How to set up time alignment like an acoustical engineer

When it comes to Audison bit ten tuning (or bit one), a large and important part of it comes down to time alignment. The speakers in your car are not in symmetrical positions, relative to where you are standing. Also, you cannot move the speakers around, like in your home. A solution for this, is to use a signal processor (like Audison Bit One / Bit Ten). Using this sound processor, you can delay the sound for different speakers, depending on the relative distance, so it hits your ears all at the same time. This will cancel some phasing issues, better center stage and less overall ear fatigue.

The audio setup were we’ll be conducting the time alignment

We are making an Audison bit ten tuning, so this means we will have 5 separate channels to work with (unlike 8 with the Audison bit one). If you compare Audison bit one vs bit ten, the process is the same, only for more channels. We can delay the sound coming from each of the individual channels. This means that we will need an amp channel for each individual speaker. And, of course, we can take advantage of the active filtering the processor offers. In our setup we have one 4 channel amp and one mono amp for the subwoofer.

Channel distribution :

2 channels for the front mid-range and tweeter. Each mid-range and tweeter share the same amp channel. A capacitor is hooked up in series with the tweeter, so no mid-range frequencies goes to the tweeter. This is a compromise solution since we only have 5 channels available on our processor. Time alignment will affect both tweeter and mid-range at the same time.

2 channels for the under-seat woofers.

1 channel for the subwoofer in the trunk.

the rear speakers are left on the factory head-unit and are cancelled all together using the fader settings.

The signal processor is out of sight, under the rear parcel shelf, but I will hook up a long USB cable for convenient access.

Equipment used

Normally, if you don’t have any equipment, you will use a measuring tape, to calculate the distances from each speaker to the listening position. This is very inaccurate and quite impossible to measure the distance from the underseat woofers and the subwoofer in the trunk. By knowing the distance, you can calculate the delay, since you know the speed of sound in the air, under normal conditions. If you want to do it the professional way, you will need several pieces of gear :

The only pieces that you are probably stressing about, are the microphone and the audio interface. They are not that expensive and you can use them for all kinds of audio measurements, which are useful if you are an audiophile.

Setting things up

Before we do the Audison bit ten tuning, we need to set the equipment and the software properly, so we can take some accurate measurements. From the picture above, you can clearly see how you need to connect the cables. The microphone needs to be placed in the position where your head will be. I took advantage of the headrest and pinned the microphone in place, using the microphone stand.

The Focusrite scarlet is USB powered, so everything is powered by the laptop’s battery. So no worries about car power adapters. Now we need to setup our software.

After you downloaded Room EQ wizard, make sure you click preferences and make the following settings:

Select the appropriate inputs and outputs.

Make a calibration file for your sound card (audio interface) and load it.

Go to the microphone tab and load your calibration file that came with the microphone.

Go to Analysis tab and select Use loopback as timing reference (this is important).

If you find this confusing, you can watch this video tutorial to get a more detailed explanation about this process.

Methodology

When we are taking the measurements, we hook up to the amplifier inputs, so we bypass the processor all together. We have to take a measurement for each individual speaker / channel. For this, all the connections stay the same, we just connect to a different amplifier input, and take another measurement.

Please make sure that you know which amplifier input corresponds to which speaker. We are going to use Room EQ wizard to make some impulse measurements. This will tell us when the sound will hit the microphone. It will show on the graph as a large spike. The large spike will always correspond to point 0 (in time). So we don’t actually know the time it took the sound to reach the microphone. That is what the loop-back is for. The loop-back will serve as reference. Since the loop-back signal will travel instantly, the difference between the 2 impulse responses will result in the number that we are after.

Measurements

After we got everything connected, we can go ahead an press the measure button, to take a measurement.

Select “both” as channel output, because the left one is used for the microphone and the right one is used for the loop-back. If you are measuring tweeters or midrange speakers, make sure you set the start frequency a bit higher, so you don’t damage your speakers. For bigger speakers, 20 Hz – 20 kHz is perfectly acceptable. After measuring each individual speaker your Room EQ should look like this :

For each individual measurement, you will see in the highlighted area, the delay and the equivalent distance in meters. To make our Audison bit ten tuning, we need the distances, so let’s write them down :

Front – left : 0.7 m.

Front – right : 1.22 m

Under-seat woofer – left : 1.18 m

Under-seat woofer – right : 2.2 m

Trunk subwoofer : 2.79 m

These measurements we will use to input in our Audison bit ten software.

Audison Bit One / Audison Bit Ten tuning

Now you have to plug in your Audison processor into your laptop, download the software from their website, and fire up the application. Select each individual channel and enter the values that we measured from the step before.

You have to select each individual channel and input the distance. When entering the distance, it can get slightly altered, because the software works in set increments. Like in the example above : I entered 70 cm and it displays 70.1 cm. You can use fine tuning to the right, the get the exact delay, but I would not bother with that. Bear in mind that the microphone is small compared to your head, you have 2 ears instead of one and you can move your head if you want. For these reasons I consider fine tuning not mandatory, but you can do it if you suffer from OCD.

After you entered all the values, the application calculates the delays. As you fill this information for each channel, the delay values will sometimes change. The reason is that it takes the furthest channel as reference and sets the delay to 0 for that particular channel. Once you finished setting the distances, the application will take as reference the subwoofer channel, since it is the furthest, and set its delay to 0. After that, it calculates the delays for the rest of the channels, so that the sound reaches your head at the same time from all sources.

Now your Audison bit ten tuning is done. At least the time alignment part. You still got the filtering part and equalizer settings, if needed. You can do these settings by ear, and sometimes it’s better to do so, but phase is tricky business.

Conclusion on the Bit Ten Audison setup

Time alignment is not the easiest thing to do, if you don’t have the proper tools. Like I said, you can do it with approximation using a tape measurement, and you should get decent results. But if you want settings with a high level of accuracy, you need some measuring equipment. Not setting the phase correctly can completely ruin your audio experience. From a subjective point of view, I find it very fatiguing when listening to a car audio system without time alignment. It puts more strain on one ear, compared to the other. So get your Audison bit ten tuning right, and enjoy some music.

References