INDIANA UNIVERSITY, DEPT. OF PHYSICS P105, Basic Physics of Sound, Spring 2010

Size: px

Start display at page:

Download "INDIANA UNIVERSITY, DEPT. OF PHYSICS P105, Basic Physics of Sound, Spring 2010"

Dwayne Boyd
6 years ago
Views:

1 Name: ID#: INDIANA UNIVERSITY, DEPT. OF PHYSICS P105, Basic Physics of Sound, Spring 2010 Final Exam Friday, 7 May 2010, 2:45 4:45 p.m. Closed book. You are allowed a calculator. There is a Formula Sheet and Equal Loudness Curves at the back of the exam Fill in the answers on the exam sheets themselves. If you run out of room, use the back of the sheets. You must show your work and give answers with units for full credit! In many cases I give partial credit try not to leave blank answers. Show your work when relevant for fill in the blank and multiple-choice questions as well. Please keep your work covered and your eyes on your own paper during the exam. The penalty for cheating is an automatic zero! 1. Warming up (3 pts) The speed of sound at 20 C is 343 m/s. What is the wavelength of a sound wave with a period of 150 µs? m. 2. Belting out a tune (a) (3 pts) A man steps right up to a microphone and sings a note at a sound pressure level of L p = 105 db at the microphone input. If the sensitivity of the microphone is S V = 52 dbv, what is the resulting voltage across the microphone? mv (note units!) 1

2 (b) (3 pts) The big clunky microphone he is singing into is a dynamic microphone. Describe how a dynamic microphone works. (c) (2 pts) He is using the most common directional microphone which is a microphone. 2. An intimate room? (a) (3 pts) A chamber orchestra performance room is 12 m long, 8 m wide, and has a ceiling height of 10 m. Calculate the reverberation time, RT, at 500 Hz for this room if the front and back walls are covered with plywood paneling (a i = 0.17), the side walls are painted concrete block (a i = 0.06), the ceiling is acoustical tile on concrete (a i = 0.76), and the floor is Terrazzo marble (a i = 0.02). RT = s. (b) (2 pts) If this is too long a reverberation time, what would one do to make it shorter? 2

3 (c) Prof. Van Kooten's son is playing violin right at the center front of the room as part of a recital. Prof. Van Kooten is listening standing in the center of the back wall of the room. What are the times for sound to reach Prof. Van Kooten's ears? (take v sound = 343 m/s) (i) (2 pts) Direct sound, t 0 = msec. (ii) (3 pts) First reflection off of the side wall, t wall = early time). msec (which is the (iii) (3 pts) What is the reflected time delay? t 1 = msec. Would the room be considered "acoustically intimate"? Why or why not? Would an echo be heard? Why or why not? 3

4 3. One of 96 trombones: (a) (3 pts) A band member is playing a note on a trombone that is the fourth harmonic, f 3 = Hz playing in "first position" of the slide. If the air inside the instrument is at 20 C so that v sound = 343 m/s, what is the effective length of the instrument? (The trumpet can be treated as a tube closed at the mouthpiece end and open at the bell end). L = m. (b) (2 pts) What is the wavelength of this sound? m. (c) (3 pts) If she now wants to play a note that is 3 semitones lower, what length would she have to add to the trombone by moving the slide? (d) (3 pts) She now returns the length of the trombone to that which was initially necessary to play the note of f = Hz. However, after she has been playing steadily for a while, the temperature of the air in the instrument has now risen to 28 C. What is the frequency of the resulting note? (e) (3 pts) What is the purpose of the bell on a brass instrument? (f) (3 pts) What is the sound intensity level of 96 trombones playing simultaneously each with an equal sound intensity level of 64 db? 4

5 4. What is a sound wave? (2 pts) A sound wave is an example of (circle one): (A) A transverse wave needing a medium in which to travel; (B) a longitudinal wave needing a medium in which to travel; (C) a transverse wave that does not need a medium in which to travel; (D) a longitudinal wave that does not need a medium in which to travel; (E) a circular wave needing a medium in which to travel. 5. "It's why we come to class!" (2 pts) When Prof. Van Kooten (finally!) broke a glass using sound, this was demonstrating the principle of. 6. Digital Sound for the future In the future, everyone will have cochlear implants tweaked to allow them to hear in the frequency range 20 Hz to 60,000 Hz. You have to design a super-duper digital sound system to satisfy these future humans. (a) (2 pts) Approximately how many octaves is this frequency range? (a) (2 pts) What sampling frequency should be used so that you can record with this system with decent fidelity for our super humans? Hz. (b) (4 pts) In addition to greater frequency range, you are also measuring each sample with 24 bits. If the signal ranges from 0 V to 10 V, what is the smallest increment of signal you can record (corresponding to 1 bit)? V What is the resulting dynamic range in db? db (b) (2 pts) One of the amplitude samples as a decimal is Express this as a base- 2 binary number that can be recorded in your system:. 5

6 (c) (2 pts) If you record two stereo channels on to some future recording media that can fit (only) 100 Gigabytes on one disk, how long a recording can you have on the disk if you record at the above rate? minutes. (d) (2 pts) Someone foolishly tries to record the sounds of a bat at a frequency of 80kHz with this system. This results in the frequencies of the bat sound being represented at falsely lower frequencies. This effect is called. 7. Shredding on Stage: the full chain (a) (4 pts) Pete Townshend (who?) plays a single note on his electric guitar at 196 Hz. If the guitar string that he is playing it on has a length of L = 64 cm strung at a tension of 15 N, what is the mass of the string? kg. (b) (2 pts) The signal from his guitar goes into an amplifier with a power gain of 40 db. If the output of the amplifier is 100 W of electrical power, what was the input? W. (c) (3 pts) The output of the amplifier goes to a speaker with an impedance of 8 Ω, what is the voltage, V, across the speaker terminals? volts. 6

7 (d) (4 pts) The efficiency of the speaker is 5%. What is the sound intensity level of the resulting sound at a distance of 20 m if we can assume that the sound spreads spherically? db. What is the loudness level in phons? (show on attached loudness level plot) phons. (e) (2 pts) Pete frets his string, resulting in a note of higher frequency of 440 Hz, with the same sound power. Is this sound perceived as louder or softer by the admiring crowd, and why? (circle one) (A) Louder (B) Softer (f) (2 pts) The band's drummer hits his cymbal with a crash. What can you say about the relative frequencies of the different modes of oscillation of the cymbal? This results in something called pitch. 8. Funky Frequency Spectra A sound waveform has the frequency spectrum shown below. 7

8 (a) (4 pts) From the formula sheet description of different waveforms, sketch the approximate waveform below. Make sure to fill in the horizontal axis! (b) (3 pts) This same sound is then input into a tube of a certain length L that is open on one end and closed at the other. Estimate the length of the tube that results in the frequency spectrum below. 8

9 9. A final earful (5 pts) Fill in the blanks in the table below: "Physics" Quantity Frequency "Perceived" (by ear & brain) Quantity Nerve Signal in inner ear Loudness Timbre Thanks for being an attentive and interesting class I've thoroughly enjoyed teaching you this semester. This is likely your last exam of the semester, so have a great summer!! Remember, "Physics Rocks". Prof. Van Kooten 9

10 Equal loudness curves: 10

Harmonics and Sound Exam Review

Name: Class: _ Date: _ Harmonics and Sound Exam Review Multiple Choice Identify the letter of the choice that best completes the statement or answers the question. 1. Which of the following is not an example