Wow, this was a confusing thread! This is my understanding so far (and I'm far from being an expert so please correct me and/or fill in any details I've missed):
I think there are two kinds of measurements being confused in this thread: nearfield measurements of the speaker itself, and measurements for room correction.
1) Speaker measurements: This is done nearfield (e.g. at 1 m distance from the speaker) and the purpose is to correct the speaker itself over its entire frequency range. Mainly done by the speaker manufacturer (or the DIY:er). The measurement must be done in an anechoic chamber, with a Klippel scanner, or using time gating while having the speaker suspended far enough from all reflecting surfaces. The results could be used to improve the design (time aligning drivers, adjusting directivity), design crossovers, and to get the overall response curve for the speaker (to be applied later; you can find such data here for many commercial speakers).
... and, once you have a speaker that sounds good all by itself:
2) Room correction: Measurements done at the listening position(s) with the speaker in its intended position in the room. Measurements are averaged over multiple positions* (e.g. using the moving microphone method) and only low frequency peaks are corrected.
Why only peaks? ―Because the nulls (troughs) cannot be corrected: if you increase the output at those frequencies, the reflected waves that cancel out the sound will also increase in amplitude and you will still get the nulls (together with a lot more distortion from your poor speakers).
Why only low frequencies? ―Because our ears and brain are very good at resolving direction of the sound at higher frequencies; in essence our brain already does this filtering for us. The speakers themselves already sound good, remember? While it is possible to equalise the average, omnidirectional room response at higher frequencies as well, your speakers won't sound right if you do!
(Also, placing the target level in REW below the average level – much below the level calculated by "Calculate target level from response" – to get rid of the troughs is at best pointless: you will compensate by increasing volume later, so it is equivalent to trying to boost the troughs. Except now you also have filters all over the place doing who-knows-what to phase etc...
*) It's also possible to do more advanced room correction taking phase into account and using FIR filters (i.e. not biquads). This requires frequency-dependent gated sweep measurements, I think, and might also be able to actively correct room mode ringing? See e.g. Barnett, "State-of-the-Art of Digital Room Correction" (and watch the video linked in part 2, if you are really interested).
I think there are two kinds of measurements being confused in this thread: nearfield measurements of the speaker itself, and measurements for room correction.
1) Speaker measurements: This is done nearfield (e.g. at 1 m distance from the speaker) and the purpose is to correct the speaker itself over its entire frequency range. Mainly done by the speaker manufacturer (or the DIY:er). The measurement must be done in an anechoic chamber, with a Klippel scanner, or using time gating while having the speaker suspended far enough from all reflecting surfaces. The results could be used to improve the design (time aligning drivers, adjusting directivity), design crossovers, and to get the overall response curve for the speaker (to be applied later; you can find such data here for many commercial speakers).
... and, once you have a speaker that sounds good all by itself:
2) Room correction: Measurements done at the listening position(s) with the speaker in its intended position in the room. Measurements are averaged over multiple positions* (e.g. using the moving microphone method) and only low frequency peaks are corrected.
Why only peaks? ―Because the nulls (troughs) cannot be corrected: if you increase the output at those frequencies, the reflected waves that cancel out the sound will also increase in amplitude and you will still get the nulls (together with a lot more distortion from your poor speakers).
Why only low frequencies? ―Because our ears and brain are very good at resolving direction of the sound at higher frequencies; in essence our brain already does this filtering for us. The speakers themselves already sound good, remember? While it is possible to equalise the average, omnidirectional room response at higher frequencies as well, your speakers won't sound right if you do!
(Also, placing the target level in REW below the average level – much below the level calculated by "Calculate target level from response" – to get rid of the troughs is at best pointless: you will compensate by increasing volume later, so it is equivalent to trying to boost the troughs. Except now you also have filters all over the place doing who-knows-what to phase etc...
*) It's also possible to do more advanced room correction taking phase into account and using FIR filters (i.e. not biquads). This requires frequency-dependent gated sweep measurements, I think, and might also be able to actively correct room mode ringing? See e.g. Barnett, "State-of-the-Art of Digital Room Correction" (and watch the video linked in part 2, if you are really interested).