Flow Impedance Measurement Module
Flow impedance is an important way of characterizing acoustical structures such as porous material, ports, and membranes.
Porous materials such as cloths or screens are commonly used in acoustical designs to provide damping. Damping is critical to control resonances. Strong resonant behavior can lead to non-uniform frequency response and large displacements that can lead to distortion. Materials are also used to prevent foreign material (dust and dirt) from getting into a device through an acoustic port. Colored and textured materials are also used to improve appearance.
An example of a porous material used to cover the tip of an ear insert headset speaker is shown below. The headset has been disassembled and to highlight these two components. To fully understand the influence of the porous material on the headset response, the impedance through the porous material cap needs to be measured and imported into the Ares modeler. The Flow Impedance Measurement device is able to perform such measurements.
porous material cap
porous material cap
Ports are typically holes made in an audio device's plastic housing to provide a means for sound to exit or enter the device. Examples are the speaker and microphones for a cell phone or a headphone.
Earpiece port microphone port
They're also used to port sound to a microphone.
Flow Impedance is the ratio of the pressure drop through an acoustical structure divided by the velocity through the structure.
If P is the pressure drop across the structure, and U is the velocity, then the flow impedance Zf is
If U is particle velocity, then Zf will have units of Pa-m/s, or MKS Rayls. (Since this normalizes the impedance by the surface area, it is also referred to as the specific acoustic impedance. )
If U is volume velocity, then Zf will have units of Pa-m^3/s, or Acoustical Ohms. (This is also referred to as acoustic impedance in some texts. )
Measuring Flow Impedance with the MAE100 Apparatus
The MAE100 apparatus can measure Zf for a variety of materials and structures. The MAE100 system is shown below.
Circular samples are cut using a die and mounted in the sample holder as shown below, in this case the gray material is the sample to be measured across a 1cm diameter area.
The MAE100 apparatus assembled for measuring the sample is shown below.
The Ares Acoustic System software is then used to perform the measurement, and an impedance curve such as that shown below is generated. The real part of the impedance is the solid line, white the imaginary part is the dashed line.
For an acoustic materials such as that shown above, the impedance of the material is usually assumed to be a simple constant real value. As can be seen from the graph, this is largely the case for this material for frequencies up to about 3000 Hz. However, after 3000 Hz, the motion of the material effects the impedance greatly, and its behavior dramatically changes.
To capture this behavior in a model, the impedance can be imported into the Acoustic Modeler's acoustic resistor or acoustic materials elements. Element #2 in the Ares model below is the acoustic material element which can import such data. Importing the exact impedance of the material will increase the accuracy of the model, and allow you to better choose a material for an application.