JiTT Questions- Exam 1

How long (in meters) should a closed tube in air be such that its fundamental frequency is 200 Hz. (This is actually a review problem for Chapter 3.) Take the speed of sound in air to be 345 m/s. a. 0.43 meters b. 0.86 meters c. 1.72 meters d. 3.45 meters

a

The idea of superposition of waves (see Section 2.4, page 47 text) is one of key physical ideas in the course. Consider Figure 2.35 (page 49) which displays this idea graphically for waves "in phase" and "out of phase" where we see as a result "constructive interference" and "destructive interference." Now suppose the phase difference between the 2 waves is somewhere between the "in phase" and "out of phase" values. Can you sketch the two waves and what you expect from their superposition (or addition)? From this superposition do you expect the resultant wave to have the same or different period and the same or different amplitude of the original wave? a. same period; different amplitude. b. different period; same amplitude. c. same period; same amplitude. d. Can't say with the information provided -- or no idea!

a

What is the wavelength of a 500 Hz tone in air? a. 0.68 m b. 1.46 m c. 5.0 m d. 6.8 m

a

A "sonogram" is a graphical display whose vertical axis is _______ and whose horizontal axis is _______, respectively. a. power, frequency b. frequency, time c. amplitude, distance d. amplitude, time

b

A nylon guitar string is 65 cm (or 0.65 m) long and has a mass density 8.3 x 10-4 kg/m (or 0.00083 kg/m) and is held at a tension of 56 N. Calculate the fundamental frequency of this string. a. 20 Hz b. 200 Hz c. 400 Hz d. 520 Hz e. 1040 Hz

b

Figure 1.7 in the text is a graph of distance (x) versus time (t). The displacements (or distances) from equilibrium at t1 = 2.0 sec and t2 = 2.5 sec are x1 = ___ and x2 = ___, respectively. (Please fill in the blanks for x1and x2 .) a. 0; 6 b. 0; 3 c. 0; -3 d. -3; 0 e. none of the above

b

For the following question, please take the speed of sound to be 345 m/s. Consider a closed tube exactly 50 cm long. Find the first 3 resonant frequencies of this tube a. 172 Hz, 344 Hz, 517 Hz b. 172 Hz, 517, 860 Hz c. 345 Hz, 690 Hz, 1035 Hz d. 345 Hz, 1035 Hz, 1725 Hz

b

How long (in meters) should a closed tube in air be such that its fundamental frequency is 100 Hz. (Take the speed of sound in air to be 345 m/s.) a. 0.43 m b. 0.86 m c. 1.72 m d. 3.45 m

b

If one adds up a number of sound waves with the designated frequencies and amplitudes listed after this sentence (a Fourier synthesis), one hears a frequency of _________. Add: Sounds waves of 400 Hz, of amplitude 1, plus 800 Hz of amplitude 1/2, plus 1200 Hz of amplitude 1/3, plus 1600 Hz of amplitude 1/4 a. 200 Hz b. 400 Hz c. 800 Hz d. 1200 Hz e. 1600 Hz

b

What is the frequency of a square wave with an amplitude of A = 3 V and a period T = 2 ms? a. 114 Hz b. 500 Hz c. 1000 Hz d. 2000 Hz

b

A stretched string has a fundamental frequency of 175 Hz. Find the new frequency if you simultaneously do all of the following 3 things: you increase the tension in the string by a factor of 4, and you double its length and you decrease the mass per unit length of the string by a factor of 9. (There are many ways to solve this problem. One way to to consider each of the three things separately, that is what change happens when you only increase the tension by a factor 4? Then do this for the length and the mass density -- and then multipy the 3 effect together. Give it a try! And we definitely will review this question in class!) a. 175 Hz b. 350 Hz c. 525 Hz d. 700 Hz e. 1575 Hs

c

At what frequency does the wavelength of sound equal the diameter of a speaker with a 4 inch diameter. Recall that when we consider whether sound waves of a particular frequency will be diffracted, we compared the diameter of an opening to the wavelength. Here you will first need to convert the diameter of the speaker in inches to meters using 1 inch = 0.0254 meters. a. 338 Hz b. 1688 Hz c. 3376 Hz d. 6752 Hz

c

Consider equations 2.1 and 2.2 (page 26, text) applied to sound traveling through the air. The speed of sound in air is approximately 340 meters/second (m/s). Suppose the sound has a frequency ( f ) of 170 Hz (well within the range of notes we can hear and many could sing). The wavelength of this sound wave (in meters) is ____. a. 0.5 b. 1.0 c. 2.0 d. 340

c

Consider further Figure 1-7 text which is a graph of position versus time. Choose the answer below which provides the time at which the graph indicates the maximum positive velocity. a. t = 1.0 sec b. t = 1.5 sec c. t = 2.0 sec d. t = 2.5 sec

c

For the following question, please take the speed of sound to be 345 m/s. Now consider an open tube exactly 50 cm long. Find the first 3 resonant frequencies of this tube. a. 172 Hz, 344 Hz, 517 Hz b. 172 Hz, 517 Hz, 860 Hz c. 345 Hz, 690 Hz, 1035 Hz d. 345 Hz, 1035 Hz, 1725 Hz

c

What is the velocity of sound in a gas that has a 2000 Hz tone with a wavelength of 0.68 m traveling in it? a. 171 m/s b. 343 m/s c. 686 m/s d. 1360 m/s

d

What is the velocity of sound in a gas that has a 2000 Hz tone with a wavelength of 0.68 m? a. 171 m/s b. 343 m/s c. 686 m/s d. 1360 m/s

d

Please look at Fig 2-43 (page 55 of textbook) to answer this question. You are on a sidewalk and observe an ambulance approaching you with its siren on. The ambulance approaches and then passes you. Using Fig 2-43 as a guide (this guide should also correspond to your experience!), how would you describe the sound that you hear (please select the best answer): a. The frequency of sound for this case (for you as a stationary observer) does not change after the ambulance passes. b. The frequency of the sound rises (that is a higher frequency) after the ambulance passes. c. The frequency of the sound drops (that is a lower frequency) after the ambulance passes d. The frequency of sound I hear as the ambulance approaches is lower than the frequency the ambulance driver is hearing.

c

Position, length, or distance can have many units including inches, feet or miles, or in the metric system centimeters (cm), meters (m), or kilometers (km). Velocity can have the units of feet/second (feet per second) or meter/second (m/sec). An object is found to move from an initial position of xi = 10 cm to a final position xf = 50 cm in a time interval from ti = 1 sec to tf = 3 sec, respectively. The velocity of the object (in cm/sec) is: a. 40 cm/s b. 25 cm/s c. 20 cm/s d. 13.3

c

The claim is that a "Fourier analysis" of a square wave of frequency 400 Hz yields components of frequencies ____, ___, and _____ with amplitudes of ____, ______, and ______, respectively. The correct entries for the blanks are listed in which of the answers below? a. 400 Hz, 800 Hz, and 1200 Hz, with amplitudes of 1, 1/2, and 1/3, respectively b. 400 Hz, 800 Hz, and 1600 Hz, with amplitudes of 1, 1/2, and 1/4, respectively c. 400 Hz, 1200 Hz, and 2000 Hz, with amplitudes of 1, 1/3, and 1/5, respectively d. 400 Hz, 1200 Hz, and 2400 Hz, with amplitudes of 1, 1/3, and 1/6, respectively

c

Two tones of different - but close- frequencies are generated, both with the same amplitude. Their frequencies are 410 Hz and 406 Hz. First look at Figure 2-37 (page 51 in the text) and check at several times "t" on the horizontal axia (you will need a ruler or a straight edge) along the two top waves to convince yourself that the lowest graph displayed is simply the sum (the superposition) of the upper two waves. Now identify for our case, when we sum the 410 Hz and 406 Hz waves, we hear a beat frequency of ________Hz, and hear a sound at a frequency of _________Hz, respectively. (Please fill in the two blanks.) a. 408, 816 b. 2, 408 c. 4, 408 d. 4, 416

c

Two trumpet players tune their instruments to exactly 440 Hz. Find the difference in the apparent frequencies due to the Doppler effect if you are the stationary observer (listener) and one plays her instrument without moving (that is at 440 Hz) while the other plays her instrument while marching towards you. Assume that 1 m/s is a reasonable marching speed. [Question just to think about - would you hear these two players as out of tune?] a. 1.3 Hz below 440 Hz b. 0.0 Hz since the assumed marching speed is small compared to the speed of sound c. 1.3 Hz above 440 Hz

c