Sound

Review Natural frequency: –The frequency or frequencies at which an object tends to vibrate What is an example of a sounding board and how does it work? Recall the activity from yesterday with the water in the glasses. What happened to the pitch of the tapping sound as the glass becomes fuller with water? –The pitch decreases (becomes lower). Why? –The glass and the water are vibrating more slowly (lower frequency) --> producing a lower note

Objectives 1.Describe resonance 2.Interference of sound waves 3.Describe beats

Natural Frequency Drop a penny and a metal rod on the floor. What do you notice? Different natural frequencies: –Vibrating of an object at it’s unique set of frequencies –this forms its special sound The natural frequency of the smaller bell is higher than that of the big bell and rings at a higher pitch.

Resonance Resonance: –Frequency of a forced vibration of an object matches the the object’s natural frequency --> increase in amplitude Examples: –Swing/pendulum –Loose end of a car –Tuning forks –Pipe and heat (demo)

Resonance with Tuning Forks and your Radio (Demo)

Resonance with Bridge Marching troops Let’s watch a dvd clip looking at resonance between a bridge and a cat??

Breaking Glass!!

Interference Can sound cancel sound? Yes! Sound waves can be made to interfere (remember constructive and destructive?) Which are which in the above diagram? Constructive: crest overlaps crest Destructive: crest overlaps trough Crest = compression Trough = rarefaction

Interference & the Loudness of Sounds Equally distant from 2 speakers (fig. a) compressions & rarefactions in step sound is louder Move to side (fig. b) paths from speakers differ out of phase Not as loud

Destructive interference usually not a problem b/c reflection aids in filling in cancelled spots Can be a problem in poorly designed theaters or gymnasiums.

Preventing Destructive Interference Speakers produce compressions in front and rarefactions in back. Do we remember what this means? If sound reaches our ears from front + back --> out of phase. What happens? It will be canceled. Speakers have boxes Forward air travels out of box, backward air stays in

Beats Beats: –periodic variation in the loudness of sound Tuning forks Walking with friend Tuning a guitar Humming into a fan

Figuring Out Beats If a tuning fork vibrates 264 times per sec and a different fork vibrates at 262 times per second, they are in step twice each second. How do we figure that out?  = 2 What is the beat frequency when a 262 Hz and a 266 Hz tuning fork are sounded together? A 262 Hz and a 272 Hz?