Do it today!
Why would you want to do that (i.e. prevent regular drum head modes)?
Eric
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
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.
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.
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.
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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.
Just trying to understand the basis of a rather messy, time consuming sanding mod, when the raw natural surface seems quite nice.
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.
X, the last thing I want to do is mislead you so if someone needs to correct me please do so.
According to the TI guys, sanding the surface actually makes the panel louder. It evidently removes the melted polystyrene skin that develops when the product is cut or extruded which seems to dampen the sound level somewhat. They did not however PVA their sanded panels. The purpose of the PVA coating was oustensibly ment to stiffen the panel and hopefully help with the High Freq output. Many who have tried it do say they think it helps but I've not seen anyone show some frequency tests of the before and after PVA treatment (and yes some have also recommended using other treatments like epoxy, shellac, other kinds of plasticiser's to stiffen the panels)
TI also said that rounding the corners of the panel did smooth the FR and was worthwhile. But, easing the edges (i.e., rounding off the edges) did not seem to do anything to effect the sound from the panel or its FR. But they did it anyways simply because it looked better cosmetically (WAF).
So maybe if you could do a freq sweep on the panel before the PVA and after, as well as before/after the sanding, we would know for sure
I personally have used a PVA treatment of 3:1 water to glue (a 25% diluted mixture) on a cheap Dollar store paper skined foam board to stiffen it. One coat on front and back and it defintely stiffens the panel a bit (but also badly warps the panel as well). Unfortunately, I dont have any way to do test measurements to see if it made any real difference and I did not try multiple coats as the panel being so warped was really not all that usable.
TI also said that rounding the corners of the panel did smooth the FR and was worthwhile. But, easing the edges (i.e., rounding off the edges) did not seem to do anything to effect the sound from the panel or its FR. But they did it anyways simply because it looked better cosmetically (WAF).
So maybe if you could do a freq sweep on the panel before the PVA and after, as well as before/after the sanding, we would know for sure
I personally have used a PVA treatment of 3:1 water to glue (a 25% diluted mixture) on a cheap Dollar store paper skined foam board to stiffen it. One coat on front and back and it defintely stiffens the panel a bit (but also badly warps the panel as well). Unfortunately, I dont have any way to do test measurements to see if it made any real difference and I did not try multiple coats as the panel being so warped was really not all that usable.
Ok, I understand the purpose now. Boiled linseed oil might be another option that won’t warp the paper skins. It reacts with air to form a hard plastic like skin. Takes days though. That might be a worthwhile and inexpensive panel. I like how light the foam core panels are. One could add some dampening ribs along strategically determined directions to get rid of the main panel “flapping” modes.
@veleric
“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?”
I will try Eric
DML’s create sound pressure differentials by transverse waves. This paper is a good explanation of how this works and is very readable-you can skip the math and still get a good understanding https://web.archive.org/web/2013031...6/files/290707 Final Report - Final Draft.pdf
If you have handled a DML exciter you will notice its suspension is very stiff and excursion is low. Compare this to a conventional driver where the entire cone moves freely backwards and forwards in a pistonic state. This works for bass and up to lower midrange after which it fails and the cone enters a different mode not unlike the action of a DML. You may well have read about the many attempts to create a cone that is rigid at all frequencies- e.g metal cones, the use of domes for the midrange driven at the edge. All these attempts to provide a truly pistonic motion at all frequencies fail eventually, another reason the bandwidth of conventional drivers are limited and we use multiple drivers. However, the mass of the cone is still ’seen’ by the voice coil and limits frequency response.
This never happens with a DML’s the sound is produced entirely by bending waves. I use the example of a hammer striking an anvil. The anvil mass is never moved by the hammer, but it imparts its energy to the anvil which rings. I guess the mass in the area local to the exciter trust pad must provide an impedance match, otherwise there would be no sound, but I believe it is a local resistance and it is not the total mass of the panel providing the impedance. Thats my best guess at the physics and no doubt a Phd will be along any second now to tell me I have it all wrong.
As an aside, the DML model does answer a puzzle that was raised in another thread on this site. The thread World' Best Midranges - SHOCKING Results & Conclusions. concerned testing a wide range of mid-range drivers to answer the question which is best. It was a double blind test so no one saw or knew which drive unit was being tested.In amongst the expensive highly engineered cones and domes just for fun a 12inch paper cone PA speaker was included. The result of the test was controversial because that old 12inch paper cone performed just as well as some very expensive kit.
Now with my experience with DML’s it seems entirely possible, in fact likely, that that 12 inch driver was operating in a DML like state.
“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?”
I will try Eric
DML’s create sound pressure differentials by transverse waves. This paper is a good explanation of how this works and is very readable-you can skip the math and still get a good understanding https://web.archive.org/web/2013031...6/files/290707 Final Report - Final Draft.pdf
If you have handled a DML exciter you will notice its suspension is very stiff and excursion is low. Compare this to a conventional driver where the entire cone moves freely backwards and forwards in a pistonic state. This works for bass and up to lower midrange after which it fails and the cone enters a different mode not unlike the action of a DML. You may well have read about the many attempts to create a cone that is rigid at all frequencies- e.g metal cones, the use of domes for the midrange driven at the edge. All these attempts to provide a truly pistonic motion at all frequencies fail eventually, another reason the bandwidth of conventional drivers are limited and we use multiple drivers. However, the mass of the cone is still ’seen’ by the voice coil and limits frequency response.
This never happens with a DML’s the sound is produced entirely by bending waves. I use the example of a hammer striking an anvil. The anvil mass is never moved by the hammer, but it imparts its energy to the anvil which rings. I guess the mass in the area local to the exciter trust pad must provide an impedance match, otherwise there would be no sound, but I believe it is a local resistance and it is not the total mass of the panel providing the impedance. Thats my best guess at the physics and no doubt a Phd will be along any second now to tell me I have it all wrong.
As an aside, the DML model does answer a puzzle that was raised in another thread on this site. The thread World' Best Midranges - SHOCKING Results & Conclusions. concerned testing a wide range of mid-range drivers to answer the question which is best. It was a double blind test so no one saw or knew which drive unit was being tested.In amongst the expensive highly engineered cones and domes just for fun a 12inch paper cone PA speaker was included. The result of the test was controversial because that old 12inch paper cone performed just as well as some very expensive kit.
Now with my experience with DML’s it seems entirely possible, in fact likely, that that 12 inch driver was operating in a DML like state.
You are correct. PVA glue mixture is simply used for stiffening the panel as well as dampening. The reason why its MIXED with WATER is because using glue straight without water will make it stiffer but it will be OVER DAMPED.
Also to Jerry or whomever I don't use capitals to shout. I use capitals to emphasize certain words to sink into thick skulls. lol
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I've always laughed at mega (cost) systems being demoed on Youtube, but since in this case we are talking about dirt cheap DMLs to hell with it, I am going to post an mp3 of my DMLs playing. It seems you can’t upload audio files to diyAudio hence the ZIP. I recorded the track ‘Walking on the Moon’ by The Yuri Honing Trio playing on the DMLs I talked about in post #500. The recording doesn't give you the complete picture of the musical presence or sound stage produced by the speakers but it will, I hope, give you some idea of the frequency range and tone of my polycarbonate DML diaphragms. I had to take an excerpt from the track and encode it at 128kbps to fit the ZIP file size limit here.
The file was played out of JRiver through a cheap Behringer UCA-202 into a Tripath TA2024 based Chinese amp in our guest bedroom. Nothing special here. It is just the panels on their own with no Subwoofer, DSP or EQ applied in JRiver. Sadly, my homemade mics and micpre are a little bit hissy. Guess I could have always turned the mics down and the speakers up.
The file was played out of JRiver through a cheap Behringer UCA-202 into a Tripath TA2024 based Chinese amp in our guest bedroom. Nothing special here. It is just the panels on their own with no Subwoofer, DSP or EQ applied in JRiver. Sadly, my homemade mics and micpre are a little bit hissy. Guess I could have always turned the mics down and the speakers up.
Can you tell me more about your polycarbonate panels: How thick? How large (length and width)? Foamed (or otherwise lightened)? Anything else of note?
Thanks,
Eric
Veleric,
The polycarbonate I used is sold as glazing polycarbonate, used for pergolas and green houses I believe. It is internally 'ribbed' and 4mm thick. It is lightweight and quite rigid, especially across the ribs. I have designed them using the Golden Ratio of Nature 1.618, as I was unaware at the time of the Golden Ratios for DML panels. They are untreated.
The polycarbonate I used is sold as glazing polycarbonate, used for pergolas and green houses I believe. It is internally 'ribbed' and 4mm thick. It is lightweight and quite rigid, especially across the ribs. I have designed them using the Golden Ratio of Nature 1.618, as I was unaware at the time of the Golden Ratios for DML panels. They are untreated.
Consider the soundboard bracing of an acoustic guitar. I saw a you tube of a guy who had his panels horizontally and sprinkled them with either salt or sugar to observe the pattern. It was interesting to see, as he modified the test frequencies and volume.
(Sorry, this isn't the first post I'd envisioned. LONG-time lurker, recent member, first-time poster.)
OH my bad I did not see your post. Rounding the corners and pva glue is part of the standard foundation for XPS/EPS. Yes you still want them rounded.
Yes correct as the back side of the panel needs at least 2ft. space from the wall. Plus I wouldn't hang the panels, instead use them as floor standers with built in stands.
I see many people talk about the Golden ratio and this is the technique I deviate from based off my own (ears) personal preference and certain principles.
My exciter placement is close to and or dab smack in the middle of the panel. I've tried the so called golden ratio and to my ears it don't sound right.
When placed in the middle it has the "BEST" midrange out put and coherency as well as the best bass and over all balance.
Principles this is based on are BMR's. No BMR is off center including the square and or rectangle shaped BMR's. All BMR's are dead center like any conventional cone driver. I have never seen a conventional cone driver with its voice coil off center from the diaphragm.
Another principle comes from my ripples in the pond analogy. If a (exciter) pebble is dropped in the middle of the pond the ripples will reach the edges evenly. If a pebble is dropped closer to one side then the other the ripples reaching the edges wont be equal as the side closer to the pebble will reach the edge before the other creating unequal wave lengths.
BUT TO EACH THERE OWN as this is the way I design my panels.
My exciter placement is close to and or dab smack in the middle of the panel. I've tried the so called golden ratio and to my ears it don't sound right.
When placed in the middle it has the "BEST" midrange out put and coherency as well as the best bass and over all balance.
Principles this is based on are BMR's. No BMR is off center including the square and or rectangle shaped BMR's. All BMR's are dead center like any conventional cone driver. I have never seen a conventional cone driver with its voice coil off center from the diaphragm.
Another principle comes from my ripples in the pond analogy. If a (exciter) pebble is dropped in the middle of the pond the ripples will reach the edges evenly. If a pebble is dropped closer to one side then the other the ripples reaching the edges wont be equal as the side closer to the pebble will reach the edge before the other creating unequal wave lengths.
BUT TO EACH THERE OWN as this is the way I design my panels.