At this point I am pretty sure most who were paying attention have probably "checked out" at this point in the whole "how am I going to measure speakers" discussion but I'm going to make one final post on the topic because I had to feed my own curiosity...
To recap where my mental state is:
Driveway ground plane measurements look quite good. They make life a bit easier because there is no need to stitch. But they do come with cons (I'll cover this later). The only issue with GP in the driveway is neighborhood kids and other random things causing distractions when I am measuring. The backyard would be ideal but as I discovered in earlier tests, the grass is too absorbent and even a 2x8 foot section of plywood wasn't enough. I either need a large concrete pad or some other area of reflective surface. As a "last ditch" effort of trying to make the backyard GP measurements work, yesterday I went to Lowe's and purchased 3 large sheets of plywood. The idea was I would try them laying in the backyard as a "large, flat, reflective surface". I could have purchased more but I thought if 3 isn't sufficient to do what I need I might as well just stick with the driveway if I am going to do GP measurements.
First I had to wet the sheets down because they were bent. I learned this trick from my skatepark/ramp building days. Wet the concave side, lay it on the concave side. I let it set overnight. This morning they were all mostly flat.
Then it was on to testing. I tried ALL sorts of configurations between measuring longways and sideways. Here's some random photos...
But, I'll spare you the results of EVERYTHING and get to the point more quickly:
First up, let's look at the data from placing the mic on a single sheet of plywood vs the "bare ground" I took a couple weeks ago vs the (windowed) ground plane measurement I conducted in my garage:
As you can see, there is some improvement in the high frequencies when using the single plywood sheet compared to the bare ground measurements. Notably, with the plywood the HF response no longer has a sharp dip it did in bare grass. But, the plywood measurement shows a dip between 100-300hz and compared to the garage GP mesurement the HF response is still not what it should be; down by about 2-3dB above 1kHz.
What happens when I add a small piece of OSB behind the mic (Purple) and then as another test I add a small sheet behind the microphone as well (orange)?
Practically zero difference when placing the small section of OSB behind the speaker and then both the speaker and the mic.
No point wasting time doing that. But, further results will all have both these scrap OSB pieces in the measurement.
Okay, so what happens when I add a second sheet of plywood to the side (brown)?
What about adding a 3rd piece of plywood to the side, with the speaker/mic in the middle (green)?
What about shifting the speaker/mic off to one piece of plywood (purple)?
Moral of this story:
Don't bother wasting money on plywood thinking if you create enough surface area you'll get accurate results. Maybe if you buy a LOT of plywood. Maybe. But then you have to take it up and put it down for test.
If you want to do ground plane measurements you need a LARGE, FLAT, concrete area. I wasted $60 on this experiment but at least I know now. So, let me save you money: Don't waste your time doing what I did with the plywood. As you can see it doesn't fix the issue entirely.
(now, maybe I could 'calibrate' the measurement for this plywood but I don't trust that.
