LAST: intensity and loudness. READ: 6.3, (6.4,5 optional),

Transcription

1 Phys 1240: Sound and Music LAST: intensity and loudness TODAY: decibels NEXT: decibels, ears, and hearing READ: 6.3, (6.4,5 optional), Coming up: 6.6 (mostly, try to make sense of Fig 6.12!) CAPA 5 due Thurs

2 intensity Intensity is Energy / (time * area) I = power / area Intensity is how we describe the "strength" of sound waves. It's a property of the wave (like its wavelength, or frequency, or amplitude) It's associated with "loudness" (sort of like frequency <=> pitch)

3 You and I are standing next to each other, listening to the exact same sound. Your ears are TWICE as big as mine We listen for the same amount of time. How does the total energy received compare? A) You receive twice what I do B) You receive half what I do C) We receive the same

4 b You and I are standing next to each other, listening to the exact same sound. Your ears are TWICE as big as mine We listen for the same amount of time. How does the power received compare? A)You receive twice what I do B)You receive half what I do C)We receive the same

5 c You and I are standing next to each other, listening to the exact same sound. Your ears are TWICE as big as mine We listen for the same amount of time. How does the intensity received compare? A)You receive twice what I do B)You receive half what I do C)We receive the same

6 old You and I are standing next to each other, listening to the exact same sound. Our ears are identical. You listen for TWICE as long as I do How does the intensity received compare? A)You receive twice what I do B)You receive half what I do C)We receive the same Power? Energy?

7 Reminder from last time (thinking about mass on spring) For any oscillation, energy ~ (amplitude) 2 => a pressure wave with just 3 times more amplitude will carry 9 times more energy! (9 times more energy passes a given area in a given time) So - Intensity grows like (amplitude) 2 too!

8 recall Amplitude is overpressure => extra force/area

9 Flute #1 generates a sound with maximum overpressure 2 N/m 2 Flute #2 makes a sound with maximum overpressure 6 N/m 2 How does the intensity of the sounds compare? I(#2) / I(#1) =. A) 3 B) 6 C) 9 D) 12 E) 36

10 Ears aren t linear If you DOUBLE the intensity hitting your ear (=> twice the power, energy every second passing through your eardrum) It does NOT seem twice as loud (!?) You need a LOT more power (or energy/sec) to feel like it s louder

11 Practice with powers of 10 What s 1,000 * 10-12? A) 1E-11 B) 1E-10 C) 1E-9 D) 1E-8 E) Something else/???

12 Practice with powers of 10 What s 1,000 * 10-12? A) 1E-11 B) 1E-10 C) 1E-9 = 1 *10-9 = 10-9 D) 1E-8 E) Something else/???

13 Listen to a VERY quiet sound, the softest a person can perceive. Intensity = I 0 =10-12 W/m 2 Crank it up. 10 times more intensity! Now at 10 I 0 Seems some amount louder. (Not 10 times! Just a bit louder) We say the sound intensity level (the SIL ) is 10 decibels (db) higher. (ADDED 10 db)

14 SIL = Sound Intensity Level We say I 0 has an SIL of 0 db. Previous slide argued sound with intensity 10 I 0 has an SIL of 10 db Now crank it up 10 times more! We re now at I=10*10 I 0 = 100 I 0 What s the SIL? This ADDED another 10 db. (It added the SAME increment) We re up to an SIL of 20 db

15 MULTIPLY the intensity by 10, => ADD another 10 db to SIL. Intensity can go WAY up, while SIL goes up just a little

16 Decibel levels SIL 0 db Intensity I 0 (10-12 W/m 2 )

17 Decibel levels SIL 0 db 10 db Intensity I 0 (10-12 W/m 2 ) 10 I 0 (10-11 W/m 2 )

18 Decibel levels SIL 0 db 10 db 20 db Intensity I 0 (10-12 W/m 2 ) 10 I 0 (10-11 W/m 2 ) 100 I 0 (10-10 W/m 2 )

19 Decibel levels SIL 0 db 10 db 20 db 30 db Intensity I 0 (10-12 W/m 2 ) 10 I 0 (10-11 W/m 2 ) 100 I 0 (10-10 W/m 2 ) 1,000 I 0 (=10 3 I 0 ) (10-9 W/m 2 )

20 SIL 0 db 10 db 20 db 30 db?? Intensity I 0 (10-12 W/m 2 ) 10 I 0 (10-11 W/m 2 ) 100 I 0 (10-10 W/m 2 ) 1,000 I 0 (=10 3 I 0 ) (10-9 W/m 2 ) 100,000 I 0 (=10 5 I 0 ) (10-7 W/m 2 )

21 SIL 0 db 10 db 20 db 30 db 50 db Intensity I 0 (10-12 W/m 2 ) 10 I 0 (10-11 W/m 2 ) 100 I 0 (10-10 W/m 2 ) 1,000 I 0 (=10 3 I 0 ) (10-9 W/m 2 ) 100,000 I 0 (=10 5 I 0 ) (10-7 W/m 2 )

22 SIL 0 db 10 db 20 db 30 db 50 db 100 db Intensity I 0 (10-12 W/m 2 ) 10 I 0 (10-11 W/m 2 ) 100 I 0 (10-10 W/m 2 ) 1,000 I 0 (10-9 W/m 2 ) 100,000 I 0 (=10 5 I 0 ) (10-7 W/m 2 )?? W/m 2

23 SIL 0 db 10 db 20 db 30 db 50 db Intensity I 0 (10-12 W/m 2 ) 10 I 0 (10-11 W/m 2 ) 100 I 0 (10-10 W/m 2 ) 1,000 I 0 (10-9 W/m 2 ) 100,000 I 0 (10-7 W/m 2 ) 100 db I 0 (10-2 W/m 2 )

24 Textbook says: 90 db => Subway train 100 db => Machine shop 110 db => Rock concert 120 db => Jet engine 500 yards away (Painful) 150 db => unbearable

25 Decibels as "relative SIL s" Difference in SIL 0 db 10 db 20 db 30 db Ratio of intensities , db 100,000 = db 10 10

26 5.2.3 There are two instruments, a trumpet and a flute. The trumpet produces a sound with10x more intensity than that of the flute. The decibel level for the trumpet is compared to the flute. a) 10 db greater b) 10 db less c) 1 db greater d) 1 db less e) Not enough information

27 5.2.5 If a sound is 0 db, is any energy transferred? a) Yes b) No c)??

28 5.2.6 At a concert, the trumpet section is playing 30 db louder than the French horn section. The trumpet section is playing with of the French horn section. a) 3x the intensity b) 10x the intensity c) 30x the intensity d) 1000x the intensity e) x the intensity

29 5.2.4 What if the trumpet is 20x more intense than the flute. Is the decibel level a) Less than 10 db higher b) 10 db higher c) Between 10 and 20 db higher d) 20 db higher e) More than 20 db higher

30 Diff in SIL 0 db Intensity ratio 1 1 db 2 db 3 db 10 db 10 (etc - see Table 5.1 in text)

31 Diff in SIL 0 db 1 db 2 db 3 db Intensity ratio (call it 1.3) db 10 (etc - see Table 5.1 in text)

32 5.2.4b A trumpet is 20x more intense than the flute. The decibel level is A) 3 db higher B) 12 db higher C) 13 db higher D) 20 db higher E) 23 db higher

33 5.2.4c A trumpet is 22 db more than the flute. The ratio of intensities, I(trumpet)/I(flute), is A) 16 B) 22 C) about 30 D) 160 E) 200

34 5.2.4d A violin is playing at 60 db. If you double the intensity of sound (e.g. by adding a second violin!) what happens to the SIL? A) Stays the same, still 60 db B) Goes to 62 db C) Goes to 63 db D) Goes to 80 db E) Goes to 120 db

35 5.3.1 There is a source in the center emitting sound outward. How does intensity of the sound at r compare to that at R? R r a) I (at r) > I R b) I (at r) < I R c) I (at r) = I R d) Not enough information

36 5.3.1 How does the total power (Energy/second) passing through the whole spherical surface at r compare to that at R? A) P r >P R B) P r <P R C) P r =P R D)?? R r 1 cm 1 cm How does the power passing through a patch of 1 cm 2 at radius r compare to that at R?

37 5.3.1 How does the total power (Energy/second) passing through the whole spherical surface at r compare to that at R? A) P r >P R B) P r <P R C) P r =P R D)?? How does the power passing through a patch of 1 cm 2 at radius r compare to that at R?

38 The area of a sphere grows like (Radius)^2 Intensity = Power / area So, intensity of sound decreases like 1/(Radius)^2.

39 5.3.1 If R=2r, what is I R /I r? a) 1 b) 2 c) 4 d).5 e).25

40 5.4.1 True or False: A background noise with a sound level of 100 db would be distracting? (what could produce this sound?) a)true b)false

41 5.4.2 Rank the following sound levels from lowest to highest: library, football game (stadium), live rock music, alarm clock. a)library, football game, live rock music, alarm clock b)library, alarm clock, football game, live rock music c)library, alarm clock, live rock music, football game d)alarm clock, library, live rock music, football game e)alarm clock, library, football game, live rock music

42 5.4.3 What do you think is the approximate sound level of thunder? a)120 db b)80 db c)50 db d)0 db e)this is immeasurable

43 5.5.1 When 2 totally independent sounds of different frequencies are combined, what should be summed to determine the resulting sound level? A. Frequency B. Amplitude C. Power D. Intensity E. Something else/none of these What if the 2 sounds are at the same Frequency, and they are in phase?