Good move... I've played around with that... Just be aware that it may add a bit of a hollow note to the sound.
Actually, I found that I had to leave a hole about 150mm dia at the exciter location to mitigate that, which obviously won't work for your purpose.
Again, I'll be interested in the results
Eucy
Actually, I found that I had to leave a hole about 150mm dia at the exciter location to mitigate that, which obviously won't work for your purpose.
Again, I'll be interested in the results
Eucy
Last edited:
Eric. What comes to mind with this is that for each damping level test, you need a frequency response trace to see the effect of the damping, as that in the end is what we need to know
Eucy
I always check the FR with no-smoothing and my 'default' view is 1/24. The peaks are revealed there and one can see how damping reduces their intensities. The impedance test is interesting though, I wish I could see that alongside my FR tests but I don't have the gear or knowledge for it.
Thanks for sharing.
I'd be interested to know what the panel is, but they don't say. Do we already know that (based on the DPK panels?), I have forgotten?
One thing I thought was interesting is the pair of "C" shaped cuts on the back panel around both drivers. I can only imagine that those are to provide some compliance to the driver support.
Eric
Eucy,
Haha, we will get to that in time, you are jumping ahead! But to be clear, that is not really the point of my post. The point is to share measurement tools that improve our basic understanding and level of discourse. Also, as I know you know, the frequency response trace alone is only one measure of performance, so I would say rather, that it is one of the things we need to know.
Eric
Concerning the impedance rig for the impedance test, I do wish someone would build and sell them. I also didn't have the knowledge for it but I managed anyway, with a little help from Christian and others.
But anyway, have no fear, as the Tap Test and Close Mic test are equally good, and even better in some ways, and I'm pretty sure you have the gear required for those. You are on the right track to look at FR without smoothing. Smoothing is basically what damping does, so when you smooth curves, you basically erase the damping effect.
Eric
From this picture on Telegram channel of Mescalatio (same people as Tefra?) I gather that they use Nomex (aramid paper honeycomb) aprox. 4 mm thick. They always used paper for sheeting, but with PU glue instead of epoxy. My panels are also made with nomex/paper, and that seems to work well.
https://t.me/mescalitoacoustics/43
PS I'm not completely sure Tefra also uses this core material. In their older models the cell size of the honeycomb was much bigger.....
BTW they use this https://doc.soundimports.nl/pdf/bra...VBDS-4/dayton-audio-ex32vbds-4-spec-sheet.pdf low freq exciter. Another clever design idea, I think. Because of the steep HF roll off they can use a simple first order high pass X-over for the "tweeter" and leave the "woofer exciter" without any filtering. On my panels I use two Xcite 4 drivers, but 2 way active X-over-ed. Bit complicated.
I might just use a thin strip across the back, with a bulge to hold the driver. If there's any advantage to a shaped back strip (air spring to reduce reverse phase components) then I might do that. Alternatively, I'll do large cutouts at carefully placed positions, and cover the back with foam or felt to slightly reduce the rear signal.
Yesterday I described the use of an Impedance Test to evaluate the level of damping in a panel speaker. In this post I will describe the use of Tap Testing for the same purpose.
Tap Testing is really a variation of what is often called an Impact Hammer Test which is commonly used by engineers to identify the vibration characteristics of structures, and in particular their natural frequencies of vibration. A Hammer test normally uses a hammer (often with a load cell in the tip) to strike the structure, and an accelerometer to measure the response of the structure. In my Tap Test, I use a stick with a rubber ball on the end as my hammer, and a microphone very close to the panel in place of the accelerometer. Here's my "hammer". (I usually use the rubber ball end but the wood ball end is also useful for driving higher frequencies.)
I described the use of this test previously, in the thread linked below. In that case, the purpose was to identify the natural frequencies of a "free" panel, and use that information to calculate the elastic moduli of the panel material.
https://www.diyaudio.com/community/...r-dml-design-and-analysis.383567/post-6954047
What I'm describing now is essentially the same test, but used instead to measure damping level. Like for the Impedance Test earlier, I evaluated the same panel in five different mounting conditions. The panel is a three layer composite with a balsa core and carbon fiber skins, about 2.1 mm thick and with dimensions of 16" x 23.5". In all tests the panel was oriented with the long direction horizontal.
The five mountings are as follows:
1.Hanging Free: Panels were hung from wires attached with small alligator clips, about 5" from the two ends of the panel.
2. Four Points 3M Indoor: Panel attached at four points to a frame using 3M Indoor double sided foam mounting tape. The four points were along the two long edges, about 5" in from the corners. The tape pieces were each approximately 3/4" square. This foam tape is quite soft and rebounds quickly when compressed.
3. Full Surround 3M Indoor: Panel attached to frame around the entire perimeter, excluding about 2" from each corner. The width of tape in contact with the frame and panel was about 0.5" wide.
4. Full Surround 3M Extreme: Similar to directly above, except using 3M Extreme double sided mounting tape. This foam tape is much firmer, and has much slower rebound than the 3M Indoor used in mountings 2 and 3 above.
5. Full Surround Poron 92: Similar to 3 and 4 above except the panel was attached to the frame using Poron 92 foam (about 0.25" thick). This foam is very soft, with very slow rebound. The width of foam in contact with the panel and frame was in this case was about 1.25" wide.
Tap Test Method and Results
The Tap Test uses REW but requires only a microphone and my rubber ball hammer for hardware. In fact, you don't even need an exciter. I actually prefer to perform the tap test without an exciter attached, but it can be performed either way. Typically I observe that the exciter will affect the first resonance peak, but has little to no impact on the other peaks. The curves shown below were generated without an exciter attached.
Rather than use the normal SPL frequency sweep measurement, I use the RTA mode of REW. Here are the steps in the test:
Discussion
I won't go into great detail, but the results show almost exactly the same thing as the Impedance Test results show. Each curve has peaks at each of the resonance frequencies of the panel assembly. The first two curves (from the top down) exhibit very sharp peaks, with a very slight increase in damping for the third curve (Full Surround, 3m Indoor). The lower two show much broader, rounder peaks, indicative of significantly increased damping. One interesting difference is that this test shows not just peaks (like the Impedance Test) but also dips in the frequency response which are likewise sharp with little damping but rounder with more damping.
With respect to the location of the mic and the tapping location, the effect is similar to that of the location of the exciter in the impedance test. That is, choosing different locations for tapping and mic-ing will simply change which modes are driven (and picked up), and hence where along the frequency range the strongest peaks are. But generally it doesn't effect how sharp the peaks are.
The aspects I like best about this test are that it shows the degree of damping equally well as the impedance test, but doesn't require the impedance rig, or even an exciter. As I mentioned above, the other feature I like is that it can also be used to determine the elastic properties of panels.
Eric
Tap Testing is really a variation of what is often called an Impact Hammer Test which is commonly used by engineers to identify the vibration characteristics of structures, and in particular their natural frequencies of vibration. A Hammer test normally uses a hammer (often with a load cell in the tip) to strike the structure, and an accelerometer to measure the response of the structure. In my Tap Test, I use a stick with a rubber ball on the end as my hammer, and a microphone very close to the panel in place of the accelerometer. Here's my "hammer". (I usually use the rubber ball end but the wood ball end is also useful for driving higher frequencies.)
I described the use of this test previously, in the thread linked below. In that case, the purpose was to identify the natural frequencies of a "free" panel, and use that information to calculate the elastic moduli of the panel material.
https://www.diyaudio.com/community/...r-dml-design-and-analysis.383567/post-6954047
What I'm describing now is essentially the same test, but used instead to measure damping level. Like for the Impedance Test earlier, I evaluated the same panel in five different mounting conditions. The panel is a three layer composite with a balsa core and carbon fiber skins, about 2.1 mm thick and with dimensions of 16" x 23.5". In all tests the panel was oriented with the long direction horizontal.
The five mountings are as follows:
1.Hanging Free: Panels were hung from wires attached with small alligator clips, about 5" from the two ends of the panel.
2. Four Points 3M Indoor: Panel attached at four points to a frame using 3M Indoor double sided foam mounting tape. The four points were along the two long edges, about 5" in from the corners. The tape pieces were each approximately 3/4" square. This foam tape is quite soft and rebounds quickly when compressed.
3. Full Surround 3M Indoor: Panel attached to frame around the entire perimeter, excluding about 2" from each corner. The width of tape in contact with the frame and panel was about 0.5" wide.
4. Full Surround 3M Extreme: Similar to directly above, except using 3M Extreme double sided mounting tape. This foam tape is much firmer, and has much slower rebound than the 3M Indoor used in mountings 2 and 3 above.
5. Full Surround Poron 92: Similar to 3 and 4 above except the panel was attached to the frame using Poron 92 foam (about 0.25" thick). This foam is very soft, with very slow rebound. The width of foam in contact with the panel and frame was in this case was about 1.25" wide.
Tap Test Method and Results
The Tap Test uses REW but requires only a microphone and my rubber ball hammer for hardware. In fact, you don't even need an exciter. I actually prefer to perform the tap test without an exciter attached, but it can be performed either way. Typically I observe that the exciter will affect the first resonance peak, but has little to no impact on the other peaks. The curves shown below were generated without an exciter attached.
Rather than use the normal SPL frequency sweep measurement, I use the RTA mode of REW. Here are the steps in the test:
- Go to RTA Mode and select the following settings: Mode (Spectrum), Smoothing (No Smoothing), FFT Length (128k), Averages (4). The Mode and Smoothing settings are most critical. Other settings for FFT length and Averages may wok equally well.
- Place the microphone very close to the face of the panel. I try for about 1/8" (3 mm). Where exactly? Well, it really doesn't matter that much for this test. For these tests I located it in front of the 0.4/0.4 position on the plate.
- Activate the RTA by pressing the dark red button on the top right.
- Strike the panel with the hammer. Where? Again, it doesn't really matter. For these tests I hit the panel very near to the microphone. How hard? A pretty light tap will work fine, but a little harder will give a stronger cleared response. Free hanging panels are the trickiest, as they start swinging around, and may strike the mic. But if you get the panel swinging just a bit, then strike the panel as is swings toward the mic, with just the right timing you can provide a good impact and keep the panel from hitting the mic. This takes a little practice, but isn't hard.
- Watch the REW RTA screen. The readings will jump up when the microphone starts sensing the panel vibrations.
- After a few seconds. the RTA curve on the screen will stabilize for a few seconds, and the peaks in the response will become very clear. A few seconds later the readings will fall as the panel vibration slows.
- Now that you know how the readings react, strike the panel again but this time when the readings on the screen stabilize at the highest, clearest level, STOP the RTA with the red button in the upper right that you used to activate the RTA in step 3. This will freeze the RTA curve at the moment you stop it.
- Press "Save Current" to save the response curve to the main REW screen.
Discussion
I won't go into great detail, but the results show almost exactly the same thing as the Impedance Test results show. Each curve has peaks at each of the resonance frequencies of the panel assembly. The first two curves (from the top down) exhibit very sharp peaks, with a very slight increase in damping for the third curve (Full Surround, 3m Indoor). The lower two show much broader, rounder peaks, indicative of significantly increased damping. One interesting difference is that this test shows not just peaks (like the Impedance Test) but also dips in the frequency response which are likewise sharp with little damping but rounder with more damping.
With respect to the location of the mic and the tapping location, the effect is similar to that of the location of the exciter in the impedance test. That is, choosing different locations for tapping and mic-ing will simply change which modes are driven (and picked up), and hence where along the frequency range the strongest peaks are. But generally it doesn't effect how sharp the peaks are.
The aspects I like best about this test are that it shows the degree of damping equally well as the impedance test, but doesn't require the impedance rig, or even an exciter. As I mentioned above, the other feature I like is that it can also be used to determine the elastic properties of panels.
Eric
HvdZ,
So you use the two exciters over different frequencies ranges (obviously). What ranges and why? Why not use them both full range?
Did you buy it or make it? I can't seem to find it commercially with paper skins already on it. I do have some old Armstrong ceiling speakers with panels like that. I'd like to try more of it if I can find it.
Thanks
Eric
If I compare the specs it looks more like they are using this PUI exciter:
https://puiaudio.com/product/exciters/asx05408-hd-r
I would also like to know what exciter are they using for the tweeter?
X over at 630 Hz, power is more or less distributed evenly between the two drivers and the crossoverpoint is still out of the "sensitive" area. When I tried more drivers full range I found it to sound smeared/less nice (to me - please don't accuse me of being a senseless audiophile) than the single exiter. This 2-way solution seems to work for me. BTW I'm now leaning to use just one driver - the extra SPL I get is not very impressive, and not worth the hassle.
Made it, because I couldn't find good materials. It was easier than I had thought. Here's a description of the process (with carbon, but the process is the same) : https://www.diyaudio.com/community/threads/dml-pa-systems.390363/post-7575541
Cheers, Hans
Last edited:
I get it.
Thanks. Using enough, but not too much epoxy is a big challenge. Maybe you have seen this video before, but they guy making this nomex "double top" guitar is super meticulous about minimizing the amount of epoxy. He applies it to the nomex only, to accomplish that. Epoxy part starts around 4 minutes in.
Eric
Hello all,
Here is the link to post #10666 showing my impedance test rig. Nothing to be afraid with. There are 2 possible schematics : one uses the output of a power amp, the second an headphone output (refer to REW manual). This second one is more simple to build. It needs "only" a 100 ohm resistor and some connectors (bananas, jack or even crocodile). The main constraint is to have an usb card with headphone out and 2 line in.
If the impedance measurement is not as crucial as with conventional loudspeakers, it is very informative as you show it Eric.
We have few of those measurements but what I have in mind is the impedance measurements I saw out of this thread (I don't remember where...) is the peaks were more similar to the poron test than the free hanging one.
To come back to the impedance test rig, send me a PM for help.
Christian
Hello,
I am probably jumping ahead to... To have something closer to what we hear, my suggestion is not to use the standard FR with its time window in hundreds of ms which includes the room reflections but more to use the spectrogram (any wavelet version). The lack of damping or the excess of resonance are easy to see on it.
Eric, one thing maybe to recall is that in the impedance test, a peak is a mode (a frequency of resonance), the height and the width are the image of the damping as you explained but all the modes are not acoustically productive. Only the odd, odd modes are on axis productive (other modes have null mean velocity over the panel surface). Out of axis or on axis in "HF" it is no more true. So at this step we can't link the smoothness of the resulting FR to the quantity of peaks in the impedance test... Maybe later or not at all?
An other remark Eric is that, recently I made tests with different suspension arrangements (sorry I didn't take the time of impedance measurement) but I had a look to 2 others views : the spectrogram (you already know it is my favorite tool) and the distortion. I have no detailed synthesis but keeping the arrangements that have a low energy storage ( no long "tails" in the spectrogram) and no distortion peaks leads to full (or maybe near full) perimeter suspension. I think the distortion aspect was previously mentioned by André. Do you have the same view?
Christian
Yes the first goal of a tap test is to evaluate the elasticity parameters of a sample. So I did it on a 3mm square flute corrugated polypropylene sample. The self damping of the material (not the level of reflection resulting from the peripheral suspension) was so high that it was impossible to detect the modes. If it is not in the top the materials here, it produces nevertheless a sound. Maybe to be classified in the "too damped material" category.
More recently, I started testing a material with a not so good reputation here : the clear polystyrene (or is it acrylic or even something in this family) used to protect picture in a frame. I directly made a test panel without taking the time of a tap test. Mistake! Some information are now missing (density, Young modulus...).
So yes to the impedance test, and to the tap test.
Christian
Just looking at this right picture, I was wondering the reason of the tape shape which is almost a disk which is not the usual shape. The answer is "it is a model with the interchangeability feature". Then came a second question : what is then the behavior in that case of the central area and the consequence on the panel. There is no more possibility of drum effect. Are such exciters able of HF?
Tried to get a decent drawing out of Fusion but not so easy :\
I did make this one earlier, it is a bit messy:
Been very busy and will be for the next couple of months. Have to get the system ready for party middle of July, and after that I'm driving down to Hungary for vacation and a gig.
Have done some testing of the plates with the subs outdoors, and will do some more before the party. Sounds amazing! X-over at 110Hz with both FR and phase looking perfect in that region.
While plates does sound great, especially with some EQ, I'm not really happy with the FR. Some 10dB swings and a dip around 200Hz which also has increased distortion, and boosting that region makes it more severe. So I think I should ideally had another iteration with the placement of the plate suspension to see if I can get rid of that issue, however I made about half the plates already. But it is not a major issue, and still very happy with how the system is turning out.
Will post videos and measurements when this summers adventures are over...