SoundChapter 13

Sounds from Saturn

https://files.oakland.edu/users/blswartz/web/Images/saturn.jpg

But wait…

“But Denzmore, I thought sound couldn’t travel through space”

Brief explanation: Molecules transmit sound waves and although interstellar

space isn’t very dense it isn’t a complete vacuum

Light waves don’t need a medium and we can sometimes “convert” them to sound waves

http://www.space.com/scienceastronomy/mystery_monday_030922.html

More eerie space sounds for you http://www-pw.physics.uiowa.edu/space-au

dio/

http://www.nasa.gov/vision/universe/features/halloween_sounds.html

What is a sound wave? Sound waves are longitudinal

(compressional) waves produced by vibrating objects Air molecules move parallel to the wave motion

Longitudinal (Compressional) Wave

Troughs: Areas of Low Density becauseThe coils are stretched

Crests: Regions of High Density becauseThe coils are compressed

Longitudinal (Compressional) Wave

Compression: The region of the longitudinal wave where the density is highest

Rarefaction: The region of the longitudinal wave where the density is lowest

Audible Sound Waves

For humans, audible sound waves have frequencies between 20 and 20,000 Hz Infrasonic Waves: Frequencies below 20 Hz

Ultrasonic Waves: Frequencies above 20,000 Hz

Uses for Ultrasonic Waves Ultrasonic waves have short wavelengths

and consequently are easily reflected off small objects That’s how ultrasounds are produced!

Frequency and Pitch

Pitch: How high or low we perceive sound to be

The frequency of a sound wave determines the pitch High frequency = High Pitch Low frequency= Low Pitch

http://phet.colorado.edu/simulations/sims.php?sim=Sound

The Speed of Sound The speed of sound depends on the

medium and the temperature of the medium

Solid- Fastest speed of sound Gas- Slowest speed of sound

Higher temps mean the speed is higher

Speed of Sound in Various Media(p.482)

Shape of a sound wave Sound waves travel away from a vibrating

source in all three dimensions Therefore sound waves are spherical waves

http://wpcontent.answers.com/wikipedia/commons/1/12/Spherical_wave2.gif

Spherical Waves

Wave front: Center of compression

Ray: Lines Perpendicular to the wave fronts that indicate the direction of motion

What is The Doppler Effect? http://www.colorado.edu/physics/2000/appl

ets/doppler2.html

Explaining the Doppler Effect Relative motion between a source and an

observer creates a change in frequency

Since pitch depends on frequency, relative motion creates a change in pitch

Illustration of the Doppler Effect

Explaining the Illustration

The car is moving toward person A. Although the frequency of the horn is staying the same, since the car is moving toward person A, the wave fronts “bunch together”. The wavelength decreases, the frequency increases and that increases the pitch.

Doppler Effect for Light

M33: Which way is it rotating?

Sound Intensity (p.487) As sound waves travel, energy is

transferred from one air molecule to the next.

The rate at which energy is transferred through a unit of area is called INTENSITY

Equation for Intensity (p. 487) Intensity of a spherical wave:

Intensity (measured in Watts/m2) decreases as distance from source increases

22 4source) from distance(4 r

PPowerIntensity

Sample Problem p. 488 #4 How much power is radiated as sound from

a band whose intensity is 1.6 x 10-3 W/m2 at a distance of 15 m?

Solve the problem Given: I= 1.6 x 10-3 W/m2

r= 15 m

Watts52.4))15(4)(106.1(4 22

32 mm

WxrIP

24 r

PI

Intensity and Frequency (p.489)

Intensity and frequency determine which sounds are audible.

Sounds below 50 Hz and above 12,000 Hz need to be relatively intense to be heard

Graph on p. 489 of your book

Decibel Level (p. 490) Decibel Level (measured in dB; also known

as relative intensity): Found by relating the intensity of a given sound to the intensity at the threshold of hearing. A difference in 10 dB means the sound is approx

twice as large. i.e. a 30 dB sound is twice as large as a 20 dB

sound

Intensity to Decibel Level p. 490 When the intensity of a

sound is multiplied by 10, the decibel level increases by 10

Sample Problem (not in book) When the decibel level of a crowded room

rises from 20 dB to 40 dB, how much louder does the noise seem? If the original intensity of the noise was 1.0 x 10-10 W/m2, what is the new intensity?

Resonance (pp. 491 – 492) Resonance: the tendency of a system to oscillate

at maximum amplitude at certain frequencies, known as the system's resonance frequencies

In other words, resonance occurs when the frequency of a force applied to a system matches the natural frequency of the vibration of the system

Tacoma Narrows Bridge