Very cool. Sorry I’ve been late to the party (as usual) but can’t wait to see oodles of data. So happy you’re here to share. Thanks!! Looking forward to it.
I purchased a mic stand over a month ago. After 2 shipping issues, I finally got the 3rd one and it was good to go. I also ordered some speaker stands and planned to use one for speaker testing. But after doing some testing I realized that was a no go. The stand was a bit shaky at above 6 feet. Plus, I wanted something a bit easier for takedown and set up. So I cannibalized my basketball goal (I need a new one anyway ). I buried the pole about 2 feet in the ground and rigged up a platform from HDPE and Plywood (to be sealed soon). The platform can also be swapped out to accommodate larger speakers (floorstanders laid on their side to capture vertical response). The pole is very sturdy and can be removed from the ground and placed in my storage area between tests.
The speaker stand is now at 8.5 feet off the ground and 9 feet away from the patio cover. Nearly everything I test will be reflection free to 9 feet. Which puts the first reflection at about 12 ms; or about 80hz. Doubling that means good resolution above 160hz. That’s good. Real good. More than adequate when you consider I will be using a ground-plane measurement for LF response and "stitch" that to the FF measurement. As for external noise; I live in out in the country. So it's mostly quiet. The software/hardware is smart enough to ignore most noises and multiple sweeps will help keep them from sneaking in to my measurements. IOW, I don't have any concerns here.
I tested hoisting up a 50# bag of sand. That wasn't easy. But was do-able. I'm gonna need a crane, though, if I ever test a floorstander that has significant weight to it. But I'll burn that bridge when I get there.
Here's a few pictures with my 6 foot ladder and myself as a reference.
The last picture is of the mic stand. This sucker is a beast. Can be stood to almost 8 feet tall and the mic can reach higher when the boom is angled.
Here's a link if anyone is wanting to see more info.
So, I finally got set up today to at least start taking some test measurements. I started with the ELAC DBR62 since it's what I sent to Amir over at ASR which he tested and posted a couple weeks ago. My results are ... different. Particularly in how Amir's measurements show a lift from about 600 - 1khz while mine shows the lift from about 1-2khz. Now, this was measured with the DUT about 8.5 feet off the ground and the first reflection didn't occur until about 13 ms (see attached). That means the first reflection does not occur until roughly 80hz (that's a good thing; a real good thing). Now, I played around with seeing what happens if I drop that window down to the standard 3ms that so many are forced to use. The trend still stays the same. You'll also notice I have a smoother response up high; no dip in the 16khz region as measured by Amir. I did play with adding foam to my mic stand. No real difference was made.
Point being, outside measurements has kind of been considered the next-best-thing to anechoic (and now NFS) measurements but there's a decent difference in what I measured (with a huge window of time being reflection free). I'm going to do this again tomorrow and if I find the same results I'm planning on contacting Klippel to see what they think.
I don't necessarily think there is anything alarming here; not yet, at least. But I wanted to share.
Note: Measurements were conducted with no ambient noise (no cars, no kids screaming, etc.) Measurements were averaged with 2-4 sweeps to help remedy any external noise, just in case, as advised by Klippel R&D.
After having shared the above post and spending the day conducting said tests, I am seriously considering overhauling my method for off-axis measurements. Right now I am spinning the mic about the baffle of the DUT as I described earlier. Easy enough. But doing it 10x today for the 0 - 90 degree measurements got old. Fast. I built a turntable a couple weeks ago but decided against it. Now I'm going to play with that idea more tomorrow again. I really wish I could afford some way to automate this stuff with one of the various heavy duty turntables out there but those are a cool $1k even for the most modest options. So for now I'll just deal with the manual method.
I also now need an 8 foot ladder because that 6 foot ladder ain't gonna cut it when trying to lug a speaker up and down it and hoisting even the little bookshelves over my head to reach the 8.5 foot pedestal height. So I'll be shopping those tomorrow; hopefully someone has some on sale.
Think a motorized turntable using a power window motor?
As long as it could be made to stop in 5 or 10 degree increments, that could work. But I don't currently have the knowledge set on how to build and code something like that up myself.
For now, I think I'm going to keep the manual solution of turning the turntable myself.
That ELAC sure prioritized overall power response over on-axis flatness. Not necessarily a bad thing, though the hump in the 1-2kHz range doesn't excite me much.
I recommend a platform ladder, you'll love the extra support at the top and a platform to put the speaker down on while you climb up.
For rotation automation you'll want a stepper or a servo motor, probably a servo as most steppers don't provide much torque.
Those are both good suggestions. I'll look in to them as possibilities.
I posted this on another site and a fellow said he uses the same.
I was thinking of this last night but hanging the speaker. Didn't consider using it as a platform. That said, do you think it would be sturdy enough to place a floorstander on top of?
I like this option because I could set the speaker/turntable/platform on top of this tray and use it to help me raise it an additional 2.5 feet which, if nothing else, makes things a bit more safe since I could load it from the ground and then raise it to measurement height.
I even looked at drywall hoists last night. What I like is they can extend up to 11.5 feet (would never go that high) and they spin. So I could buy one, modify it with a platform adapter and mark it for polar response measurements. This thing looks practically perfect. But... I don't know how stable it is. Some videos I watched make it look kind of wobbly. With a speaker at the center it might be OK. This may be one of those "buy and try" things.
You can see what I mean about it being wobbly in this video. But I'm wondering if placing a platform on it and simply weighing it down with a sandbag or cinder block behind the speaker would help stabilize it.
My drywall jack is like the one shown. Saved my bacon building our house and after using one, I will never resort to laying sheetrock by hand again. Anyhow, the higher it goes the more unsteady it gets. It sways like a Willow tree in a stiff wind when fully extended. The wobble is in the part that extends as the base spreads out in maybe a +6' circle and it's pretty darn solid. Depending on how heavy the speaker is, lifting it with the winch may be tiring. Lifting 12' x 4' sheetrock is ok but you're not going to want to do it a lot and I'm not sure how much a 12x4 sheet weights. The angle of the sheet can also be pivoted but, IIRC, it does not tilt to a straight vertical position. Would need to take that into account when manu a speaker platform. Personally, I would be fine with placing smaller and medium sized speakers on it as long as they were solidly strapped down, but something Statement sized ... that's another thing. If something that big and heavy fell on someone, it could kill em'.
The reason I was interested is because I'm starting back up doing reviews and thought having the NFS 'anechoic' measurements would be a big help in determining my measurement method for reviews. My initial plan was to measure outdoors for far-field mid/high frequency and merge with a ground-plane measurement for low frequency. So, here's the first measurement...
Test # 1) Speaker on platform ~ 8.5 feet off the ground; window to about 13ms with accuracy down to approximately 100hz.
Speaker flush with front of platform (picture was taken before I began tests).
Pros: This method has the benefit of having very high resolution as you go above 200hz.
Cons: being subject to the elements (cold, wind, rain; external noise not really an issue in my area) and having to hoist a speaker up 8.5 feet in the air and risk throwing out my shoulder should I ever test a floorstanding speaker.
Here's a photo of the setup (I am using thumbnails on purpose; don't want to swamp this page with oversized images).
Here's the result compared to Amir's. You can see some decent differences namely above 1khz. Notably the 1-2khz region.
Now, I wouldn't say that test was 'final'; it was just a "proof of concept" and I suspect that I could probably get better accuracy with an additional test and higher # samples that feed the average. Given the hassle of setting the speaker up high I thought I would try a few other methods out to see how they compare.
Ok. That brings you up to speed to today's testing. Where I tried a number of things. I'll list the method, the pros/cons and provide a picture of the setup as well as my result vs Amir's result. Note: The stand was thrown together from scrap wood; if I were to use this it would change. But I didn't want to waste too much time because storms are setting in this afternoon and I needed to get to work. Also, ignore levels for now. I used a mic calibration file for FR but haven't checked the level with the pistonphone; I only care about the general trends for this process.
Test # 2) Measurement on a 5 foot stand in my garage; gating was about 4 ms.
Same stand used below but inside my garage with a ceiling height of about 10 feet. The speaker was positioned about 5 feet above the ground with the tweeter at about 10" off the platform.
Pros: Don't have to worry about the elements nor do I have to hoist the speaker 8 feet in the air.
Cons: Poor resolution below 1khz. I don't think I would even bother posting this data. For a single drive unit it's fine. But, speaking from experience, I need good resolution to determine where to merge LF response data of ported loudspeakers.
I didn't take a picture of this setup.
Here's the result compared to Amir's:
Test # 3) Outdoors on stand. Same stand as before. Window ~ 7ms.
Pro: Better resolution than garage measurement since ceiling reflection isn't there and thus gating is extended an additional 3ms or so.
Con: Outoors; weather/environment. Still not as good as the 8.5 foot platform or ground plane measurement (see below).
Here's the result compared to Amir's:
Test # 4) Ground Plane Measurement: Speaker Angled ~ 8 degrees; window out to 40ms before first reflection.
Speaker was angled at about 8 degrees so that the tweeter would be pointing on-axis with the microphone (well, as on-axis as it can be without burying the mic and pointing the mic at the DUT). Note that while the response was gated here, if I were to actually use this method I would move to the back yard where I could get the nearest reflection about 40 feet away which should get me down to around 20hz. So, this was more a sanity check test.
Pro: Invulnerability to reflection; I can get ever further out in my backyard. High resolution in to the very low frequency region.
Con: Outdoors. Diffraction effect of speaker changes due to the baffle doubling (via the mirror image effect). HF > 10khz seems to be a pain. From my research that's generally accepted as questionable. Though, I had decent results.
Phone placed at tweeter on baffle and used in 'selfie' mode to make sure the microphone was at the center of the image (trick I learned on another website).
At face value, each of my methods look pretty reasonable. And all are pretty much "industry standard". You can go to a website or magazine and see any number of reviewers using any of the above methods. But I wanted to do a comparison myself and see if there was any one method that is better than the other. Unfortunately, there isn't. I was hopeful the Ground-Plane method would yield the utmost accuracy (aka: match what Amir has). But it's missing the 600 hz - 1khz bump Amir's data shows and there's a slight difference in the LF region. Potentially diffraction effects; but playing around in EDGE and VirtuixCAD I didn't see the influence. Otherwise, it looks pretty dang good.
There's two possible reasons I'm seeing differences: 1) none of the methods I use are fool-proof or 2) I shouldn't put all my trust in to the NFS (not saying it's wrong; just saying it may also have a few things that make 97% accurate instead of 100% accurate; keeping in mind that an anechoic chamber has its own issues). I think the answer may actually be a large heaping of #1 and a small portion of #2. All of that said, once you combine these possibilities with the level of detail I'm viewing this in... really, none of my results are extremely different from the NFS results. They're just different in some areas and alike in others. I mean, I'm usually within 2dB difference. Which, yea, it's not minimal but in the grand scheme of things, I'd say that's not too bad. I want to be closer, though.
Ultimately, this is where I am at:
I still want to re-test on the 8.5 foot platform. Maybe I can do that tomorrow.
Measuring in the garage is convenient. Measuring outdoors provides better resolution, especially in the area where I would need to perform low-frequency response stitching and that is useful in determining how best to align the two sets of measurements.
I would prefer to find a method that yields the best accuracy without the need for additional post-processing. Though, when I started this venture my plan was to use the outdoor farfield measurement for > 200hz response and merge that with a ground-plane measurement for low frequency.
But if I could figure out how to calculate the diffraction impact then I could just remove it from the result. I could also use the near-field technique but if I were to get an oddly shaped port (flared) or something that's harder to easily measure and quantify then that would impact accuracy as well. There's also the "mic in the box" method but it has the issue of being able to fit a mic; some speakers (like this Elac) won't allow my mic inside the slot port.
If you've read this far I assume you have some vested interest in these results. So, I welcome your opinions, as either or both fellow testers and reviewers. Would you say the ground plane measurement is "close enough"? Is there a formula or set of formulas I can incorporate in to my Matlab script (which post-processes the data) to account for diffraction in the ground plane measurement? Would you stick with the original plan, combining "speaker in the sky" (Test #1) for mid/high frequency with ground plane for LF?
Haven't measured anything under 300hz, so definitely interested in the low freq measurements. The pro and cons are good. Looks like NFS might be the viable option for a lot of us.
Do you have a comparison of the NFS vs groundplane and the 8.5 feet. Would like to know how they compare below the 200hz or so.
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.
Now that Amir has tested the Buchardt S400, which I just received yesterday (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.
Before I show data let me say that this is NOT A FINAL TEST. This is simply an ad-hoc, "let's see if this looks logical" test to compare against Amir's data. My actual test results will be full range with higher resolution in the lower frequencies. Phew... OK. Read on.
Picture (the garage door was opened just to get some light for the photo; was shut during testing). The mic is ~ 2 meters from the speaker. The speaker itself is angled so that the tweeter has a direct line of sight with the microphone. I don't know what the angle is; I used my camera is selfie mode to put the speaker in the center of the frame and called it good enough.
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 nice. 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.
Hard to tell from the little pictures but on the ground plane measurement is the mic laying on the driveway?
I tested it both as laying flat as well as tilted. There's consistently been about 1dB difference above 6kHz between the two measurements. And the NFS results lies right between these two in the S400's case. 1dB really is not bad and I wouldn't consider it an appreciable source of error in the grand scheme. But I plan to test this a little bit more.
Hard to tell from the little pictures but on the ground plane measurement is the mic laying on the driveway?
I tested it both as laying flat as well as tilted. There's consistently been about 1dB difference above 6kHz between the two measurements. And the NFS results lies right between these two in the S400's case. 1dB really is not bad and I wouldn't consider it an appreciable source of error in the grand scheme. But I plan to test this a little bit more.
When you do the testing on the garage floor how much distance around the speaker do you need? My garage is 2 cars deep but only one car wide. Would that be enough room to measure a small bookshelf?
Hard to tell from the little pictures but on the ground plane measurement is the mic laying on the driveway?
I tested it both as laying flat as well as tilted. There's consistently been about 1dB difference above 6kHz between the two measurements. And the NFS results lies right between these two in the S400's case. 1dB really is not bad and I wouldn't consider it an appreciable source of error in the grand scheme. But I plan to test this a little bit more.
When you do the testing on the garage floor how much distance around the speaker do you need? My garage is 2 cars deep but only one car wide. Would that be enough room to measure a small bookshelf?
It depends on how low you're trying to measure response. Not sure if you know what "gating" and "windowing" is. Assuming you do, if you're limited to only 3ms or something like that then you're not going to get any sort of high accuracy below 1khz, IMHO (I realize that 3ms ~= 300hz). If you are not familiar with these terms let me know and I'll give you an explanation.
I can get around 7ms and that's not high enough resolution for me. I will be testing outside when I can and worst case, will likely be merging outdoor with indoor for HF accuracy if wind/noise is of concern.
Hard to tell from the little pictures but on the ground plane measurement is the mic laying on the driveway?
I tested it both as laying flat as well as tilted. There's consistently been about 1dB difference above 6kHz between the two measurements. And the NFS results lies right between these two in the S400's case. 1dB really is not bad and I wouldn't consider it an appreciable source of error in the grand scheme. But I plan to test this a little bit more.
When you do the testing on the garage floor how much distance around the speaker do you need? My garage is 2 cars deep but only one car wide. Would that be enough room to measure a small bookshelf?
It depends on how low you're trying to measure response. Not sure if you know what "gating" and "windowing" is. Assuming you do, if you're limited to only 3ms or something like that then you're not going to get any sort of high accuracy below 1khz, IMHO (I realize that 3ms ~= 300hz). If you are not familiar with these terms let me know and I'll give you an explanation.
I can get around 7ms and that's not high enough resolution for me. I will be testing outside when I can and worst case, will likely be merging outdoor with indoor for HF accuracy if wind/noise is of concern.
I would like to measure down to 100 - 200hz just to see what bottom end response looks like on my speakers - my living room that I measure in gets messy at about 450hz. I have an omnimic so I can adjust my gate time to see how it pics up the low end.
I would like to design some 3 ways with side firing subwoofer, crossing over at 150 or lower, through i can measure and try to blend the response with the mids higher around the 500-600 hz mark, would be good to validate the XO sim lower around the 100hz mark to see how well reality meets the simulation
Okay, back to the ground plane measurements. Ground/grass surfaces are not ideal. I'll explain.
I re-tested the Buchardt S400 in my driveway. Here's a picture of the setup:
Here is the result compared to the results Amir posted and the factory curve posted on Buchardt's website:
That's pretty good correlation within those two sets (and notice those two both used the NFS and don't even match which is being discussed in the S400 thread).
Back to my opening statement that the the grass is not ideal...
When the 'rules' of ground plane measurements say you need a "hard" surface they mean it. Grass is NOT a good place to conduct ground plane measurements. And I understand why now. Rather than keep this information to myself I wanted to share in case it might be useful for someone here in the future. If you already knew this, congrats. If you didn't, let me be your example of why not to bother.
First, here's a picture of the backyard. Where the black chunk is where the speaker and mic were placed; very, very low grass. You can see it's obstruction free for a good bit; about 40 feet from the center is the fence and my house. Nothing else in the way.
In the below graphic you can see my driveway measurement mentioned above compared to (2) different backyard measurements: The blue graph is with the speaker and mic on the ground with nothing between. Just bare ground/grass. BOTH were taken with a 2x3 foot mirror placed under the microphone, thinking that would be enough reflective area. The red graph is with a long piece of plywood running the stretch of space between the microphone and the speaker. You can see the ground between the mic and speaker is responsible for a LOT of absorption. I don't know if it's the grass because my backyard is kept cut VERY low and it is pretty bare as you can see in the photo above. But, regardless of the exact influence of grass vs ground the results are eye-opening. The plywood also is not reflective enough alone.
I am going to buy concrete pavers and see if lining them up in a 2x8 foot section between the speaker and mic will help get me results that mimic the driveway. I am hopeful it will. If it doesn't then I will either have to test in the driveway or consider pouring a small concrete slab in the backyard. I like measuring in the backyard because it's a traffic-free area. My kid has a lot of friends in the neighborhood and they are always in and out of my garage space and in the driveway in the summer. Plus I want to avoid weird stares from my neighbors.
Anyway, like I said, I didn't necessarily have to share this and I know some don't give a rip. But if it helps at least one of you then I've done my job.
Comments
Just an update...
I purchased a mic stand over a month ago. After 2 shipping issues, I finally got the 3rd one and it was good to go. I also ordered some speaker stands and planned to use one for speaker testing. But after doing some testing I realized that was a no go. The stand was a bit shaky at above 6 feet. Plus, I wanted something a bit easier for takedown and set up. So I cannibalized my basketball goal (I need a new one anyway
). I buried the pole about 2 feet in the ground and rigged up a platform from HDPE and Plywood (to be sealed soon). The platform can also be swapped out to accommodate larger speakers (floorstanders laid on their side to capture vertical response). The pole is very sturdy and can be removed from the ground and placed in my storage area between tests.
The speaker stand is now at 8.5 feet off the ground and 9 feet away from the patio cover. Nearly everything I test will be reflection free to 9 feet. Which puts the first reflection at about 12 ms; or about 80hz. Doubling that means good resolution above 160hz. That’s good. Real good. More than adequate when you consider I will be using a ground-plane measurement for LF response and "stitch" that to the FF measurement. As for external noise; I live in out in the country. So it's mostly quiet. The software/hardware is smart enough to ignore most noises and multiple sweeps will help keep them from sneaking in to my measurements. IOW, I don't have any concerns here.
I tested hoisting up a 50# bag of sand. That wasn't easy. But was do-able. I'm gonna need a crane, though, if I ever test a floorstander that has significant weight to it. But I'll burn that bridge when I get there.
Here's a few pictures with my 6 foot ladder and myself as a reference.
The last picture is of the mic stand. This sucker is a beast. Can be stood to almost 8 feet tall and the mic can reach higher when the boom is angled.
Here's a link if anyone is wanting to see more info.
So, I finally got set up today to at least start taking some test measurements. I started with the ELAC DBR62 since it's what I sent to Amir over at ASR which he tested and posted a couple weeks ago. My results are ... different. Particularly in how Amir's measurements show a lift from about 600 - 1khz while mine shows the lift from about 1-2khz. Now, this was measured with the DUT about 8.5 feet off the ground and the first reflection didn't occur until about 13 ms (see attached). That means the first reflection does not occur until roughly 80hz (that's a good thing; a real good thing). Now, I played around with seeing what happens if I drop that window down to the standard 3ms that so many are forced to use. The trend still stays the same. You'll also notice I have a smoother response up high; no dip in the 16khz region as measured by Amir. I did play with adding foam to my mic stand. No real difference was made.
Point being, outside measurements has kind of been considered the next-best-thing to anechoic (and now NFS) measurements but there's a decent difference in what I measured (with a huge window of time being reflection free). I'm going to do this again tomorrow and if I find the same results I'm planning on contacting Klippel to see what they think.
I don't necessarily think there is anything alarming here; not yet, at least. But I wanted to share.
Note: Measurements were conducted with no ambient noise (no cars, no kids screaming, etc.) Measurements were averaged with 2-4 sweeps to help remedy any external noise, just in case, as advised by Klippel R&D.
I also now need an 8 foot ladder because that 6 foot ladder ain't gonna cut it when trying to lug a speaker up and down it and hoisting even the little bookshelves over my head to reach the 8.5 foot pedestal height. So I'll be shopping those tomorrow; hopefully someone has some on sale.
As long as it could be made to stop in 5 or 10 degree increments, that could work. But I don't currently have the knowledge set on how to build and code something like that up myself.
For now, I think I'm going to keep the manual solution of turning the turntable myself.
I posted this on another site and a fellow said he uses the same.
I was thinking of this last night but hanging the speaker. Didn't consider using it as a platform. That said, do you think it would be sturdy enough to place a floorstander on top of?
Something else I looked at was this:
https://www.harborfreight.com/high-position-motorcycle-lift-99887.html
I like this option because I could set the speaker/turntable/platform on top of this tray and use it to help me raise it an additional 2.5 feet which, if nothing else, makes things a bit more safe since I could load it from the ground and then raise it to measurement height.
Same goes for something like this:
https://amzn.to/3br2OQp
I even looked at drywall hoists last night. What I like is they can extend up to 11.5 feet (would never go that high) and they spin. So I could buy one, modify it with a platform adapter and mark it for polar response measurements. This thing looks practically perfect. But... I don't know how stable it is. Some videos I watched make it look kind of wobbly. With a speaker at the center it might be OK. This may be one of those "buy and try" things.
https://www.harborfreight.com/drywall-panel-hoist-69377.html?cid=paid_google|*PLA+-+All+Products|All+Products|69377&utm_source=google&utm_medium=cpc&utm_campaign=*PLA+-+All+Products&utm_content=All+Products&gclid=Cj0KCQjws_r0BRCwARIsAMxfDRh1sqOqaCA4R5jKf-Gd-DCXN6YGWhXBpe-pPH8yDDyh46qSusKFJo4aAjHtEALw_wcB
You can see what I mean about it being wobbly in this video. But I'm wondering if placing a platform on it and simply weighing it down with a sandbag or cinder block behind the speaker would help stabilize it.
About a month ago I purchased a pair of Elac DBR62 speakers and sent it to Amir at AudioScienceReview so he could measure it with his Klippel Near Field Scanner. Results here:
Elac Debut Reference DBR-62 Speaker Review | Audio Science Review (ASR) Forum
The reason I was interested is because I'm starting back up doing reviews and thought having the NFS 'anechoic' measurements would be a big help in determining my measurement method for reviews. My initial plan was to measure outdoors for far-field mid/high frequency and merge with a ground-plane measurement for low frequency. So, here's the first measurement...
Test # 1) Speaker on platform ~ 8.5 feet off the ground; window to about 13ms with accuracy down to approximately 100hz.
Speaker flush with front of platform (picture was taken before I began tests).
Pros: This method has the benefit of having very high resolution as you go above 200hz.
Cons: being subject to the elements (cold, wind, rain; external noise not really an issue in my area) and having to hoist a speaker up 8.5 feet in the air and risk throwing out my shoulder should I ever test a floorstanding speaker.
Here's a photo of the setup (I am using thumbnails on purpose; don't want to swamp this page with oversized images).
Here's the result compared to Amir's. You can see some decent differences namely above 1khz. Notably the 1-2khz region.
Now, I wouldn't say that test was 'final'; it was just a "proof of concept" and I suspect that I could probably get better accuracy with an additional test and higher # samples that feed the average. Given the hassle of setting the speaker up high I thought I would try a few other methods out to see how they compare.
Ok. That brings you up to speed to today's testing. Where I tried a number of things. I'll list the method, the pros/cons and provide a picture of the setup as well as my result vs Amir's result. Note: The stand was thrown together from scrap wood; if I were to use this it would change. But I didn't want to waste too much time because storms are setting in this afternoon and I needed to get to work. Also, ignore levels for now. I used a mic calibration file for FR but haven't checked the level with the pistonphone; I only care about the general trends for this process.
Test # 2) Measurement on a 5 foot stand in my garage; gating was about 4 ms.
Same stand used below but inside my garage with a ceiling height of about 10 feet. The speaker was positioned about 5 feet above the ground with the tweeter at about 10" off the platform.
Pros: Don't have to worry about the elements nor do I have to hoist the speaker 8 feet in the air.
Cons: Poor resolution below 1khz. I don't think I would even bother posting this data. For a single drive unit it's fine. But, speaking from experience, I need good resolution to determine where to merge LF response data of ported loudspeakers.
I didn't take a picture of this setup.
Here's the result compared to Amir's:
Test # 3) Outdoors on stand. Same stand as before. Window ~ 7ms.
Pro: Better resolution than garage measurement since ceiling reflection isn't there and thus gating is extended an additional 3ms or so.
Con: Outoors; weather/environment. Still not as good as the 8.5 foot platform or ground plane measurement (see below).
Here's the result compared to Amir's:
Test # 4) Ground Plane Measurement: Speaker Angled ~ 8 degrees; window out to 40ms before first reflection.
Speaker was angled at about 8 degrees so that the tweeter would be pointing on-axis with the microphone (well, as on-axis as it can be without burying the mic and pointing the mic at the DUT). Note that while the response was gated here, if I were to actually use this method I would move to the back yard where I could get the nearest reflection about 40 feet away which should get me down to around 20hz. So, this was more a sanity check test.
Pro: Invulnerability to reflection; I can get ever further out in my backyard. High resolution in to the very low frequency region.
Con: Outdoors. Diffraction effect of speaker changes due to the baffle doubling (via the mirror image effect). HF > 10khz seems to be a pain. From my research that's generally accepted as questionable. Though, I had decent results.
Phone placed at tweeter on baffle and used in 'selfie' mode to make sure the microphone was at the center of the image (trick I learned on another website).
Here's the result compared to Amir's:
Conclusion:
At face value, each of my methods look pretty reasonable. And all are pretty much "industry standard". You can go to a website or magazine and see any number of reviewers using any of the above methods. But I wanted to do a comparison myself and see if there was any one method that is better than the other. Unfortunately, there isn't. I was hopeful the Ground-Plane method would yield the utmost accuracy (aka: match what Amir has). But it's missing the 600 hz - 1khz bump Amir's data shows and there's a slight difference in the LF region. Potentially diffraction effects; but playing around in EDGE and VirtuixCAD I didn't see the influence. Otherwise, it looks pretty dang good.
There's two possible reasons I'm seeing differences: 1) none of the methods I use are fool-proof or 2) I shouldn't put all my trust in to the NFS (not saying it's wrong; just saying it may also have a few things that make 97% accurate instead of 100% accurate; keeping in mind that an anechoic chamber has its own issues). I think the answer may actually be a large heaping of #1 and a small portion of #2. All of that said, once you combine these possibilities with the level of detail I'm viewing this in... really, none of my results are extremely different from the NFS results. They're just different in some areas and alike in others. I mean, I'm usually within 2dB difference. Which, yea, it's not minimal but in the grand scheme of things, I'd say that's not too bad. I want to be closer, though.
Ultimately, this is where I am at:
I still want to re-test on the 8.5 foot platform. Maybe I can do that tomorrow.
Measuring in the garage is convenient. Measuring outdoors provides better resolution, especially in the area where I would need to perform low-frequency response stitching and that is useful in determining how best to align the two sets of measurements.
I would prefer to find a method that yields the best accuracy without the need for additional post-processing. Though, when I started this venture my plan was to use the outdoor farfield measurement for > 200hz response and merge that with a ground-plane measurement for low frequency.
But if I could figure out how to calculate the diffraction impact then I could just remove it from the result. I could also use the near-field technique but if I were to get an oddly shaped port (flared) or something that's harder to easily measure and quantify then that would impact accuracy as well. There's also the "mic in the box" method but it has the issue of being able to fit a mic; some speakers (like this Elac) won't allow my mic inside the slot port.
If you've read this far I assume you have some vested interest in these results. So, I welcome your opinions, as either or both fellow testers and reviewers. Would you say the ground plane measurement is "close enough"? Is there a formula or set of formulas I can incorporate in to my Matlab script (which post-processes the data) to account for diffraction in the ground plane measurement? Would you stick with the original plan, combining "speaker in the sky" (Test #1) for mid/high frequency with ground plane for LF?
Am I off my rocker?
Do you have a comparison of the NFS vs groundplane and the 8.5 feet. Would like to know how they compare below the 200hz or so.
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.
Now that Amir has tested the Buchardt S400, which I just received yesterday (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.
Before I show data let me say that this is NOT A FINAL TEST. This is simply an ad-hoc, "let's see if this looks logical" test to compare against Amir's data. My actual test results will be full range with higher resolution in the lower frequencies. Phew... OK. Read on.
Picture (the garage door was opened just to get some light for the photo; was shut during testing). The mic is ~ 2 meters from the speaker. The speaker itself is angled so that the tweeter has a direct line of sight with the microphone. I don't know what the angle is; I used my camera is selfie mode to put the speaker in the center of the frame and called it good enough.
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 nice. 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.
I tested it both as laying flat as well as tilted. There's consistently been about 1dB difference above 6kHz between the two measurements. And the NFS results lies right between these two in the S400's case. 1dB really is not bad and I wouldn't consider it an appreciable source of error in the grand scheme. But I plan to test this a little bit more.
I can get around 7ms and that's not high enough resolution for me. I will be testing outside when I can and worst case, will likely be merging outdoor with indoor for HF accuracy if wind/noise is of concern.
Okay, back to the ground plane measurements. Ground/grass surfaces are not ideal. I'll explain.
I re-tested the Buchardt S400 in my driveway. Here's a picture of the setup:
Here is the result compared to the results Amir posted and the factory curve posted on Buchardt's website:
That's pretty good correlation within those two sets (and notice those two both used the NFS and don't even match which is being discussed in the S400 thread).
Back to my opening statement that the the grass is not ideal...
When the 'rules' of ground plane measurements say you need a "hard" surface they mean it. Grass is NOT a good place to conduct ground plane measurements. And I understand why now. Rather than keep this information to myself I wanted to share in case it might be useful for someone here in the future. If you already knew this, congrats. If you didn't, let me be your example of why not to bother.
First, here's a picture of the backyard. Where the black chunk is where the speaker and mic were placed; very, very low grass. You can see it's obstruction free for a good bit; about 40 feet from the center is the fence and my house. Nothing else in the way.
In the below graphic you can see my driveway measurement mentioned above compared to (2) different backyard measurements: The blue graph is with the speaker and mic on the ground with nothing between. Just bare ground/grass. BOTH were taken with a 2x3 foot mirror placed under the microphone, thinking that would be enough reflective area. The red graph is with a long piece of plywood running the stretch of space between the microphone and the speaker. You can see the ground between the mic and speaker is responsible for a LOT of absorption. I don't know if it's the grass because my backyard is kept cut VERY low and it is pretty bare as you can see in the photo above. But, regardless of the exact influence of grass vs ground the results are eye-opening. The plywood also is not reflective enough alone.
I am going to buy concrete pavers and see if lining them up in a 2x8 foot section between the speaker and mic will help get me results that mimic the driveway. I am hopeful it will. If it doesn't then I will either have to test in the driveway or consider pouring a small concrete slab in the backyard. I like measuring in the backyard because it's a traffic-free area. My kid has a lot of friends in the neighborhood and they are always in and out of my garage space and in the driveway in the summer. Plus I want to avoid weird stares from my neighbors.
Anyway, like I said, I didn't necessarily have to share this and I know some don't give a rip. But if it helps at least one of you then I've done my job.