I know a lot of car-audio upgrades are performed using the OEM head unit, and without testing the signal coming out of it. It’s depressing, but it happens all the time.
Back when I started in car audio, and dinosaurs roamed the earth, we were selling in-dash cassette decks. The wider the frequency response of the tape deck, the better the highs and lows sounded. The flatter the response of the tape deck, the less it imparted its own “filter” to the sound, and the more neutral it was. High-fidelity was considered to be this rarely-attained 20 Hz to 20kHz range, +/- 3dB. The 3dB deviation was because 3dB is the smallest volume difference we can hear reliably as a volume difference.
A good cassette deck was 50-15k +/- 3dB, like a good FM tuner:
A great cassette deck was 30-18kHz, +/- 3dB:
Then we got more-affordable CD players – the difference was amazing!
Let’s compare those to some car audio OEM receiver responses, measured on the electrical outputs:
BMW 1-Series base, 2010:
Honda Accord, 2019, non-amplified:
2011 Hyundai Tucson (non-amplified)
2019 Honda Passport
If we were selling an aftermarket head unit with this kind of signal out, we would send it back as defective.
So, if you aren’t testing this signal, you have no idea how bad it is, and you have no idea if your customer should be spending money on correcting it. Don’t present yourself as an expert if you don’t do this testing. Don’t tell your customer they should spend money on great speakers and amplifiers if you don’t make sure they get great signal first.
Maybe you don’t have the gear. Well, none of us had voltmeters, or routers, or laptop computers – until we did.
There is no question that testing is essential for good sound. The question is, are you selling good sound? Or are you just taking their money and hoping it all works out?