I spent some time this morning using the Room Equalization Software (REW) and a calibrated microphone to analyze the frequency response in the M4's cabin, and to tweak the equalizer settings to try and flatten the response.
For those of you not familiar with REW, it is a shareware audio measurement system that runs on a laptop. It uses a calibrated USB microphone as input, and outputs test tones over a 3.5mm stereo cable plugged into the BMW's AUX IN audio port in the console. REW sends user-definable frequency sweeps to the vehicle's audio system, which are then picked up by the mic, and displayed as a response graph on the PC.
I took care to place the measurement mic at exactly the spot where my ears are (27" above the driver's seat, and approximately 7" forward from the seatback). I sat in the passenger's seat, closed all doors, and rolled up the windows in order to simulate a typical driving condition.
I started by turning Logic 7 off, and centering all the tone controls to their flat positions. A "reference" measurement was taken with these settings. I then conducted a series of measurements while adjusting each of the frequency band sliders to different positions in order to get a feel for how the various sliders affected overall frequency response. Finally, after the familiarization phase, I started zeroing in on slider settings that produced the "best" response (in my opinion, of course). The following graph shows the beginning "reference" response, and the ending "optimized" response, which also has Logic 7 turned back on.
The first thing you will notice is that the two measurement curves are not very different. On the positive side, this means that the audio system is reasonably tuned for the vehicle in its flat settings, but on the down side, it means that the frequency sliders are only marginally effective in altering the sound characteristics.
Here are the settings I used:
100Hz +4
200Hz +7
500Hz +0
1Khz +4
2Khz +5
5Khz +5
10Khz +5
I then played some familiar music CD's to assess the results. In general, I was reasonably pleased with the sound of the system. It sounds a little less bass-heavy than I was used to in my 335i, but I think it is just a matter of getting used to a different system. Unlike other opinions I have seen in this forum, I don't think the H-K audio system is a disappointment.
If you try the settings I posted, I would enjoy your feedback. Remember, audio is highly subjective, so experiment!