demos

• Vernier microphone, logger pro, physics with computers, voice program

• measure voice, chilandi plates, tuning forks, anyone with perfect pitch, meter stick?

• Open/closed tubes

• Take data on chilandi plates, compare to sand patterns with wave driver applied.

Thanksgiving Week

• Ch.12 Problem 33a-c, 35

• No labs this week

• Wed. assignment handout today will go over in class on Monday

• Test 2 is in testing center until Wed. Nov. 25 at 5pm

12 Sound

• Homework

• Problem 33a-c, 35

• Sound waves and spectrum

• Sound speed and material parameters

• Sound Intensity Level

• Beats and Doppler Effect

• (omit sections 4, 5, 6, 9)

Sound Waves

• pressure/density waves

• compressions

• rarefactions

• reflects

• refracts (similar to light)

• diffracts (bends around corners)

Speed of Sound

Sound speed increases when stiffness increases.

Sound speed decreases when density increases.

Exs: Aluminum 5100 m/s, Water 1500 m/s, Air 343 m/s

density

stiffnessv

Sound Spectrum

• ultrasonic (f > 20kHz)

• (human) audible (20Hz < f < 20kHz)

• infrasonic (f < 20Hz)

• dogs, cats (50Hz < f < 45kHz)

• bats up to 120kHz

• elephants as low as 5Hz

Sound Intensity Level

2 :amplitude of square to

alproportion isIntensity

AI

hearing of threshold theis W/m10 where

logdB10 Level Sound

212-0

0

I

I

I

Example: Intensity

• point source of sound, 0.010 watts

• I at 10 meters: = power/area

= 0.010watts/(4102m2)= 7.96x10-6 watt/m2.

• = 10log(7.96x10-6/10-12) = 69dB

Beats

21 :sfrequencie sound

in two difference isfrequency Beat

fff

Excel file example of beat frequencies

Beat frequencies are heard up to about 15 Hz

21 fff

Doppler Effect

10

ss

oo f

vv

vvf

source offrequency

source of speed

observer of speed

waveof speed

observedfrequency

s

s

o

o

f

v

v

v

f

Motion of a sound source causes higher frequency of waves on front side (and lower frequency on back side) applet

Motion of an observer toward a sound source causes observer to hear a higher frequency (motion away causes lower frequency)

Using Doppler Equation

11

Hzfvv

vvf s

s

oo 436400

10343

20343

m/s. 343 vSound observer. toward10m/sat moving is

h siren whic 400Hz toward20m/sat drivesObserver

If observer is moving toward source +voIf observer is moving away from source –vo

If source is moving toward observer –vsIf source is moving away from observer +vs

Hzfvv

vvf s

s

oo 411400

10343

20343

observer. fromaway 10m/sat moving is

h siren whic 400Hz toward20m/sat drivesObserver

Summary

• Sound waves and spectrum

• Sound speed and material parameters

• Sound Intensity Level

• Beats

• Doppler Effect

13

Height vs. Time

0

0.5

1

1.5

2

2.5

0 5 10 15 20 25

Time (s)

Y (

m)

values of “A” and “f”?

14.6 Musical Instruments and Sound Characteristics

Standing waves can also exist in tubes or pipes, such as woodwind and brass instruments. Organ pipes are fixed in length; there is one (or more) for each key on the keyboard.

14.6 Musical Instruments and Sound Characteristics

The pitch of woodwind instruments can be varied by covering and uncovering holes in the tube.

Sound PhenomenaIf two sounds are very close in frequency, we perceive them as “beats”—variations in sound intensity.

The beat frequency is the difference of the two frequencies:

14.6 Musical Instruments and Sound Characteristics

In general, the way we perceive sound is related to its physical properties, but depends on other factors as well.

14.6 Musical Instruments and Sound Characteristics

The sum of the fundamental frequency and the overtones gives the final waveform.