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.
beat frequency
the number of times per second the the amplitude of a sound wave fluctuates. f2-f1
basic frequency
the average frequency (f1 +f2)/2
doppler effect
a shift in the frequency of a sound wave resulting from the movement of a sound source, the movement of an observer (listener), change in medium, or a combination of these factors.
heard as an increase in the frequency o an approaching sound source followed by a similar decrease in frequency o a departing sound source.
high frequency as it gets close
low frequency as it passes
fo=f [(c+vo)/c]
shock wave
medium deformation during the supersonic motion of a projectile.
Mach number.
the ratio of the speed of an aircraft to the speed of sound
sonic boom
heard as a very loud thunder, thud, or thump noise
if an object travels with the speed of sound, the shock wave travels with the object and a listener located in front of the object will hear nothing until the object arrives.
sound field
any area where sound waves are present
Near field
part of a sound field that is close to the sound source
there is a large variability in pressure produced by the various parts of the vibrating source.
Far field
part of the sound field that is far from the sound source
the nonuniform changes in pressure disappear
decreasing sound pressure follows the inverse square law
the near field to far field boarder is about 1M for a small sound source
indirect field
reverberate sound field
a sound field in which sound energy reflected from space boundaries dominates the sound energy propagating directly from the sound source.
acoustic system
enclosed spaces, filled with medium, in which standing waves occur.
helmholtz resonator
the simplest acoustic system with a single resonance frequency
like a mass on spring
reduces noise in Hvac system
signal-to-noise ratio
SNR
the ratio of the intensity of a signal, such as speech, to the intensity of a noise, such as ambient (background) noise
reverberation and echo
when sounds are reflected form a boundary, a listener receives direct (Do) and reflected (D2) sounds.
acoustic effect
a phenomenon that affects sound waves traveling through a medium including absorption, refraction, reflection, interference, and diffraction.
anechoic chamber
a specialized room with absorbing walls, ceiling, and floor that absorb all the arriving sound (each anechoic chamber has a lowest cutoff frequency below which it cannot be considered truly anechoic)
critical distance
the distance from the sound source at which the intensity of the direct and reflected sound fields are equal.
diffuse sound field
an area filled with sound energy coming from all possible directions ( with equal probability) and in which the sound pressure level is the same at all locations.
direct (sound) field
the sound field closest to a sound source where the sound energy is caused primarily by the sound source itself rather than reflections of the sound off barriers.
dispersion (diffusion)
a sound wave reflection in which sound waves are reflected in many different directions due to a curve and/or rough wall surface.
end correction
an additional length that must be added to the mass of vibration air to determine the resonance frequency o an acoustic system, which has the form of a tube (or other space) with an open end.
free (sound) field
a sound field in which sound wave propagation is not affected by space boundaries or objects; free-field sound pressure decreases 6dB per doubling of distance from the sound source.
incident wave
a sound wave that arrives at a boundary.
reflected (sound) wave
sound wave that has “bounced” off a barrier back into the original medium in the opposite direction but at the same angle as an incidence wave.
refracted (sound) wave
a sound wave that has bent around a boundary or has bent due to changes in the medium properties.
reverberation chamber
closed spaces with highly reflective surfaces that result in multiple strong reflections of sound energy from the space boundaries.
shadow zone
a zone into which sound waves do not enter orin which the sound intensity is significantly decreased. (wind bends sound waves up or down)
Snell’s Law
the relationship between the speed of wave propagation and the angle of refraction.
sound propagation
movement of sound through a medium.
affected by a number of wave phenomena that affect the speed, direction, intensity, and phase of sound waves.
thermal effect
the effect of bending sound waves away from the ground when warmer air is closer to the ground and colder air is above it.
