17.2 Speed of Sound, Frequency, and Wavelength
A teacher wants to demonstrate that the speed of sound is not a constant value. Considering her regular classroom voice as the control, which of the following will increase the speed of sound leaving her mouth?
- Submerge her mouth underwater and speak at the same volume.
- Increase the temperature of the room and speak at the same volume.
- Increase the pitch of her voice and speak at the same volume.
- I only
- I and II only
- I, II and III
- II and III
- III only
All members of an orchestra begin tuning their instruments at the same time. While some woodwind instruments play high frequency notes, other stringed instruments play notes of lower frequency. Yet an audience member will hear all notes simultaneously, in apparent contrast to the equation.
Explain how a student could demonstrate the flaw in the above logic, using a slinky, stopwatch, and meter stick. Make sure to explain what relationship is truly demonstrated in the above equation, in addition to what would be necessary to get the speed of the slinky to actually change. You may include diagrams and equations as part of your explanation.
17.3 Sound Intensity and Sound Level
In order to waken a sleeping child, the volume on an alarm clock is tripled. Under this new scenario, how much more energy will be striking the child’s ear drums each second?
- twice as much
- three times as much
- approximately 4.8 times as much
- six times as much
- nine times as much
A musician strikes the strings of a guitar such that they vibrate with twice the amplitude.
- Explain why this requires an energy input greater than twice the original value.
- Explain why the sound leaving the string will not result in a decibel level that is twice as great.
17.4 Doppler Effect and Sonic Booms
A baggage handler stands on the edge of a runway as a landing plane approaches. Compared to the pitch of the plane as heard by the plane’s pilot, which of the following correctly describes the sensation experienced by the handler?
- The frequency of the plane will be lower pitched according to the baggage handler and will become even lower pitched as the plane slows to a stop.
- The frequency of the plane will be lower pitched according to the baggage handler but will increase in pitch as the plane slows to a stop.
- The frequency of the plane will be higher pitched according to the baggage handler but will decrease in pitch as the plane slows to a stop.
- The frequency of the plane will be higher pitched according to the baggage handler and will further increase in pitch as the plane slows to a stop.
The following graph represents the perceived frequency of a car as it passes a student.
- If the true frequency of the car’s horn is 200 Hz, how fast was the car traveling?
- On the graph above, draw a line demonstrating the perceived frequency for a car traveling twice as fast. Label all intercepts, maximums, and minimums on the graph.
17.5 Sound Interference and Resonance: Standing Waves in Air Columns
A common misconception is that two wave pulses traveling in opposite directions will reflect off each other. Outline a procedure that you would use to convince someone that the two wave pulses do not reflect off each other, but instead travel through each other. You may use sketches to represent your understanding. Be sure to provide evidence to not only refute the original claim, but to support yours as well.
Two wave pulses are traveling toward each other on a string, as shown below. Which of the following representations correctly shows the string as the two pulses overlap?
A student sends a transverse wave pulse of amplitude A along a rope attached at one end. As the pulse returns to the student, a second pulse of amplitude 3A is sent along the opposite side of the rope. What is the resulting amplitude when the two pulses interact?
- 2A, on the side of the original wave pulse
- 2A, on the side of the second wave pulse
A student would like to demonstrate destructive interference using two sound sources. Explain how the student could set up this demonstration and what restrictions they would need to place upon their sources. Be sure to consider both the layout of space and the sounds created in your explanation.
A student is shaking a flexible string attached to a wooden board in a rhythmic manner. Which of the following choices will decrease the wavelength within the rope?
- The student could shake her hand back and forth with greater frequency.
- The student could shake her hand back in forth with a greater amplitude.
- The student could increase the tension within the rope by stepping backwards from the board.
- I only
- I and II
- I and III
- II and III
- I, II, and III
A ripple tank has two locations (L1 and L2) that vibrate in tandem as shown below. Both L1 and L2 vibrate in a plane perpendicular to the page, creating a two-dimensional interference pattern.
Describe an experimental procedure to determine the speed of the waves created within the water, including all additional equipment that you would need. You may use the diagram below to help your description, or you may create one of your own. Include enough detail so that another student could carry out your experiment.
A string is vibrating between two posts as shown above. Students are to determine the speed of the wave within this string. They have already measured the amount of time necessary for the wave to oscillate up and down. The students must also take what other measurements to determine the speed of the wave?
- The distance between the two posts.
- The amplitude of the wave
- The tension in the string
- The amplitude of the wave and the tension in the string
- The distance between the two posts, the amplitude of the wave, and the tension in the string
The accepted speed of sound in room temperature air is 346 m/s. Knowing that their school is colder than usual, a group of students is asked to determine the speed of sound in their room. They are permitted to use any materials necessary; however, their lab procedure must utilize standing wave patterns. The students collect the information Table 17.6.
|Trial Number||Wavelength (m)||Frequency (Hz)|
- Describe an experimental procedure the group of students could have used to obtain this data. Include diagrams of the experimental setup and any equipment used in the process.
- Select a set of data points from the table and plot those points on a graph to determine the speed of sound within the classroom. Fill in the blank column in the table for any quantities you graph other than the given data. Label the axes and indicate the scale for each. Draw a best-fit line or curve through your data points.
- Using information from the graph, determine the speed of sound within the student’s classroom, and explain what characteristic of the graph provides this evidence.
- Determine the temperature of the classroom.
A tube is open at one end. If the fundamental frequency f is created by a wavelength λ, then which of the following describes the frequency and wavelength associated with the tube’s fourth overtone?
A group of students were tasked with collecting information about standing waves. Table 17.7 a series of their data, showing the length of an air column and a resonant frequency present when the column is struck.
|Length (m)||Resonant Frequency (Hz)|
- From their data, determine whether the air column was open or closed on each end.
- Predict the resonant frequency of the column at a length of 2.5 meters.
When a student blows across a glass half-full of water, a resonant frequency is created within the air column remaining in the glass. Which of the following can the student do to increase this resonant frequency?
- Add more water to the glass.
- Replace the water with a more dense fluid.
- Increase the temperature of the room.
- I only
- I and III
- II and III
- all of the above
A student decides to test the speed of sound through wood using a wooden ruler. The student rests the ruler on a desk with half of its length protruding off the desk edge. The student then holds one end in place and strikes the protruding end with his other hand, creating a musical sound, and counts the number of vibrations of the ruler. Explain why the student would not be able to measure the speed of sound through wood using this method.
A musician stands outside in a field and plucks a string on an acoustic guitar. Standing waves will most likely occur in which of the following media? Select two answers.
- The guitar string
- The air inside the guitar
- The air surrounding the guitar
- The ground beneath the musician
This figure shows two tubes that are identical except for their slightly different lengths. Both tubes have one open end and one closed end. A speaker connected to a variable frequency generator is placed in front of the tubes, as shown. The speaker is set to produce a note of very low frequency when turned on. The frequency is then slowly increased to produce resonances in the tubes. Students observe that at first only one of the tubes resonates at a time. Later, as the frequency gets very high, there are times when both tubes resonate.
In a clear, coherent, paragraph-length answer, explain why there are some high frequencies, but no low frequencies, at which both tubes resonate. You may include diagrams and/or equations as part of your explanation.
A student connects one end of a string with negligible mass to an oscillator. The other end of the string is passed over a pulley and attached to a suspended weight, as shown above. The student finds that a standing wave with one antinode is formed on the string when the frequency of the oscillator is f0. The student then moves the oscillator to shorten the horizontal segment of string to half its original length. At what frequency will a standing wave with one antinode now be formed on the string?
- There is no frequency at which a standing wave will be formed.
A guitar string of length L is bound at both ends. Table 17.8 shows the string’s harmonic frequencies when struck.
- Based on the information above, what is the speed of the wave within the string?
- The guitarist then slides her finger along the neck of the guitar, changing the string length as a result. Calculate the fundamental frequency of the string and wave speed present if the string length is reduced to 2/3 L.