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Chapter 13 - Sound 13.1 Sound Waves

Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

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Page 1: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Chapter 13 - Sound

13.1 Sound Waves

Page 2: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

The Production of Sound Waves

Page 3: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

The Production of Sound Waves

Compression: the region of a longitudinal wave in which the density and pressure are greater than normal

Rarefaction: the region of a longitudinal wave in which the density and pressure are less than normal

These compressions and rarefactions expand and spread out in all directions (like ripples in water)

Page 4: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

The Production of Sound Waves

Page 5: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Characteristics of Sound Waves

The average human ear can hear frequencies between 20 and 20,000 Hz.

Below 20Hz are called infrasonic waves Above 20,000 Hz are called ultrasonic waves

– Can produce images (i.e. ultrasound)– f = 10 Mhz, v = 1500m/s, wavelength=v/f = 1.5mm– Reflected sound waves are converted into an

electric signal, which forms an image on a fluorescent screen.

Page 6: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Characteristics of Sound Waves

Frequency determines pitch - the perceived highness or lowness of a sound.

Page 7: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Speed of Sound

Depends on medium– Travels faster through solids, than through gasses. – Depends on the transfer of motion from particle to

another particle.– In Solids, molecules are closer together

Also depends on temperature– At higher temperatures, gas particles collide more

frequently– In liquids and solids, particles are close enough

together that change in speed due to temperature is less noticeable

Page 8: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Speed of Sound

Page 9: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Propagation of Sound Waves

Sound waves spread out in all directions (in all 3 dimensions)

Such sound waves are approximately spherical

Page 10: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Propagation of Sound Waves

Page 11: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

The Doppler Effect

When an ambulance passes with sirens on, the pitch will be higher as it approaches you and lower as it moves away

The frequency is staying the same, but the pitch is changing

Page 12: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

The Doppler Effect

The wave fronts reach observer A more often thanobserver B because of the relative motion of the car

The frequency heard by observer A is higher thanthe frequency heard by observer B

Page 13: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

HW Assignment

Section 13-1: Concept Review

Page 14: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Chapter 13 - Sound

13.2 - Sound intensity and resonance

Page 15: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Sound Intensity

When you play the piano– Hammer strikes wire– Wire vibrates– Causes soundboard

to vibrate– Causes a force on

the air molecules– Kinetic energy is

converted to sound waves

Page 16: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Sound Intensity

Sound intensity is the rate at which energy flows through a unit area of the plane wave– Power is the rate of energy transfer– Intensity can be described in terms of power– SI unit: W/m2

intensity =ΔE / Δtarea

=P

area

intensity =P

4πr 2=

(power)

(4π)(distance from the source)2

Page 17: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Sound Intensity

Intensity decreases as the distance from the source (r) increases

Same amount of energy spread over a larger area

intensity =P

4πr 2=

(power)

(4π)(distance from the source)2

Page 18: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Intensity and Frequency

Human Hearing depends both on frequency and intensity

Page 19: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Relative Intensity

Intensity determines loudness (volume) Volume is not directly proportional to intensity Sensation of loudness is approximately

logarithmic The decibel level is a more direct indication of

loudness as perceived by the human ear– Relative intensity, determined by relating the

intensity of a sound wave to the intensity at the threshold of hearing

Page 20: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Relative Intensity

•When intensity is multiplied by 10, 10dB are added to the decibel level•10dB increase equates to sound being twice as loud

Page 21: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Forced Vibrations

Vibrating strings cause bridge to vibrate

Bridge causes the guitar’s body to vibrate

These forced vibrations are called sympathetic vibrations

Guitar body cause the vibration to be transferred to the air more quickly– Larger surface area

Page 22: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Resonance

In Figure 13.11, if a blue pendulum is set into motion, the others will also move

However, the other blue pendulum will oscillate with a much larger amplitude than the red and green– Because the natural frequency matches the frequency of the

first blue pendulum Every guitar string will vibrate at a certain frequency If a sound is produced with the same frequency as one of

the strings, that string will also vibrate

Page 23: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

The Human Ear

The basilar membrane has different naturalFrequencies at different positions

Page 24: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Chapter 13 - Sound

13.3 - Harmonics

Page 25: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Standing Waves on a Vibrating String Musical instruments, usually consist of many

standing waves together, with different wavelengths and frequencies even though you hear a single pitch

Ends of the string will always be the nodes In the simplest vibration, the center of the

string experiences the most displacement This frequency of this vibration is called the

fundamental frequency

Page 26: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

The Harmonic Series

Fundamental frequency or first harmonicWavelength is equal to twice the string length

Second harmonicWavelength is equal to the string length

fundamental frequency = f 1 =vλ1

=v2L

f n =nv2L

n = 1, 2, 3, . . .

Page 27: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Standing Waves on a Vibrating String When a guitar

player presses down on a string at any point, that point becomes a node

Page 28: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Standing Waves in an Air Column Harmonic series in an organ pipe

depends on whether the reflecting end of the pipe is open or closed.

If open - that end becomes and antinode

If closed - that end becomes a node

Page 29: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Standing waves in an Air Column

f n =nv2L

n=1, 2, 3, . . .

The Fundamental frequency can be changed by changing the vibrating air column

Page 30: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Standing Waves in an Air Column

Only odd harmonics will be present

f n =nv4L

n=1, 3, 5, . . .

Page 31: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Standing Waves in an Air Column Trumpets, saxophones and

clarinets are similar to a pipe closed at one end– Trumpets: Player’s mouth

closes one end– Saxophones and clarinets: reed

closes one end Fundamental frequency formula

does not directly apply to these instruments– Deviations from the cylindrical

shape of a pipe affect the harmonic series

Page 32: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Harmonics account for sound quality, or timbre Each instrument has its own characteristic

mixture of harmonics at varying intensities Tuning fork vibrates only at its fundamental,

resulting in a sine wave Other instruments are more complex because

they consist of many harmonics at different intensities

Page 33: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Harmonics account for sound quality, or timbre

Page 34: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Harmonics account for sound quality, or timbre The mixture of harmonics

produces the characteristic sound of an instrument : timbre

Fuller sound than a tuning fork

Page 35: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Fundamental Frequency determines pitch In musical instruments, the fundamental

frequency determines pitch Other harmonics are sometimes

referred to as overtones An frequency of the thirteenth note is

twice the frequency of the first note

Page 36: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Fundamental Frequency determines pitch

Page 37: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Beats

When two waves differ slightly in frequency, they interfere and the pattern that results is an alternation between loudness and softness - Beat

Out of phase: complete destructive interference

In Phase - complete constructive interference

Page 38: Chapter 13 - Sound 13.1 Sound Waves. The Production of Sound Waves

Beats