Hi Christian...
It wants to curl in the transverse direction more than longitudinally, so I place a stick about 8-10mm thick (vary as appropriate) lengthwise under the panel along the centre, then use clamps to just catch the tippy toe edges of the panel without blocking the coating too much, bending it over the stick. I also use barbell weights when needed. Coat the panel, wait until dry, release the clamps and check for straightness. Clamping on a flat plane will also help but the reverse bend is the most effective.
Any minor missed spots in the coating can be touched up easily in subsequent coats by moving the clamping positions (I do about 3 light coats all up on each side).
You get a feel for what's needed after a bit of practice.
Cheers
Eucy
So I tried the 1/2" Foamular and it is indeed considerably louder than the plywood. It also retains good bass extension, staying fairly flat to around 70 Hz or so. There was a huge dip from 8kHz - 10.5kHz which is odd and could not be eliminated by adjusting exciter position (I was using two). There is also an anomaly at 1500 Hz which I have seen for different panels and exciter placements; it can be reduced with exciter movement but not completely eliminated.
The below is from a cell phone RTA with a waterfall plot underneath. Based on everything I have seen so far this one is decent. Note though that the scale is huge so this is not as flat as it may look.
The below is from a cell phone RTA with a waterfall plot underneath. Based on everything I have seen so far this one is decent. Note though that the scale is huge so this is not as flat as it may look.
I meant to mention it sounds like there is some ringing that I did not hear with the plywood. I do not know whether or not to attribute that to unattached foam edging and/or exciter; nothing right now is permanently attached to the foam. It could also be the Foamular itself since it definitely rings more than plywood when tapped. I do have very little foam around the edge, say less than 10% of the overall perimeter, so I could add quite a bit if needed.
I should also say that sound distribution across the panel is perhaps a bit more than with the plywood but nothing drastic.
I should also say that sound distribution across the panel is perhaps a bit more than with the plywood but nothing drastic.
More data... I set up and took some preliminary measurements with the Foamular only. The area under and around the panel was enclosed with Corning 703 2" x 24" x 48" sound panels with foam prisms on the surface. Measurements were taken with an ECM8000 pointed 90 deg to the panel, about 1 ft below the panel, at three positions along the length (1/3, 1/2, and 2/3). Two sweeps per measurement. The results are below and the REW file is attached for those who want to examine it.
This is a preliminary measurement. The actuator position is not optimized (visible in the photo below).
Measurement equipment
Mic shown with acoustic panels (Corning 703 2" thick)
This is a preliminary measurement. The actuator position is not optimized (visible in the photo below).
Measurement equipment
Mic shown with acoustic panels (Corning 703 2" thick)
I bought one of those years ago. Now I want to do some more serious work I regret it so bought one of Dayton's yesterday. 🙁 Just cause I want to take spl measurements especially dbZ. I went for the phantom powered one as do have 2 methods of driving it. A preamp and a usb audio interface with that type of input. Otherwise the USB mic would make more sense,
You may need to hang the panels well off the floor as most do. Some distance away from any rearward wall.
Curious - what exciters are you using?
Hi Deude
The vertical scale you used makes it look worse than it most likely sounds... How about trying a more normal scale like 50/55 to say 105. I'm surprised the psych smoothing looks no better than the 1/6, but that's probably scale again
How does it look if you copy the scale of your first sweep graph?
Did you coat the panel at all?
Cheers
Eucy
Hi Earbourne
I discovered that ACT Foam+Rubber stock VH grade EPS sheets 1200x625x10mm @$10/ sheet...I'm in Canberra at the moment so I'll go and check it out .
10mm will probably be thick enough in the high density foam
Deude,
Glad to see you got REW running. That's a great start.
I took a look at your measurements (including the data file). My first comment is that taking the measurements at 1 ft away is a little unfamiliar to me. I usually take measurements at 1 meter or father, if I'm looking for the far field response. I also do measurements with the mic very close (say 1-3 mm), but only if I am trying to assess the natural frequencies or mode shapes, for say determining the elastic moduli of the panel or confirming a finite element model. So I don't know how exactly to interpret measurements "in between" distances.
And for a panel as large as your panel is, you probably need to be at 2 meters or more to be approaching "far field" I would think.
The drop off in frequency response above 3k is common in my experience for PS foam.
Looking closely at your data, the distortion looks very high. But that might be from how the exciters are temporarily mounted to the panels. If they are just resting by their own weight, they can bounce around and cause a lot of distortion. Using a "test" exciter and double sided tape for a temporary mount will cure that if that's the real issue.
Looking at the Wavelet spectrogram, there is some ringing at 80 Hz and a lot of ringing below that. Using more foam around the perimeter might help reduce it, though it seems the low frequency resonances are the hardest to damp. It might also just be the nature of a panel that large.
But before doing anything drastic, figure out how to turn the panel on its side securely, so you can get measurements at say 2 meters away.
Eric
REW is a great tool still; it has been years since the last time I used it (to DSP tune a car system).
I expected comments about the measurement setup 🙂
Short of testing in a certified full anechoic chamber with freshly calibrated equipment there will be issues.
Long story short bad weather means no outdoor testing for a few days. What you see is what I have.
The panel is not modified other than removal of the clear plastic sheet on both sides.
The exciters are these
The REW file is attached to the above post if you want to rescale it. These are the first sine sweeps I have done on the DML panel; the previous test was an RTA from a cell phone.
I expected comments about the measurement setup 🙂
Short of testing in a certified full anechoic chamber with freshly calibrated equipment there will be issues.
Long story short bad weather means no outdoor testing for a few days. What you see is what I have.
The panel is not modified other than removal of the clear plastic sheet on both sides.
The exciters are these
The REW file is attached to the above post if you want to rescale it. These are the first sine sweeps I have done on the DML panel; the previous test was an RTA from a cell phone.
I definitely didn't intend to be critical. I'm sure my practices and equipment have lots of room for improvement.
Me neither. Just a bit niffed having saved money some time ago and then bought better but to suite other kit I have bought.
I have been considering a mic with a serialized calibration file, like the UMIK-1, but am going to stay with the ECM8000 for now. The M-Audio interface is a nice unit with XLR inputs and phantom power so its good enough.
Before I purchased the Dayton calibrated mic, I was using a RODE NT3 studio mic for measurements. It was the flattest-response mic I had lying around.. It's a cardioid pattern, its response is indeed very flat for a studio mic, but it's not ruler-flat, and has +/-2db peaks and dips between 1k and 20k.
But taking the same measurements with the calibrated Dayton (with and without the calibration files) there's hardly any discernable difference in readings. Mic position, speaker/room position and room furnishings, and probably the weather, seem to make a larger difference than (reasonably decent) microphones do.
Without access to decent anechoic chambers, and repeatable measuring environments I don't think we should get too hung up on nitty-gritty detail?
But taking the same measurements with the calibrated Dayton (with and without the calibration files) there's hardly any discernable difference in readings. Mic position, speaker/room position and room furnishings, and probably the weather, seem to make a larger difference than (reasonably decent) microphones do.
Without access to decent anechoic chambers, and repeatable measuring environments I don't think we should get too hung up on nitty-gritty detail?
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Deude, I suspect that HF anomaly comes from the cellphone mic itself. I have exactly the same problem when measuring background noise in different locations with several different phones.
I'm not sure whether it's a characteristic of the microphone itself or of the physical dimensions of the cellphone casing (kinda inverse baffle diffraction?? )
The question is why? Is it only EPS and density related, or XPS as well. I've lost my early XPS sweep results so I'll have to check again as I switched to ply and didn't continue with the XPS. I don't remember the reason being hf fall off though
I thought Steve had strong results extending out to 16k+ with EPS??
All the more reason to test the VH grade EPS
Eucy
There it is ....VH grade must be the go to get hf extension. See post #9,410 https://www.diyaudio.com/community/...s-as-a-full-range-speaker.272576/post-7272318
Edit:..hmm.. reading further on, it seems that Steve's material is PU grade 36 kg/m3 whereas VH grade is 28 kg/m3
Still... VH grade sounds promising
Eucy
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I came across info like this - part way down the page
https://www.cross-spectrum.com/weblog/2009/07/
I have an interest in building a subwoofer and ideally need a mic that will go lower than the ECM8000 anyway. Testing would have to be done outdoors but I have read of another method where the mic is placed rather close to the speaker. It claims far field predicted to 1dB.
DML I want to look at aspect ratio, size and thickness of rather small panels in as many materials as I can easily get. I don't expect much in the line of a bass response. 🙂 I now have as much kit as I think I need but still delayed due to a PC problem.
I have a soundblaster usb unit with ~120db dynamic range but have also bought a cheap interface by ESI, range ~100db weighted. What that means can be determined by linking it's inputs to outputs and using the software. Might be £60 wasted but on Linux it has some advantages compared with using the SB as I can't run any of creative's software.
Not sure how low you want to go. The ECM8000 is "ruler-flat" down to 20Hz with the calibration file (can you get individualised calibration files for these mics??) Without the calibration I think you get a 2 or 3db drop in response at 20hz. Do you need a sub to go that low?
Yes, that's nearfield measurement. You put the mic just outside the excursion limit of the dust cap. 5mm should be enough for safety should you accidently plop an input.
Then you also need a measurement with the mic in the port if it's a vented box, and you need to splice those two measurements together. Make sure the vent is not pushing too much air otherwise the mic capsule could pick up wind noise.
DMLs can go very low indeed, the limit is the panel dimension. But do not expect a transient "kick" like a cone woofer.
https://dmlspeakers.com/articles/Does Size Really Matter
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Hello,
I had a look too to your measurements. In the same way Eric wrote, even if I have made a lot of measurements with REW, I stays modest in what I can say!
I agree with the previous comments mainly about the distance. We can learn from very short or "long" distance from the panel, seems not from intermediate with in addition a large panel.
I would add I am surprised by the cell phone result or let say the difference between cell phone and REW in the HF. The results from REW are what I have observed too with the HF roll off (material or exciter mounting method?). I won't trust cell phone results without making some additional test like using an external mike (iMM6?) and checking that the electrical and software path is "neutral" by injecting REW pink noise at the external mike input. I think I already posted about a possible method.
When re-scaled the FR are not unusual. The IR seem also not unusual. I mean seems there is no unexpected additional signals (like noise) so the set up seems correct.
I should probably go back to my own measurements but I am surprised by the quite short decay time (opposite of Eric's reaction) with such material without extra damping (PVA coating)... to see.
As Eric mentioned; having a look to the spectrogram in for example 1/6 oct wavelet is informative to see what are room reflections, panel modes.
I think it is also interesting in the REW set to include the electrical impedance measurement. I did it only recently. It is informative about the first modes (to separate them from room effects in the same range), the importance of the inductance of the exciter (inductance that participates to the HF roll off).
Christian