As I mentioned, I have a way to optimize the low frequency measurements a bit more. I just dread having to put this monster on the stand and spend the 4+ hours to measure it again.
If we take the
technical specification from the Revel website as a basis and assume that the information given is close to reality,
@amirm's near-field measurements of the BR port and a low-frequency driver can be used to calculate the approximate low-frequency response.
Since we unfortunately lack the absolute phase frequency responses, we calculate as an approximation with the minimum phase for BR port and woofer.
For the BR-port the minimum phase is inverted.
Then the near field measurements of BR port and woofer are summed. I corrected the sound pressure of the BR-port so that the frequency response at -10dB is about 24Hz according to the specification. Then the baffle-step correction is performed and the approximation for the low frequency range of the F328Be is finished.
In the diagram below
@amirm's NFS measurement is shown in
purple and
Harman's in light green. In green and blue you can see Amir's nearfield measurements of BR port and woofer chassis.
In
red you can see the calculated approximation for the low frequency range of the F328Be (calculated as described above).
Down to 60Hz the agreement between NFS measurement (purple) and calculated low frequency response (red) is very good. Harman's measurement deviates already clearly.
At even lower frequencies there are strong deviations and I think the calculated measurements of the NFS are a little off - not only 1dB, but loosely about >10dB@25Hz.
But of course, the more I overestimated the BR-Port sound pressure, the more my approximation approaches the NFS measurement.
On the other hand it is very likely that the NFS measurement in the 20-30Hz range has problems to calculate the sound pressure "correctly".
What is also clear is that the -3dB@35Hz in the technical specification even after Harman's own measurements are not true.
The -3dB point is extremely high at about 70Hz and should result in a rather slim low bass performance when the speaker is placed freely.
This behavior in the low bass range could also explain the unusually low harmonic distortion in the low bass range.
In order to achieve 96dB sound pressure, as in Amir's distortion measurement, the three woofers only need to displace very little due to the fast low frequency decay.