[keep_hope_alive]

I measured the factory EQ curve so I can correct for it as I build the replacement sound system.

The images attached are
1. the factory EQ before correction,
2. the correction EQ overlaid with factory, and
3.the result of the EQ correction.

The other images are
4. the tweeter frequency response and
5. the overall system frequency response, without subwoofer.

As you can see, a good sound system isn't built by just replacing speakers and adding a subwoofer, because the factory system was tuned to match the factory drivers. GM has a long history of having a fairly wicked EQ curve.

The screenshots you're seeing are from JL Audio Tun software, the same software that is used to tune the amplifiers. This software allows me to see the response while I adjust the EQ.

[keep_hope_alive]

How I measure this:
First, I have a USB drive with a Pink Noise track loaded in the car, playing on repeat.
Second, I have the output of the amplifier routed to a USB audio interface, allowing me to see the signal out of the factory radio.

My JL MV600/6i amplifier has six amplified channels and six preouts, all with dedicated EQ and customizable signal routing. I route the input to an output so I have a dedicated EQ for correction. Then I route that output back into the amplifier so I have a corrected signal that can be further tweaked for each channel.

Third, I adjust the EQ while watching the factory signal, correcting it in real time.

Fourth, I place a measurement microphone at the headrest and make EQ adjustments to individual channels while watching the real-time RTA. When using a single mic, you will have peaks and valleys due to a single point measurement. The best option is a microphone array, something I will do later. I didn't have a lot of time last night to get an initial tune done.

[ctrlz]

Thanks, keep_hope!
This is the first frequency response curve I have seen posted for this car!

Here is a direct quote from you, so readers know which system this is for (they will all be different):
"I ordered the 1SS package because it included the 8” infotainment IOS interface but without Bose. As a result, I have a full-range signal out of the front that I can use."
He has a 2023 1SS.

[Msquared]

To clarify, this is the analog front-channel output from the factory radio module, right? I'd be curious what the output for the rears actually looks like, just to settle the discussion about it not being flat. Also, the digital output that can be intercepted by interfaces like NavTV and Axxess is flat? Or is the EQ being applied upstream of that, too? Just curious about all this.

[keep_hope_alive]

Originally Posted by Msquared

To clarify, this is the analog front-channel output from the factory radio module, right? I'd be curious what the output for the rears actually looks like, just to settle the discussion about it not being flat. Also, the digital output that can be intercepted by interfaces like NavTV and Axxess is flat? Or is the EQ being applied upstream of that, too? Just curious about all this.

This is the electrical signal out of the front speaker outputs.

I hid the rear wires when I was acoustically treating the car, making accessing them a bit inconvenient for now. For sure, they are also not flat and are greatly attenuated.

Because I have the 1SS non-Bose, there is no digital signal to intercept. That EQ is inside the head unit module.

[keep_hope_alive]

I will be redoing this effort soon. I will check both the left and right channels separately as I realized that I only measured the left channel and I suspect there are different EQ for each. Additionally, I will run an Impulse Response on both channels as I suspect there is time delay (based on the factory sound stage differences in each seat).

Next, I'll be running full Impulse Response measurements for each of the new channels to properly adjust delays. I'll document that process in my build log. i'll update this thread with new measurements of the stock system.

[Msquared]

Originally Posted by keep_hope_alive

This is the electrical signal out of the front speaker outputs.
...
Because I have the 1SS non-Bose, there is no digital signal to intercept. That EQ is inside the head unit module.

Well dang. All this time I've been thinking that the DSP interfaces were actually interfacing with a digital signal from the radio module, not just the analog outputs. I kind of half-assed thought the extra connectors would plug into some unused port on the module. I never even stopped to really look at the connectors in their harness. My mistake!

[keep_hope_alive]

Here's a wiring diagram for how I measure the factory signal and correct for it in the same amp.