I tried to tap one in to the frame but it was too long ,so I'll have to saw it down a bit.
While doing this I forgot about the dome😱
I now have a concave dust cover come tweeter .
I've drilled a hole at the back of the spine so I can try and push it back again, but the frequency response is unchanged ,so as long as it does not produce bad sounds like the harder materials , it could stay this way,we shall see.
Picture of concave and response.
Steve.
While doing this I forgot about the dome😱
I now have a concave dust cover come tweeter .
I've drilled a hole at the back of the spine so I can try and push it back again, but the frequency response is unchanged ,so as long as it does not produce bad sounds like the harder materials , it could stay this way,we shall see.
Picture of concave and response.
Steve.
Attachments
Steve,
These sorts of painting stretcher frames have mitered tongue and groove joints at the corners held together only by the compression of the canvas stretched over the frame. No glue. No fasteners. Likely invented in Titian’s workshop (I always like a little art history). The keys give you some adjustment.
Of some additional possible interest for your DML experiments…
Before the development of hydro carbon based acrylic/plastic glues and sizing, painters prepared canvases by:
(1) stretching material, typically cotton canvas or linen (much stronger than cotton so you can use finer and lighter weight material) over wooden stretcher frames, then
(2) sizing the front of the canvas or linen with warm to hot rabbit skin glue.
The rabbit skin glue had several interesting properties, it:
- shrunk the support material making it taunt.
- created an impervious barrier to the oil (typically linseed &/or clove) based grounds &/or paints that would, by themselves, degrade the material support over time if they directly contacted the canvas or linen and (this gets interesting),
- served as a humidity regulator, which stabilized the tautness of the canvas or linen over time, and thereby
- expands and contracts in relationship with the support.
Applying rabbit skin glue to both sides did not work well. The canvas or linen would rot or degrade pretty quickly. A painting needs to breath to live a long life.
Well prepared painting supports have their own kind of dynamic response to their environs. Some have lasted for 600 years (so far) even when exposed to wide extremes of temperature and humidity.
When acrylic (think PVA) dries it becomes impervious to moisture, this sets up tension within the, essentially, composite panel because the materials have different expansion coefficients. The early developers of these acrylics for paints, e.g: Dow Chemical and Rhom & Hass might take exception to this assessment, but practitioners and art restorers have real concerns about this.
We have reinforced concrete only by the luck that steel rebar and concrete have the same expansion/contraction coefficients over environmental temperatures.
If someone tried using aluminum or titanium rebar the building would fall down.
I would expect PVA (especially diluted PVA) to develop micro fissures over time, which would likely change the sound of a DML panel. Of course it might take a decade. I don’t think anyone really knows.
HMMM…?
It could prove very interesting to try fine linen or even silk with rabbit skin glue.
It might also prove interesting to try a synthetic material closer in chemical composition to acrylic paints/glue stretched over a wooden painting frame then treated with PVA.
- Andreas
These sorts of painting stretcher frames have mitered tongue and groove joints at the corners held together only by the compression of the canvas stretched over the frame. No glue. No fasteners. Likely invented in Titian’s workshop (I always like a little art history). The keys give you some adjustment.
Of some additional possible interest for your DML experiments…
Before the development of hydro carbon based acrylic/plastic glues and sizing, painters prepared canvases by:
(1) stretching material, typically cotton canvas or linen (much stronger than cotton so you can use finer and lighter weight material) over wooden stretcher frames, then
(2) sizing the front of the canvas or linen with warm to hot rabbit skin glue.
The rabbit skin glue had several interesting properties, it:
- shrunk the support material making it taunt.
- created an impervious barrier to the oil (typically linseed &/or clove) based grounds &/or paints that would, by themselves, degrade the material support over time if they directly contacted the canvas or linen and (this gets interesting),
- served as a humidity regulator, which stabilized the tautness of the canvas or linen over time, and thereby
- expands and contracts in relationship with the support.
Applying rabbit skin glue to both sides did not work well. The canvas or linen would rot or degrade pretty quickly. A painting needs to breath to live a long life.
Well prepared painting supports have their own kind of dynamic response to their environs. Some have lasted for 600 years (so far) even when exposed to wide extremes of temperature and humidity.
When acrylic (think PVA) dries it becomes impervious to moisture, this sets up tension within the, essentially, composite panel because the materials have different expansion coefficients. The early developers of these acrylics for paints, e.g: Dow Chemical and Rhom & Hass might take exception to this assessment, but practitioners and art restorers have real concerns about this.
We have reinforced concrete only by the luck that steel rebar and concrete have the same expansion/contraction coefficients over environmental temperatures.
If someone tried using aluminum or titanium rebar the building would fall down.
I would expect PVA (especially diluted PVA) to develop micro fissures over time, which would likely change the sound of a DML panel. Of course it might take a decade. I don’t think anyone really knows.
HMMM…?
It could prove very interesting to try fine linen or even silk with rabbit skin glue.
It might also prove interesting to try a synthetic material closer in chemical composition to acrylic paints/glue stretched over a wooden painting frame then treated with PVA.
- Andreas
Steve,
When tapping in keys, start with a gentle tap of one key in one corner, then one in the opposite corner diagonally across the panel. Move to either of the nearest corners and again tap only one key, then the one on the opposite corner. Repeat with the second key in each corner following the same path around the panel.
Repeat as necessary to tighten the panel.
When tapping in keys, start with a gentle tap of one key in one corner, then one in the opposite corner diagonally across the panel. Move to either of the nearest corners and again tap only one key, then the one on the opposite corner. Repeat with the second key in each corner following the same path around the panel.
Repeat as necessary to tighten the panel.
Steve,
I didn't have a voice recorder app on my phone, so I just now downloaded one. After I changed the positioning of my DMLs from hanging the support poles to standing them, they knocked terribly on low notes, so I put little squares of melamine sponge between the ply and the spline to prevent that. Now they have seem to have lost all range below about 200 Hz. For now, I am just supplementing the sound with my Nautaloss and XKi and sub. I had to modify the support poles to stand them up, so I can't go back to the hanging method. Bummer. They sound ok, but I'm a bit cheesed that they lost so much quality of sound. Between all the speakers and the DMLs, it sounds really good, but I can tell the lack of bass from the DMLs. I didn't put the capacitors in line with the exciters to limit the bass, so that is probably why they knocked so badly. (I was checking them with a deep bass song). It is hard to do the modifications to my system, so it takes me a while to get around to doing it. Too many irons in the fire, I guess. I'll get around to putting the capacitors in line with the exciters and recording them for you. Just don't expect it any time too soon. I haven't forgotten your request for a recording of the DMLs, though.
I didn't have a voice recorder app on my phone, so I just now downloaded one. After I changed the positioning of my DMLs from hanging the support poles to standing them, they knocked terribly on low notes, so I put little squares of melamine sponge between the ply and the spline to prevent that. Now they have seem to have lost all range below about 200 Hz. For now, I am just supplementing the sound with my Nautaloss and XKi and sub. I had to modify the support poles to stand them up, so I can't go back to the hanging method. Bummer. They sound ok, but I'm a bit cheesed that they lost so much quality of sound. Between all the speakers and the DMLs, it sounds really good, but I can tell the lack of bass from the DMLs. I didn't put the capacitors in line with the exciters to limit the bass, so that is probably why they knocked so badly. (I was checking them with a deep bass song). It is hard to do the modifications to my system, so it takes me a while to get around to doing it. Too many irons in the fire, I guess. I'll get around to putting the capacitors in line with the exciters and recording them for you. Just don't expect it any time too soon. I haven't forgotten your request for a recording of the DMLs, though.
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Thanks Andreas,it will be easier to tap the keys in now the dome isn't poking out anymore.
Do you stand the panel up and tap down the key,does it actually stay in the groove without pinging straight out again?
Jaxboy.
For recording and uploading straight from the phone I use the free version of recforge 2 , I have three other apps that I like as well but you have to use your computer to convert them to mp4.
The free version only let's you record for three minutes then pauses .
I used to be able to press pause and it would continue recording,but now it seems to switch to mono now 😕
But now I record 256kbps I only have just over 4mins anyway.
Was the panel knocking because you turned it up the other way of because you took off the caps ?
Steve.
Do you stand the panel up and tap down the key,does it actually stay in the groove without pinging straight out again?
Jaxboy.
For recording and uploading straight from the phone I use the free version of recforge 2 , I have three other apps that I like as well but you have to use your computer to convert them to mp4.
The free version only let's you record for three minutes then pauses .
I used to be able to press pause and it would continue recording,but now it seems to switch to mono now 😕
But now I record 256kbps I only have just over 4mins anyway.
Was the panel knocking because you turned it up the other way of because you took off the caps ?
Steve.
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Steve,
Get all the keys going in a bit at a tim and the tension of the stretched material will begin to hold them, then it will hold them fast.
I've often done it putting a panel front face down on a table (usually covered with linen), any table will do.
Andreas
Get all the keys going in a bit at a tim and the tension of the stretched material will begin to hold them, then it will hold them fast.
I've often done it putting a panel front face down on a table (usually covered with linen), any table will do.
Andreas
Canvas DML Testing
Hi y'all 🙂
I want to continue doing some testing on the Canvas DML Concept using 30x40cm and 40x50cm Cotton Canvases (260g/m²).
Spedge, you were using a 4x6'' plywood panel for a 12x16'' canvas?
Hi y'all 🙂
I want to continue doing some testing on the Canvas DML Concept using 30x40cm and 40x50cm Cotton Canvases (260g/m²).
Spedge, you were using a 4x6'' plywood panel for a 12x16'' canvas?
- Does anyone know, in what way different panel sizes/surface areas relative to a fixed canvas size/surface area effect the outcome?
- What is a reasonable panel size range relative to a fixed canvas size?
Bernard.
Yes my panel is approximately 12x16inches , I believe it is pretty close to the patent panel size, except the 2mm ply is larger and does not have an separating layer.
At the moment I am only using a 50x50 pva mix to start with on a blank canvas(no ply) just to see what happens, it would be interesting to see what would happen with larger panels.
Off hand I can't remember the size of the small panel on the patent panel I posted.
My naked canvas(without ply panel) frequency drops below 125hz as opposed to the 6x4inch ply 40hz
Would a stiff but lighter panel or a coating such as fiberglass be better ?
I'm afraid it's all a bit ,suck it and see at the moment.
At the moment I only have the 6x9inch veneer panel and the art panel playing together, they have a very similar frequency response above 125hz and both sound very good together.
I'm not sure if the pva has helped the sound, but I think it might have ,but will not know until I make an identical panel without pva for comparison.
Not sure if a neet pva coating will help or hinder, I think it's more about the stiffness in the central area and how to implement it?
If xo above 150hz I'd probably go for a 6x9inch veneer panel and leave it at that, but if a full range panel is needed the canvas is essential , but a bit more work and larger.
Don't be put off by my small canvas panel , it drives my room well down to 40hz you would be as surprised as I was ,I'm sure.
It would be interesting to try multiple art panels to see the effects of low end coupling?
Steve.
.
Yes my panel is approximately 12x16inches , I believe it is pretty close to the patent panel size, except the 2mm ply is larger and does not have an separating layer.
At the moment I am only using a 50x50 pva mix to start with on a blank canvas(no ply) just to see what happens, it would be interesting to see what would happen with larger panels.
Off hand I can't remember the size of the small panel on the patent panel I posted.
My naked canvas(without ply panel) frequency drops below 125hz as opposed to the 6x4inch ply 40hz
Would a stiff but lighter panel or a coating such as fiberglass be better ?
I'm afraid it's all a bit ,suck it and see at the moment.
At the moment I only have the 6x9inch veneer panel and the art panel playing together, they have a very similar frequency response above 125hz and both sound very good together.
I'm not sure if the pva has helped the sound, but I think it might have ,but will not know until I make an identical panel without pva for comparison.
Not sure if a neet pva coating will help or hinder, I think it's more about the stiffness in the central area and how to implement it?
If xo above 150hz I'd probably go for a 6x9inch veneer panel and leave it at that, but if a full range panel is needed the canvas is essential , but a bit more work and larger.
Don't be put off by my small canvas panel , it drives my room well down to 40hz you would be as surprised as I was ,I'm sure.
It would be interesting to try multiple art panels to see the effects of low end coupling?
Steve.
.
Steve,
I'm not sure if the response suffered because of inverting the DMLs. There were multiple things going on when I reconnected them after turning them upside down. 1. The metal tab connecting the pole to the spline was now contacting the exciter, and I couldn't get a sponge spacer between them. 2. I did not transfer the capacitors to the new wires when I changed the configuration, and I was playing a deep bass song. 3. I put a melamine sponge between the spline and the ply panel to limit the thump caused by the heavy bass, and to prevent the metal tab touching the exciter. As a result of all these things, I now have limited response below mid-range. I think at least some of that is due to the sponges limiting the exciter travel. I am running thin coax wire and its own amp, so it will be a little bit of a hassle to insert the capacitors into the system, and I have several other projects ongoing right now. It's an electrical nightmare on my desk, as both amps, the junction boxes and other wiring are all sitting exposed, as I will need to make a new box to house it all. As I am a perfectionist, that will take a while. Meanwhile, I am running the DMLs with the Nautaloss I and XKi systems with my sub, and it sounds quite nice. I can really notice the clarity of the mid-range from the DMLs. It is quite startling at times, in fact. I am hearing notes and tings and guitar squeaks that I never heard before, but in a good way.
I'm not sure if the response suffered because of inverting the DMLs. There were multiple things going on when I reconnected them after turning them upside down. 1. The metal tab connecting the pole to the spline was now contacting the exciter, and I couldn't get a sponge spacer between them. 2. I did not transfer the capacitors to the new wires when I changed the configuration, and I was playing a deep bass song. 3. I put a melamine sponge between the spline and the ply panel to limit the thump caused by the heavy bass, and to prevent the metal tab touching the exciter. As a result of all these things, I now have limited response below mid-range. I think at least some of that is due to the sponges limiting the exciter travel. I am running thin coax wire and its own amp, so it will be a little bit of a hassle to insert the capacitors into the system, and I have several other projects ongoing right now. It's an electrical nightmare on my desk, as both amps, the junction boxes and other wiring are all sitting exposed, as I will need to make a new box to house it all. As I am a perfectionist, that will take a while. Meanwhile, I am running the DMLs with the Nautaloss I and XKi systems with my sub, and it sounds quite nice. I can really notice the clarity of the mid-range from the DMLs. It is quite startling at times, in fact. I am hearing notes and tings and guitar squeaks that I never heard before, but in a good way.
Jacoby.
Yes dml can be quite startling sometimes, squeals squeaks and bumps,tapping feet.
So much detail, the down side is if the recording is poorly produced ,you will hear it.
Over use of echo bad recording techniques, I could even hear a faulty echo machine slightly crackling away into the distance,I had to listen carefully on headphones to be able to make sure it was not a fault with my dml panel.
This sort of thing happens all the time, hearing noises and thinking something is wrong with the panel,only to find it is in the music recording.
Steve.
Yes dml can be quite startling sometimes, squeals squeaks and bumps,tapping feet.
So much detail, the down side is if the recording is poorly produced ,you will hear it.
Over use of echo bad recording techniques, I could even hear a faulty echo machine slightly crackling away into the distance,I had to listen carefully on headphones to be able to make sure it was not a fault with my dml panel.
This sort of thing happens all the time, hearing noises and thinking something is wrong with the panel,only to find it is in the music recording.
Steve.
Jaxboy.
One time I spent all day stripping down my system and eps panel only to find with very very careful listening on different headphones that the distorted sound was in the actual recording 😱
I had to walk away and leave the mess for a day or so ,as it was a nightmare to put it all back together again😕
Sounds that seem to be masked deep in the recording are heard as bright as day .
It's not that the odd sounds are louder ,although they seem to be ,it is that they are so coherent and obvious, no mushyness.
A bit like the running water,which can on say multiple driver speakers sound unrealistic and odd.
This was the first thing I noticed with dml ,real sounds sounded so real, the machine gun for instance was staggeringly realistic in my room, I could not stop laughing it was so unbelievable.
The downside is some old or badly recorded cds can sound bad,but some old simply recorded recordings can sound excellent ?
So dml can be very revealing of certain LPs for instance , this is why I will always have my standard speakers to use on those recordings ,as these were the type of speaker they were set up to be played on.
Steve.
One time I spent all day stripping down my system and eps panel only to find with very very careful listening on different headphones that the distorted sound was in the actual recording 😱
I had to walk away and leave the mess for a day or so ,as it was a nightmare to put it all back together again😕
Sounds that seem to be masked deep in the recording are heard as bright as day .
It's not that the odd sounds are louder ,although they seem to be ,it is that they are so coherent and obvious, no mushyness.
A bit like the running water,which can on say multiple driver speakers sound unrealistic and odd.
This was the first thing I noticed with dml ,real sounds sounded so real, the machine gun for instance was staggeringly realistic in my room, I could not stop laughing it was so unbelievable.
The downside is some old or badly recorded cds can sound bad,but some old simply recorded recordings can sound excellent ?
So dml can be very revealing of certain LPs for instance , this is why I will always have my standard speakers to use on those recordings ,as these were the type of speaker they were set up to be played on.
Steve.
Geosand.
I 've only just had a proper listen to the video that you posted of the xps panels.
The first thing I have to say is I really don't like the sound of 1inch thick XPS especially untreated.
The coated 5mm XPS I have used in the past is much more pleasing to listen to.
The violins sounded very harsh ,on the other hand the guitar sounded quite nice.
The sub was rather loud but I know how hard it is to mix the sound with one microphone in a room.
The guy that posted the design on parts express visited the NXT RUBBISH site over on audiocircle and quickly made the project for parts express.
I can't say that I wasn't a little annoyed at this ,as it was more a HOW NOT to build a DML panel 🙁
Well there you go 😕
Too much haste and a very cheap project.
But everyone follows this including tech ingredients, it seems EPS is too hard to work with ?
Let alone others good materials.
A little knowledge can do a lot of damage.
But maybe a lot of people have built this and thought ,hey this actually works, how can I build one that sounds good?
Steve.
I 've only just had a proper listen to the video that you posted of the xps panels.
The first thing I have to say is I really don't like the sound of 1inch thick XPS especially untreated.
The coated 5mm XPS I have used in the past is much more pleasing to listen to.
The violins sounded very harsh ,on the other hand the guitar sounded quite nice.
The sub was rather loud but I know how hard it is to mix the sound with one microphone in a room.
The guy that posted the design on parts express visited the NXT RUBBISH site over on audiocircle and quickly made the project for parts express.
I can't say that I wasn't a little annoyed at this ,as it was more a HOW NOT to build a DML panel 🙁
Well there you go 😕
Too much haste and a very cheap project.
But everyone follows this including tech ingredients, it seems EPS is too hard to work with ?
Let alone others good materials.
A little knowledge can do a lot of damage.
But maybe a lot of people have built this and thought ,hey this actually works, how can I build one that sounds good?
Steve.
I decided to listen to some music a couple of days ago, after a lag of many months not listening to anything, and fired up the turntable. An enormous short dumped enough power into the exciters to fry the voice coil on one of the panels (the other panel was saved thanks to a disconnected phono lead). I have no idea how it happened.
Finding a replacement Dayton Audio DAEX25FHE-4 is proving nigh on impossible. The Tectonic exciters Steve mentioned a few pages ago are now discontinued. However, I have a pair of Monacor EX-40/4 exciters on backorder which should arrive in a couple of weeks.
I'll be taking the opportunity to replace the 3mm ply panels with 10mm EPS ones (they are 400 x 550mm - silver ratio). After 50/50 PVA treatment, the EPS panels are still a fraction of the weight of the ply panels. Theoretically, this should give a bit more efficiency and maybe high frequency extension.
As usual, I'll be using the minidsp hd for EQ and XO duties, crossing over to a Monacor 15 inch bass driver.
I'm tempted to try a coat of Plastikote on the panels. When dry, it makes for a pretty hard but thin coating. I used it when doping 'ordinary' drivers.
It does seem that audio exciters are currently in very short supply, thanks to the pandemic.
Finding a replacement Dayton Audio DAEX25FHE-4 is proving nigh on impossible. The Tectonic exciters Steve mentioned a few pages ago are now discontinued. However, I have a pair of Monacor EX-40/4 exciters on backorder which should arrive in a couple of weeks.
I'll be taking the opportunity to replace the 3mm ply panels with 10mm EPS ones (they are 400 x 550mm - silver ratio). After 50/50 PVA treatment, the EPS panels are still a fraction of the weight of the ply panels. Theoretically, this should give a bit more efficiency and maybe high frequency extension.
As usual, I'll be using the minidsp hd for EQ and XO duties, crossing over to a Monacor 15 inch bass driver.
I'm tempted to try a coat of Plastikote on the panels. When dry, it makes for a pretty hard but thin coating. I used it when doping 'ordinary' drivers.
It does seem that audio exciters are currently in very short supply, thanks to the pandemic.
Aurora has been around a few years now ,but I only discovered her a month or so ago on YouTube.
Aurora - It Happened Quiet (Live at The Current) - YouTube
Plenty of creaking noises on this very delicate and intimate recording session.
Just had a thought that I might replace my small surround speakers with small card panels ,temporarily anyway,as the dog will surely destroy them.
I have already modified the very small surround speakers about 4inches in size , with a G A Briggs (wharfedale ) idea , which now sound very nice, but it would be very interesting if the panels would be an improvement ?
Actually I could just do the two front speakers using my veneer panel and the card panel ,mmmm we shall see.
Steve.
Aurora - It Happened Quiet (Live at The Current) - YouTube
Plenty of creaking noises on this very delicate and intimate recording session.
Just had a thought that I might replace my small surround speakers with small card panels ,temporarily anyway,as the dog will surely destroy them.
I have already modified the very small surround speakers about 4inches in size , with a G A Briggs (wharfedale ) idea , which now sound very nice, but it would be very interesting if the panels would be an improvement ?
Actually I could just do the two front speakers using my veneer panel and the card panel ,mmmm we shall see.
Steve.
Lordtarquin.
Sorry to hear about your mishap .
Don't forget to scuff the surface before you apply the 50x50 mix ,the surface should feel quite fluffy, if only to stop the thin mixture from running off😀
This will replace the bad sounding surface and increase rigidity helping spread the sound across the panel.
I would be very cautious about adding damping of any sort ,the panel will be very efficient and dynamic very much like a horn speaker.
damping will do as it says and reduce volume and soften the sound,unless you prefer a softer sounding speaker that uses doping of course.
I don't know what grade eps you have but you should reach at least 10k before any roll off occurs.
Steve.
Sorry to hear about your mishap .
Don't forget to scuff the surface before you apply the 50x50 mix ,the surface should feel quite fluffy, if only to stop the thin mixture from running off😀
This will replace the bad sounding surface and increase rigidity helping spread the sound across the panel.
I would be very cautious about adding damping of any sort ,the panel will be very efficient and dynamic very much like a horn speaker.
damping will do as it says and reduce volume and soften the sound,unless you prefer a softer sounding speaker that uses doping of course.
I don't know what grade eps you have but you should reach at least 10k before any roll off occurs.
Steve.
Alright so I think you guys have convinced me to try canvas.
My 3 questions are:
- Is it necessary to mount the exciter to a wood panel and then onto the canvas? I am fine with that, I did that with my styrofoam panels.
- Seems like center placement is the norm.... doesn't that create resonant nodes? OR is that irrelevant with a DSP (which I will be using)?
- How low can I get with 2'x2' panels? I saw that spedge is getting down to 40Hz with a 12x16 canvas reinforced by a 4x6" wood panel.
I am getting a little frustrated with my styrofoam panels. They are just not efficient down low and clip at pretty moderate volumes, even when high passed at 100Hz. Above 200Hz or so they sound pretty good though.
My 3 questions are:
- Is it necessary to mount the exciter to a wood panel and then onto the canvas? I am fine with that, I did that with my styrofoam panels.
- Seems like center placement is the norm.... doesn't that create resonant nodes? OR is that irrelevant with a DSP (which I will be using)?
- How low can I get with 2'x2' panels? I saw that spedge is getting down to 40Hz with a 12x16 canvas reinforced by a 4x6" wood panel.
I am getting a little frustrated with my styrofoam panels. They are just not efficient down low and clip at pretty moderate volumes, even when high passed at 100Hz. Above 200Hz or so they sound pretty good though.
Steve, yes I gave the panel a decent sanding on both sides and rounded the corners, then applied the watered down PVA. I'm trying 70 grade - not the highest density, but if I like the sound signature, I'll go for the ultra-high density stuff from Mr Polystyrene on ebay. My plywood panels didn't really fall off too much until around 17k.
I notice quite a bit about canvas panels now. How would you say they compare with EPS ones?
I notice quite a bit about canvas panels now. How would you say they compare with EPS ones?
First of all, it would be in order if anyone did compare those styrofoam panels of the video with canvas panels like you have used?
Second is how push wood under the frame to tighten it up?
Third is what size panels are we talking about?
Has anyone tried dual exciters, perhaps one limited to lower frequencies?