Material Acoustics – Absorption2micMethod

This wizard will help you through the connections and in situ impedance measurements under normal and oblique angles of incidence.

Click the Next > button to continue.

Connect the equipment to your Sound Card:

1. Connect the loudspeaker to sound card output ch. 1 (left output).

2. Connect microphone 1 to sound card input ch. 1 (white, left input).

3. Connect microphone 2 to sound card input ch. 2 (red, right input).

Note: It is important that you sound card gives exactly the same delay time for each measurement. E.g. the VXpocket may give 1 sample deviation and should not be used.

The probe is directional. Therefore the orientation is of importance.

To ensure proper measurements be sure that the microphone is facing the loudspeaker.

Todo: Add picture

If you use one mic, you need to measure twize with the microphone position changed. The advantage is that you don't need two microphones and no calibration is required.

If you use two microphones, only one measurement is required but the microphones must be calibrated. How to do this is explained in the next wizard step.

Follow the procedure in the word document for the micropone calibration.

To do: include this document here.

Position the probe and loudspeaker as shown in the picture.

Be sure that the mark on the probe is facing the loudspeaker.

Important: It should be no sound reflecting objects within 1.5 m (5 ft) to ensure semi-anecoic conditions.

Go to Toolbars→Levels (and mixer) and make sure it is checked to display the Levels (and mixer) dialog.

To test the levels, select Measurement→Test Levels, push F2 or click the blue Test button. You should see something like in the picture.

Set the volume so the input level is in the range -40 to -3 dBFS. Use the Esc key or the red STOP button to stop.

The level must not be 0 dBFS (which means digital clipping and a red lamp is lit to indicate this).

Go to Measurement→Start to start a measurement or click the Go button shown in the picture.

You should now hear a sine sweep signal from the loudspeaker.

Check that the lower plot (impulse response) after the measurement if finished, looks as shown in the wizard picture (displayed to the left of this text).

The lower limit should be set just before the impulse, the upper limit should be appox. one meter after the lower limit.

This will exclude the room reflections.

Use the mouse to move the upper and lower limits. The impulse reponse before the lower limit should be zero.

We will now set this free field measurement as the reference.

Go to File→Advanced Save→Frequency Domain Ref. Measurement .

The upper plot should now be a flat line with the value 1 as shown in the picture.

Go to Measurement→Start to perform a new measurement. Do not change the measurement setup.

Check that the plots displayed after the measurement is finished, looks as shown in the wizard picture (displayed to the left of this text).

From the picture we see that the absorption coefficient should be higher than 0.95.

Position the loudspeaker approx. 40 cm away from the sensor.

Position the sensor as close as possible to the acoustic material to be measured.

Use the picture for illustration.

The default values are: Distance = 0 mmAngle = 0 degrees

If you want to change the default settings, go to Measurement→Saving Defaults… and type in the desired values as shown in the picture. It must be written exactly as shown in the figure.

THE DEPENDENCY OF DISTANCE IS NOT YET IMPLEMENTED. IT IS CURRENTLY SET TO ZERO.

We will now measure the absorption coefficent of the Microflown suitcase.

Put the probe on your Microflown suitcase as shown in the picture (the marking should face the loudspeaker).

Go to Measurement→Start to start a measurement or click the Go button.

A typical result is shown in the figure.

Curves can be overlaid using Plot→Overlay Curves or the button shown to the right in the picture.

Curves can be deleted using Plot→Erase Curves… or using the button shown to the left in the picture.

To display the magnitude of the impedance, go to Plot→Plot Type Settings→Frequency Response–Spectrum. In the dialog box select Magnitude as shown in the picture.

As shown in the figure you can also select Real Part and Imaginary Part.

In the same dialog box you can set the vertical axis to a dB scale (Linear is default).

The plot can be scaled in several ways using the mouse or the toolbar at the lower left.

For more information, please click the help button positioned below to the left.

To display the impedance phase, we must change the plot type.

To do this, go to Plot→Plot Type\Routing\Test… and select Phase Response–Function as shown in the picture.

The curve data can be exported using File→Export Data… or Edit→Copy Data.

Before exporting data, click in the upper plot to set it active (indicated by a thin frame around the plot).

The data can now be opened in a spreadsheet, e.g. Microsoft Excel.

Use File→Export Plot… to export the plot.

If you want to restart this Wizard, do this from Measurement→Wizard→Restart.

You can easily edit this Wizard to include your own specific instructions from Measurement→Wizard Editor….

If you want WinMLS to remember your settings, store the setup from Setup→Store….

Click the Finished button to exit this wizard.