chapter 13 - sound 13.1 sound waves. the production of sound waves
TRANSCRIPT
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)
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.
Characteristics of Sound Waves
Frequency determines pitch - the perceived highness or lowness of a sound.
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
Speed of Sound
Propagation of Sound Waves
Sound waves spread out in all directions (in all 3 dimensions)
Such sound waves are approximately spherical
Propagation 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
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
HW Assignment
Section 13-1: Concept Review
Chapter 13 - Sound
13.2 - Sound intensity and resonance
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
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
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
Intensity and Frequency
Human Hearing depends both on frequency and intensity
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
Relative Intensity
•When intensity is multiplied by 10, 10dB are added to the decibel level•10dB increase equates to sound being twice as loud
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
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
The Human Ear
The basilar membrane has different naturalFrequencies at different positions
Chapter 13 - Sound
13.3 - Harmonics
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
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, . . .
Standing Waves on a Vibrating String When a guitar
player presses down on a string at any point, that point becomes a node
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
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
Standing Waves in an Air Column
Only odd harmonics will be present
f n =nv4L
n=1, 3, 5, . . .
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
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
Harmonics account for sound quality, or timbre
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
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
Fundamental Frequency determines pitch
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
Beats