Hi there,
Someone needed with knowledge in speaker measurements.
I am using this bible by Kimmo Saunisto for measurements speaker simulation: https://kimmosaunisto.net/Software/VituixCAD/VituixCAD_Measurement_REW.pdf
Most of it is clear and made for newbies, like me, but some parts of it still are confusing, one of it is related to nearfield measurements. In page 9 of it there is one sentence, which is very straightforward, but still leaves more questions than answers:
"Locate woofer cone close to floor or wall to make half space conditions at LF."
What does it mean? I measure farfield measurements of speaker with REW at home, with "good enough" conditions - driver at ~1.2m height from floor, and distance is around the same or more from nearest walls and big objects. Usually it is 1 meter distance from source to mic, as in most instructions. Gating is ~4-4.5ms. Then I move microphone stand close to the driver, at ~0.005m distance and measure it. The same for the BR vent. Then I do importing into Vituix and start sim.
EDIT: Yes, I do the merger and everything else as in this instruction
The question is - do I have to move speaker close to the wall when doing nearfield measurements??? That instruction says that, but I haven't found it mentioned nowhere except there. If I simply put it close to the wall at 90 degree angle, I am getting results of big SPL boost at ~150-500 Hz and drop below 150 Hz. When I import it into Vituix and do exactly the same with both nearfield measurement options the crossover needs to be adjusted a little. That is for 2-way. For 3-way or 4-way speakers the data for biggest woofers will get skewed even more. And yes, with near wall measurements the simulation looks uglier. I don't care much about the "beauty" but more about accuracy.
Can someone with experience comment about this?
Thank you.
Someone needed with knowledge in speaker measurements.
I am using this bible by Kimmo Saunisto for measurements speaker simulation: https://kimmosaunisto.net/Software/VituixCAD/VituixCAD_Measurement_REW.pdf
Most of it is clear and made for newbies, like me, but some parts of it still are confusing, one of it is related to nearfield measurements. In page 9 of it there is one sentence, which is very straightforward, but still leaves more questions than answers:
"Locate woofer cone close to floor or wall to make half space conditions at LF."
What does it mean? I measure farfield measurements of speaker with REW at home, with "good enough" conditions - driver at ~1.2m height from floor, and distance is around the same or more from nearest walls and big objects. Usually it is 1 meter distance from source to mic, as in most instructions. Gating is ~4-4.5ms. Then I move microphone stand close to the driver, at ~0.005m distance and measure it. The same for the BR vent. Then I do importing into Vituix and start sim.
EDIT: Yes, I do the merger and everything else as in this instruction
The question is - do I have to move speaker close to the wall when doing nearfield measurements??? That instruction says that, but I haven't found it mentioned nowhere except there. If I simply put it close to the wall at 90 degree angle, I am getting results of big SPL boost at ~150-500 Hz and drop below 150 Hz. When I import it into Vituix and do exactly the same with both nearfield measurement options the crossover needs to be adjusted a little. That is for 2-way. For 3-way or 4-way speakers the data for biggest woofers will get skewed even more. And yes, with near wall measurements the simulation looks uglier. I don't care much about the "beauty" but more about accuracy.
Can someone with experience comment about this?
Thank you.
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"Accurate" pseudo-anechoic low frequency measurements in a room is a nearly impossible task. And it is quite questionable whether it really makes much sense to do so.
I suppose kimmo's suggestion to do nearfield measurements near a wall (or floor) gives more useful results and might just reflect the "standards" we are used to, for better comparability.
The suggestion leaves questions. The usage of 'half space conditions' hasn't been defined here that I might understand the intent.
Do you have in mind my conditions? Of course no half space conditions. I measure the exact way as suggested in the same document - normal living room with some distance from sound source and mic. This is the same way most of us do our "good enough" diy measurements. I am reading this document almost every time before/during/after measurements, as I am far from being automatic. I doubt for now only about this one sentence.
Yes. I do EVERYTHING as in this document, not by word, but by letter. If I do merge the results with the "Locate woofer cone close to floor or wall to make half space conditions at LF" - that is placing speaker near wall at 90 angle, I get big bump. My guts and ears say that this is not the right way. I trust my ears, as the results differ in the voice zone, and you will hear 2-4dB raise. But this raise is more or less in the simulation only. The another way is probably to place driver parallel to the wall, with whole speaker cabinet back being near the wall, but I think I will get smth very similar.
If I merge the results by measuring speaker in the middle of the room - I get more realistic curve, everything seems just right
I am lost in translattion a little...
AllenB, in your personal opinion and experience: all the measurements - farfield and nearfield should be done with speaker being in the middle of the room and just move the mic to the 1m and ~0.005m distance??
Yes..
At the same time I know people have their own interpretations. Sometimes there is more than one good way to do something. It would be interesting to hear from Kimmo.
At the same time I know people have their own interpretations. Sometimes there is more than one good way to do something. It would be interesting to hear from Kimmo.
My merge area is above the bump - the difference is BELOW merging point. I am pretty sure about that. Merging midpoint is ~360Hz, I checked this case.
Made gif with differences in raw woofer responses. I was a bit wrong - all the difference is pretty much in sub 300Hz area, not 500Hz.
Maybe it would me more clear for all of us if someone else tries this, as I am still a human and do silly mistakes. Also I got 1 faulty mic, 1 mic which measures pure nonsense, amp which plays no bass (up to 400Hz) and so on. There are plenty of variables which can go south.
I will. I would like to have whole section of subsection of Hornresp, REW and VituixCAD, as they are de-facto standards now. Single thread for all the answers soon becomes a junkyard... Maybe for other opensource and free-for-diy tools also.
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YES! I measured both near wall, just turned speaker cabinet around.
But his was NOT pure 2pi conditions, I just tired to be as close as possible, for 2pi you need to be flush with wall. Or to put back close to (infinite) wall and so on. Both options are not possible for me. If my general knowledge in math is right, 90 degree near the wall is more or less very similar in wave propagation to being flush with wall: twice the reflection from one side and "zero" reflection from the other. Perhaps you can try more options?
stv, you are way ahead in audio knowledge, so YMMV and probably you are closer to answer what Kimmo meant with this sentence.
p.s. Perhaps I will re-measure with some other options, like back to the wall
Yes, but at low frequencies a small distance to the wall has low influence. Generally the near field responses should be very similar for mid-room and near wall measurements.
Your gif shows a huge difference below 200 Hz, just as if either the vent was not included or it was included but out of phase (?).
Also the higher output at the low end (40 Hz) for the near-wall version would indicate that.
maybe the room influence is further reduced with quasi-2pi-measurement? it should be very small anyway for nearfield measurement ...
Not necessarily, seeing that it's modal. Say a wave goes 10cm across half the baffle and 20cm back to the wall before returning. In theory the 6dB that half space gives in the bigger picture is going to drop to only 3dB at just over 140Hz. That doesn't even get you out of the Schroeder region.