Phase check in a Loudspeakers system

NOTE: In Sample Champion version 2.8 and above, a "Correlation Meter" and a "Stereo Check" functions are available for measuring very easily the phase between the 2 channels.

When an audio system has a large number of loudspeakers, the possibility of wrong connections is always high. A friend of mine got a new car, but the loudspeaker system provided by the manufacturer sounded a little strange and he asked me to investigate. Here are the results...

For the test I used a 250 Hz sine signal, played from the car CD player. Specifically I used the CD Audio Test & Demonstration CD by Stephen Court & Alan Parson (track 48, infinite repeat mode), but any other is suitable, also a self-made recorded CD with some sine signals.

VERY IMPORTANT: For phase tests I recommend to use a signal with a low frequency (long wavelength) so that small displacement errors in the microphone positions will not be interpreted as phase errors.
250-500 Hz could be a good value. Higher frequencies require more attention in the microphone positioning.

The system in the car analyzed here has 6 loudspeakers: 4 in front (1 woofer and 1 large-band tweeter in every channel) and 2 in rear (1 woofer in every channel).

Sample Champion settings for this measurement:

- 2 channels input
- 4K samples buffer
- Repeat mode (4096 cycles)
- Scope window opened, both channels shown (Red and Green)

First step:

I connected a microhone to each of the 2 input channels of the soundcard, selected one of the 6 loudspeakers as reference (the left woofer in front) and placed both microphones very near to this loudspeaker.

Starting the measurement, following waves are shown:

The Scope plot is:

The measured waves (left channel in green, right channel in red) are not pefectly overlayed because the microphones are in different positions (although very near). Using a low frequency test tone this difference should be small.

Then I moved one of the 2 microphones near to another loudspeaker (left front tweeter), leaving the other near the reference one.

The measurement shown the following:

Here a small wave displacement can be observed, but this can be due to physical factors such as a crossover or a different distance of the loudspeaker from the microphones (behind the grid). So the phase between these 2 loudspeakers is correct.

Measurement in the right channel front loudspeakers; right front tweeter:

Right front woofer:

So all 4 front loudspeakers are correctly wired...
Let's go to rear loudspeakers.

Left rear loudspeaker:

BAD BAD BAD!!! Phase inverted! Here is the cause of the bad sounding audio system! The waves (red and green in the plot) are clearly displaced.

Finishing the mesurements, we go now to the right rear loudspeaker:

This is correct.

So the solution is correcting the phase of the left rear loudspeaker.

Trick: when playing sine tones, it could be useful to change the fader and balance settings, when possible, in order to minimize the effect of the sound from all other loudspeakers.

Another trick: when the second microphone is placed in front of another loudspeaker, it could be necessary to adjust the input gain on the audio card mixer or on an external microphone pre-amplifier (if present), and turn ON and OFF the Y auto-scale button of the Scope window, in order to have on the plot the 2 waves with the same amplitude.

We would like to thank also a little guy that helped us and gave some useful suggestions during the measurements:

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