hear loudly due to constructive interference? (b) What is the lowest frequency that
maximally destructively interferes at the location of the listener?
7. The middle C string on a piano vibrates at a fundamental frequency of 440Hz. Suppose one such string
is 63 cm long, and is tied at both ends under a tension of 944N. What is the linear density of the string?
(Linear density = mass per unit length.)