Phasing speakers and EQ curve adjusting

[91AWDAstro]

To get the most of out of your audio system you need to properly set gains, check speaker phasing and if you have an EQ to adjust it for the acoustical environment of the vehicle.
When using multiple equipment such as a head unit, crossover, and EQ you need to first match the gains as needed between the equipment. With this system there is an AudioControl EQ-S which is easy to adjust per the instructions. Typically you want to keep the signal high through the audio chain and then lower the amp’s gain if it has one. This will insure minimum background noise.

After all wiring was complete I used an AC volt meter and a test disc playing a 1 kHz test tone to match the gains on the amps. I put the meter across the speaker terminals and made sure all channels had the same voltage reading. This was done since there are independent R and L gains on these particular amps.

Next is speaker phasing. It is very important to ensure phasing of the speakers is correct. If the drivers are out of phase (meaning right and left channels hooked up with opposite polarity) your bass will suffer and the highs will sound imbalanced. There are some instances where you want this, but I will explain that later. For low frequency such as mid bass or subs, it’s easy to check phasing using a small AAA battery. Simply remove the speaker connections at the amp and hook the positive of the battery to the + of the speaker wire and tap the negative to the other wire. Have someone observe the speaker cones while you do this. You will notice that the cones are moving in or out. If you swap the polarity of the battery the direction of the cone movement will change. You want to make sure that both right and left channels are the same with the same polarities.

For tweeters and some midranges you can’t use this method because the speaker does not have the excursion to visualize cone movement. The only way is to use a scope. I use the scope method for checking all my phasing’s. To do this you will need a test disc with various frequencies or an audio signal generator. Also a 2-channel oscilloscope, microphones and a preamp. I use an old cassette deck with mic inputs for my preamp. If you use the test disc it has to be a mono signal. If not just use a Y cable to mix the inputs at the amp. I use the audio generator for this so I have easier control of the signal. I unplug the inputs to the amp and plug the output of the generator to it. Hook the outputs of the cassette deck to the inputs of the scope. Right on one channel and left on the other. Here is a picture of my test setup.

[91AWDAstro]

Be sure your system is off while making connections! Start with a midrange or mid bass and set the generator or disc to track with 1 kHz tone. First set both mics next the same speaker to ensure your phasing is the same on the microphones. Then you can move one mic in front of the right speaker and the other in front of the left. If the speakers are in phase the signals will overlap or look like the first picture. If they are out of phase the will be out 180 degrees and look like the second picture. Simply swap one speakers polarity and it should correct the problem. Now for the tweeters you will have to change the frequency to around 15 kHz. You can also check phasing between tweeters and mids, but you will have to pick a frequency close the crossover point so both drivers will be responding. For subs use 30Hz or 80Hz.

In phase

Out of phase


If you are using a speaker crossover that uses a linkwitz-riley filter, then the tweeters should be wired out of phase with the midranges. This will also be seen on the scope. Left and right tweeters and mids should be in phase with their counter part of the other channel.

[91AWDAstro]

After the phasing is complete then you can move on to the EQ curve. I use a SPL meter set to “C” weighting. You could also use a standard mic and preamp too. I use a tripod to set the microphone input at approximately where the middle of the driver’s ears would be.


You will need a spectrum analyzer with at least 1/3 octave increments. I use a program I got off the net called IPR real-time analyzer. There are also stand alone analyzers you can rent too. I like the program because you can also do a waterfall plot wich shows decay as well. The only downfall to the program on my laptop is the back ground noise of the internal sound card. Just make sure you have the volume high enough to be well over the noise floor. You will need a test disc with pink noise. If your head unit has a repeat track function you will want this on. Plug your sound meter or mic into the input of your analyzer or sound card. Make sure the sub out if you have one is set to a low setting and tone controls flat. Run the pink noise track. You can now adjust the EQ settings to get a flat curve or what I call an ear adjusted response. I use the ear adjusted response, because the human ear does not hear flat at lower SPL levels. This curve has a peak in the mid band at 500hz. Which ever you choose you just don’t any big peaks or dips at one frequency. Try to curve so that it’s smooth between the bands. If you find that you have a 500 Hz problem such as I had here, but don’t have an adjustment for that band you can adjust the settings above and below down to try and compensate. In my system I have what’s known as a constructive interference which is difficult to control. I wound up adding a series notch filter to the passive crossover to help control it.

Screen shot of IPR analyzer. Note the 500hz peak.

Here's the EQ-S used for setting up the system. Man I miss my EQ-T's that were real 1/3 octive adjstment.

[GEJ]

=D> =D> Great thread Chris.I relate to the work your doing indirectly because of my work with PA rock systems.We used pink/white noise to try to EQ systems.
We used 1/3 Uri eq's.I remember when tuners just came out for guitar's.It was so pure that most players thought they where still out of tune because they where so used to being out of tune. I think you do know that you will adjust it some for your own wants and needs.I got to admit that I am suprized to see a 30 cycle note out of such small boxes.But then again if your out of audio for 2 mins like I have been,it like being out of it for yrs.How about the rest of the van body?.You doing anything to it to kill off the noises from the low end of the system?.

Man you sure have got,(a old rock head banger), me thinking about a system for Ole Yellar and for the RV. :-k :-k All and all I really do like in your project the fact you have covered the V8 conversion and the sound system for it too. Great work Chris.

[LaGrasta]

great thread, interesting.

[WVKayaker]

One of the most informative threads for setting up stereo gear I have ever seen. Awesome post!