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Terminology in Acoustic and Piezoelectric Analysis

1. Acoustic Impedance

Is used as a boundary condition for piezoelectric analysis and acoustic analysis.

Piezoelectric analysis: acoustic impedance = pressure/speed

Acoustic analysis: acoustic impedance = sound pressure/particle velocity

The unit is pa.s/m.

 

2. Radiation Impedance

Is output to the result table of the acoustic-harmonic analysis.

Radiation impedance = (force applied on the driving face)/(vibration velocity of the driving face)

 

The unit is Ns/m.

3. Specific Acoustic Impedance

Is not an output item in Femtet but an important quantity mentioned in Help of Femtet.

 

Specific acoustic impedance = sound pressure of the plane wave / particle velocity of the plane wave

 

The unit is pa.s/m. This value is a quantity specific to a medium and given by [density of medium x propagation speed of wave].

If this value is set for the acoustic impedance boundary condition, the incident plane wave perpendicular to the boundary is not reflected.

See Acoustic Analysis Example 7: Acoustic Impedance for example.

 

Example: Calculation of specific acoustic impedance of air

Air density: 1.144[kg/m3]

Air speed: 340[m/s]

Specific acoustic impedance: 388.96[kg/m2/s] (or[Pa.s/m])

4. Sound Pressure Level

Sound pressure level [dB] = 20 log(|P|/P0)

 

|P|: Amplitude of sound pressure Given by selecting [Sound Pressure [Pa]] for the field and [Absolute] for the phase in the respective boxes on the result tab.
P0: Reference sound pressure P0 = 1.41x20x10-6 [Pa] The coefficient 1.41 is the square root of 2. It is used to convert the effective value to the amplitude.