# Chapter 13 - Sound 13.1 Sound Waves.

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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 Also depends on temperature
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 than observer B because of the relative motion of the car The frequency heard by observer A is higher than the frequency heard by observer B

HW Assignment Section 13-1: Concept Review

13.2 - Sound intensity and resonance
Chapter 13 - Sound 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

Sound Intensity Intensity decreases as the distance from the source (r) increases Same amount of energy spread over a larger area

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 natural
Frequencies at different positions

Chapter 13 - Sound 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

Fundamental frequency or first harmonic
The Harmonic Series Fundamental frequency or first harmonic Wavelength is equal to twice the string length Second harmonic Wavelength is equal to the string length

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

Standing Waves in an Air Column
Only odd harmonics will be present

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