Now, the only question I still have is... how do you *know* your result is A-OK?
Don't want to be a pain, as I've suggested this a few times, but the easiest way to check would be to calculate the low frequency response from near field measurements of woofer and BR ports.
As
@napilopez has proven with his measurements and explained in
his howto, if the measurements are done correctly and then calculated, the accuracy is quite good.
With a little practice, the calculation is done in less than 15 minutes, even with complex speakers - depending on how accurate you are with the delay of the individual sound sources and their baffle step correction - I'm sure
@napilopez can confirm this.
How accurate the calculated frequency response is (see turquoise curve below) can be seen nicely when comparing the different measurements. Please keep in mind that as a rough rule of thumb, the calculated FR is only valid up to about 200-300Hz (it's depending on the chassis and cabinet size), in this frequency range the sound pressure level comparison should be made with the NFS measurement.
The calculated frequency response is a good indicator to determine outliers in the low frequency measurements of the NFS.
At 30Hz, the deviation of the old NFS measurement from the calculated FR is 9dB. The new NFS measurement, on the other hand, shows no deviation from the calculated FR at 30Hz.
The calculated low frequency response at 30Hz is between the Audioholic's measurement (ground plane) and the manufacturer's measurement.
Which is another indication that the frequency response calculated from the near-field measurements is quite reliable - which is not surprising, since at the large wavelengths simplified physical models still achieve very good results.
In order for the near-field measurements to be as accurate as possible, a few conditions must be met.
- When measuring the sound sources of a loudspeaker, the measurement distances should always be the same - to ensure that the sound pressure level summation is correct
- The measurement distance depends on the sound source dimension. Since the cabinet influence is calculated later (baffle step correction), it is sufficient to use driver and BR port dimensions:
Measuring distance < 0.11m x source dimensions → error <1dB
Source:
Arta Handbook
The measuring distance for an 8'' woofer would thus be <2cm (<<1'').
Since this is not possible with the NFS measuring device, it is best to do this manually with an extra microphone cable (using the Klippel software).
The Klippel software, as far as I know, is also controllable via a scripting language. It would make sense to write a script that, for example, starts with a twenty second delay, does not move the robot arm, and takes measurements every twenty or thirty seconds, for a total of five.
For the HDI-3800, for example, this would complete the near-field measurements in two minutes.
If the best possible accuracy is not required, you can probably even hold the measurement microphone by hand.