ap physics b - waves and sound

7/29/2019 AP Physics B - Waves and Sound

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Waves and Sound

AP Physics B


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What is a wave

A WAVE is a vibration or disturbance in space.

A MEDIUM is the substance

that all SOUND WAVES travelthrough and need to have inorder to move.


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Two types of WavesThe first type of wave is called Longitudinal.

Longitudinal Wave- A fixed point will move parallel with the wave motion

2 areas

Compression- an area of high molecular density and pressureRarefaction - an area of low molecular density and pressure


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Two types of WavesThe second type of wave is called Transverse.

Transverse Wave- A fixed point will move perpendicular with the wave motion.

Wave parts(recall demo for simple harmonic motion )- crest,trough, wavelength, amplitude, frequency, period


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Wave Speed

You can find the speed of a wave by multiplying the waveswavelength in meters by the frequency (cycles per second). Since a

cycle is not a standard unit this gives you meters/second.


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ExampleA harmonic wave is traveling along a rope. It is observed that theoscillator that generates the wave completes 40.0 vibrationsin 30.0 s. Also, a given maximum travels 425 cm along a rope

in 10.0 s . What is the wavelength?

===cycles

f30

40

sec1.33 Hz

===

==

=

f

vfv

t

xv

wavewave

10

25.4

0.319 m

0.425 m/s


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Standing WavesA standing wave isproduced when a wave

that is traveling isreflected back upon itself.There are two main parts

Antinodes Areas ofMAXIMUM AMPLITUDE

Nodes Areas of ZEROAMPLITUDE.


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Sound WavesSound Waves are a common type of standing wave as theyare caused by RESONANCE.

Resonance when a FORCED vibration matches anobjects natural frequency thus producing vibration,sound, or even damage.

One example of this involves shattering awine glass by hitting a musical note that ison the same frequency as the naturalfrequency of the glass. (Natural frequencydepends on the size, shape, and

composition of the object in question.)Because the frequencies resonate, or arein sync with one another, maximum energytransfer is possible.


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Sound WavesThe production of sound involves setting up a wave in air. To set upa CONTINUOUS sound you will need to set a standing wavepattern.

Three LARGE CLASSES of instruments

Stringed - standing wave is set up in a tightly stretched string

Percussion - standing wave is produced by the vibration of solidobjects

-

OPEN or CLOSED

Factors that influence the speed of sound are density of solids orliquid, and TEMPERATURE


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Closed PipesHave an antinode at one end and a node at the other. Each soundyou hear will occur when an antinodeappears at the top of thepipe. What is the SMALLEST length of pipe you can have tohear a sound?

You get your first sound or encounteryour first antinode when the length ofthe actual pipe is equal to a quarter of awavelen th.

This FIRST SOUND is called theFUNDAMENTAL FREQUENCY or theFIRST HARMONIC.


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Closed Pipes - HarmonicsHarmonics areMULTIPLES of thefundamental frequency.

In a closed pipe, you have a NODE atthe 2nd harmonic position, therefore NO

SOUND is produced


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Closed Pipes - HarmonicsIn a closed pipe you have an ANTINODE at the 3rdharmonic position, therefore SOUND isproduced.

CONCLUSION: Sounds in CLOSED pipes areproduced ONLY at ODD HARMONICS!


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Open PipesOPEN PIPES- have an antinode on BOTH ends of thetube. What is the SMALLEST length of pipe youcan have to hear a sound?

You will get your FIRST sound when the

length of the pipe equals one-half of awavelength.


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Open Pipes - HarmonicsSince harmonics are MULTIPLES of the

fundamental, the second harmonic of an

open pipe will be ONE WAVELENGTH.

The picture above is the SECOND harmonic or the FIRST OVERTONE.


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Open pipes - HarmonicsAnother half of a wavelength would ALSO produce an

antinode on BOTH ends. In fact, no matter howmany halves you add you will always have an

antinode on the ends

The picture above is the THIRD harmonic or the SECONDOVERTONE.

CONCLUSION: Sounds in OPEN pipes are produced at ALLHARMONICS!


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ExampleThe speed of sound waves in air is found to be 340

m/s. Determine the fundamental frequency (1stharmonic) of an open-end air column which has a

length of 67.5 cm.

= lfv 2

=

=

f

f)675.0(2340

251.85 HZ


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ExampleThe windpipe of a typical whooping crane is about

1.525-m long. What is the lowest resonantfrequency of this pipe assuming it is a pipe closed

at one end? Assume a temperature of 37C.

=+331376.0 353.2 m/s

=

=

=

f

fv

lfv

)525.1(4

4

57.90 Hz

ap physics b - waves and sound

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