This thread may spur some discussion, or it may not. I often see people ask about what final "curve" they should tune their system to. You see responses range from things like: flat, "smiley face", ELC (equal-loudness contour), gentle slope downward, the "JBL Curve" (provided by Andy W.), etc. It just so happened I decided to do a bit of research on the topic while I had some down-time recently and wanted to share my findings. Everything below was obtained from simply using Google Search.
So, let's start with the curves written out. I ended up settling on five different curves. Any explination/logic I found about why these curves exist will be shared below said curve.
1.) JBL/Andy W. Curve:
-60-160hz: Transition to 0
160-3khz: Flat (0)
3khz+: Gradual Roll-off to -6db @ 20khz
* Andy has stated this target response is the "ideal response for a small listening environment" with a bump on the low-end to compensate for typical listener preference.
2.) Crutchfield Labs Curve:
-125-200hz: Transition to 0
200-12.5k**: Flat (0)
12.5-16khz: Roll-off to -3db
16-20khz: Roll-off -6db (-9db total)
**This range includes a +1db bump at 400hz, a -1db dib at 10khz, and a +1db bump at 12.5khz.
* This target curve was in a recent edition of Crutchfield Labs. Here is the only comment I found to the logic behind it, "This represent's Jeff's ideal sound curve, one that helps overcome some of the limitations that appear when you listen to music in a moving vehicle."
Crutchfield Car Stereo Proving Ground
3.) Audyssey Curve:
20-1khz: Flat (0)
1-3khz: -3db dip
3-10khz: Roll-off to -1.5db
10-20khz: Roll-off -3db (-4.5db total)
* This "popular" curve is the closest I found to flat that some people recommend. I also thought the logic behind the curve was interesting. The roll-off on the top-end is performed as a gated measurement. This eliminates reflections being added to the measured response. Ungated response will be "whatever it is" after reflections are accounted for, but it should be close to flat. This leaves the small dip. It was explained that because of the popularity for 8"/1" and 6.5"/1" speaker combos (both in home and car), we have become accustomed to and prefer the sound of a directivity mismatch, and the dip in power response, and therefore Audyssey "tunes it in" even if it isn't needed. They also flatten the response of the sub up to around 3khz, add the crossover at 80hz, and allow the end-user to adjust sub output if they perfer a boost on the bottom end (similar to all the other graphs)
4.) Audio Control Curve
-40-250hz: Transition to +1db
-250-2khz: Transition to 0db
-2-20khz: Roll-off to -5db
* It seems this curve was included in the manual with the old Audio Control RTA's. The manual basically said, "Flatten response as much as possible (you won't like it), and then adjust to something approximating this curve." I found this information on diyaudio.com I believe if you want to dig further.
5.) B&K Curve
-160-2khz: Transition to 0db
-2-20khz: Roll-off to -3db
* There is a long PDF about this curve which I've attached. It seems to be well-received in mimimum phase systems (which the car ISN'T), but I wanted to include it because it also represents the people who like a smooth and steady roll-off.
And for those of you who are visual learners...a graphical representation of all the curves and an average of all the curves: