Surface impedance calculator for Matlab

Roland Kruse, Ping Rong

Oldenburg University, Faculty V. Institute of Physics. Germany

Dec. 2006

Introduction

This collection of Matlab m-files is used to calculate the acoustic surface impedance from a level difference, excess attenuation or field impedance spectrum with a given measurement geometry.

Theoretical background

Sound field model

[1] Nobile, M. A. (1985). "Acoustic propagation over an impedance plane." J. Acoust. Soc. Am.78(4): 1325-1336.

Efficient deduction of the surface impedance

[2] Taherzadeh, S. and Attenborough K. (1999). "Deduction of ground impedance from measurements of excess attenuation spectra." Journal of the Acoustical Society of America105(3): 2039-2042.

Surface impedance from field impedance spectra

[3] Lanoye, R. et al. “A practical device to determine the reflection coefficient of acoustic materials in-situ based on a Microflown and a microphone sensor”.

[4] Lanoye, R. et al. (2006). “Measuring the free field acoustic impedance and absorption coefficient of sound absorbing materials with a combined particle velocity-pressure sensor.” Journal of the Acoustical Society of America119(5): 2826-2831.

[5] Kruse, R. and Mellert, V. (2006). In-situ Impedanzmessung mit einem kombinierten Schnelle- und Drucksensor. DAGA 2006. Braunschweig

Program files

The program files are stored in three zip files:

Impedance_Main.zip

GUImpedance.mGUI for the main program

GUImpedance.figCorresponding figure

Impedance_Master4gui.mMain programused by GUImpedance.m

Impedance_Master.mVersion without GUI

Impedance_Estimate_EA0.mSubprograms to calculate the impedance

Impedance_Estimate_EA.mat one frequency

Impedance_Estimate_LD0.m

Impedance_Estimate_LD2.m

Impedance_Estimate_LD.m

Impedance_Estimate_PU0.m

Impedance_Estimate_PU.m

Field_Z_Grad.mUsed by Impedance_Estimate_PU.m

GetQg.mUsed by Impedance_Estimate_LD2.m

Impedance_Misc.zip

octave_average.mAverages data over nth octave bands

binomial.m3rd party m-files from Mathworks User file exchange

cef.mcalculates w(z) = exp(-z²)*erfc(-iz)

naninterp.m

Impedance_Examples.zip

FreeFieldCal.txtExample of a free field PU-probe calibration

TubeCal.txtExample of a standing wave tube PU-probe calibration

LoudspeakerCorr.txtExample of a loudspeaker directivity pattern

To open B&K PULSE ASCII files the GetPulseAsciiFile.dll is needed (PULSE must be installed on the computer).

Data files

All files are tab delimited ASCII files. Please keep in mind that Matlab expects the decimal separator to be a “.”.

Format of input data

1.Column: Frequency

2.Column: Real part of the transfer function, excess attenuation or field impedance

3.Column: Imaginary part

Format of a free field calibration (only needed for field impedance data)

The measured transfer function p/u will be multiplied with the correction function to obtain the “real” field impedance.

1.Column: Frequency

2.Column: Real part of the correction function

3.Column: Imaginary part

Format of a standing wave tube calibration (only needed for field impedance data)

1.Column: Frequency

2.Column: Sensitivity of the microphone [V/Pa]

3.Column: Sensitivity of the microflown [V/(m/s)]

4.Column: Phase between pressure and velocity signal

Format of a loudspeaker directivity correction file

A correction for the loudspeaker directivity pattern can only be used for the predefined geometries.

1.Column: Frequency

2. – 9.Column: Sound pressure relative to the on axis sound pressure (linear, no dB)

for the angles 4°, 9°, 14°, 18°, 24°, 28°, 39°, 45°

Usage

1)Choose the input file that contains the level difference, excess attenuation or field impedance spectrum. The spectrum will be displayed in the window below.

2)Select the type of the input data.

3)Select the desired calculation type:

  1. Plane wave like solution
  2. First term only solution
  3. Asymptotic series of N terms (N ≥ 2)

4)If you chose a field impedance you need to select the calibration type:

  1. Standing wave tube calibration
  2. Free field calibration
  3. Datasheet
  4. No calibration, input is already a field impedance

5)Choose a geometry.

6)Select options if desired:

  1. Plot result Display a figure with the estimated surface impedance
  2. Plot Absorption Coeff Display a figure with the estimated (plane wave) absorption coefficient
  3. Directivity Corr. Correct for the loudspeaker directivity pattern (Not a spherical source).
  4. Lim (80 – 4kHz)Limit calculation to 80 – 4000 Hz
  5. LD Octave OrderAverage the input data over nth octave bands
  6. Imp. Octave orderAverage the surface impedance over nth octave bands

7)Press GO