TUTORIAL ANSWERS 210-225

 

 
 

Your Answer to Q210.  Sorry, your answer is not correct.  What is it that determines the repeat time?

Help: Fundamentals of Sound, Secs. 7-B, 7-D.

Or would you like a HINT?

You should try to work out the answer on your own, but if you insist on reading it the correct answer is here

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Hint for Question 210:  The fundamental is the slowest in returning to its initial shape. When it finally has, the second harmonic has returned twice, the third three times, etc.
 
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Your Answer to Q210. Congratulations, your answer is correct.

If you would like, you can compare your answer to the "official" correct answer

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Correct Answer to Question 210:  The fundamental is the slowest in returning to its initial shape. When it finally has, the second harmonic has repeated twice, the third three times, etc. Thus the mininum time one has to wait is the period of the fundamental, which is indeed the repeat time of the wave.

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Your Answer to Q215.  Sorry, your answer is not correct. Think about the case where the frequencies present are 20 Hz and 30 Hz with a missing fundamental of 10 Hz.

Help: Fundamentals of Sound, Secs. 7-B, 7-D.

Or would you like a HINT?

You should try to work out the answer on your own, but if you insist on reading it the correct answer is here

Return to Question 215.

 

 

Hint for Question 215:  Say you have just the second and third harmonics present and no fundamental. What is the first time that the waves will all be back to their starting shapes?
 
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Your Answer to Q215. Congratulations, your answer is correct.

If you would like, you can compare your answer to the "official" correct answer .

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Correct Answer to Question 215:  Say you have just the second and third harmonics present and no fundamental. The first time that the waves will all be back at their starting shapes so the wave begins repeating is still the period of the fundamental, even though it is not actually present in the wave. Suppose the fundamental is 10 Hz, so that the second harmonic is 20 Hz, and the third, 30 Hz. Then after the period of the second harmonic (1/20 s) the third will have repeated 1.5 times and will not be at its starting shape. The first time both the second and third harmonics will begin repeating simultaneously is after the period of the fundamental (1/10 s). Thus the statement is false because on some occasions, at least, it does repeat at the frequency of the fundamental.

However, there are cases where the wave repeats more often than the fundamental frequency. An example is a wave that is made up of the second harmonic (20 Hz) plus the fourth (40 Hz). This wave repeats every 1/20 s, the period of the second  harmonic. This is because the fourth harmonic is exactly twice the second, which acts as if it were a fundamental frequency. (We might call it an "effective fundamental" or "false fundamental.")

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Your answer to Question 220:  Sorry, your answer is not correct.  The frequency spectrum gives you several pieces of information. What information do you need to draw a waveform?

Help: Fundamentals of Sound, Sec. 7-A.

Or would you like a HINT?

You should try to work out the answer on your own, but if you insist on reading it the correct answer is here .

Return to Question 220.

 

 

Hint for Question 220:  Look at the different complex waves in Figures 6-4 and 6-5 of the Fundamentals of Sound. What information do you need to draw the waveforms shown?
 
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Your Answer to Q220. Congratulations, your answer is correct.

If you would like, you can compare your answer to the "official" correct answer .

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Correct Answer to Question 220:  To draw the pressure versus time graph of a complex wave you need to know the amplitude, frequency, and phase of the constituent sinusoidal waves. The phase refers to the exact position of the wave (usually relative to other waves), e.g., whether, say, it is now at a trough or a crest or elsewhere. Without this information you are unable to draw the wave shape. a) is the correct answer.

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Your Answer to Q225.  Sorry, your answer is not correct. What determines pitch? What determines sound quality?

Help: Fundamentals of Sound, Sec.6-F.

Or would you like a HINT?

You should try to work out the answer on your own, but if you insist on reading it the correct answer is here .

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Hint for Question 225:  Pitch is determined by the repeat frequency of the complex wave, but sound quality is determined by the frequency spectrum.
 
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Your Answer to Q225. Congratulations, your answer is correct.

If you would like, you can compare your answer to the "official" correct answer .

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Correct Answer to Question 225:  Pitch is determined by the repeat frequency of the complex wave, but sound quality (of a sustained note) is determined by the frequency spectrum . Thus, if the notes all have the same repeat frequency, we hear the same pitch, however, what they differ in is the amplitudes of the different constituent sinusoidal frequencies making up the note, i.e., the nature of the frequency spectra. a) is correct. b) is wrong because the ear is not sensitive to phase. c) is also correct because the harmonic series is the same for each wave if they have the same repeat frequency. d) is wrong because, if the amplitudes of all frequency components were the same for all three notes, then they would have the same frequency spectra and would have precisely the same sound quality. The overall correct answer is e), two of the statements are true, a) and c).

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