超薄防护性网布和防水透气膜的声学建模【PPT版】

Presented by Dr.Jason McIntosh and Mr. Dest Zhao

On mesh Generations/Meshdeveloped trend

Purposeof acoustic materials

• Protect audio components from foreignmaterial

• solidmaterials (dust, dirt, debris, rocks, fingers, etc.)
• liquids(water, ear wax, oils, etc.)
   

• Dampen acoustic modes

• mode frequently detract from a desirable “flat” frequency

• response  resistive damping of the materials are commonly used to dampen these modes

• Allow sound to pass through

• most designs try to achieve an “acoustically transparent” condition

• however there are times when the complex reactance can be used to enhance  the acoustical behavior

Acoustic Impedance

Thin acoustic materials are characterizedby their acoustical impedance

If dP has units of Pa, and V has units of m/s,then Z will have units of MKS Rayls

If V has units of m3/s (volume velocity or U), then Z willhave units of Acoustic Ohms

Effects of Material Motion

An acoustic wave can pass through thematerial in one of two ways

Ro: Characterization as a simple resistor

Viscous shearing of the flow aroundfibers result in energy loss expressed as a resistance to the flow.

ZPM  : Plate and Membrane Behavior

Plates: when a plate is deformed, the restoring force comes from its internalstructure

Membranes:  when a membrane is deformed, the restoringforce comes from the tension T placed onto themembrane

Plateand Membrane Behavior

A material typically has either “plate”or “membrane” behavior, however it’s possible for it to have both, hence theimpedance through the material will be called

 ZPM  =impedance of Plate and/or Membrane

These modes have no net air displacementand so do not contribute to the impedance.

Effectsof Material Motion

Ares model for circular membrane showing ZPM

Anti Resonances

Circular Mode shape

Nodal line moves inward as frequency increases.

Just below Fo, the– and + volumetric displacements are equal and the net displacement is 0.

“Anti Resonances”

Effects of Material Motion

Mathematically these two paths arerepresented as a parallel combination of the viscous resistance through thefibers (Ro), and the impedance of theplate/membrane behavior of the material (ZPM). 

The net material impedance (ZMAT) is

        ZMAT = Ro || ZPM

Zmat= Ro || Zpm

The parallel impedance

    ZMAT = Ro || ZPM

Has the property that

   |ZMAT| £ |Ro|

and

   |ZMAT| £ |ZPM|

Modeling in Ares – Circular shapes

Modeling in Ares – Rectangular shapes

Experimental Measurements vs Model  

Mesh

Comparison between measured and simulatedreal and imaginary acoustic impedance of Acoustex 080 in a typical speakerconfiguration (left) and in an extreme configuration (right) in order tohighlight plate/membrane contribution in the acoustic impedance

Experimental Measurements vs Model

Membrane

Acoustic impedance of a “genericmembrane” measured and simulated with a typical area for MEMS protectionapplication. (left) Real and imaginary acoustic impedance. (right) Magnitude in dB.

Experimental Measurements vs Model

Hybrid material

Acoustic impedance of H68T02 (woven meshwith “leaky membrane” attached) measured and simulated with a typical area forMEMS protection application. (left) Real and imaginary acoustic impedance.(right) Magnitude in dB

Conclusions

• Material motion is important indetermining the acoustical behavior of thin materials

• Resonances and “Anti-resonances” dominatethe behavior of the material

• Resonances are controlled by internal damping

• “Leaky” resistive paths through thematerial can be used to control the “anti-resonances”

• Ares “FEA” material elements model theseeffects with good correlation to experiments