Ch. 17: waves (sound)

Outline for 4/20:

• Announcements• Nature of sound• demos• Speed of sound• Interference of sound waves• HITT quiz

Announcements

• Final exam Sat, April 28. Covers all material,slight emphasis on Chs. 13,15-17(10:00am to noon)

Rooms: Carlton 100: Aa-MaFlint Hall 50: Mc-PoMatherly 0018: Ra-RyWeil 270: Sa-Vo

McCarty B G086: Wa-Zz

Reviews: Hirschfeld/Korytov NPB1001 Apr 27 2nd-3rd periods Kahya NPB 2165 Apr 24 7pm-9pmBomstad NPB 1101 Apr 26 2nd-3rd periodsWhiting NPB 2205 Apr 25 7pm-Shankar NPB2205 Apr 26 7pm -

Announcements cont’d

• Final WebAssign HW12 due Tues, Apr. 24 at 11pm.

Sound

Sound as pressure wave

Speed of sound

HRW derives

Relation between pressure wave and density wave

Interference of 2 sound waves

Problem 1

Sound intensity

Intensity scale - decibels

HITT Quiz

A B

Fiendish IFAS scientists expose a helpless alligator to pure sine wave sound coming from two speakers each 5 m from the alligator.Speaker A emits wave which are 180 degrees out of phase with speakerB, each with source power 250 W. What intensity does the alligator hear, in W/m2? (Note: this is a “point alligator”).

A. 10/p B. 20/p C. 50/p D. 100/p E. 0

HITT quiz soln

Ans: E. Since the sources are 180 degrees out of phase, thesound waves arriving at the alligator cancel. The alligatoris then at a node of the standing wave, and intensity is zero.