- Dec 1, 2018
- Land O’ Lakes, FL
Others should take note that Amir has posted the Apparent Power graphs, but interpreted them incorrectly. He pointed to where the highest expansive flattened out, but instead it is where the Total Apparent Power flattens, and also didn’t choose an appropriate frequency.In a nutshell, the NFS is just a tool to obtain free-field (anechoic) measurements without having to have an anechoic chamber. The results from the NFS allow us to predict what the sound field radiated by a loudspeaker at any distances* and directions in 3D space (in the free field/anechoic condition).
The NFS can tell us at what distance far field will begin for the speaker. When I say "far field", I am referring to the term's meaning in acoustics, which is at what distance the loudspeaker begins to behave like a point source (i.e. the listener is far enough away from the loudspeaker that it looks small and integrated). Very often people confuse the terms "near field" with "direct field" and "far field" with "reverberant field". (See http://www.sengpielaudio.com/DirectFieldAndReverberantField.pdf)
The following posts explains how the NFS determines at which distance far field begins.
Note: * At any distances further than the measurement locations.
Not sure if you watched Erin’s (@hardisj, just to tag him) talk with Klippel (1hr 23min mark):
You don’t look at the highest order to determine far-field, you look at Total Power. Look at how huge your y-axis is, that is throwing off the intuitive nature, we don’t need the 13th spherical harmonic to be -200dB down from the monopole. He states (as well as one of Klippel’s PDFs) that the far-field limit is when Total Power reaches 0.5dB from being monopole.
Also, showing it at 400Hz just shows it at 400Hz, not it and above. So at 400Hz it’s not 1.54m but around 30cm (your scale is too huge to see 0.5dB increments). This makes sense as it’s mostly the woofer playing that frequency. You stated as you went lower in Hz that the distance increased, which makes sense as this is a rear-ported speaker so the sound field is more complex in the bass and you need to be further away for the port and woofer to sum.
The guy at Klippel said that they are working on generating far-field transition distance for all frequencies, but that it needs work.
However, due to my understanding, there is a manual way. You first look at the Radiated Sound Power graph:
View attachment 117966
And look at where the monopole nature is most reduced, in this case around 1500Hz.
I am willing to bet if you look at the Apparent Sound Power for any frequency for this speaker above that ~1500Hz will have the furthest distance to be considered far-field.
Assuming the Apparent Sound Power data can be exported, can it only be exported as 1 frequency (what the graph is limited to), or can you export it for all frequencies? Because if so and you are willing, I can find the far-field distance.