Sound System – Detect Feedback

This setup will help you detect feedback frequencies.

The sound from your sound system is recorded with a microphone.

The frequency response is plotted and the feedback frequencies can be read out. After a measurement is peformed or opened, the cursor is set to the frequency that has the highest level. This should detect the main feedback frequency.

Click the Next > button to continue.

If you have not loaded the settings for your sound card, go to Setup→Load… and select your sound card in the list shown in the picture.

Click the Load button to load the sound card settings. After doing this, you must reload this setup.

If you cannot find your sound card on the list, click help. The help file will give you instructions on how to set up your sound card.

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

If the dialog looks the same as shown in picture (without any volume sliders), WinMLS does NOT control the Window mixer and you should note the following:

It is not possible to set any volumes from WinMLS. Set volumes either from an external software or hardware mixer.

Click help for more info.

If WinMLS is set to control the Windows mixer, the mixer shown in the figure is displayed and you can use the sliders to set the volumes. If not, set volumes either from an external hardware mixer or your soundcard hardware controlls/software mixer (if any).

Set the volume so the input level is in the range -40 to -3 dBFS.

You should avoid that the level is as high as 0 dBFS (which means digital clipping and a red lamp is lit to indicate this).

Connect the measurement microphone to sound card input channel 1 (left).

Your microphone might need a pre-amplifier and make sure the phantom power is turned on if your microphone is using phantom power.

To measure the correct absolute levels, the input chain must be calibrated. If you only need to compare relative levels, you may skip this part.

Calibration can be done in different ways as explained if you click help. Here we assume that you are using a microphone and have a calibrator.

Then what you need to calibrate is to click the Cal button on the standard toolbar as shown in the figure. Follow the procedure to complete the calibration.

To perform a measurement, select Measurement→Start, push F3 or click the green Go button.

You should see something like in the picture.

If the curve is not visible, this can be because the curve is outside the plot area. To auto-scale, use the buttons shown in the picture (they are found in the bottom of WinMLS).

After clicking the auto-scale button(s), click it once more to turn it off. The auto- scaling makes it harder to see the curve when continuous measurements are performed.

The text displaying the cursor frequency and level in the plot can be formatted and positioned by the user.

The cursor can then be removed from Plot→Layout→Cursor.

The cursor field can be removed from Plot→Layout→Cursor Value.

On how to position or change the formatting of the cursor field, click help.

If you have purchased a higher level WinMLS license, you may update the curve faster.

To set this, go to Measurement→Settings…. In step 8. Mode make sure Multi is selected as shown in the picture.

Note: This may not work on older PCs, if the PC hangs or crashes, change back from Multi to Single.

If you have problems stopping a measurement, push the Esc key several times. It also helps if "Background services" is given priority (this can be set in the Windows OS settings).

As default the plot display is updated every second or faster and no averaging is done.

This means that if you set the equalizer, this change will be completely updated in the curve within one second after the change is made.

If you have a higher WinMLS license you can change the length of recorded data from Measurement→Settings… as shown in the figure.

Increasing the length of the recorded data will increase the frequency resolution. In the next wizard step another method is discussed.

The frequency resolution can be increased if you need a more accurate frequency read-out.

Go to Plot→Plot Type Settings→General Frequency Domain… and increase the zeropadding length as shown in the picture.

From the picture we see that for the selected zeropadding, the frequency separation is 0.37 Hz.

If you have purchased a higher level WinMLS license, you may turn off the continuous mode. Alternatively you may load another setup that does not have continuous update.

If you want to turn off the continuous mode, go to Measurement→Settings… and in 8. Mode, make sure Single is selected.

Then go to Measurement→Tasks… and make sure that Automatic start of measurement after new measurement is NOT checked as shown in the picture.

If you want to store a curve, use the Measurement Panel toolbar as shown in the figure. For more info, click help.

If you want to add new curves without deleting the previous curves, make sure Overlay is checked on the Plot menu.

To find a feedback frequency, position the mouse pointer above the peak. Then the cursor will lock to the maximum value as shown in the picture and thus the exact frequency will be detected.

You may use the mouse to zoom in on the peak by dragging a rectangle from upper left to lower right. To zoom out again, drag a rectangle in any other direction.

The plotting range can also be set from Plot→Axis range.

The maximum detection can be limited to a frequency range by filtering out the frequencies outside of the desired range.

To detect feedback frequency in the range 500-2000 Hz, use a BP filter at 1 kHz with a width of 2 octave as shown in the picture.

Filtering is set from Plot→Plot Type Settings→Time Data…. Make sure the check box Perform filtering is checked and click the Options button to set the type of filter as shown in the picture.

If you need to display feedback frequencies in different frequency ranges, it may be useful to display several windows each covering a frequency range as explained in the previous wizard step.

Add a new window from Window→New Window…. Use for example Window→Tile Horizontal to group the windows.

To monitor several microphones simutaneously, first set the number of channels at the upper right in Measurement→Settings….

They you may route each channel to a separate window. The previous wizard step explained how to add multiple windows.

To set the routing, go to Plot→PlotType /Routing/Test… as shown in the picture.

From the bottom that dialog you may also use the Reference measurement routing if you want to plot the difference in spectrum between two channels.