Very cool Zvu,
I wish I could get confidence in sims down low like you, but i still lug all my subs out onto the driveway / yard. Always seem to find a few anomolies...
Heck yea Art, dang wind.
It's a long open westward run, here on the lake i live at. Sometimes I have to wait weeks to get still air when I want to make some fine measurements.
When my girlfriend hears me yell "surfs up", she knows it's my haha substitute for "no wind",.... and Mark is about to make measurements
I bet you got some real wind at your place also.. plus dry air to contend with i figure ?? (not that dry air is an up close issue)
Bill, i've found when wind is up even a little, dual channel and swept sine are both bogus. But for a while i was thinking REW's swept sine was doing OK, until I started comparing successive measurement loopback times. Appears swept sine just catches a swept mic Lol
I wish I could get confidence in sims down low like you, but i still lug all my subs out onto the driveway / yard. Always seem to find a few anomolies...
Heck yea Art, dang wind.
It's a long open westward run, here on the lake i live at. Sometimes I have to wait weeks to get still air when I want to make some fine measurements.
When my girlfriend hears me yell "surfs up", she knows it's my haha substitute for "no wind",.... and Mark is about to make measurements
I bet you got some real wind at your place also.. plus dry air to contend with i figure ?? (not that dry air is an up close issue)
Bill, i've found when wind is up even a little, dual channel and swept sine are both bogus. But for a while i was thinking REW's swept sine was doing OK, until I started comparing successive measurement loopback times. Appears swept sine just catches a swept mic Lol
Mark,
No more dry air since moving from New Mexico to Florida, but still plenty of wind. At the apartment I live at now, the road noise from I-95 is really high, though it's down at least 3dB since "social distancing"..
Art
Alright, guys. I slept on this a bit and thought, "what if I take the merging approach and apply it to a ground plane measurement both indoors and outdoors" with the notion being that indoor GP measurement would be a way to help shore up any concern over HF response when measuring outdoors due to wind/noise/etc.
Below is a picture of the setup for the MF/HF GP portion:
(that tandem bike rocks!)
The nearest reflecting point was about 6 feet away. Here is the impulse response gated:
And here is the result compared against the NFS as well as my driveway GP measurement conducted yesterday where I was concerned some wind could be causing issues. Note: The SPL isn't an exact match because the distance may have been off a couple inches in either measurement; I'm just looking for trends.
And this is the result of the speaker on an 8.5 foot stand vs my GP measurement today and the NFS measurement:
All of these results of mine look pretty close to the NFS results. However, the striking difference is in the 600-1kHz region where the NFS shows approximately a +2dB difference over both my GP and outdoor 8.5 foot stand measurements.
So, do I quit obsessing over this difference? Do I call the GP method adequate and just carry on; potentially using the indoor GP measurement for mid/high frequency merging when conditions are a bit more windy/noisy? I am willing to do (2) sets of ground plane measurements if it saves me from breaking my back and/or wallet worrying over hoisting large speakers in to the air. And based on what I'm seeing in my results, the GP measurement matches as well as my high-stand mounted speaker measurements do to the NFS, other than the diffraction effect due to the mirror-image baffle. The only concern I currently have with the GP measurement is off-axis measurements. Does setting the speaker on a turntable that is lifted off the ground matter? I suppose I could just make a long 'table' to align with the turntable that runs the length of the path between speaker --> mic and place the mic on it at the other end. The height would alter the response some but if I'm only using the indoor measurement to avoid any concerns over very high frequency (>8kHz) then that is not of concern, either. I'll test this out later.
Below is a picture of the setup for the MF/HF GP portion:
(that tandem bike rocks!)
The nearest reflecting point was about 6 feet away. Here is the impulse response gated:
And here is the result compared against the NFS as well as my driveway GP measurement conducted yesterday where I was concerned some wind could be causing issues. Note: The SPL isn't an exact match because the distance may have been off a couple inches in either measurement; I'm just looking for trends.
And this is the result of the speaker on an 8.5 foot stand vs my GP measurement today and the NFS measurement:
All of these results of mine look pretty close to the NFS results. However, the striking difference is in the 600-1kHz region where the NFS shows approximately a +2dB difference over both my GP and outdoor 8.5 foot stand measurements.
So, do I quit obsessing over this difference? Do I call the GP method adequate and just carry on; potentially using the indoor GP measurement for mid/high frequency merging when conditions are a bit more windy/noisy? I am willing to do (2) sets of ground plane measurements if it saves me from breaking my back and/or wallet worrying over hoisting large speakers in to the air. And based on what I'm seeing in my results, the GP measurement matches as well as my high-stand mounted speaker measurements do to the NFS, other than the diffraction effect due to the mirror-image baffle. The only concern I currently have with the GP measurement is off-axis measurements. Does setting the speaker on a turntable that is lifted off the ground matter? I suppose I could just make a long 'table' to align with the turntable that runs the length of the path between speaker --> mic and place the mic on it at the other end. The height would alter the response some but if I'm only using the indoor measurement to avoid any concerns over very high frequency (>8kHz) then that is not of concern, either. I'll test this out later.
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Funny you say that. I actually have on hand an Adam S2V sent to me which was also tested by Amir. I plan to do some testing tomorrow on that for this reason.
Once I square away the method I want to use then I'll begin the *real* process of testing.
Funny story about that Tandem bike. I sold it to a fellow locally. My 9yo daughter had seemed to lose interest and I wasn't going to ride it by myself. As I was closing the deal I brought the speaker out of the garage to clean. Kid asked why I was cleaning. I explained. She broke down in tears stating that she had "big plans" for us to ride it this summer when she was out of school. So, needless to say, the bike was not sold. LOL
Once I square away the method I want to use then I'll begin the *real* process of testing.
Funny story about that Tandem bike. I sold it to a fellow locally. My 9yo daughter had seemed to lose interest and I wasn't going to ride it by myself. As I was closing the deal I brought the speaker out of the garage to clean. Kid asked why I was cleaning. I explained. She broke down in tears stating that she had "big plans" for us to ride it this summer when she was out of school. So, needless to say, the bike was not sold. LOL
I would not pay too much attention if you find out there are some differences betweeen your and Amir's measurements of the same loudspeaker. Although it is very advanced Klippel measuring system, it works with algorithms and depending on their accuracy the end result will suffer more or less. Large manufacturers with substantial knowhow are still measuring either in anechoic chambers or outside or in large hangars with speakers suspended (usually combination of two mentioned). That is not because they can not afford the NFS system and ditch the more expensive and time consuming ways to do the job. It is because they need utmost accuracy because of mass production since they need to factor in tolerances in production drivers and crossovers. Almost all of mass production manufacturers are working with averaged responses from large number of drivers and averaged transfer functions for crossovers.
That being said, i think that results shown here (if we disregard the influence of loudspeaker stand) could well be more accurate than NFS.
That being said, i think that results shown here (if we disregard the influence of loudspeaker stand) could well be more accurate than NFS.
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Now that Amir has tested the Buchardt S400, which I just received yesterday as well (not the exact same speaker he tested; just the same model), I took the opportunity to do a little bit more testing. It has been raining off and on this morning so I proceeded with the garage-based ground plane measurements.
Picture (the garage door was opened just to get some light for the photo; was shut during testing).
Nearest reflection point from either the mic or the DUT was about 7 feet. Window of IR is about 8 ms wide.
I ran a series of tests but I'm providing what matters most. Note, I smoothed all data to 1/24 octave resolution.
Here is the FR comparison:
And to better see the difference, here is the NFS ÷ Ground Plane.
The delta is really quite good. The resolution of my measurement isn't as high due to the 8ms window so that explains some of the low-mid frequency deviance. I was specifically looking for differences in the 600-2kHz region because that's where my Elac DBR62 differences showed up. I am seeing a similar trend here; my 600-1kHz region is a little bit lower, while my 1-2kHz region is a little bit higher. I mean, these are nitpicking levels. But assuming this is a trend caused by the NFS hardware itself then I'm OK with that conclusion and am happy to continue on. Assuming, of course, my off-axis measurements reveal similarly good matches with the NFS results.
**Of course, my results do not show the same 520Hz high-Q peak the NFS shows but that's simply because this, once again, is an indoors measurement with a relatively low window. Assuming my speaker has the same quality it will/should show up once I perform the GP test outdoors and can set my window to 40 ms. **
There could also be some very minor differences in HF due to imprecise aiming on my part; I didn't get the laser level out. I just eyeballed it to be on-axis horizontally. But the vertical reference axis was the tweeter itself.
Overall, however, this additional data set gives me a pretty good feeling about using the ground plane measurement. It's a really close match to the NFS result. I'll test outdoors when the weather permits and also conduct polars to see if there are any glaring issues.
Have to thank Amir again for posting his data. It's a really good way for me to sanity check my own methods and provide me even further confidence that what I am getting is quite good.
Picture (the garage door was opened just to get some light for the photo; was shut during testing).
Nearest reflection point from either the mic or the DUT was about 7 feet. Window of IR is about 8 ms wide.
I ran a series of tests but I'm providing what matters most. Note, I smoothed all data to 1/24 octave resolution.
Here is the FR comparison:
And to better see the difference, here is the NFS ÷ Ground Plane.
The delta is really quite good. The resolution of my measurement isn't as high due to the 8ms window so that explains some of the low-mid frequency deviance. I was specifically looking for differences in the 600-2kHz region because that's where my Elac DBR62 differences showed up. I am seeing a similar trend here; my 600-1kHz region is a little bit lower, while my 1-2kHz region is a little bit higher. I mean, these are nitpicking levels. But assuming this is a trend caused by the NFS hardware itself then I'm OK with that conclusion and am happy to continue on. Assuming, of course, my off-axis measurements reveal similarly good matches with the NFS results.
**Of course, my results do not show the same 520Hz high-Q peak the NFS shows but that's simply because this, once again, is an indoors measurement with a relatively low window. Assuming my speaker has the same quality it will/should show up once I perform the GP test outdoors and can set my window to 40 ms. **
There could also be some very minor differences in HF due to imprecise aiming on my part; I didn't get the laser level out. I just eyeballed it to be on-axis horizontally. But the vertical reference axis was the tweeter itself.
Overall, however, this additional data set gives me a pretty good feeling about using the ground plane measurement. It's a really close match to the NFS result. I'll test outdoors when the weather permits and also conduct polars to see if there are any glaring issues.
Have to thank Amir again for posting his data. It's a really good way for me to sanity check my own methods and provide me even further confidence that what I am getting is quite good.
Very nice work !
I've always been amazed at how well the speaker on floor, aimed at mic on floor, works.
I learned it from a transducer designer, on one of the prosound forums.
It also makes me wonder about the 3x baffle distance measurement rule, whether the rule matters....or if it is only about simple reasonable triangulation focusing.
If you ever try polars with this method, pls post them.
It's still on my to do list...
I've always been amazed at how well the speaker on floor, aimed at mic on floor, works.
I learned it from a transducer designer, on one of the prosound forums.
It also makes me wonder about the 3x baffle distance measurement rule, whether the rule matters....or if it is only about simple reasonable triangulation focusing.
If you ever try polars with this method, pls post them.
It's still on my to do list...
Hi, and thanks for starting this thread, and discussing this in detail. And, of course, thanks to all who participated. I am starting to make measurements, but I find relatively little information on the "methods" themselves: most of the threads here are about some aspect or other of the tools used, and not on the specificities of the methods. I know the basics, but, being of engineering training, "basics" never seem good enough; I want to know how things "really" work, and why.
I do have one question, if I may (newbie here, please excuse me if it's dumb): about the "speaker and mic on the ground" method, what is the problem (there must be one...) with using a sort of "anechoic mat" on the strip of ground between speaker and mic? Would it not remove unwanted reflections from the ground, making the measurement more "open-air-like"?
Thanks, to you and all the others who continue to educate us newbies.
DF
I do have one question, if I may (newbie here, please excuse me if it's dumb): about the "speaker and mic on the ground" method, what is the problem (there must be one...) with using a sort of "anechoic mat" on the strip of ground between speaker and mic? Would it not remove unwanted reflections from the ground, making the measurement more "open-air-like"?
Thanks, to you and all the others who continue to educate us newbies.
DF
I would think that rule still applies. Seems the ground plane method is just another way of measuring albeit with some pros/cons like any other method. I would expect the 3x rule (or whichever method you prescribe to that defines true far-field) would also apply here as well.
And I also agree that this method works well. I've only seen people use it for subwoofers so in my head I just assumed ground plane was not good for full range measurements. I am extremely happy, after having exhausted my energy building and testing all these different ways, to find that the GP method works as well as the other methods. I hope to take some polar measurements tomorrow of the Elac and see how that aligns to my previous speaker stand at 8.5 feet measurements as well as eyeball them to the NFS results. I will definitely post the results.
I am hopeful those are just as "in the ballpark" as this singular set of on-axis measurements because it makes testing so much easier than lugging a speaker up and down a ladder. Not to mention the money it saves me that I was going to spend on a new, taller ladder and potentially a hoist. But, again, all of this depends on how well the polars work, but I am feeling good about the state of things. Once again, thanks for the feedback here, folks.
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Excellent thread! I had no idea GP measurement could be used across the whole bandwidth. This is not covered in the publications that I've read including Joseph D'apolito's book on Loudspeaker Measurement where it's strictly in the context of low frequency measurement. Your tests clearly show this is a valid method! The only trick would be for speakers that don't have a flat front baffle or horns that do not have a flat front face that allows the mirror image to continue into the floor.
Was all your measurements done with Db-LAB? I personally use ARTA since it seems very revealing of any faults. I've found other measurement software such as REW to embellish the results which as a speaker designer is not what I wanted.
Was all your measurements done with Db-LAB? I personally use ARTA since it seems very revealing of any faults. I've found other measurement software such as REW to embellish the results which as a speaker designer is not what I wanted.
Making a mat that is anechoic to low frequencies is not very easy, this is the same problem that anechoic chambers face, most are not anechoic below 80Hz as it needs a big space with a lot of absorbent.
Having a fully reflective surface helps with the acoustic mirror image that ground plane measurements are based on. That is why bikinpunk went with the mirror between.
Great thread, I have considering some of the same questions myself, what is the best way to measure with the space and options I have available.
There have been a couple of threads started to discuss building a Near field Scanner type device. Lots of good information but no real actionable result so far.
Bikinpunk, i guess you are HardisJ in ASR forum. I see lots of dust rose because of resonance in S400 at 500Hz followed by lots of explanation of measurement technique. I really don't want to get in that discussion - to crowded for me and i get the feeling my post would be swamped by other post.
If you have S400 with you, just do nearfield measurement (max 10mm mic from the cone). It should be valid up to 800Hz for 6.5" woofer. Move the gate to 50-60ms and look if there is any clue of resonance. If it is not there - then it is the artefact of NFS or merging technique.
And i'd made impedance sweep of raw woofer in that cabinet. Crossover and notch filters can hide them sometimes. If there is real resonance at 500hz, it will show in raw (no crossover attached) woofer impedance measurement.
If you have S400 with you, just do nearfield measurement (max 10mm mic from the cone). It should be valid up to 800Hz for 6.5" woofer. Move the gate to 50-60ms and look if there is any clue of resonance. If it is not there - then it is the artefact of NFS or merging technique.
And i'd made impedance sweep of raw woofer in that cabinet. Crossover and notch filters can hide them sometimes. If there is real resonance at 500hz, it will show in raw (no crossover attached) woofer impedance measurement.
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I am.
I had planned to do that but after I verified if my speaker shows that same peak. I haven't had a chance to do a backyard ground plane test to get the window out far enough to provide the resolution needed to see that very high-Q peak. But after seeing he posted that both his samples has this I went ahead and tested. I'll post the results there.
Here you go! I made this post on ASR so I'll just copy/paste it from there. I specifically bolded a portion of it that I think you'll want to keep in mind...
I performed my own spin on a DIY'd turntable. Now, this is, once again, not something I would consider *FINAL*. It was an indoors ground plane measurement and therefore the window of reflection free response is only about 6ms long which means accuracy only down to ~166hz, and the DUT was lifted about 2 inches off the ground as it was placed on a turntable and therefore, comparing back to back against the "direct to floor" method, there is some variation, notably below 1kHz. The plans for future tests is to conduct ground plane measurements outside so I get a longer gate and bury the turntable so it sits flush with the ground so there is no misleading data due to it being on a 2" platform. But I believe the response >1kHz is at least useful as a comparison within itself (IOW, comparing on/off axis response within my own dataset).
The center of rotation is the front baffle. Some reviewers place the DUT at the center of the turntable, thus making the center of the speaker itself the rotation point. My rotation point is the baffle. You'll see what I mean in these pictures:
This is the 0 degrees (on-axis) setup:
This is the 90-degrees off-axis setup:
And here are the results of my horizontal. Again, keeping in mind the notes I mentioned above.
Cool. I've just read what you wrote. It looks to me like side panel resonance. I'd measure the impedance of woofer without crossover, and then i'd take clamp to make some pressure on the sides to see how impedance changes. Move the clamp across the panels until the impedance is clear of any large anomalies (other than breakup) that's the place where brace should be put.
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