# Course:PHYS341/2018/Assignments/A2

## Part 1

Basic Concepts. For questions 1-2, fill in the gaps in each paragraph, choosing from the set of words/phrases at the end of each question (each of which may be used more than once) to form the most correct, precise statements. In the case where two words can be interchanged with no change in meaning, place them in alphabetical order (the code cannot handle more than one correct answer!).

1.

 The acoustic phenomenon of beating occurs when two sound waves very close in with each other. This is because the two waves periodically get in and out of with each other and the rises and falls. The frequency of the beats equals the difference between the of the two waves. amplitude, amplitudes, frequencies, frequency, information, interact, loud, phase, power, wavelength(s), unwanted.

2.

 A small sound source radiates sound waves at one frequency equally in all directions. The is determined by the sound speed in the medium. The sound speed is determined by the stiffness and density of the medium. Stiffer media (all else being equal) have sound speeds. More dense media (all else being equal) have sound speeds. As the wave radiates outward the may change (but only if the medium does), but the does not. As the wave radiates outward the becomes smaller - even without damping - as the sound is spread over an ever growing area. amplitude, frequency, higher, lower, power, velocity, unchanged, wavelength

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## Part 2

For the following questions, select all statements that are correct; there may be more than one. When you have completed the quiz, I suggest to pay attention to all the comments, including those for wrong answers, even if you got it right first time.

1. When a sound wave travels down an narrow air-filled pipe, how are the air molecules moving?

 (a) They migrate down the pipe from one end to the other → They can do this, but this motion is not sound, it is flow (b) They oscillate back and forth in the direction of the wave → Partly right (c) They oscillate back and forth across the diameter of the pipe → In principle this is possible but usually usually only at very high frequencies; the oscillation in the direction of the pipe is usually much larger (d) Random thermal motion → Partly right (e) They oscillate back and forth in the direction of the wave and undergo random thermal motion → Correct (f) They oscillate back and forth across the diameter of the pipe and undergo random thermal motion → Wrong

2. A sound wave radiating out isotropically from a small source is detected at two positions, 10m and 20m. How will the 10m signal compare with the 20m signal?

 (a) Four times the intensity. → Correct (b) Four times the frequency. → No, the frequency has to be that of the source (c) Four times the wavelength. → No - only if the medium changes, which I would have specified (d) A quarter of the intensity. → Is the sound getting louder or softer the closer you get to a source? (e) A quarter of the frequency. → No, the frequency has to be that of the source (f) A quarter of the wavelength. → No - only if the medium changes, which I would have specified

3. Longitudinal waves

 (a) can occur only in gases → Not only (b) can occur only in liquids and gases → Not only (c) can occur in solids, liquids and gases → Correct (d) can transmit energy permanently from one place to another → Correct (e) can transmit matter permanently from one place to another → No, the matter of the medium only oscillates to and fro

4. Beats are an acoustic phenomenon in which

 (a) you hear the individual pressure oscillations of a sound wave → No (b) two sound waves with vastly different frequencies add together to make a sound with an audibly oscillating amplitude → No, they have to be close in frequency (c) two sound waves very close in frequency add together to make a sound with an audibly oscillating amplitude → Correct (d) two sound waves go in and out of phase with each other causing the amplitude to rise and fall → Correct (e) two sound waves interfere with each other → Not very specific, but correct

5. If I blow over the top of a beer bottle I produce sound at a certain frequency. If I add some water to the bottle and repeat, the frequency will

 (a) rise → Correct (b) fall → No (c) stay the same → Correct

6. (following on from the previous question) because

 (a) the water is less dense than the glass → No, the glass is not resonating here (b) the length of the air column in the bottle is reduced → No (c) the volume of air in the bottle is reduced → Correct (d) the mass of air in the neck has not changed → It has not changed, but that is not the reason

7. (following on from the previous question) and

 (a) it is easier to squeeze a smaller body of air a certain amount than a larger one by the same amount → No, because the pressure goes much higher for a small body of air than a big one. (b) it is harder to squeeze a smaller body of air a certain amount than a larger one by the same amount → Correct (c) it is no harder, or easier, to squeeze a smaller body of air a certain amount than a larger one by the same amount → No

8. An object (string, air column, soundboard etc.) has a natural frequency.

 (a) If it is vibrated equally over a range of frequencies it will vibrate with a largest amplitude at the natural frequency. → Right, that is what resonance is (b) If it is vibrated at any other single frequency it will still vibrate at its own natural frequency. → No, things only respond with the same frequency with which they are excited (c) If it is struck (with a solid object or a jet of air) it it will vibrate at the natural frequency. → Right, because impulses and air jets contain a wide of frequencies and the object will pick out its own natural frequency (d) The natural frequency is a property of the object alone. → Right (e) The natural frequency is a property of the object and whatever has excited it. → No, the excitation mechanism doesn't affect the natural frequency of the object (f) An object can have only one natural frequency. → No, it can have many; it is highly unusual only to have one.

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