A Study of DMLs as a Full Range Speaker

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#10,922
Audiofrenzy.
Thanks for the pm.
My original plan about 15 years ago, was to mount my 2inch bandor full range drivers in an open baffle with the DML panel on the top.
This was so I could switch between the two units or run them together.
Hopefully for the best of both worlds and type of music.
You mentioned having the tweeter, if needed, on the side.
This is a good idea I think as you can time align the tweeter with your panels depending on your seating position and which side you have your tweeter on.
This I did with my rigid ply panels for a while.
As i have said I have been building great sounding DML panels for 15 years , but to some I am still an amateur (you know who I mean) and do not know what I am talking about.
People have always said how good my recordings sound, not one has said they sounded sh#t.
After I sent my youtube site to my neighbour, he now wants to come and listen to them because he thought they sounded very good.
And he owns a very eye watering expensive pair of B and W speakers plus the rest of his kit.
He never mentioned the recording was bad.
Unless everyone who listens to my panels is tone deaf ,I must be doing something right 🙄
Steve.

Sorry for the amateur drawing 😅
 

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#10,924
deanznz said:
DML's used as tweeters crossed at 4,500 hz

the guys blog post is here:
https://www.engineeringradio.us/blog/2021/12/distributed-mode-loudspeakers/

Distributed mode loudspeakers demonstration​

Click to expand...
Thanks for sharing that, deanznz. I think he probably could have gotten away with much smaller panels if he's only using them above 4500 Hz.

I may have to try that solid poplar, in retrospect I'm surprised to realize that I have not already. I've played with poplar plywood, yes, but never the solid poplar.

Eric
 
#10,927
Andre Bellwood said:
Hi Christian,
I can concur. During testing I have experimented with two exciters separated by 100mm on a single panel. There's no evidence of audible comb filtering. At all. Not vertically, not horizontally, not in the measurements, and no matter what signal sources are used (music, guitar, piano, keyboard, orchestra, white noise, pink noise, swept sines, spot sines etc etc etc.)
(I think I saw some inexperienced DIYer's a while ago referring to the graphic of an RTA mode pattern on a measured panel as "comb-interference." But, even making allowance for their measuring environment, I suppose such expertise should just be ignored. )

Yes indeed, comb filtering is unavoidably manifested in the acoustic medium with a coherent wave-front (in air for a standard pistonic driver,) but I'm sure you agree that DML panels are probably as different to cones as what F1 race cars are to bicycles.

To confirm:
Speed of sound in air is around 343m/s.
Speed of sound in a (polycarb) DML Panel is around 2250m/s.
In air, two pistonic drivers separated by 100mm will start combing at 3,4kHz. Very audible, with beaming and lobing, and leading to a tiny listening sweet-spot only a foot or two wide.
On a DML panel, two exciters separated by 100mm will start "combing" (if that's the correct word) at 22,5kHz if AND ONLY IF the DML panel delivers a coherent, single wave-front. Which it does not. And which is probably why audible combing does not exist in DML sound.

I'm currently testing with 8 x 40W drivers (8-ohms, 320WRMS) on 1200 x 500mm panels. I found the (panel) sweet spots for the drivers by using various combinations of Chladni patterns with spot sine-waves, and real-time RTA FR using white noise. This eventually resulted in the drivers being spread out in a mirrored configuration, and I suspect that a maximum number of panel modes are activated.

Golden-Ears-Jerry was here over the weekend, and we set up two such DML panels out in the garden. No matter where you stand, the musical reproduction was like giant head-phones: Smooth. Full stereo image everywhere. No sweet spot. No combing.

Good article in Wiki, that.

If anybody else is interested...
Single pistonic speakers that attempt to produce a full range response are especially susceptible to IMD because (low-excursion) HF signals in such a cone are directly superimposed on the (large-excursion) movements in the same cone which is required to reproduce LF signals. This means that the HF signals directly undergo a Doppler effect producing signals that are not present in the source signal, and, besides HF beaming—leading to combing, hot-spots and dead spots—these are all unavoidable issues especially in pistonic full-range cone drivers. These are physical principles that cannot be addressed by any amount of engineering or signal processing without adding band-width-limited domes or cones... Enter multiple drivers, cross-overs, phase shifts and complexity. 🤦‍♂️

Agreed!
A DML panel requires much much less membrane movement to produce LF than what a cone driver does. Therefore IMD is very significantly reduced in principle, and might be reduced even further by careful design and optimisation of vibration modes.

On a slight tangent, I confirmed a while ago that multiple drivers per panel do not necessarily produce higher SPL the more drivers are added. The best configuration seemed to be 4 drivers per panel, carefully placed, wired as a 4-ohm load, and additional drivers need to be split onto independently-damped membranes if SPL is to be increased.
But I need to test this again since I now have a power amp that can drive low impedances with constant voltages. I suspect that the only reason I did not previously get a concomitant SPL increase with multiple drivers was because the amp PSU was sagging under a 4-ohm load.


The surprising thing is that, probably because of the very high speed of sound in a panel, carefully-placed multiple drivers do tend to transform across the panel to make it vibrate in such a way that the DML action is not degraded, and pin-sharp imaging is still manifested across a massive listening sweet-spot whether-or-not multiple drivers are installed across a single panel or even across multiple panels.

Obviously the confusion arises from inexperienced beginners mistakenly thinking about the DML wave-front as a single wave-front, such as produced by pistonic cones, and this confusion leads to more confusions about things like "combing." But DML speakers produce a diffuse wave-front and which appears to be a holographic signal, and which is why it does not manifest many of the unavoidable problems such as found in traditional cone speakers.

Interesting Indeed.
Thanks Christian, it's always instructional to get such solid engineering input from your discussions.
Click to expand...
Eric.
You were right, it was not amateur it was inexperienced diyers, it was in this very long and incorrect opinion of how a DML panel operates.
I look in now and again to see if there is any new and exciting ideas and maybe give some advice.
I hope that is alright with you ?
I'm sorry to say I have been very disappointed with what has been going on .
No amount of maths and scientific babble is going to tell me how a material is going to sound!
As I have said before, it is like searching for the holy grail, when it is right in front of you.
If you are going to investigate why a certain panel material sounds so good compared to other materials, that is a different matter.
But I have given up on testing new materials myself on this site as I think it is pointless .
If I was building a pro speaker I would use multiple exciters and thousands of Watts.
But at home this is totally unnecessary.
Pro work is about quantity over quantity.
Home audio is about quantity over quantity.
I have never had to use EQ to flatten any of my DML panels responses.
If it is that bad , put it in the bin !
I shall continue to pop in now and again to hopefully give advice to the friendly people on this forum 🙄 but only if the want it.

Steve.
 
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#10,929
spedge said:
Jaxboy.
Thanks, nice to hear (see) a friendly voice.
Pm me , any time 👍
I will always answer.
Steve.
Click to expand...
Steve,
You are still my guru for DMLs. In fact, I almost left with you, but I hung around in the shadows, just in case there was a new breakthrough. While there are many other reasoned voices on here, none of them have your experience . Thanks for that offer, should I ever need to.
 
#10,931
I think English is not the native language of most players
Maybe the grammar of the words is not so precise
Over time, it may lead to a lot of confrontations
I think everyone is free to share or not share.
I find that there are not many people who often speak up.
They are all regular people on the forum.
The more confrontational atmosphere in a group becomes worse, the fewer people will speak.
If you like DML, I hope you will think more before posting and not be scornful, slanderous or confrontational.
Hope you all have fun
 
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#10,933
Veleric said:
I think he probably could have gotten away with much smaller panels if he's only using them above 4500 Hz.
Click to expand...
Might be worth pointing out that his zobel network is probably actually impedance correction and needed when driven via a variable resistor.

Size ? Maybe SPL figures but he doesn't really show how the bracing is attached. Just what area is vibrating. There is a wiki link. One of the comments on that has always interested me.
Broad frequency range (100 – 18000 Hz for 0.6m2 panel).
The ones I listen to daily are a bit under 0.2m^2, Similar frequency range with an SPL of 87db.
 
#10,940
It appears that he uses a dedicated crossover like with the Bose 901s. I wonder how they sound with just the music going to it. He is not very good at running a site, with the music completely drowning out his speech. They do sound good, though at least on my system. Look good, too.
 
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