NXT Exciter On Mylar Diaphragm?

borispm

Member
2010-05-16 4:15 am
Hi everyone,

I am playing with the idea of trying to fit the largest NXT exciter that Parts-Express carries to a mylar sheet instead of foam board or so. Any idea on that? Will it work or not? I am thinking of getting something with wave propagation like Quad ESL's main panel does. I know it's a bit different from NXT's recommendation on stiff panels, but I'm hoping for accidental discoveries.

Regards,
Boris
 
Hello Boris,

a stretched thin foil will reproduce sound if excited.

Problem with the "Dayton" style exciter sold at PE is
they are designed to operate as "seismic mass" without
being attached to a frame themselves.

They use their "sticky spider legs" to attach to the
membrane itself to hold their own mass.

If you do that to a thin foil, you add very much mass
in relation to the membrane, which lowers the resonance
of the stretched diaphragm.

The "legs" will also influence wave propagation
- i expect kind of 'hampering' - and its weight will
pull on the upper area of the foil.

Attaching the exciter with its magnet to a frame
to hold it's own weight and attaching only the
moving part of the exciter to the membrane
is the better way when using foil.

Kind Regards
 
ive often though about the validity of doing this myself, and since i havent i couldnt criticise. i would really love to try this with a large piece of PVDF(?) but the obvious expense stopped me. I feel a little wary of the exciters, since many require a 'solid' diaphragm, but if you fixed the rear of the exciter, alllowing only the motor and mylar to move, then im sure youd be getting somewhere. mylar tension, and the concentricity of the exciter, would then be about the only 'tweaks', or problems. i.e. film resonance dependant on the tension, and carefull tensioning of the film to retain VC concentricity. im assuming the exciters yours talking about are 'dynamic' and not 'piezo'.

i might still have a go myself, 2x1 foot ellipse, with the boggo standard brass piezo mono morph fixed to rear battening, and bonded to a tensioned mylar film radiator. with enough thought i think a decent baffle mid-HF driver could be made. maybe no ESL but possibly better or at least cheaper alternative, minus the 'doubling' of a more basic amateur ESL attempt.
 
Hi,

It simply won't work, you'd be better off stripping down a normal driver.
they work best, unsurprisingly in the way intended. I had a pair of
decent Missions + sub, Wharfedale still do a possible package with
far larger satellites, called loudpanel, two drivers per panel, the
panels do 160Hz upwards.

WHARFEDALE LoudPanel Speakers BNIB on eBay (end time 21-Dec-10 21:24:21 GMT)

Best you can hope for , with my Missions, is fairly neutral speech quality.
Surprisingly good though when used for AV rather than hifi.

rgds, sreten.
 

borispm

Member
2010-05-16 4:15 am
Hi Sreten,

Is it the the Mission M-Cube that you have?
My idea is a bit different from the NXT panel, instead of allowing resonance, I am actually trying to damp the edges of the panel so that it behaves like a whirlpool in an infinitely large body. I hope by reducing the resonance rather than allowing it to occur, I may have a bit more low end but with even lower efficiency. Efficiency will be even lower than the NXT panel, but with my current idea I may end up with a virtual point source that is quite a distant away from the actual speaker.

Right, tearing a motor from a normal driver should work even better!
 
I think I might give this a try. I don't have high hopes for it, but I will be on vacation for 2 weeks and it will give me something to do. It will be the final exciter experiment I do before tossing them in the trash.

I remember coming across a diagram on this forum that showed how to stretch mylar for an ESL using only tape and a flat surface. It basically just showed the sequence that the different edges and corners should be stretched and taped to get even tension. Does anyone know where I can find this?
 

borispm

Member
2010-05-16 4:15 am
Hi the anonymous one,

Good to know that you're going to give it a try. I do not have high hopes for that too, I guess the frequency response will be similar to those 1" to 2" drivers, maybe slightly better. But I'm actually expecting far better stereo imaging than the NXT design as the foam at the edge can absorb and reduce resonance, and hence stopping the "total chaos" of the panel. Instead, it will be a well controlled vibration like a whirlpool in calm water. I am looking forward to trying mylar sheets with different thickness and amount of damping. As far as my memory can recall, there is a psycho-acoustic theory suggesting the amount of delay and decay of the delayed signal can affect the listener's perceived source of sound, and this may results in different amount of "ambience" or "soundstage" or whatever you call it.

Regards,
Boris
 
Hi,

In another life where I actually got a job in speaker design this would have
been something I wanted to look into. I wanted to look at something like
a 13"x8" open panel atop force cancelling boxed midwoofers.

My assumption was that the thin mylar diaphragm would actually not
be flat, thought I'd have tried that, but somewhat curvilinear to give
some 3d stiffness, the idea is essentially a cheap Mangar type driver,
trying to control the wavefronts eminating from the voice coil.

I surmised a typical suspension would be too heavy, I envisaged a
circle of posts with wire running between them in a pattern to
touch the coil as a form of lightweight, low audibility suspension.

There was an American speakers built along these lines, with open
baffle woofers I recall, I recall the problem was keeping the coil
and/or ferrofluid in the gap, it was never fully developed.

Alas no-one wanted an EEE with more ideas than real common sense
with loudspeakers at the time. Manafacturuers are very conservative.

rgds, sreten.
 
Last edited:

SoundRight

Member
2010-07-28 10:47 am
.."My goal is to get fairly even response from 150Hz-15kHz with an A4 sized diaphragm. Is that too high an expectation?"

Don't bother making one yourself, investigate the Micro Pro from Sound Leisure Industries (UK)...a 25watt exciter on an A4 diaphragm. Yes, around 150Hz to 20KHz!

I use a stacked pair of Mini Pro DMLs myself (Fane Mini Pro as was). They can sound superb crossed over to a dipole or cardoid sub bass unit.

Don't know what people want really..They have no cabinet, no crossover, benign impedance, 95db/watt stacked, 110 x 120 degrees dispersion up to 10k..less vertically stacked I imagine and only one failing really. They are not flat in amplitude response..well, that's a problem isn't it!!

Mine are 24db/oct rolled at 100Hz and using dual gated response analysis, corrected to be flat from 100 to 20kHz, worst case +/- 3dB and very very flat from around 500Hz upwards...all EQ is done in the digital domain so I go from CD digital out to digital EQ to DAC magics to analog active crossovers..DBX for the panels and a separate feed and digital EQ with it's own DAC magic to the power amp for the cardoid sub woofer..just a simple cheap Behringer analog active with 0-2ms second delay for the rear facing woofer....( 2 x 18" Kilomax units..I design loudspeakers as a profession)

Could have gone with a loudspeaker management system but this would have negated using a digital signal chain all the way to the 192Khz/24bit DACs at the end of the chains..only one conversion to analog then before the power amps!

Works for me!
 

borispm

Member
2010-05-16 4:15 am
Hi SoundRight, the Micro Pro seems like a decent DML but it's not exactly what I want to try. DMLs have vague soundstage and are not very precise point sources, they are like a lot of point sources vibrating at different times. I am looking for something that is not based on this working principle, so what I'm trying to use is mylar, not something very stiff. But if I can find a pair I would definitely buy them for fun.

Sreten: I suspect the conventional speakers sound reasonable enough so no one's adventurous enough to spend money and R&D and try something new, especially in the current economy.
 

ashok

Member
2002-06-06 4:43 am
3RS
.............. I suspect the conventional speakers sound reasonable enough so no one's adventurous enough to spend money and R&D and try something new, especially in the current economy............

I think that isn't true . We have lots of guys trying all kinds of funky things on this forum. There are many other forums also !
Remember the small dipole with dual film separated with a small air gap and driven by a small speaker at the bottom ? Meant ( at that time ) for computer speaker use ?
Many guys were interested in that and we had some threads on that long ago.
You can be sure that not everyone who tries experiments puts up results on the forum or anywhere on the web just because they think it isn't good enough yet ( ....and might get laughed at ... or just rudely shot down ...? ) !
 

borispm

Member
2010-05-16 4:15 am
I think that isn't true . We have lots of guys trying all kinds of funky things on this forum. There are many other forums also !
Remember the small dipole with dual film separated with a small air gap and driven by a small speaker at the bottom ? Meant ( at that time ) for computer speaker use ?
Many guys were interested in that and we had some threads on that long ago.
You can be sure that not everyone who tries experiments puts up results on the forum or anywhere on the web just because they think it isn't good enough yet ( ....and might get laughed at ... or just rudely shot down ...? ) !

Ashok,

First, thanks for your reply. I am sure you are a wise fella and thanks for reminding me that possibility. There are many experiments that I've done and I do not think it's good enough to be published too.

Secondly, what you've quoted from me is a reply to what Sreten said earlier. Sreten said many manufacturers are too conservative and do not dare to try something new, and I actually stated that it's because in the current economy no one dares to risk his money on R&D.

Thanks for your reply anyway.

Regards,
Boris
 

SoundRight

Member
2010-07-28 10:47 am
I am sorry borispm "DMLs have vague sound stage" is just plain wrong!

In some ways it is the myths that have grown around this technology that have prevented it from not being more universally adopted...why, because it is much cheaper than conventional loudspeakers. We have constantly seen the market protect itself over the years..and propagate the myths that people want to hear.

I am a professional loudspeaker designer..my conventional designs have been very favourably reviewed in both professional journals by Hugh Robjohns and Hi Fi magazines by Ivor Humphries and David Allcock. My monitors were more than favourably compared with PMC and ATC..and I was working with relatively cheap Fane drivers...a lesson here perhaps?

Now..DML's ..unlike so many that comment, I have worked with these designs and in collaboration with the UK manufacturers for many years. I am also old enough to have come from stacked Quads/Kelly Ribbons and my own ATC designs.

Fiction..DML's don't image!!

Fact..they have constant directivity at virtually all frequencies and can be easily flattened in amplitude response. The have no crossovers and no cabinets.

Fact.. they are not pressure sources like conventional piston loudspeakers and therefore do not excite room resonances in the same way..this is why they are so good in complex architectural structures..

Fact.. they actually image very precisely..I know I am looking at a stacked pair now(!) What they don't do is insist on you sitting in one hot spot to get an artificial image. They actually produce a stable 3D image with very precise instrument location..which you can virtually walk about in!!

So much research has gone into professional sound systems...bass steering, cardioid responses, run times..to ensure that an audience gets the most even coverage it can regardless of where they are seated or standing.

Virtually none of this knowledge has trickled down to the Hi Fi industry that would still rather sell you an expensive length of cable for an unbalanced system than tackle the real issues.

We have cheap digital processing now at 96kHz/24bit but so few use it. The problem with "purism" is that it is just so wrong!! There are ways of correcting and balancing system/environment performance and just sticking two loudspeakers in a room sadly does not do it..

Actually, although so few dare mention it, the biggest problem with stereo...is the other speaker(!) and how they both interfere with each other..particulary at low frequencies.

Other problems include comb polar response, beaming at higher frequencies, phase shift at crossover frequencies, varying polar tilts..and monopole pressure sources (sealed, TL or reflex loudspeakers) causing peaks and troughs throughout the listening area...maximum pressure at the boundaries and often cancellation in the middle of the room..and at differing frequencies!

No sir! DML's (and a single dipole or preferably cardoid sub woofer source combining the benefits of velocity source at higher frequencies and pressure source at very low frequencies) can come a lot closer to the "truth" than most traditional conventional set ups.

I think it was Linkwitz that said he had no problem with those that choose to spend fortunes on cables and amplifiers (created from a solid block of aluminium!) ..and nor do I, but let us not pretend that it gets you closer to a more authentic sound reproduction.

Feel free to enjoy the hobby in any way one chooses, but the Hi Fi world has been hood winked into not absorbing what the pro's have known for a long time..and lets be honest if you are the editor of a magazine and a manufacturer advertises with you who just happens to sell 10K amplifiers and 1.5K lengths of cable..is it really going to be in his interest not to say how fantastic it all sounds..and of course it probably is a thing of beauty without a doubt.

But objective measured reviewing went out nearly 30 years ago now..I can remember, I have been in the Hi Fi and live music industry since the 70's!!

So, finally!! Going right back to the original point..please do the research and check out the Peter Mapp article re:DMLs and so many others that have been written and available from the AES library. They are not perfect loudspeakers of course but in terms of producing low distortion, flat response and a 3D soundstage wherever you sit, they are quite difficult to beat!

Cheers..and Merry Christmas!!

David
 
Hi,

The Mini Pro's look interesting but a pair is not cheap, nevermind a stacked pair.

My little Missions (+sub) were superb for AV, but not really flat enough for hifi.

The Wharfedale loudpanels look the best bet for domestic experimentation,
will full digital EQ perhaps the major limitation (IMO) could be removed.

rgds, sreten.