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Low Frequency Damping

Page history last edited by Randy Coppinger 12 years, 8 months ago

These are my notes from the presentation.  If anything here is confusing, inspiring or absolutely incorrect your comments would be much appreciated.  Thanks!


T9 - Damping of the Room Low-Frequency Acoustics

Reza Kashani


Wave propagation is a function of time and space.


Irregularly shaped rooms are evaluated using Finite Element Analysis tools.


Large obstacles (couch, rack, desk) in a room actually change the frequency of the room modes.


Walls that vibrate (ie - drywall) will create a pair of modes above and below what would be present for a wall that does not vibrate.


Standing waves linger past the end of the original sound, a la reverb. My take away: Trying to control acoustical room response w/ EQ ignores the time delay ("after ring") of the resonance. Treat acoustics not signal!


The microphone always picks up a combination of the sound source and the reflections off the boundaries. If you can reduce the reflections you can get more of the original sound source: acoustic damping.


Right at the wall surface you have maximum potential energy and minimum kinetic energy. Sound absorbing materials remove kinetic energy. [This is why mounting away from a wall spreads the absorption lower].


Acoustic velocity at the surface is zero and maximum at ¼ wavelength from the wall - middle of the room for the first mode.


Reactive absorbers are damping mechanisms that convert potential energy (highest at the surface of the wall) to kinetic energy (velocity) and then dissipate that kinetic energy.


Helmholtz Resonator is, mechanically speaking, a flat surface on a spring. The neck (the shaft inside the hole) resonates, which converts the potential energy into kinetic energy. Absorption inside should be just next to the neck, not actually in the neck, for maximum effectiveness.


Cavity Backed Perforated Panels may be thought of as a collection of Helmholtz Resonators. They are used to quiet jet engines!


Panel absorbers – panels/plates that resonate (they are tuned) convert pressure to velocity and then absorb inside the cavity. Problem: these surfaces have more than one mode!


The tuning frequency of two Helmholtz resonators can be the same, but their effectiveness may not be shared if there is a significant difference in size. Effective ones are typically bulky in size.


Without enough damping in the reactive absorber, the target mode gets split (as in the vibrating drywall).  So you can’t just tune a panel/plate, you have to make sure that you have enough absorption to dissipate the kinetic energy.


Impedance of an absorber = pressure / velocity.


Reactive Acoustic Absorber! Can be tuned. Can be retuned. Similar in size to a subwoofer. A mic picks up the pressure, a speaker plays back into the room. You can dampen two frequencies (more not advisable) using the same mic and speaker. Basically noise cancellation headphones for a room.


It does not take a lot of energy to remove energy; the energy is already there. Placement is important! Walls, probably corners are best for location.


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