Upload
tracey-hart
View
225
Download
3
Embed Size (px)
Citation preview
ACOUSTICS
Sound in a Medium Sound Wave Phenomena Sound Fields Earphones Resonance and Standing Waves
Sound in a Medium
Vibrating object displaces molecules in medium
molecules move back and forth
“bump” into others transmitting vibration thru medium
In the Medium:
We have both OSCILLATION of particles and TRANSMISSION of energy (or
propagation)
Particle Motion
In Air, in line with transmission--LONGITUDINAL
On Water, perpendicular to transmission--TRANSVERSE
Displacement of Molecules in the Medium creates areas of more molecules --increased density--CONDENSATION and areas of fewer molecules --decreased density--RAREFACTION
Because We have Transmission:
We can talk about how fast sound travels in the medium
= SPEED OF SOUND or c Depends on medium, temperature, density,
state
In Air = 344 meters/sec or 1100 feet/sec
Sound Travels Out From the Source• In All Directions• (at the same speed)• So, Until Sound Encounters some object, • the “wavefront” is spherical
We Can Also Talk About:
Distance Traveled during each cycle = WAVELENGTH
= c/f
• Wavelength = speed of sound / frequency
Wavelength Questions:
• What is the wavelength in meters of a 1720 Hz sound traveling in air?
• What is the wavelength in meters of an 86 Hz sound traveling in air?
Question 1:
• Freq = 1720 cyc/sec, c = 344 m/sec• wavelength = c/f
• =344m/sec /1720 cyc/sec
• =0.2 m/cyc
Question 2:
• Freq = 86 cyc/sec, c = 344 m/secwavelength = c/f
= 344m/sec /86 cyc/sec
= 4 m/cyc
EXAMPLE OF SOUND WAVES
http://rustam.uwp.edu/GWWM/sound_waves.html
When Talking about Amplitude:
• Remember Power is Rate at which Work is done
(Work /Time = Power)• But the power in sound doesn’t all travel
the same direction• Only some of it reaches you.
Therefore, we are more interested in:
• How much Sound Power there is in a given area
• (e.g., the opening of ear canal, microphone)
• New term: INTENSITY = Power/Area
Remember :
• Sound Power is spread over the Wavefront• So the farther you are from the sound
source:• the larger the area over which power is
spread• the smaller the intensity
Intuitively, we all know this
• The closer you are, the louder the sound
• The farther away you are, the softer the sound
The Physics of the Situation:
• The relation between distance and intensity is an example of
• THE INVERSE SQUARE LAW
• Intensity = 1/distance2
WHY?
• Surface area of sphere = 4 Pi r2
• In this case r = distance• The area is proportional to distance squared
Change in Intensity
= old d2 / new d2
EXAMPLE:
Moving from 100 m to 200 m away from source
Delta I = 100 2/200 2
= 1 x 104/4 x 104
• = 1/4• =0.25
Sound Wave Phenomena
• Reflection-bouncing off an object• Absorption-sound trapped (absorbed) by
an object• Diffraction-spreading of sound into area
beyond an object• Refraction-bending of sound waves in a
medium
Sound Encountering an Object:
• Transmission-setting object into vibration• Reflection-sound bounces back• Absorption-sound becomes trapped in gaps
of surface of object
Reflected and Incident Sound Meet
• Producing INTERFERENCE
• Where the two waves meet in phase, the intensity doubles --Constructive Interference
• Where they meet out of phase, cancellation
--Destructive Interference
Getting around an Object:
depends on size of object and wavelength of sound
when > object’s diameter, sound passes by
when < object’s diameter, sound blocked Area of reduced or no sound energy is
“sound shadow”
Diffraction
• Sound passing an object will spread to fill in area beyond it.
Refraction
• the bending of the sound’s path produced by changes in medium
• e.g., temperature changes will bend path of sound propagation
Sound Fields
• FREE FIELD = no objects in medium
• ANECHOIC CHAMBER = room with highly absorptive walls; an attempt to create a free field.
Sound Fields (cont’d)
• SOUND TREATED ROOM = has somewhat absorptive walls, produces some reflections
• REVERBERATION ROOM = highly reflective walls set at odd angles; many reflections and complex interactions. Creates a uniform (diffuse) sound field.
Reverberation:
• Persistence of sound in a sound field after the source is turned off
• = time taken for intensity to drop to 1 millionth of initial value
• Reverberation
ROOM VOL./ABSORPTION COEF.
Reverberation Time
• Least for Anechoic Chamber• Most for Reverberation Room
• Longer for larger rooms with reflective walls
Earphones
• Miniature loudspeakers to introduce sound into the ear.
• Supra-aural (sits on the pinna)• Insert (sits within external canal)• Calibrated in “artificial ears” (6cc or 2cc
couplers)
Resonance
• Helmholtz Resonators simulate influence of mass and compliance (stiffness) on resonance. Tube and Cavity.
• Mass component--inversely proportional to resonant freq
• Compliance component--directly prop. to resonant freq
• Resistance -- doesn’t affect resonant freq, but produces broader tuning
Standing Waves
• Interaction between incident and reflected waves
• Produces areas of :• constructive interf. --ANTINODE• destructive interf. --NODE
Standing Wave Illustration
Standing Waves (cont’d)
• Intensity varies with position• Position of nodes, antinodes depends on
frequency
Pipes produce standing waves
closed pipes —antinode at open end and node at closed end
open pipes — antinode at each open end closed pipe, length = ¼ l open pipe, length = ½ l
Standing Waves in Pipes
A closed pipe only produces odd harmonics. Frequency of harmonics = (n c)/4 L, Where n=1, 3, 5, ... c = speed of sound L is the length of the pipe.
In music, harmonics are called overtones.