Originally Posted by John Robert
Flattest measured in room or flattest anechoic response?
Anechoic. We shouldn't change the source and the equipment needs to faithfully reproduce the source. There have been many studies that show we prefer a flat frequency response including those by Toole and Olive as mentioned.
Let's say you are talking with a friend you can easily notice if their voice sounds different. Let's say they sound nasal due to a cold, or hoarse from a sore throat you immediately notice it. It doesn't matter what type of room you are in you can still notice their voice is different. An inaccurate speaker is altering the source and the studies show that people recognize this even with material they are unfamiliar. Just like many can guess when a stranger has a cold or sore throat.
Speakers are typically measured anechoic or quasi-anechoic (outside in an open area like a field) so room problems aren't part of the speaker design and the source remains accurate.
Rooms play a huge role in quality of sound but the source still needs to be accurate. You will get the best sound quality if you can treat the room and leave EQ as the very final tweak. I am not saying EQ isn't beneficial but sometimes it used where it shouldn't or and often things are over EQ'd. Studies have shown that EQ does not always make sound better.
Speaking of a flat response, let's talk about flat in room response. In most home sized rooms high frequencies start to roll off at ~6-8khz and will be down several DB by 20khz. Should we correct for this? Well it depends, many experts will say no as it causes the sound to be not natural and we recognize it. I agree with this and find it fatiguing over time.
However, if I were to play material and AB it many listeners would like the boost. It makes things like cymbals or glass breaking more pronounced and in a simple AB test listeners feel they are hearing more detail. But over time listeners change their mind as this gets fatiguing. Just like the other poster stated some sounds will get more fatiguing over time.
If you look at many auto EQ systems they will often have a 'flat' setting or some other choice like theater, natural, or whatever they like to call it which is often the better choice. When users AB these modes they often choose flat because in that short period of their testing it sounds more detailed.
Another thing to keep in mind is these frequencies are out of our main vocal range where it is easiest for us to hear unnatural timbres. So changes in this upper range is tougher for us to notice with short testing times.
Good acoustics starts with a faithfully reproduced source and once in the room it all comes down to location, location, location. Location of seating, location of speakers/subs, location of acoustic treatments. Use EQ judiciously and as the final tweak.
The big problems with speakers is that is difficult to get any useful measurements from manufacturers. Some pro companies provide this information but few consumer speaker companies do.
Hope this helps.