PHYS 130 Exam #2 3/26/2009

Transcription

1 PHYS 130 Exam #2 3/26/2009 Equations you might need: p = F/S v = d/t F = ma vt = (T 20) v = f λ W = FD (f1 f0)/f0 ±V/v nλ = L1 L2 fb = f1 f2 P = E/t I = P/S (Ax/Ay) 2 = Ix/Iy SILx SILy Table 5.1 Level Difference SIL1 SIL2 = 0 db Intensity Ratio 1.0 = I1/I2 1 db db db db db db db db db db db db n db 10 n Show your work in order to receive full credit.

2 Name: ID: 1. There are two sources of sound in a room. One produces a sound level of SIL1 = 69 db, and the other one produces a sound level of SIL2 = 75 db. The threshold of hearing has an intensity of I0 = W/m 2. a. Find the intensity I1. b. Find the intensity I2. c. Find the intensity ratio of I2/I1 using SIL1 and SIL2, without using the answers in (a) and (b). d. What would be the combined intensity (Itotal) if the frequencies of these two waves are widely different? Express you answer in I1. e. What is this combined sound level (SILtotal)? f. What is the amplitude ratio A2/A1? 1

3 If these two waves have frequencies very close to each other, they will interfere. g. What is the amplitude (Ac) of the constructive interference (in phase)? Express your answer in A1. h. What is the intensity ratio of Ic/I1, where Ic is the intensity of this constructive interference? i. What is the sound level (SILc) of this constructive interference? j. What is the amplitude (Ad) of the destructive interference (out of phase)? Express your answer in A1. k. What is the intensity ratio of Id/I1, where Id is the intensity of this destructive interference? l. What is the sound level (SILd) of this destructive interference? 2

4 2. Figure 6.7 shows the just noticeable difference (JND) in sound level as a function of initial sound level. Use the figure to answer the following questions: a. Starting at 8 khz and 65 db, how much must you change the sound level to make a noticeable difference? b. What percentage change in intensity does the answer to 2a represent? 3. Figure 6.8 shows the just noticeable difference (JND) in frequency as a function of initial frequency. The solid curve represents musically useful levels, roughly at 60 to 90 db. Use this to answer the following questions: a. How much must you change the frequency of a sound to make it recognizably different if you start from 200 Hz? b. How much must you change the frequency of a sound to make it recognizably different if you start from 2000 Hz? 4. Figure 6.5 shows how the average threshold of hearing loss shifts for 100 people with no history of industrial noise exposure. Men of various ages in years are indicated by solid lines, and women of various ages in years are indicated by dashed lines. From this figure: a. Do older people suffer more hearing loss at low or high frequency? b. Do men suffer more or less hearing loss than women? 5. Briefly descript what acoustic reflex means. 3

5 6. What is the frequency of the note two octaves higher than A440? What is the frequency of the note one octave lower than A440? What is the frequency of the note one semitone above A440 in the equal temperament chromatic scale? What is the frequency of the not two semitones below? 7. An outdoor air raid siren produces a sound level of 110 db at a distance of 15 m. a. What would be the sound level at a distance of 30 m? b. How far away must you go to find a level of 92 db? 8. Use Figure 6.12 to answer the following questions: a. What must be the intensity level of a 100 Hz tone in order for it to sound as loud as a 1000 Hz tone played at 10 db? b. Find the loudness level (in phons) of a 100 Hz, 1000 Hz, and 3000 Hz tone played at 60 db, respectively. Which one of them will sound the loudest? c. What must be the intensity level of a 500 Hz tone in order for it to be perceived at the same loudness as a 10,000 Hz tone played at 60 db? 4

6 Use Figure 6.12 and 6.13 to answer the following two questions: 9. Find the loudness in phons and in sones for the following sine waves: a. 500 Hz at 30 db b. 400 Hz at 80 db c. 50 Hz at 70 db 10. For the frequencies listed below, find out what sound level in db is required for a 1-sone loundness. a. F = 1000 Hz b. f = 4000 Hz c. f = 50 Hz 11. If A 6 (1760 Hz) is the eighth harmonic in a series, what is the fundamental frequency? What is the frequency for the fifth harmonic? 12. Briefly describe the functions of the following: a. Outer ear b. Middle ear c. Inner ear 5

7 13. Suppose one bird makes a sound of 30 db. a. What would the sound intensity level be from two birds chirping together? b. How many birds are required to have a sound intensity level of 50 db? 14. A steady source of sound is emitting sound waves outwards isotropically. If you measure the intensity of the sound wave as 320 W/m 2 when you are 1 meter away from the source, what is the intensity if you are 2 meters away? 3 meters away? 4 meters away? 6