Chapter 6: Sound Propagation Flashcards
reverberation
multiple reflections of sound energy from the boundaries of an enclosed space.
open space
outdoor sound field with no, or relatively few boundaries.
closed space
indoor space; space enclosed in boundaries.
internal absorption
absorption of sound wave energy by the medium through which the sound wave is traveling.
external absorption
absorption of sound energy y a space boundary.
absorption coefficient
the measure of the effectiveness of a medium and its boundaries in absorbing sound wave energy.
a = I abs/I
a= absorption coefficient
I = the sound intensity arriving at the boundary
I abs = the sound intensity absorbed by this area
sound reflection
when a sound wave reaches a boundary part of its energy is absorbed. the remaining part of sound energy is reflected back into the sound field.
B= I ref/I
B= reflection coefficient
I = the sound intensity at the boundary
I ref = the energy reflected back from the boundary to the space.
a + B = 1
The sum of the absorption coefficient and the reflection coefficient must equal 1.
sound refraction
the bending of sound waves (changes in direction) when the sound enters a medium with a different density or stiffness.
as a sound enters a new medium, the speed of sound changes.
total internal reflection
beyond a certain angel of incidence (about 13 degrees from air to water) the wave will bend so much that it will not enter the barrier.
temperature gradient
a change in temperature with altitude (or depth)
sound refraction that can occur when sound waves travel through a single medium that changes in temperature.
wind shear
wind speed generally increases as altitude increases causing a wind shear phenomena.
reverberation time (RT)
time it takes for a brief sound to decrease in sound pressure by 60 dB.
**depends on volume of room and absorption properties.
RT= .161 v/A
RT=reverberation in seconds
v=volume
A=Total absorption of bounties
sound diffractions
waves bending around objects and through openings in boundaries.
low frequencies bend easier than high frequencies
acoustic shadow
area behind the obstacle that the direct sound wave doesn’t enter or in which the sound intensity is greatly reduced.
sound interference
the combination of two or more propagating sound waves; if overlapping waves of the same wavelength are in-phase, CONSTRUCTIVE INTERFERENCE occurs, and if the two waves are out of phase, DESTRUCTIVE INTERFERENCE occurs,
standing waves
stationary waves
special case of wave interference when sound is bouncing between two reflected parallel surfaces producing nodes and antinodes that are identical in nature to those observed in the mechanical systems (from Ch. 4 strings, rods, plates)
a special case of wave interference in which the wave is stationary and does NOT propagate outward from a space.
caused by reflection , the incident wave hitting the reflected wave and causing this standing wave.
beats
slow periodic amplitude fluctuations caused when two sine waves that are very close in frequency interfere with one another.