ErinsAudioCorner

[hardisj]

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 which surprised me, though it did make a difference.

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.

 

[hardisj]

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.


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.

 

[hardisj]

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. I will need to consider this a bit more. What I've read is that aiming the mic down is best to get it on the same plane but I worry about the reflection from the holder so I will try to come up with a way to do this. Maybe something as simple as a thin block wedge where there is no empty space and therefore no chance for a reflection. If I re-test this aspect I will post back the results.






***** 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).

 

[hardisj]

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/

 

[napilopez]

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):



View attachment 61322



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:

View attachment 61345

View attachment 61348

And when the mic is flat, there is just a small gap between the mic and the surface:
View attachment 61349




Here are the results:

View attachment 61350


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.

View attachment 61346

View attachment 61347


Results:

View attachment 61351

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:

View attachment 61352


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).View attachment 61353

Thank you for all this! As you may know, I usually do my bass measurements in the nearfield and do summations/compensate for bafflestep, but I've been considering trying to do ground plane measurements for sanity checks/another point of data. So it's handy to know what works and what doesn't.

I don't have a yard being in NYC and all, but I do have access to two large rooftops across the street from my place at a building owned by the same landlord, so I've been considering trying some ground plane measurements there to compare against my nearfield ones. The good news for me is that the bass is quite similar in all your measurements.

It's actually a bit of an interesting setup, since the building is shaped like an 'H' and I could actually measure from one rooftop to the other with a big gap and 6 storey drop between them, essentially eliminating the floor reflection. I wonder whether that might approximate free-field conditions.

As for your high-frequency combing, the S400 is actually a bit of a tricky test case for this, as the data from buchardt does show this combing. Mine does too, and though my data was from before they changed the crossover, the combing presumably comes from the physical design, so it should apply.

It might be better to test that particular detail out with another speaker.

 

[Juhazi]

I didn't know that ground plane works so well above subwoofer frequencies too!

Yep, I believe that mic head as close to ground as possible is best. And preferably the speaker sideways sot that all driver units are as close to ground as possible. Problem is how to rotate it for off-ax and to keep drivers at same height. There shoud be a cavity for the box...

Anyway normal quasi-anechoid is pretty reliable down to ~1000Hz even indoors.

 

[hardisj]

As for your high-frequency combing, the S400 is actually a bit of a tricky test case for this, as the data from buchardt does show this combing. Mine does too, and though my data was from before they changed the crossover, the combing presumably comes from the physical design, so it should apply.

Yea, looking back at my previous ground plane measurement in the driveway I see that same trend there, too. The mic was angled in that version.

I will need to consider this a bit more. What I've read is that aiming the mic down is best to get it on the same plane but I worry about the reflection from the holder so I will try to come up with a way to do this. Maybe something as simple as a thin block wedge where there is no empty space and therefore no chance for a reflection. I'll edit my post because that assessment may have been made in haste. Thanks for pointing that out.

 

[hardisj]

Anyway normal quasi-anechoid is pretty reliable down to ~1000Hz even indoors.

Yea, but that's pretty abysmal. And when you are trying to line up a ground plane measurement with a heavily windowed result that gets you resoltuion out to only a few milliseconds it makes life much harder. Imagine you had made the assumption that a grass ground plane measurement was sufficient for making up the difference below the 1kHz you acehived indoors. You'd have made a terribly inaccurate response between those two because the grass measurement shows a pretty big dip in the midrange. The person would have just assumed it's real and merged it with the far-field and called it a day.

Aside from that, I wanted a singular method. For bookshelf speakers, stitching nearfield to farfield is fine because you can pretty much assume all angles of far-field will line up to the nearfield in the same manner. But for a taller or more complex speaker you need high resolution to a lot lower frequency to see where the divergence occurs as you go pretty far off-axis. That's why I spent all this time trying to understand the limits of the ground plane measurement and based on my results compared to results using the 8.5 foot platform I am quite convinced that a large, flat concrete surface ground plane measurement will yield as good or better results than a quasi-anechoic measurement; and be easier because you have one set of data representing each measurement than two (or more if you have multiple ports).




As for off-axis measurement, I made mention of it in the Buchardt S400 thread but forgot to post it here. So here's a copy/paste:

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:

View attachment 60548


This is the 90-degrees off-axis setup:

View attachment 60549




And here are the results of my horizontal. Again, keeping in mind the notes I mentioned above.

View attachment 60546

View attachment 60547

 

[BYRTT]

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/

Donated a few mirrors ...

 

[hardisj]

Donated a few mirrors ...

View attachment 61382

Thanks a lot! I really appreciate it!

 

[hardisj]

Please, allow me to vent...


Yesterday the weather was nice. No wind. The neighborhood was quiet. So, I set out to measure the S400 and get it done. I ran horizontal and vertical measurements. About 30 total measurements.

But, I made a stupid mistake. Given that it seemed the "mic angled down" measurements were a close match to what was expected, I decided to go this route but I made a foam "shroud" to prevent any reflections from the mic holder altering the result. Like so:

My mistake was: I didn't test the 'shroud' first. So, I completed all of these measurements, went and looked at the results and saw this:

Man, that is a LOVELY sight to see. Look at that high resolution! Oh, but wait.... Above 3kHz the data is junk!!!! Sonuva!!!!..

So, today I measured in the garage (ground plane, 2 meters, gated to about 8ms): 1) with shroud and 2) without shroud. The "with shroud" reduces output above 3kHz by about 1-1.5dB.

I am about to lose my ever-loving mind here.

I really want accurate data out of a single measurement (ie, no 'splicing' or 'blending'). My issue with splicing is it requires nearfield measurements of each woofer/vent and results in a nearfield bump. My issue with blending is it requires additional modeling that would also increase lead time. Plus, there's the concern that you really can't just assume a NF mic measurement of the different sources will behave the same off-axis; especially when you don't have high resolution due to the window size. And then imagine splicing a single NF set of measurements with multiple off-axis measurements. But from this little bookshelf alone I can already tell you the polars wouldn't be correct and the resolution/accuracy in the midrange would also suffer. Proof? Look at my above set of horizontal measurements; see the split begins about 100-200hz? On-axis NF + FF stitching won't show you that. I want that data.

A single ground plane measurement would give me data down to < 30Hz; that's resolution better than some anechoic chambers. It also means no need to blend or splice. That also means there is less chance for error in complicated enclosures where there are different sound sources on different sides of a large enclosure (ie, woofer on front, vent on back). Cons of the ground plane measurement are simply that it is outside and I have to contend with weather and neighborhood kids. Not to mention having to walk 40 feet to go turn the speaker (and I have played with different automated designs but they all elevate the speaker on a platform which only alters the data).


I have one more thing I want to try. If that is no bueno I'm gonna have to really think this over and consider the pros/cons of the above. I keep falling back to the driveway ground plane measurement but again, that has its own set of cons. I think my only solace in all of this is that "if it were easy, everyone would do it". I just didn't realize how much legwork I was going to be putting in to get confidence in my measurements. I almost wish I didn't know what the data should look like. I tell ya', ignorance is bliss, folks.


If any of you think I am being too complicated about this: I am. I know it. But that's because I want this data to be accurate. I could easily do some 5ms windows in my garage, do some NF measurements, slap 'em together and call it a day. And that's not a knock against others who don't have a choice but to do that. It is simply that I know I can do better... but how badly do I want to do better is the real question. Hey, if nothing else, maybe I can help you guys avoid the same mistakes I am making.

I think I just need someone to hug me and tell me it'll be OK. LOL

/rant

 

[napilopez]

Oof, I feel you. But as you implied, I guess there's a reason I've never heard of anyone only using ground plane measurements to measure a speaker (other than subs). Good luck!

 

[DDF]

Hey Erin,
To try and avoid the hassle of a ground plane measurement, have you considered Fincham's (KEF) old technique? Here's a paper by Benjamin refining it.

Basically, pre-equalize the stimulus with a high pass filter that gets the impulse response of the speaker to decay to low levels within the window you can support, and then use the exact inverse filter to post eq the measurement to correct the measured response. Its all just number crunching, no special hardware needed. Allows you to get much longer impulse response measurement windows using smaller spaces.

 

[thewas]

But, I made a stupid mistake. Given that it seemed the "mic angled down" measurements were a close match to what was expected, I decided to go this route but I made a foam "shroud" to prevent any reflections from the mic holder altering the result. Like so:

Wouldn't you need a 45° mic calibration for such?

Oof, I feel you. But as you implied, I guess there's a reason I've never heard of anyone only using ground plane measurements to measure a speaker (other than subs). Good luck!

Prof. Goertz (the one who does also the S&R monitor reviews and measurements) seems to use them https://www.fidelity-online.de/kef-reference-1-messungen/

 

[Zvu]

Hi Hardis,

I wouldn't be so exclusive about merging measurements. Given all the data you have provided here my take is that some kind of hybrid approach is a must. I'd do merging but with a twist. Definitely both ground plane and 3m in the air have their advantages. Ground plane has very good resolution and it delivers you from calculating different responses for port, pr and woofer. Gated in the air gives insanely good resolution above a certain point. I would align the two measurements and look for resolution changes and merge it at the place where lifted speaker starts to loose its resolution.

I think that if ultimate accuracy is wanted, no single measurement will suffice and some merging will be necessary.

 

[hardisj]

I think that if ultimate accuracy is wanted, no single measurement will suffice and some merging will be necessary.

I don't entirely agree. I have already proven that ground plane measurements are just as useful for HF as they are for LF. It's just working with measurements outdoors that become tricky (it gets hot in Alabama, wind, kids, etc). But, no matter what, I'll have to measure outdoors for LF response. And if I have to do that, I might as well use it for HF as well.

I am not saying I may not default to merging or use it occasionally, in a pinch. But all the reasons I mentioned above are the reasons I am trying to stay away from it.

 

[QMuse]

I don't entirely agree. I have already proven that ground plane measurements are just as useful for HF as they are for LF.

Indeed you have - her's the difference between your gound plane measurement of Buchardt S400 relative to what @amirm measured with Klippel:

 

[hardisj]

Hey Erin,
To try and avoid the hassle of a ground plane measurement, have you considered Fincham's (KEF) old technique? Here's a paper by Benjamin refining it.

Basically, pre-equalize the stimulus with a high pass filter that gets the impulse response of the speaker to decay to low levels within the window you can support, and then use the exact inverse filter to post eq the measurement to correct the measured response. Its all just number crunching, no special hardware needed. Allows you to get much longer impulse response measurement windows using smaller spaces.

I looked at this last night. Admittedly, I skipped some here and there and focused on the intro, loudspeaker testing and conclusion. It looks like a promising method but the author notes the work isn't finished. Also, it seems the low-frequency resolution is still suspect. I see they used an 80hz 3rd order filter to essentially emulate the time needed to capture low(er) response but the impression I got is response in to the low-bass area wasn't replicated. I didn't see a 'final solution' that I could readily use at this moment. Maybe I missed it?

 

[hardisj]

Wouldn't you need a 45° mic calibration for such?


Prof. Goertz (the one who does also the S&R monitor reviews and measurements) seems to use them https://www.fidelity-online.de/kef-reference-1-messungen/

Not 100% sure about the need for the calibration file. But, it's a worthwhile point. That said, I think I am actually more trusting of the flat mic measurements. That was my initial gut feeling.

Thanks for that link. I didn't know about his site. I'll dig around there a bit.

 

[Zvu]

I don't entirely agree. I have already proven that ground plane measurements are just as useful for HF as they are for LF. It's just working with measurements outdoors that become tricky (it gets hot in Alabama, wind, kids, etc). But, no matter what, I'll have to measure outdoors for LF response. And if I have to do that, I might as well use it for HF as well.

I am not saying I may not default to merging or use it occasionally, in a pinch. But all the reasons I mentioned above are the reasons I am trying to stay away from it.

Oh, you'll get consistent results, just not entirely accurate all the time in my opinion.

Hornloaded loudspeakers (horn on the cabinet) or the ones with complex cabinet design - how feasible will it be to measure ground plane ?

It works with straight egde loudspeakers very well, as you proved. With some of the B&W, Devialet, Wharfedale Jade or some stepped baffle speakers - i would be extra cautious about diffraction problems that maybe would not exist with other measurement techniques.

I mentioned Kef doing measurements by putting the mic in the ground and suspending the speaker face down from 8m height. They don't do that for the fun of it.