L05 & 06 Flashcards
How is the trace matching at walls of bending waves described mathematically?
k_B = k sin( Theta_i)
It determines the wavennumber of the bended waves.
How do Bubble curtains change the speed of sound in water?
Due to the air volume fraction factor Beta it increases the impedance of the media drastically so that the speed of sound is decreased significantly.
Enclosed Spaces:
Which phenomenon appears when a diffuse isotropic sound field is applied to an enclosed space?
The sound field is distributed equally at each location in the room, and the direction of the sound source is not hearable anymore.
In reality, sound fields are anisotropic and depend on the inventory, media and measures of the room.
Which measure is important to take into account when optimizing the sound quality in enclosed spaces? (Flutter echo)
The Reverberation time after Sabine’s reverberation formula:
Time after turning off the src until the sound level decays by 60dB (not hearable in most of the cases)
Depends on the mean absorption factor alpha:
If alpha «_space;1 : on air holds after Sabine reverberation formula:
T60 = -0.163 s/m * V/(alpha_mean*A)
A higher reverberation time increases the echo in rooms.
In case the Area is increased we get less Reverberation due to more reflection and absorption.
In case the Room volume is increased the Reverberation is decreased due to less reflection.
Reverberation is shown in an Echogram: p(t) increases (direct sound - reflection time tau - early reflections - Reverberant tail)
What do we have to take into account when determining positions in rooms with clear sound reception?
The critical radius from the source:
rc = mean absorption * Area / 16pi
Within that radius, the rms value of sound pressure is low and the sound source is perceived as localized and focused.
Beyond that radius, the source is harder to localize and we get morw hall.
What does the Dispersion relation say?
The wave vector can be spatially distributed into three dimensions.
The pressure is described then by the 1/8 th of the sum of all wave Amplitudes in each direction
(A* exp( wlmnt - kxll - kym*m - kzn * n))
- Why do we have to calculated the Eigenfrequencies in a room to optimize the sound characteristics?
- How do we do that?
- By the wave equation in 3D
1/c^2 * 2nd deriv.(p) in t =
2nd der in space( p )
And the Helmholtz equation derived by it - The eigenmodes are oscillations at the natural frequencies a system can follow with when exposed to a frequency
Sound propagation:
What is the shallow water Sound channel?
Its the Area in the sea (appr. 1km from the coast) where the sound pressure is much more attenuated than in the deep ocean.
When emitting a pulse in the deep ocean the reflections of the sound are much longer measurable.
What is the Sound speed Profil in water?
It visualizes the speed of sound depending on the depth.
It contains the
surface layer (1500m/s)
-> lower pressure, higher temperature
seasonal thermocline (500m, 1490m/s) &
Main thermocline (1000m, 1470 m/s)
-> pressure higher, but temperature as well
And the deep isothermal layer above.
-> pressure higher, temperature const.
Horizontal changes in c appear only with splash and other when rivers end.
How is the transmission loss of a source calculated?
For spherical waves?
TL = 10log_10(I(ro)/I(r)) or
20log_10(p(ro)/p(r)) 1mikroPa in water (20in air)
Extrapolation from the far field to near field (r»ro)
Spherical waves:
Geometrical spreading:
TL = 20log_10(r/ro)
How is the spreading when c is constant?
Rays (paths perp. to wavefronts) propagate in a multipath scenario straight,
Are reflected @ surfaces (sea bottom, water surface)
What’s the spreading characteristics with the sound speed profile in water?
The Sound speed change leads to a quasi continuous change of the wave vector k.
The Sound rays are bended and lead to a cylindrical spreading (horizontally).
Sound channels like the SOFAR Channel appear. The Acoustical Energy is bounded mostly and so the efficiency of sound propagation in that layer is increased.
How does the Time-of-Arrival Estimation in TL - Experiment look for transmission in shallow waters?
It’s done by using a matched filter to estimate the desired signal and efficiently cancel out noise:
The amplitudes of sound pressure are measured, filtered and visualized in an r-t plot:
r(m) up to 11km on y axis
delta t on x
HFM pulse (T= 1s) @ 1-4kHz emitted
-> pressure peak travels in t&r
What does the shallow water sound channel estimate?
How is the Transmission loss contributed?
TL = 10log_10(r/r0) (spherical)
- 10log_10(H/pi)
-> cylindrical spreading, more efficient than spherical propagation
-> even more efficient for lower frequencies
How does the absorption coefficient behave with increased frequency?
It increases linear by f^2.
We have as well the viscosity influence and the influence of impurities like boric acid and magnesium sulfate (plus water) but at same temperature, Salinity and depth it increases with f^2.
