Maybe it doesn't have to be anechoic, as such. All it has to do is reflect sound *away* from both the speaker and the mic. A 45º sawtooth profile, reflecting sound upwards towards the sky (in the open, of course...)?
DF
Here you go! I made this post on ASR so I'll just copy/paste it from there. I specifically bolded a portion of it that I think you'll want to keep in mind...
I performed my own spin on a DIY'd turntable. Now, this is, once again, not something I would consider *FINAL*. It was an indoors ground plane measurement and therefore the window of reflection free response is only about 6ms long which means accuracy only down to ~166hz, and the DUT was lifted about 2 inches off the ground as it was placed on a turntable and therefore, comparing back to back against the "direct to floor" method, there is some variation, notably below 1kHz. The plans for future tests is to conduct ground plane measurements outside so I get a longer gate and bury the turntable so it sits flush with the ground so there is no misleading data due to it being on a 2" platform. But I believe the response >1kHz is at least useful as a comparison within itself (IOW, comparing on/off axis response within my own dataset).
The center of rotation is the front baffle. Some reviewers place the DUT at the center of the turntable, thus making the center of the speaker itself the rotation point. My rotation point is the baffle. You'll see what I mean in these pictures:
This is the 0 degrees (on-axis) setup:
This is the 90-degrees off-axis setup:
And here are the results of my horizontal. Again, keeping in mind the notes I mentioned above.
Those plots look great to me !
Well done !
Confirms off-axis ground-plane technique is no problem ...if we can figure out the right setup to rotate with.
Okay, back to the ground plane measurements. Ground/grass surfaces are not ideal. Everything I've read about measuring in the ground plane says you need a hard reflective surface. I didn't realize how important this was. 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 (BOTH were taken with a 2x3 foot mirror on the ground at the mic): The blue graph is with the speaker and mic on the ground with nothing between. Just bare ground/grass. 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 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. I don't think it matters that the area next to the speaker and mic are not hard reflective surfaces but I don't know for sure until I measure. 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.
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 (BOTH were taken with a 2x3 foot mirror on the ground at the mic): The blue graph is with the speaker and mic on the ground with nothing between. Just bare ground/grass. 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 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. I don't think it matters that the area next to the speaker and mic are not hard reflective surfaces but I don't know for sure until I measure. 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.
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I very much appreciate you having shared this. Since reading about ground plane measurements many years ago, I had always been skeptical, and therefore never even tried it. The information you shared here is invaluable, especially with the examples of both good and less successful measurements.
I second Shaun's statement: this is wonderful information, and I'm sure there are many of us who, like me, want to know how to do things right, to get useful results. Your methodical and well documented efforts probably saved quite a few hours wasted on flawed measurements, by many more than one person.
So, thank you very much indeed. Now, I'm off to try to find out *why* the surface has to be reflective...
DF
So, thank you very much indeed. Now, I'm off to try to find out *why* the surface has to be reflective...
DF
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. I don't think it matters that the area next to the speaker and mic are not hard reflective surfaces but I don't know for sure until I measure. 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.
Very cool and thank you!
You've made me very glad the only speakers I've pulled off the driveway, out into yard to measure, have been subs....well, for the most part anyway
Hey, maybe a good ole shuffleboard court could serve double duty for you and family / neighbors...
Grass is both reflective and absorptive dependent on angle of incidence to the blade, my guess is the 2.5kHz vs the 4kHz dips are resultant from out of phase reflected energy at the mic location.
My guess is there will be virtually no difference between the effect of pavers and plywood other than the their height above the grass.
As far as set up time, several sheets of plywood elevated above the grass height with corner pavers would be a lot easier than laying out the same area with pavers..
Art
It is based on the theory of acoustical mirror images, by measuring a speaker on the reflective surface with the mic on the reflective surface in the same plane that boundary is removed from the measurement. Like measuring two speakers one on top of the other, so you get a 6dB level increase and baffle effects that look like it is twice as big as it really is.
Here's a Linkwitz Diagram to visualize it
The less reflective the surface the less this holds true, interference happens and the result is less accurate.
Erin's measurements show that it is still good below 500Hz. It's only when you want to measure full range that the surface becomes critical.
Attachments
I agree with this statement. Bikinpunk, you are doing groundbreaking work... Groundbreaking in the sense that you are demonstrating (with setup photos and measured evidence) a practical low cost method of doing ground plane measurements. Everything I have ever read about ground plane measurements has been theoretical... but with frustratingly few details. Now we have some real guidance to work with.
Bravo!
Jim
bikinpunk
"Here's the result compared to Amir's. You can see some decent differences namely above 1 khz. Notably the 1-2 khz region."
Hi, I believe that your measurements with Amir cannot very accurately correspond.
To do this, you need to calibrate all the microphones in one laboratory
and if microphones are calibrated there in 1/3 octave, then the comparison of measurements in 1/24 octave will be incorrect.
"Here's the result compared to Amir's. You can see some decent differences namely above 1 khz. Notably the 1-2 khz region."
Hi, I believe that your measurements with Amir cannot very accurately correspond.
To do this, you need to calibrate all the microphones in one laboratory
and if microphones are calibrated there in 1/3 octave, then the comparison of measurements in 1/24 octave will be incorrect.
I think that would be the least of the causes of variance. Most microphones, the good ones anyway, need little calibration even at their worst points where they become directional and experience breakup. My microphone (Earthworks M23) is extremely flat even without a calibration file. I believe it is below 1dB difference at the highest correction point, referenced to 1kHz. Even if Amir is not using a calibration file I wouldn't expect the differences in response to be attributed to that; I believe the measurement methods would be the overarching cause for differences. At least below 10kHz.
I honestly didn't view my posts as being that enlightening but I appreciate the kind words and encouragement.
I measured in the backyard again but this time with pavers. I made a run about 1.5 x 8 feet long of 1.7" thick pavers purchased from Lowe's this morning. Here's a photo of the setup:
Here's the pavers result (blue) overlaid with the driveway measurement conducted a couple days ago (black) and the plywood measurement (red):
I then decided to try a few things...
So, first I laid a strip of plywood ON TOP of the pavers, running the length of the pavers (blue). I didn't take a picture of this but it is literally the same as above just with plywood laying on top of all the pavers.
I also tried laying the plywood crossways in front of the microphone (green).
I think this combination of data is showing me that I need more than just a length of pavers between the mic and the DUT. I need a large area. Something like a driveway or patio where is more concrete area.
Since I was already in my backyard I decided to do a quick sanity check to make sure nothing was wrong with the hardware or software by measuring the speaker on my back patio like you see below. This measurement was gated due to the wall you see in the photo, so it does't have the high resolution of the other measurements.
Conclusion:
From the above you can see the patio sanity check works out; it matches the driveway response fairly well (with less resolution due to the ~ 4ms gating). So, it is confirmed, the pavers/plywood/grass backyard method cannot be used. I (you/we?) need a much larger slab of concrete than a 16 inch wide set of pavers and/or possibly multiple layers and much more area of plywood.
I honestly thought the pavers would make up the difference. But they did not. I don't know if the result is because the pavers are not dense enough to fully reflect sound, if it's the fact that there is grass around, that the pavers are not 100% flat (but lined up well enough I would have thought), or that my backyard has a gradual slope. I'm just lost on the explanation here but maybe it's the culmination of different things. Not only does the top end still experience a large dip around 6kHz but there's also a dip in the midrange compared to the driveway measurement.
So, I will be using my driveway, patio or an empty parking lot for any ground plane measurements. Based on the data, I encourage you guys to do some testing of any non-paved space you'd like to use and compare it against a measurement in a parking lot or driveway.
And, yes, I realize this may seem like I am obsessing. I am. I simply want to get the most accurate measurement I can but I also want to understand what makes other methods inaccurate.
Here's the pavers result (blue) overlaid with the driveway measurement conducted a couple days ago (black) and the plywood measurement (red):
I then decided to try a few things...
So, first I laid a strip of plywood ON TOP of the pavers, running the length of the pavers (blue). I didn't take a picture of this but it is literally the same as above just with plywood laying on top of all the pavers.
I also tried laying the plywood crossways in front of the microphone (green).
I think this combination of data is showing me that I need more than just a length of pavers between the mic and the DUT. I need a large area. Something like a driveway or patio where is more concrete area.
Since I was already in my backyard I decided to do a quick sanity check to make sure nothing was wrong with the hardware or software by measuring the speaker on my back patio like you see below. This measurement was gated due to the wall you see in the photo, so it does't have the high resolution of the other measurements.
Conclusion:
From the above you can see the patio sanity check works out; it matches the driveway response fairly well (with less resolution due to the ~ 4ms gating). So, it is confirmed, the pavers/plywood/grass backyard method cannot be used. I (you/we?) need a much larger slab of concrete than a 16 inch wide set of pavers and/or possibly multiple layers and much more area of plywood.
I honestly thought the pavers would make up the difference. But they did not. I don't know if the result is because the pavers are not dense enough to fully reflect sound, if it's the fact that there is grass around, that the pavers are not 100% flat (but lined up well enough I would have thought), or that my backyard has a gradual slope. I'm just lost on the explanation here but maybe it's the culmination of different things. Not only does the top end still experience a large dip around 6kHz but there's also a dip in the midrange compared to the driveway measurement.
So, I will be using my driveway, patio or an empty parking lot for any ground plane measurements. Based on the data, I encourage you guys to do some testing of any non-paved space you'd like to use and compare it against a measurement in a parking lot or driveway.
And, yes, I realize this may seem like I am obsessing. I am. I simply want to get the most accurate measurement I can but I also want to understand what makes other methods inaccurate.
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Ok, now that the decision on where to measure has been made, I want to investigate how to measure:
1) Mirror vs no mirror
2) Mic angled vs mic flat
Note: Any photos I show were taken today in the sunlight to show you how I measured LAST NIGHT. They are just for reference; the measurements were all made at the same time as the datasets above.
All of the following measurements were done at 1 meter to help me get a little bit more reflection-free time but it did not effect the relative trends that I measured at 2 meters. Here is the comparison (note: the mic was flat on the ground as you can see in section #1 below):
Okay, so the mic will be placed at 1 meter for all following test results. Moving on...
#1) Let's look at the first concern, with and without a mirror:
For setup reference, here are the two setups:
And when the mic is flat, there is just a small gap between the mic and the surface:
Here are the results:
As you can see, the comparison with and without mirror laying flat on the ground/mirror are practically the same. So, assuming the surface you are measuring on is a hard surface like concrete I would say you can go without using a mirror.
Now, just for the heck of it, let's see if adding a mirror and angling the mic changed anything...
Pictures of setup. The mic is angled toward the ground and the shell is touching the actual ground.
Results:
Not a real surprise that the difference here is essentially null. We saw in the above when the mic was flat the mirror made no difference. Same thing here with the mic angled.
Now, that means we can assume that the results With Mirror == Without Mirror. Regardless of mic position.
#2) Mic angled vs mic flat:
Comparing "without mirror", referencing the above photos for setup, here is the overlaid results:
These results show higher frequency combing. I can't say how much of this is the result of the mic holder causing reflections or if this is literally all caused by the mic being angled. I would need to remeasure with foam or something that wouldn't cause a reflection to hold the mic at an angle to know for sure. But it looks to me like the mic laying flat on the ground is likely more accurate.
***** Extra Info! Wind matters, duuuuhhhhhhh.... but I still wanted to see how much. *****
And for those who may wonder about wind, I measured twice yesterday. All the data I showed for the pavers results were done in the morning and then again at night. It was quite windy yesterday morning with gusts up to about 15mph. So I scrapped that and decided to wait until the winds subsided. But I saved those results and have plotted them vs the nighttime measurement below. "Wind" is red. "No wind" is black. Again, these are with pavers and intended only to show the effect the 10-15mph winds had on my mesurements (2 meter distance).
1) Mirror vs no mirror
2) Mic angled vs mic flat
Note: Any photos I show were taken today in the sunlight to show you how I measured LAST NIGHT. They are just for reference; the measurements were all made at the same time as the datasets above.
All of the following measurements were done at 1 meter to help me get a little bit more reflection-free time but it did not effect the relative trends that I measured at 2 meters. Here is the comparison (note: the mic was flat on the ground as you can see in section #1 below):
Okay, so the mic will be placed at 1 meter for all following test results. Moving on...
#1) Let's look at the first concern, with and without a mirror:
For setup reference, here are the two setups:
And when the mic is flat, there is just a small gap between the mic and the surface:
Here are the results:
As you can see, the comparison with and without mirror laying flat on the ground/mirror are practically the same. So, assuming the surface you are measuring on is a hard surface like concrete I would say you can go without using a mirror.
Now, just for the heck of it, let's see if adding a mirror and angling the mic changed anything...
Pictures of setup. The mic is angled toward the ground and the shell is touching the actual ground.
Results:
Not a real surprise that the difference here is essentially null. We saw in the above when the mic was flat the mirror made no difference. Same thing here with the mic angled.
Now, that means we can assume that the results With Mirror == Without Mirror. Regardless of mic position.
#2) Mic angled vs mic flat:
Comparing "without mirror", referencing the above photos for setup, here is the overlaid results:
These results show higher frequency combing. I can't say how much of this is the result of the mic holder causing reflections or if this is literally all caused by the mic being angled. I would need to remeasure with foam or something that wouldn't cause a reflection to hold the mic at an angle to know for sure. But it looks to me like the mic laying flat on the ground is likely more accurate.
***** Extra Info! Wind matters, duuuuhhhhhhh.... but I still wanted to see how much. *****
And for those who may wonder about wind, I measured twice yesterday. All the data I showed for the pavers results were done in the morning and then again at night. It was quite windy yesterday morning with gusts up to about 15mph. So I scrapped that and decided to wait until the winds subsided. But I saved those results and have plotted them vs the nighttime measurement below. "Wind" is red. "No wind" is black. Again, these are with pavers and intended only to show the effect the 10-15mph winds had on my mesurements (2 meter distance).
Shameless plug, but if any of you guys have a few extra bucks and found this information useful enough to do so and you don't mind helping me out, I spent about $30 on the pavers & $20 on the mirror and have no real use for them now that I've shown they don't help but unfortunately cannot return them. So if you don't mind contributing a little bit to the cause I would definitely appreciate it. It's not that I wouldn't have done all this anyway. It would just be appreciated and help me offset my costs a little bit if you deem it worthwhile.
https://www.erinsaudiocorner.com/contribute/
https://www.erinsaudiocorner.com/contribute/
I guess I need to point out that everything that I’ve read in the books indicates what you need is a large flat reflective area. I basically just did the roundabout way of proving that to be true. On one hand I feel rather silly and dumb For not having understood and trusted what was meant. But I know me well enough to know that if I didn’t see this through on my own that I would not have benefited from the experience. So, ultimately I guess I just proved to myself that what I’ve read is accurate. The issue I had was that I saw so little talk about testing speakers full range; everything seem to be dedicated toward subwoofer and low frequency response only. Now I feel more confident in using this method for full range testing. You live and learn.