(Edit: Sorry about the extra large photos: I'll re-size if I have the time)
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). Mic at 1m; signal 2.83v. Note: I don't care that the mic stand can cause a reflection for this measurement; this is just a test of a test.
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:
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. Mic at 1m; signal 2.83v.
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.
Mic at 1m; signal 2.83v. Note: I don't care that the mic stand can cause a reflection for this measurement; this is just a test of a test.
Pro: Better resolution than garage measurement since ceiling reflection isn't there and thus gating is extended an additional 3ms or so. Don't have to hoist the speaker up 8 feet in the air.
Con: Outoors; weather/environment. Still not as reflection-free 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). Mic at 2m; signal 2.83v. Note: Though 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.
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:
Thoughts:
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 at this point:
I still want to re-test on the 8.5 foot platform. Maybe I can do that tomorrow.
Measuring in the garage is convenient. But 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?
- Erin
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). Mic at 1m; signal 2.83v. Note: I don't care that the mic stand can cause a reflection for this measurement; this is just a test of a test.
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:
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. Mic at 1m; signal 2.83v.
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.
Mic at 1m; signal 2.83v. Note: I don't care that the mic stand can cause a reflection for this measurement; this is just a test of a test.
Pro: Better resolution than garage measurement since ceiling reflection isn't there and thus gating is extended an additional 3ms or so. Don't have to hoist the speaker up 8 feet in the air.
Con: Outoors; weather/environment. Still not as reflection-free 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). Mic at 2m; signal 2.83v. Note: Though 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.
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:
Thoughts:
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 at this point:
I still want to re-test on the 8.5 foot platform. Maybe I can do that tomorrow.
Measuring in the garage is convenient. But 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?
- Erin
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I wish the review would have shown a picture of the measurement conditions for the speaker. Something that comes to mind is that no measurement math can inherently differentiate a speaker from whatever it is sitting on, i.e., determine where the cabinet ends and the stand or shelf or platform or whatever begins.
IMO a very few dB in speaker measurement terms really isn't a lot, particularly when the speaker will actually get used in a room rather than be dangled in infinite open space. That can get into numbers that are interesting but not super relevant to actual use.
IMO a very few dB in speaker measurement terms really isn't a lot, particularly when the speaker will actually get used in a room rather than be dangled in infinite open space. That can get into numbers that are interesting but not super relevant to actual use.
He uses the Near Field Scanner:
Near Field Scanner System (NFS)
Here's a video from Warkwyn showing it in action:
YouTube
I agree. I think at this point I'm just so "deep in it" that I can't get past the multiple overlaps but also the portions of each method that don't quite line up. And it's annoying me. Your reply, believe it or not, takes a little weight off my shoulders.
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I have some comments about your setup. Let's look at this picture for example:
You have a huge yard. Why not use it? What is important is the closest distance of a reflecting surface. There is a shadow in the foreground that makes me assume you are next to your house or other building. That's a reflecting surface. Get some long extension cords and make your measurement halfway between your house and the fence int the background.
Next up: the speaker stand and what lies below. Note that the stand is set up on a patio or concrete pad. That is an excellent reflecting surface. This is probably limiting your resolution: the difference in time from speaker to mic (direct thru air) and the time it takes for sound travel from speaker to pad and back up to mic. If you could elevate your speaker even more and/or put the mic closer to the speaker, that would delay when the first reflection arrives and you will get better resolution. You only need to have the mic at least 3 times the larger of the baffle width or height away I think.
Also: if this was set up above the grass instead of a concrete pad you would get less high frequency reflection because grass and earth will partially adsorb higher frequencies. You can also put something like pillows on the ground where the reflection will take place to help with this.
You mention that the stand might be an issue. I agree. No need to have that top late or shelf that is wider than the base of the speaker. It's probably causing some of that ripple you see at higher frequencies in your measurement. Can you modify it or make a slimmer one? One option is to use the top step of a 10' ladder. The top is usually slim enough to not protrude past the speaker baffle and you can walk up to place the speaker. Do a stability check first!
If you have to use that stand (old basketball hoop?), I would try to place the speaker on top so that the baffle plane is aligned with the edge of the shelf. That will certainly be better than the exposed "lip" sticking out past the baffle that you have now. And if you cannot go way out into the grass, you could at least do the measurement from the other side, with the mic stand in the grass, looking back at the home/structure that is generating the shadow.
Honestly you are already doing many things right. The above advice should help you get the most out of this kind of measurement.
One comment, though. You said:
You have a huge yard. Why not use it? What is important is the closest distance of a reflecting surface. There is a shadow in the foreground that makes me assume you are next to your house or other building. That's a reflecting surface. Get some long extension cords and make your measurement halfway between your house and the fence int the background.
Next up: the speaker stand and what lies below. Note that the stand is set up on a patio or concrete pad. That is an excellent reflecting surface. This is probably limiting your resolution: the difference in time from speaker to mic (direct thru air) and the time it takes for sound travel from speaker to pad and back up to mic. If you could elevate your speaker even more and/or put the mic closer to the speaker, that would delay when the first reflection arrives and you will get better resolution. You only need to have the mic at least 3 times the larger of the baffle width or height away I think.
Also: if this was set up above the grass instead of a concrete pad you would get less high frequency reflection because grass and earth will partially adsorb higher frequencies. You can also put something like pillows on the ground where the reflection will take place to help with this.
You mention that the stand might be an issue. I agree. No need to have that top late or shelf that is wider than the base of the speaker. It's probably causing some of that ripple you see at higher frequencies in your measurement. Can you modify it or make a slimmer one? One option is to use the top step of a 10' ladder. The top is usually slim enough to not protrude past the speaker baffle and you can walk up to place the speaker. Do a stability check first!
If you have to use that stand (old basketball hoop?), I would try to place the speaker on top so that the baffle plane is aligned with the edge of the shelf. That will certainly be better than the exposed "lip" sticking out past the baffle that you have now. And if you cannot go way out into the grass, you could at least do the measurement from the other side, with the mic stand in the grass, looking back at the home/structure that is generating the shadow.Honestly you are already doing many things right. The above advice should help you get the most out of this kind of measurement.
One comment, though. You said:
The resolution is the same at all frequencies. FFT data comes in "bins" of a fixed width. So the resolution of the first bin is the same as the last. As you go up in frequency there are more bins per octave (that's a log scale based measurement). The measurement program (ARTA?) draws a continuous line but in reality the data is more like a histogram.
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Charlie, thanks for the reply. Though, I think you missed some comments I made in my OP.
I stated that if I decided to go with the ground plane measurement route, I would measure in the backyard for ground plane measurements.
Yes, but it would be of little consequence. The window is already about 13ms, as I mentioned. That’s the first reflection based on the PLD from speaker/mic vs speaker/ground/mic. 13ms is about 80hz; doubling that provides me a lot of flexibility above 200hz.
You mention that the stand might be an issue. I agree. No need to have that top late or shelf that is wider than the base of the speaker. It's probably causing some of that ripple you see at higher frequencies in your measurement. Can you modify it or make a slimmer one? One option is to use a 10' ladder. The top rung is usually slim enough and you can walk up to place the speaker. Do a stability check first!
Mentioned this in my OP as well. 😉
Thanks. This post was more of a “I tried various things; all differ from the NFS “anechoic” measurement in some form or another. Do I sweat it, or just pick one and go with it? I don’t like “good enough”, but I also don’t know what’s truly realistic to expect. I have exhausted my resources and my testing helps to show that. So, I think I’ve done the best I can and the results still show deviation from the NFS. But I won’t call it an “error” from the NFS measurements… I feel pretty good about what I’ve done thus far.
What I mean is, with the window the floor bounce effect subsides as you go past the initial bounce time. I should have been more clear.
Ultimately, I guess I'm a bit unclear on what your suggestion is:
Are you suggesting go to the center of the yard, accept the ground bounce and roll with it?
I was thinking a hybrid of that + the ground plane for LF measurement portion.
And, you mentioned measuring from the other side of the stand. If the stand stays in play then that's what I'll do. The reason I was measuring from the concrete was because I was rotating my mic stand about the speaker in that initial test. That was no bueno. Which is why I built a wooden turntable stand. So, the idea is that I would place the new turntable stand on the concrete and put the mic stand in the grass. Again, if I don't go with a ground-plane measurement for the whole shebang. I just don't know if I trust the ground plane measurement for a full-range measurement; above 10khz seems to get dicey from what I'm seeing in the pro-audio sites. I have to say, though, the GP measurement I posted looks quite good and nearly perfectly matches the NFS data except for the 600hz to 1khz region. And unfortunately my other measurements I did today match that bump in the NFS data. So it's a "six-of-one, half-a-dozen of the other" situation... :/
Are you suggesting go to the center of the yard, accept the ground bounce and roll with it?
I was thinking a hybrid of that + the ground plane for LF measurement portion.
And, you mentioned measuring from the other side of the stand. If the stand stays in play then that's what I'll do. The reason I was measuring from the concrete was because I was rotating my mic stand about the speaker in that initial test. That was no bueno. Which is why I built a wooden turntable stand. So, the idea is that I would place the new turntable stand on the concrete and put the mic stand in the grass. Again, if I don't go with a ground-plane measurement for the whole shebang. I just don't know if I trust the ground plane measurement for a full-range measurement; above 10khz seems to get dicey from what I'm seeing in the pro-audio sites. I have to say, though, the GP measurement I posted looks quite good and nearly perfectly matches the NFS data except for the 600hz to 1khz region. And unfortunately my other measurements I did today match that bump in the NFS data. So it's a "six-of-one, half-a-dozen of the other" situation... :/
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Understood, but that method doesn't isolate the drivers' output from the effects of the baffle or speaker box sides, nor would we want it to; neither does it isolate from surfaces the box might be sitting on, the difference is entirely a matter of human definition. In essence anything (barring very effective directivity) within some portion of a wavelength becomes effectively part of the speaker. The measurement definition of what is speaker and what is environment, even with NFS, depends on how software, gear, and unit-under-test are set up.
In a related note, some speakers are designed to be used near relevant surfaces like corner or back wall. Almost all home speakers are intended to be used near a floor. All those things are proper "parts of the speaker".
Also, to add to what Charlie said, the distances (from surfaces) you need to consider are not just related to the speaker -- but also to the microphone. Which is where that shadow from the house catches notice.
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Understood. I knew what you meant regarding the ability of the the measurement system to make the distinction between the DUT and the measurement rig (though, I would assume Klippel has somehow taken this in to account). You mentioned wishing you had pictures of the measurement conditions so I didn't know if you caught the NFS part. IME, people tend to pick and choose what they read.
The mic was also 9 feet away from any surface (nearest wall = 9 feet perpiducular); everything else like the patio behind it and the trampoline to the other side > 9 feet.
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I re-tested the bookshelf on the 8.5 foot stand.
The result follows closely with Amir's but, again, I still have a discrepency in the 600-2khz region; where the NFS shows a bump from 600 to 1khz, my result is a bit flat and where the NFS results flatten from 1-2khz, my results show a mild bump. In the grand scheme, these differences are less than a couple dB. At this point, I think I'm willing to simply chalk this up to measurement method and call it a day. Even the early NFS measurements didn't overlay exactly on top of the Harman graphs. But I will attempt a closer measurement to see if this changes the trends I am getting in the response.
The result follows closely with Amir's but, again, I still have a discrepency in the 600-2khz region; where the NFS shows a bump from 600 to 1khz, my result is a bit flat and where the NFS results flatten from 1-2khz, my results show a mild bump. In the grand scheme, these differences are less than a couple dB. At this point, I think I'm willing to simply chalk this up to measurement method and call it a day. Even the early NFS measurements didn't overlay exactly on top of the Harman graphs. But I will attempt a closer measurement to see if this changes the trends I am getting in the response.
Bikinpunk, thanks for posting this, such measurement methods are seldom discussed in detail.
I've spent a great deal of time trying various outdoor measurements too.
So here's my 2 cent comments ....
First, I need to say I don't understand how NFS is able to get around the "measure no closer than 3x greatest baffle dimension" rule of thumb.
I'm not doubting that NFS works...I don't begin to have the expertise to question it and Kippel...just wondering how it does it....
Anyway, I've tried speaker in air with mic in air and with mic on ground.
I've tried speaker on ground with mic on ground, and with mic in air.
I've got a 3-mic rotating boom off a deck that opens to a lake, that in combination with a spinorama can grab polars pretty easy. Albeit with more reflections than getting out in the yard, as it only has a 3m measuring distance.
I guess the measurements I've come to use and trust the most, are speaker on ground, mic on ground, generally at 4 meters.
I like to use a gentle hill where the speaker center focuses uphill to the mic. Or on flat ground, tilt the speaker like you did.
I've read that the mic should be on a very smooth surface, like glass, for best VHF readings. And that the capsule should be as close to the glass as possible without touching, tilting down.
But frankly, I don't much care about that kind of VHF accuracy because I keep a readily adjustable high shelf filter in play all the time simply for needed source material adjustment.
Sometimes I'll go to the trouble of speaker in air. I've found a Global Truss ST-132 is a great way to get them 13 ft up. I still think mic on ground is the way to go then too, but speaker tilt can become an issue.
And this is just me, but i refuse to gate.
Throws away data for looks sake, imo/ime.
I just work at getting as reflection free as possible, and live with that.
I've spent a great deal of time trying various outdoor measurements too.
So here's my 2 cent comments ....
First, I need to say I don't understand how NFS is able to get around the "measure no closer than 3x greatest baffle dimension" rule of thumb.
I'm not doubting that NFS works...I don't begin to have the expertise to question it and Kippel...just wondering how it does it....
Anyway, I've tried speaker in air with mic in air and with mic on ground.
I've tried speaker on ground with mic on ground, and with mic in air.
I've got a 3-mic rotating boom off a deck that opens to a lake, that in combination with a spinorama can grab polars pretty easy. Albeit with more reflections than getting out in the yard, as it only has a 3m measuring distance.
I guess the measurements I've come to use and trust the most, are speaker on ground, mic on ground, generally at 4 meters.
I like to use a gentle hill where the speaker center focuses uphill to the mic. Or on flat ground, tilt the speaker like you did.
I've read that the mic should be on a very smooth surface, like glass, for best VHF readings. And that the capsule should be as close to the glass as possible without touching, tilting down.
But frankly, I don't much care about that kind of VHF accuracy because I keep a readily adjustable high shelf filter in play all the time simply for needed source material adjustment.
Sometimes I'll go to the trouble of speaker in air. I've found a Global Truss ST-132 is a great way to get them 13 ft up. I still think mic on ground is the way to go then too, but speaker tilt can become an issue.
And this is just me, but i refuse to gate.
Throws away data for looks sake, imo/ime.
I just work at getting as reflection free as possible, and live with that.
Thank you for the reply. Great information.
Likewise, I have had trouble finding discussion on measurement methods. It seems people just pick one and go with it but don't go through the extra effort to test in different manners to explore differences to weigh out the pros/cons.
The NFS uses holography; it measures nearfield in thousands of points and then uses math (above my pay grade) to ‘extract’ that to farfield conditions.
My concern with the ground plane measurement is the HF response. From my readings, the area above 10kHz is often subject to the environment. I want to experiment with this in my backyard this afternoon now that it has dried up a bit. I’ll post the data back. I, too, have read that a highly reflective surface is imperative to the ground plane measurement so I’ll dig around the house and see if I can find something to use. My plan was that if I had good results that I felt were worth pursuing then I would purchase a large mirror to place the mic on.
I’m hoping the GP measurement testing bears more fruit when I do this. Because it is a lot easier to set the speaker on the ground than it is to lift it 8+ feet in the air. Especially large speakers. Which brings up this… The truss you are using… that is one of the ones I was looking at last night. There’s also the Ultimate Support TS-110B stand which looks really interesting. It’s air assisted to help with the lifting and I can purchase a speaker stand adapter plate to mount a solid board to in order to place a speaker on top. Are you happy with it? Is it stable up to 8-9 feet with a heavier (50#+) speaker? For bookshelf speaker testing I’m not concerned but with floorstanding speakers I’m having a hard time finding a good solution.
I have even considered getting a lift like this to place my speaker/turntable stand on and then simply use that to lift it an additonal 5 feet off the ground. Sure would make life easy if speaker-in-the-air measuring is the route I go. But that's a good deal of money that I do not have.
Likewise, I have had trouble finding discussion on measurement methods. It seems people just pick one and go with it but don't go through the extra effort to test in different manners to explore differences to weigh out the pros/cons.
The NFS uses holography; it measures nearfield in thousands of points and then uses math (above my pay grade) to ‘extract’ that to farfield conditions.
My concern with the ground plane measurement is the HF response. From my readings, the area above 10kHz is often subject to the environment. I want to experiment with this in my backyard this afternoon now that it has dried up a bit. I’ll post the data back. I, too, have read that a highly reflective surface is imperative to the ground plane measurement so I’ll dig around the house and see if I can find something to use. My plan was that if I had good results that I felt were worth pursuing then I would purchase a large mirror to place the mic on.
I’m hoping the GP measurement testing bears more fruit when I do this. Because it is a lot easier to set the speaker on the ground than it is to lift it 8+ feet in the air. Especially large speakers. Which brings up this… The truss you are using… that is one of the ones I was looking at last night. There’s also the Ultimate Support TS-110B stand which looks really interesting. It’s air assisted to help with the lifting and I can purchase a speaker stand adapter plate to mount a solid board to in order to place a speaker on top. Are you happy with it? Is it stable up to 8-9 feet with a heavier (50#+) speaker? For bookshelf speaker testing I’m not concerned but with floorstanding speakers I’m having a hard time finding a good solution.
I have even considered getting a lift like this to place my speaker/turntable stand on and then simply use that to lift it an additonal 5 feet off the ground. Sure would make life easy if speaker-in-the-air measuring is the route I go. But that's a good deal of money that I do not have.
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]Good stuff
One thing with HF measurements that might not be fully apparent with REW, is how much a little wind effects the measurement.
Using a dual channel FFT where the measurement is in continuous real time, the upper end of the phase trace starts waving like a flag with very little wind.
I never see that with REW, cause it just grabs what it grabs. Need to look at loopback times for variance due to wind,.... if variance gotta kinda ignore VHF...
I've use the ST-132 with speakers up to 80 lbs. I lay the stand down on it's side, and stick the speaker on the pole, then walk it upright like raising a flag pole. It's the easiest, cheapest way to get 13 ft high I've found. Only con is it's heavy...but then again that equals safety and capacity and height....
I gone mine from idjnow.
I also have some Ultimate 90B's. They are nice, but i do wish they had gas support like the 110b's. But they aren't in the same league at all, with the ST-132.
One thing with HF measurements that might not be fully apparent with REW, is how much a little wind effects the measurement.
Using a dual channel FFT where the measurement is in continuous real time, the upper end of the phase trace starts waving like a flag with very little wind.
I never see that with REW, cause it just grabs what it grabs. Need to look at loopback times for variance due to wind,.... if variance gotta kinda ignore VHF...
I've use the ST-132 with speakers up to 80 lbs. I lay the stand down on it's side, and stick the speaker on the pole, then walk it upright like raising a flag pole. It's the easiest, cheapest way to get 13 ft high I've found. Only con is it's heavy...but then again that equals safety and capacity and height....
I gone mine from idjnow.
I also have some Ultimate 90B's. They are nice, but i do wish they had gas support like the 110b's. But they aren't in the same league at all, with the ST-132.
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Aah, i put top hats in all my DIY builds.
If i had a store bought to measure, i guess i'd make a shelf/bracket and mount in on something like this...ProX X-SSMP Speaker Stand Mounting Plate | IDJNOW
If i had a store bought to measure, i guess i'd make a shelf/bracket and mount in on something like this...ProX X-SSMP Speaker Stand Mounting Plate | IDJNOW
If you want to get the most accurate result (although dependant of outside conditions) you need a crane or some kind of construction that is 8m high. You turn the loudspeaker face down, and flush mount the mic in the ground. That will give you resolution to about 20Hz. That is how they do it in Kef when they want really high resolution measurements and very long gate to get the most accurate results.
I'm doing essentially what you are doing but in nearby school - at their basketball court. It costs about 20 euros and they let us use it from 22:00h to 05:00h. This pic is made this winter when temperature outside was -3 degrees Celsius. Loudspeaker is at height of 3 meters which gives me about 11-12ms gate time and good resolution down to 80Hz.
I'm doing essentially what you are doing but in nearby school - at their basketball court. It costs about 20 euros and they let us use it from 22:00h to 05:00h. This pic is made this winter when temperature outside was -3 degrees Celsius. Loudspeaker is at height of 3 meters which gives me about 11-12ms gate time and good resolution down to 80Hz.
As for really low stuff - under 100Hz, i just do the simulation in one of sim programs (Unibox, Winisd) and merge it to my measured data. Simulators are very precise down low. As Andrew Jones said - if you give me ts parameters, i'll calculate your response under 100Hz with 0.5dB error.
About mic stand reflections. I did mount some damping material around it and made measurement before and after. The results were so similar that i gave up on taping it to the mic stand every time i do the measurements. Here are fer pics of my measurements:
Kef R300 woofer raw in cabinet - very hires baffle step effect visible 2dB grid resolution
Kef LS50 midwoofer raw in cabinet on and off axis 2dB grid resolution
Kef LS50 tweeter raw in cabinet on and off axis 2dB grid resolution
Small three way speaker i worked on 0-10-20 degrees 1dB grid resolution
0 and 90 degrees off axis 5dB grid resolution
About mic stand reflections. I did mount some damping material around it and made measurement before and after. The results were so similar that i gave up on taping it to the mic stand every time i do the measurements. Here are fer pics of my measurements:
Kef R300 woofer raw in cabinet - very hires baffle step effect visible 2dB grid resolution
Kef LS50 midwoofer raw in cabinet on and off axis 2dB grid resolution
Kef LS50 tweeter raw in cabinet on and off axis 2dB grid resolution
Small three way speaker i worked on 0-10-20 degrees 1dB grid resolution
0 and 90 degrees off axis 5dB grid resolution
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Mark,
Wind gradients alone can easily cause well over 360 degree phase rotations at 16kHz measured real time with a dual channel FFT.
On top of the wind gradient refractive effects, the ST-132 at full extension is literally "waving like a flag", that movement can easily be several wavelengths long at 16kHz.
In the photos below, looking up the ST-132 at full height, it is easy to see the deflection backwards (toward the speaker and mic cord) and forward. Those static deflections simply from the slight COG (center of gravity) difference between the test speaker (SynTripP) with and without it's horn extender are small compared to the 100+ millimeter rotations visible with relatively light wind gusts.
Art
Attachments
Thanks for the info.
With outdoor measurements the biggest issue I've been battling is the wind.
The extraneous noise is minute and sparse enough to not be a big issue in my case. But the HF response is the area I'm most concerned with when it comes to outdoor GP measurements; namely due to the wind. Your post makes me wonder if I could take the stitching approach here as well. Something like this:
1) Measure typical GP to for LF/MF.
2) Run an additional test with a "wall" to block out wind. Think of a tunnel. Maybe a foot or two wide. Enough to ensure <1kHz gating. (or just measuring in my garage, for that matter)
3) Merge the two together to ensure accuracy of HF response.
I suppose it's at least worth a shot. I made a trip to Lowe's earlier to purchase a 2x2 section of raw mirror to place the mic on and plan to test it this afternoon. So I will also see if the 'tunnel' approach helps at all with the high frequency response resolution.
I have a large enough back yard where GP measurements allow resolution down to 20hz or so. And it sure is a lot easier to measure on flat ground than it is to suspend a large speaker in the air. Especially when you are trying to rotate the DUT with the center point being at the baffle which complicates matters even more wrt the stand/support.
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Wind variations can be reduced by using a swept-sine (log style) stimulus. The trouble with basic dual FFT using program material or noise stimulus is that the HF is applied over the entire 'burst' time. As weltersys mentioned, the times where the HF gets shifted out of phase from the phase at other times will effectively cancel the HF sensed over the full burst time. But with a swept sine, particularly the 'log-swept' (where speed of frequency change is proportional to the frequency), the stimulus is applied and analyzed over shorter time periods so HF cancellation has less effect.
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Ok. FWIW, this is the module I am using for these measurements.
https://www.klippel.de/fileadmin/klippel/Bilder/Our_Products/R-D_System/PDF/S7_TRF.pdf
I also plan to run an indoor ground plane test and do some comparisons as discussed in my previous post. If nothing else, it will scratch an itch.
https://www.klippel.de/fileadmin/klippel/Bilder/Our_Products/R-D_System/PDF/S7_TRF.pdf
I also plan to run an indoor ground plane test and do some comparisons as discussed in my previous post. If nothing else, it will scratch an itch.