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Old 13th June 2010, 12:40 PM   #521
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Hi Oliver,

OK i think i understand ...

When you look at the laser holographs from the manger transducer,
you see clearly that it operates in a modal fashion, even at the upper frequencies.

The lowest resonances are even visible in the impedance plot.
Very similar behavior like a rigid panel transducer.

What is the main difference between the bending stiff panels
called "DML" and a transducer like Manger ?

I think it is the coincidence frequency beeing above the
audible band in case of the manger, because the propagation
speed of the bending waves is lower than on a rigid panel.
This allows for high modal density on a small membrane area.

But there are also bending wave transducers in rigid panel style
introduced with coincidence frequency at the upper limit of
hearing.

The membrane not excited in the center may be a further "DML"
feature.

And the Manger having a surround which strongly reduces
reflection ...

But a difference in principle of operation ?

It is merely a difference in dimensioning and materials used:

- where do you want to adjust the Q of your modes ?
- what about modal density and modal overlap ?

For good sound quality, you need modal overlap to be high
enough, so that single modes are not audibly detectable.

The undamped rigid panel style DML which has damping only
via sound radiation is IMO not suited for high quality audio
reproduction. How should it be ?

As i have seen in recent papers the tendency goes towards
increasing the coincidence frequency and/or to account for damping
in those cases, where sound quality plays a role.

Whether this coincides with the initial approach of rigid panel
DML is a wording problem IMO and should not be our problem
in diyAudio at least ...
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Last edited by LineArray; 13th June 2010 at 01:08 PM.
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Old 13th June 2010, 01:35 PM   #522
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Quote:
Originally Posted by el`Ol View Post
Wouldn't that prevent a modal operation? This is more what the Manger is intended to do.
I think there is an intermediate transition from what is "modal" behavior
to a "statistical" behavior, characterised by modal density and overlap
being as high, that the modes cannot be identified anymore.

When it comes to energy storage: Which decay time is tolerable ?

If you would state - which i do not believe - that a DML must have
identifyable modes and a long decay, then a DML would be
d e f i n e d by its poor quality.

The NXT developers will probably not be amused from such
a definition.

Is there a decay time limit separating "conventional bending transducers"
from "DML" ?

I think in fact a DML also aims for "hiding" the modal character by
statistical behavior (high modal density and overlap).

Which decay time is tolerable may be a point worth discussing.

But i think there is also an efficiency vs. quality conflict.

A panel lacking any damping might go very loud even when
using a weak and cheap exciter.

Quality increases manufacuring cost ... i guess this observation is
not entirely new and also applies to other kinds of stuff.

Kind Regards
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Old 13th June 2010, 02:57 PM   #523
el`Ol is offline el`Ol  Germany
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Quote:
Originally Posted by LineArray View Post

Is there a decay time limit separating "conventional bending transducers"
from "DML" ?
My personal definition of conventional bending wave operation is that the wave doesn't reach the edge.

What I am interested in is the frequency-dependence of damping. How is it in air? And what materials come close?
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Old 13th June 2010, 04:38 PM   #524
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Quote:
Originally Posted by el`Ol View Post
My personal definition of conventional bending wave operation is that the wave doesn't reach the edge.
...
In the early results from the prototypes i posted the waves are far from
being absorbed before reaching the edges and there is also reflection.

If e.g. in the Manger transducer there was no reflection from the
edges, there would not be visible resonances in the impedance
curve.

With a perfect termination at the edges the waves would look like
propagating beyond the edges and disappear without reflection,
like waves in an infinite large pool, where the form of the panel is
just a picture frame restricting your view. A perfect termination would
simulate an infinitely large panel.

Simply speaking this is why also a rigid panel style DML gets better
the larger it is - i see your point but i feel that the distinction is
rather artificial.

Using your definition even an upsized DML would not be a 'DML' anymore.

Maybe it is the term itself which is not very instructive.

But we are all on thin ice here, because there are no tools or
established quality measures to describe sane limits for the
bahaviour of these kind of speakers, be it
"conventional bending wave" or "DML".

There is still work to do i feel.

Btw: What kind of fish is the Göbel transducer in your personal opinion ?
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Old 13th June 2010, 05:40 PM   #525
el`Ol is offline el`Ol  Germany
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Quote:
Originally Posted by LineArray View Post

Using your definition even an upsized DML would not be a 'DML' anymore.
It woud be a DML up to a lower frequency.
The importance of distinguishing between DML and not-DML is lobing. In the frequency range where one according to my defintion has a true bending wave transducer with multiple exciters one has the same lobing problems as with pistonic speakers.
About the Goebel:
My impression was that it has ultra-high resolution at low levels (even higher than in reality), but gets into trouble at higher levels.
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Old 14th June 2010, 11:24 AM   #526
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Quote:
Originally Posted by el`Ol View Post
..
About the Goebel:
My impression was that it has ultra-high resolution at low levels (even higher than in reality), but gets into trouble at higher levels.
Would you classify it as "DML" or "Conventional Bending Wave Transducer"
according to your definition ?
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Old 14th June 2010, 11:46 AM   #527
el`Ol is offline el`Ol  Germany
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I did't see a vibratometric image of the Goebel, I just listened to it. But as far as I know (patent) it has two exciters, so it would have lobing problems in conventional bending wave operation mode.
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Old 14th June 2010, 02:00 PM   #528
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Assuming extremely high damping of the panel or whatever used as membrane,
a multiactuator panel would at high frequencies in fact show similar behavior
like multiple pistonic tweeters mounted apart, given that the exciters cover
the same frequency range.

But also - and especially - with a low damped panel you have disturbing
interference effects of the bending waves on the panel itself, when exciters
radiating the same frequency are only a few halfwavelenghts
(here: bending wavelengths on the panel) apart. That effect gets even
worse with a "low loss" panel.

This is why it is mostly disregarded to have more than one exciter for the
highs. As i remember you told me once, that the highs of the podium did not
meet your taste ... and this might possibly be one of the reasons.

I assume the "Podium" is a true "DML" according to your classification ?
Made of thick and rigid honecomb core, Shelley Katz states he loves the
material because it "rings like a bell" ...

The art is IMO to adjust damping of membrane and suspension in a way to maintain
bending wave propagation on the membrane to support frequency independent
dispersion but also have a smooth and fast spectral decay.

There is no perfect recipe how to achieve that, many solutions are possible.
A panel and its suspension having extremely low loss is only good for making
a mechanical reverberator (plate reverberator) - as we know such devices
have been in use in the past decades for studio reverberation - but are not
suitable as quality loudspeakers.

I personally do not think it is useful to make a distinction between different
bending wave speaker concepts as if they were different principles.

To me a Podium is a Podium, a Manger is a Manger and a Linaeum is a Linaeum
for example.

There are differing technical solutions applied (and often patented) by each
manufacturer to achieve the best compromise or the "optimum" due to ones
philosophy.

I would summarise those speakers simply as "bending wave speakers" or as
"constant velocity speakers", because the average velocity magnitude across
the membrane area is nearly frequency independent, whereas the conventional
pistonic speaker is mainly a "constant acceleration" speaker.
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Last edited by LineArray; 14th June 2010 at 02:21 PM.
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Old 20th June 2010, 01:40 PM   #529
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Hi guys,

I'm sort of back, been very busy with work related stuff.

I took a look again on the Audiocircle forum and appearantly I ticked off a guy named "zygadr" (at least I think it was me). He feels that there are people (like me) questioning his results and ask the same questions over and over again. He refers to diyaudio around the date that I started my replies.

I think the same questions keep popping up due to little "evidence". If I state that a certain material is THE BEST THERE IS, OMG I CAN'T BELIEVE MY EARS!! Then people like me are interested in reproducing this sound and try to get into the details to how this sound could be so good/bad.
Please post pictures and detailed information of the build and preferably with measurements. If these lack, someone else can never rebuild them.

People tend to forget that a sound which sounds great to one, may sound awfull to the other.
Let's not forget that I am interested in NXT tech. and do not think they are toys, but I question certain claims and would like to see them verified by measurements (and I don't care about the straightness of a graph, a graph is a handy tool).

So, zygadr if I ticked you off, sorry mate
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Old 20th June 2010, 02:53 PM   #530
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Quote:
Originally Posted by mikesbmw View Post
...
Let's not forget that I am interested in NXT tech. and do not think they are toys, but I question certain claims
...
What are those claims you question particularly ?

What feels a a little strange to me is that NXT has managed somehow,
that all bending wave related technology is widely associated with
their name. Be it "flexurally rigid" panels or not.
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