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A Study of DML's as a Full Range Speaker
A Study of DML's as a Full Range Speaker
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Old 4th November 2019, 04:24 PM   #721
Veleric is offline Veleric  United States
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Originally Posted by xrk971 View Post
I really need to stop by HD on my way home and pick up a couple of XPS panels and 2x4’s and build this baby.
Do it today!

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Originally Posted by xrk971 View Post
I am also thinking a trapezoid shape might be good to prevent regular drum head modes.
Why would you want to do that (i.e. prevent regular drum head modes)?

Eric
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Old 4th November 2019, 04:45 PM   #722
Veleric is offline Veleric  United States
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Originally Posted by BurntCoil View Post
DML Attributes

Bending wave/Transverse wave operation

I have noted one of the benefits of this mode of operation above but there are further benefits that should be noted.

1. DML’s do not move pistonicaly and therefore have no moving mass. This removes a constraint on loudspeaker performance regarding high frequency reproduction.
2. All frequencies down to the bass cut-off are transmitted equally across the entire surface of the panel giving the typical DML ‘window on the performance’ effect. Although in principle this also happens with electrostatics because they are dipoles and not bipoles this advantage is lost to them through phase cancelation.
3. Due to its mode of operation edge reflection is a potential problem with DML’s but is very easily dealt with compared to the complex problem of dealing with mechanical vibration in drive units and cabinets. To be clear, it IS a problem, but low order compared to other systems. For balance, electrostatics are free of these problems entirely.
Burnt,
Love your summary. There were just two points (1 and 3 above) that I would like you (or anyone) to clarify further. Concerning point 1, clearly there is indeed moving mass in a DML. the exciter is a moving mass, and the panel is a moving mass. If they don't move, there's no sound! So you must mean it in a way that I don't understand. Can you clarify?

Concerning point 3: Why do you say "edge reflection is a potential problem"? What exactly is the problem with edge reflection? How does it negatively impact DML performance?

Thanks,
Eric
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Old 4th November 2019, 05:20 PM   #723
geosand is offline geosand  United States
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Originally Posted by Veleric View Post
Burnt,
Love your summary. There were just two points (1 and 3 above) that I would like you (or anyone) to clarify further. Concerning point 1, clearly there is indeed moving mass in a DML. the exciter is a moving mass, and the panel is a moving mass. If they don't move, there's no sound! So you must mean it in a way that I don't understand. Can you clarify?

Concerning point 3: Why do you say "edge reflection is a potential problem"? What exactly is the problem with edge reflection? How does it negatively impact DML performance?

Thanks,
Eric
For point 3. The problem is one of a large resistance (Impedence?) between the vibrations on the very edge of the panel vs the air its trying to move into. Thus a good bit of the vibrations then get reflected back into the panel thus potentially causing a source of distortion. There are ways to reduce this edge reflection...simply by clamping the edges around the periphery of the panel front/back or even by lining the edges with a soft foam to try to absorb the edge energy to reduce its reflection. Other methods can be thought of as well...and no, clamping does not automatically imply a frame but thats how its usually used.

In one youtube video a guy used hot water pipe soft foam insulation around the entire edge of his panel, it just slipped right on and provided dampening for both the edge of the panel and the edges of the front/back of the panel. Looks kind of funky but did work without the use of a frame. YMMV
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Old 4th November 2019, 05:28 PM   #724
xrk971 is online now xrk971  United States
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A Study of DML's as a Full Range Speaker
Quote:
Why would you want to do that (i.e. prevent regular drum head modes)?
Those modes force some frequencies to be in resonance and this non-flat vs frequency spectrum.

Pentagonal is another good one.
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Old 4th November 2019, 05:39 PM   #725
geosand is offline geosand  United States
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I really need to stop by HD on my way home and pick up a couple of XPS panels and 2x4’s and build this baby. I am thinking of going with the foam edge suspension idea. I think the melamine scrubber sponge pads that I use for sound dampening (same as BASF Basotect) hot melt glued to the edges of the XPS and mounted in a frame might be the way to go. I am also thinking a trapezoid shape might be good to prevent regular drum head modes.
If your thinking of using a trapezoid shape, why not go all out and use a trapezium shape where all four side are non-parrellel?

If I'm not mistaken, for our Brit friends, the words trapezoid and trapezium have the oppisite meaning.
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Old 4th November 2019, 05:46 PM   #726
jerryo is offline jerryo  Isle of Man
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An interesting explanation of the action of DML’s can be found on YouTube. Search for....

Tectonic Plates in reverberant spaces.
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Old 4th November 2019, 09:38 PM   #727
xrk971 is online now xrk971  United States
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Ok, I just picked up qnty 2 panels 24x24in x1in thick of the pink XPS. Some pine 3x1 strips for the frame and steel L brackets. Now just need to find time to make it.

How important is it to sand it to get a rough finish and why does that make a difference?

I am considering the beer can inverted dome aluminum tweeter mod that OffGrid did - but on XPS.

Click the image to open in full size.
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Last edited by xrk971; 4th November 2019 at 09:42 PM.
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Old 4th November 2019, 11:09 PM   #728
CraigSu is offline CraigSu  United States
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The sanding is prep for the PVA/water wash to seal the panel. Some folks are doing 2 coats on the exciter side and 1 coat on the front.
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Old 5th November 2019, 01:03 AM   #729
xrk971 is online now xrk971  United States
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A Study of DML's as a Full Range Speaker
So why do we need PVA to seal a shiny closed cell foam surface? Is there something about the microstructure of the foam surface that makes it sound better? It is the PVA, a stiff coating that enhances the sound quality? Or is it that it provides damping?

Just trying to understand the basis of a rather messy, time consuming sanding mod, when the raw natural surface seems quite nice.
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Old 5th November 2019, 01:12 AM   #730
CraigSu is offline CraigSu  United States
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Judging from the opinions of others in this thread more experienced than me, the PVA coating is de rigeur for using the XPS panels. Prep sand with 80-100 grit before applying the PVA to allow better bonding/sealing. The PVA both stiffens and dampens. Since you have 2 panels you could always prep one and compare them with/without.

X, the last thing I want to do is mislead you so if someone needs to correct me please do so.
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