AES Section Meeting Reports

Taiwan - August 9, 2018

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The Key technologies: A perfectly integrated system, advanced analog technology, Class G amplifier, adaptive output impedance, compensated phase response

*Basic theories
-Room modes pressure distribution

*Direct sound:
-Enables to identify the sound source, qualitatively as well as locate the origin in space
-Must be as accurate as possible
*Indirect sound:
-Enables to apprehend the environment
a. Sum of all reflected sounds
b. Little impact on direct sound if it is close enough in time (Haas effect)
c. Decreases with each reflection and absorption factor
-Reverberation time: RT60
a. RT60 = 0.16*room volume / equivalent absorption (witha=1)
b. Depends on type of audio message: Church 5 to 10 seconds; Studios 0.2 to 0.5 seconds
c. Must be "similar" over all frequencies for good listening conditions

*Room modes:
-Room dimension = x (wave length/2)
-Standing wave with little diffusion
-Peaks and null always at the same place
-Long reverberation time, long RT60
-Against a (rigid) wall, acoustic pressure is highest and acoustic velocity is lowest
*Room modes are a real pain:
-Create peaks and nulls in frequency response
-Linger on longer than other frequencies in the room (long RT60)
-Mask higher frequencies (muddy masking effect)
-Very difficult to control (generally between 20 and 150 Hz)
-Cause errors for the sound engineer

*How to control reflection to reach adequate RT60
-Sound effect mechanism:
a. Against a rigid wall: acoustic velocity = 0
b. Kinetic energy from acoustic velocity is transformed into potential energy (pressure), created high pressure and acceleration in opposite direction: reflexion
-3 solutions to avoid/reduce reflections:
a. Modify wall geometry
b. Slow down acoustic velocity in front of wall
c. Let the wall move to dampen/absorb acoustic velocity
*Slow down acoustic velocity
-Acoustic absorption through porous material:
a. Transform velocity into heat through friction
b. Important to slow down velocity where it is highest
c. A thickness of porous material equivalent to 1/4 of wave length is needed
*Let the wall (or part of it) move
-Dampen the acoustic velocity:
a. The wall (or part) moves to dampen or absorb the acoustic velocity
b. A resonator will move easier on frequencies that correspond to its own natural mode Membranes, Helmholtz, etc.
-Make the wall (or part of it) move very accurately without emitting any sound

*How does the AVAA(Active Velocity Acoustic Absorber) work?
-The AVAA imposes a low acoustic impedance on its surface (between 15-150Hz)
-It sucks in low frequencies over 1 to 1.5m
-The equivalent absorption area is up to 25 times the surface of the AVAA
*Tests and results
-Time: Decay time is reduced and more even
-Frequency: Frequency is more even with less "peaks" and "nulls"
-Space: Sound source location is more accurate (less humming)

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