# Chapter 15 Sound - PowerPoint PPT Presentation

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Chapter 15 Sound. If a tree falls in the forest and no one is there to hear it, does it make a sound ?. A sound wave is a pressure variation that is transmitted through matter. Sound is a longitudinal and a mechanical wave. Chapter 15 Sound. The speed of sound in air at 0º C is 330 m/s

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Chapter 15 Sound

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### Chapter 15 Sound

If a tree falls in the forest and no one is there to

hear it, does it make a sound?

A sound wave is a pressure variation that is

transmitted through matter.

Sound is a longitudinal and a mechanical wave

### Chapter 15 Sound

The speed of sound in air at 0º C is 330 m/s

and increases at .6 m/s for each degree increase

in temperature.

In general, the speed of sound is faster in solids

and in liquids because the molecules are closer

together.

The frequency and the wavelength are related

by the equation v = f.

### Chapter 15 Sound

Loudness: The loudness of a sound depends

primarily on the amplitude of the pressure wave.

Loudness is measured in decibels.

Frequency: The number of cycles per second.

The human ear can detect frequencies between

20 and 20,000 Hz.

Frequency and loudness are not related.

### Chapter 15 Sound

Find the frequency and the period of a sound

wave moving in air at room temperature (20º C)

with a wavelength of .8 m.

v = f or f = v/

f = (330 m/s + (.6 m/s)(20))/.8 m

f = 342 m/s /.8 m= 427.5 Hz

### Chapter 15 Sound

If a sound is made and hear the echo from a

wall 3 seconds later, how far away is the wall?

The temperature is 30º C.

d = vt

d = (330 m/s + (.6 m/s)(30) ( 1.5 s)

d = (348 m/s)(1.5 s) = 522 m

### Chapter 15 Sound

Doppler Shift: A change in sound frequency

due to the relative motion of either the source

or the detector.

Demonstration

### Chapter 15 Sound

Resonance: When the

frequency of a sound

matches the natural

frequency of an object.

Animation

### Chapter 15 Sound

Raising and lowering the

tube changes the length of

the air column. When the

air column is in resonance

with the tuning fork, the

sound is the loudest,

### Chapter 15 Sound

• Musical Instruments

• String -Vibration of a string resonate a sounding board.

• Brass - Vibration of lips

• Reed - Vibration of a column of air

• Percussion - Vibration of an object, vibrates a column of

• air.

### Chapter 15 Sound

Columns of /4, 3/4, 5/4, … will all be in

resonance with a tuning fork.

Demonstration

### Chapter 15 Sound

Columns of /2, , 3/2, 2, … will all be in

resonance with a tuning fork.

### Chapter 15 Sound

A tuning fork with a frequency of 392 Hz

is found to cause resonances in an air column

Spaced by 44.3 cm. The air temperature is

27º C. Find the velocity of sound in air at that

temperature.

Resonances are spaced by one-half

wavelength so  = 2l

f = 392 Hz

l = 44.3 cm

V = f = f(2l) = (392 Hz)(.866m) = 347 m/s

### Chapter 15 Sound

The frequency of a tuning fork is unknown. A

student uses an air column at 27°C and finds

resonances spaced by 39.2 cm. What is the

frequency of the tuning fork?

Resonances are spaced by one-half

wavelength so  = 2l

v = 347 m/s

l = 39.2 cm

F=v/  = 347m/s/.784 m = 443 Hz

### Chapter 15 Sound

Higher frequencies which are

Odd-number multiples of the

fundamental frequency are

called Harmonics.

### Chapter 15 Sound

The ear consists of three basic parts -the outer ear, the middle ear, and the inner ear.Each part of the ear serves a specific purpose in the task of detecting and interpreting sound.

The outer ear serves to collect and channel sound

to the middle ear. The middle ear serves to transform

the energy of a sound wave into the internal vibrations

of the bone structure of the middle ear and ultimately

transform these vibrations into a compressional wave in

the inner ear. The inner ear serves to transform the energy

of a compressional wave within the inner ear fluid into nerve impulses which can be transmitted to the brain.

### Chapter 15 Sound

Sound Quality

• Chord Several pitches played together.

• Dissonance An unpleasant set of pitches.

• Consonance A pleasant set of pitches.

• Octave The interval between two

• frequencies with a ratio of 2:1

Pythagoras noted that pleasing sounds occurred

when the strings have lengths in small

whole-number ratios, 1:2, 2:3, etc.

### Chapter 15 Sound

Beat Notes When two frequencies

are close together

interfere to

produce high and low sounds.

Fbeat= |fa - fb|

Beats