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University Physics Volume 1

Conceptual Questions

University Physics Volume 1Conceptual Questions

Conceptual Questions

17.1 Sound Waves


What is the difference between sound and hearing?


You will learn that light is an electromagnetic wave that can travel through a vacuum. Can sound waves travel through a vacuum?


Sound waves can be modeled as a change in pressure. Why is the change in pressure used and not the actual pressure?

17.2 Speed of Sound


How do sound vibrations of atoms differ from thermal motion?


When sound passes from one medium to another where its propagation speed is different, does its frequency or wavelength change? Explain your answer briefly.


A popular party trick is to inhale helium and speak in a high-frequency, funny voice. Explain this phenomenon.


You may have used a sonic range finder in lab to measure the distance of an object using a clicking sound from a sound transducer. What is the principle used in this device?


The sonic range finder discussed in the preceding question often needs to be calibrated. During the calibration, the software asks for the room temperature. Why do you suppose the room temperature is required?

17.3 Sound Intensity


Six members of a synchronized swim team wear earplugs to protect themselves against water pressure at depths, but they can still hear the music and perform the combinations in the water perfectly. One day, they were asked to leave the pool so the dive team could practice a few dives, and they tried to practice on a mat, but seemed to have a lot more difficulty. Why might this be?


A community is concerned about a plan to bring train service to their downtown from the town’s outskirts. The current sound intensity level, even though the rail yard is blocks away, is 70 dB downtown. The mayor assures the public that there will be a difference of only 30 dB in sound in the downtown area. Should the townspeople be concerned? Why?

17.4 Normal Modes of a Standing Sound Wave


You are given two wind instruments of identical length. One is open at both ends, whereas the other is closed at one end. Which is able to produce the lowest frequency?


What is the difference between an overtone and a harmonic? Are all harmonics overtones? Are all overtones harmonics?


Two identical columns, open at both ends, are in separate rooms. In room A, the temperature is T=20°CT=20°C and in room B, the temperature is T=25°CT=25°C. A speaker is attached to the end of each tube, causing the tubes to resonate at the fundamental frequency. Is the frequency the same for both tubes? Which has the higher frequency?

17.5 Sources of Musical Sound


How does an unamplified guitar produce sounds so much more intense than those of a plucked string held taut by a simple stick?


Consider three pipes of the same length (L). Pipe A is open at both ends, pipe B is closed at both ends, and pipe C has one open end and one closed end. If the velocity of sound is the same in each of the three tubes, in which of the tubes could the lowest fundamental frequency be produced? In which of the tubes could the highest fundamental frequency be produced?


Pipe A has a length L and is open at both ends. Pipe B has a length L/2 and has one open end and one closed end. Assume the speed of sound to be the same in both tubes. Which of the harmonics in each tube would be equal?


A string is tied between two lab posts a distance L apart. The tension in the string and the linear mass density is such that the speed of a wave on the string is v=343m/s.v=343m/s. A tube with symmetric boundary conditions has a length L and the speed of sound in the tube is v=343m/s.v=343m/s. What could be said about the frequencies of the harmonics in the string and the tube? What if the velocity in the string were v=686m/sv=686m/s?

17.6 Beats


Two speakers are attached to variable-frequency signal generator. Speaker A produces a constant-frequency sound wave of 1.00 kHz, and speaker B produces a tone of 1.10 kHz. The beat frequency is 0.10 kHz. If the frequency of each speaker is doubled, what is the beat frequency produced?


The label has been scratched off a tuning fork and you need to know its frequency. From its size, you suspect that it is somewhere around 250 Hz. You find a 250-Hz tuning fork and a 270-Hz tuning fork. When you strike the 250-Hz fork and the fork of unknown frequency, a beat frequency of 5 Hz is produced. When you strike the unknown with the 270-Hz fork, the beat frequency is 15 Hz. What is the unknown frequency? Could you have deduced the frequency using just the 250-Hz fork?


Referring to the preceding question, if you had only the 250-Hz fork, could you come up with a solution to the problem of finding the unknown frequency?


A “showy” custom-built car has two brass horns that are supposed to produce the same frequency but actually emit 263.8 and 264.5 Hz. What beat frequency is produced?

17.7 The Doppler Effect


Is the Doppler shift real or just a sensory illusion?


Three stationary observers observe the Doppler shift from a source moving at a constant velocity. The observers are stationed as shown below. Which observer will observe the highest frequency? Which observer will observe the lowest frequency? What can be said about the frequency observed by observer 3?

Picture is a drawing of a moving source that emits a sound wave with a constant frequency, with a constant wavelength moving at the speed of sound. Source moves from the stationary observer 2 to the stationary observer 1 and passes next to the stationary observer 3 on its way.

Shown below is a stationary source and moving observers. Describe the frequencies observed by the observers for this configuration.

Picture is a drawing of a stationary source that emits a sound wave with a constant frequency, with a constant wavelength moving at the speed of sound. Observer one is moving to the source with the speed v1, Observer 3 is moving to the point located in a close proximity of the source with the same speed. Observer 2 located on the side opposite to the observer 1 moves to the source with the speed v2 that is twice of v1.

Prior to 1980, conventional radar was used by weather forecasters. In the 1960s, weather forecasters began to experiment with Doppler radar. What do you think is the advantage of using Doppler radar?

17.8 Shock Waves


What is the difference between a sonic boom and a shock wave?


Due to efficiency considerations related to its bow wake, the supersonic transport aircraft must maintain a cruising speed that is a constant ratio to the speed of sound (a constant Mach number). If the aircraft flies from warm air into colder air, should it increase or decrease its speed? Explain your answer.


When you hear a sonic boom, you often cannot see the plane that made it. Why is that?

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