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Old 6th April 2010, 08:41 AM   #21
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Interestingly the vibrational modes of the crash cymbal look very
similar to that of the "Manger" transducer e.g.

Why not use a crash cymbal as a loudspeaker membrane ?

To achieve a balanced response of a bending wave transducer, the modal
density - the number of modes in a given frequency interval - has to be
much higher in a good bending wave transducer than in most musical
instruments.

A musical instrument has typically a low number of high Q resonances in
a given frequency interval, while in a bending wave tranducer we ask for
a high number of resonances having also a lower Q.

Depending on the Q of the resonances and their distribution over frequency,
it is widely held as a rule of thumb, that a bending wave transducer
should have at least 10 modes per octave, to make a balanced response
possible. A mode is a vibrational pattern spreaded along the membrane
area of the transducer. Every mode is associated to a certain resonant
frequency af the system.

Instead of trying to avoid resonance, the design of a bending wave
transducer is about increasing the number of modes to make the device
behave "statistical" by modal overlap:

Once the modal overlap is high enough, there is no resonant behavior
audible.

I explain a bending wave transducer sometimes to myself as some kind of
"universal" musical instrument. If the modal density and further acoustical
properties are chosen well, it is able to "emulate" existing musical instruments:

An observed vibrational mode of our crash cymbal e.g. or a violin to be reproduced,
falls into a frequency range, where a whole set of modes of the bending
transducer is excited. If built well, the characteristic spectral power
distribution of the instrument can be approximated very closely.

To achieve this, it is not necessary for the vibrational modes itself to
look similar to those of the instrument to be reproduced. What matters is -
like for conventional loudspeakers too - a balanced frequency response of
the radiated sound pressure and the acoustic output power.
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Last edited by LineArray; 6th April 2010 at 08:50 AM.
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Old 6th April 2010, 10:27 AM   #22
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Some examples for manufacturers of bending wave loudspeakers:

Transducers based on wave propagation on thin foil

Manger Schallwandler

Airfoil Loud Speakers


Conical shaped membrane driven at the apex

Ohm Speakers

German Physiks - High End Technology Loudspeaker Manufactur - DDD Driver - Home


Flexurally (more or less) rigid flat panels

TERRA-SP3000 of Teragaki-Labo?????????????????TERRA???????????

Welcome to GBEL - Revolutionary Bending Wave Technology

NXT Sound - Home

http://futuremusic.com/blog/2007/12/...hile-speakers/


Special case

MBL - Unique High End Audio

I assume bending here is only intended for the lowest mode,
higher order modes seem to be circumvented by using smaller
membranes for higher frequencies. So these may not be
"bending wave speakers" in the same way as the examples
mentioned above.
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Last edited by LineArray; 6th April 2010 at 10:35 AM.
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Old 6th April 2010, 01:24 PM   #23
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When looking at the manifold of possibilities in designing
bending wave transducers, there seems to be only one thing
in common:

Radiation of sound by a membrane able to modal vibration.

A classification of a given bending wave transducer could
be according to

Generation of driving force
... dynamic, piezolectric ...

Shape of the coupling area to the membrane
... small circular area, ring, line ...

Number of actuators (exciters) driving the membrane
... single actuator, multiple actuators, muliple actuators but differing in transmitted frequency range ...

Kind of membrane due to form
... flat rectangular, flat circular, high/low aspect ratio, curved ...

Kind of membrane due to material properties
... rigid, soft (correlates with propagation speed of bending waves and coincidence frequency* ) ...

Ability to whole body motion
... whole body motion at low frequencies, modal behaviour only ...

and so on.

None of those design desicions make the transducer good or bad in itself.
But every decision has implications for its properties like

- efficiency
- usable frequency range
- size needed for sufficient modal density
...



Examples for historic roots of bending wave loudspeakers:

- 1929 Patent for exciting a showcase to transmit music and speech for advertising.
If you klick on "Volldokument laden" button on top right you can view the full
document including figures, which are quite self explaining.

http://depatisnet.dpma.de/DepatisNet...E000000465189A

- 1924 (about) "Ibach Tonspiegel", "Sound Mirror" named Transducer from the german piano manufacturer Ibach.
This device also has been patented.

Tonspiegel Radio Ibach, Berlin W


Not to mention the traditional knowledge of instrument makers,
which can at least partially be applied to bending wave
lousdpeakers. The "Tonspiegel" is an early example for that
kind of knowledge transfer.

-------------------------
* Since the propagation speed of bending waves on the membrane rises with
frequency due to dispersion, there is a frequency for each membrane where
the propagation speed of bending waves equals the speed of sound in air.
At coincidence and above the efficiency of a bending wave transducer is
higher than below. Some concepts have a coincidence frequency
above the upper frequency limit of hearing (e.g. Manger) and some within.
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Last edited by LineArray; 6th April 2010 at 01:29 PM.
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Old 7th April 2010, 07:29 PM   #24
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This chapter briefly summarizes the internal physics and the
"in room behaviour" of panel type distributed mode loudspeakers.

Loudspeaker and headphone handbook - Google Bcher

------------
"The Loudspeaker and Headphone Handbook", John Borwick, Focal Press
Third edition 2001

Chapter 4 "The Distributed Mode Loudspeaker" by Graham Bank
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Last edited by LineArray; 7th April 2010 at 07:34 PM.
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Old 8th April 2010, 08:57 AM   #25
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Summarized and translated excerpt from an study conducted by NXT:

The pictures show a comparison in interaction
of direct and reflected sound between a panel type
bending wave loudspeaker and a conventional 2-way loudspeaker.

Both speakers were positioned about 50cm from a wall.
A moving microphone like shown in figure 11 was used
for measurement of sound pressure as a function of angle.

2-way loudspeaker: 10cm cone bass-midrange unit and 19mm dome tweeter
Panel loudspeaker: 58cm x 66cm, carbon cover on aluminium honeycomb core

The main radiation axis of both speakers was parallel to the wall.
Signal used was white noise.

x-axis shows angle between microphone and the wall
y-axis shows frequency
color shows sound pressure level

The strong interference of direct and reflected sound field caused by the
conventional speaker is visible in figure 12 by angle dependent dark cues
in the sonagram.

The panel type bending wave loudspeaker in figure 13 shows less comb filtering,
which is due to far lower correlation between direct and reflected sound.

------------------------------
All 3 figures (downsized) were taken from:

"DML - Distributed Mode Loudspeaker
Ein neuer Schallwandler,akustische Eigenschaften und
die Konsequenzen fr den praktischen Einsatz" (Original title)

by Charalampos Ferekidis

Own translation of title:
"DML - Distributed Mode Loudspeaker
A new transducer, acoustic poperties and
consequences for practical use"

Link to the article in german language:
http://www.wvier.de/texte/NXT_Tonmeistertagung.pdf
Attached Images
File Type: jpg ExperimentalSetting.JPG (45.8 KB, 590 views)
File Type: jpg Conventional.JPG (239.5 KB, 522 views)
File Type: jpg DML.JPG (248.0 KB, 504 views)
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Last edited by LineArray; 8th April 2010 at 09:00 AM.
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Old 10th April 2010, 10:39 AM   #26
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Example step responses from own earlier prototypes.

Not directly comparable to each other due to varying conditions.
Attached Images
File Type: jpg StepResponse_StampedMetal.JPG (235.4 KB, 484 views)
File Type: jpg StepResponse_Composite_I.JPG (277.6 KB, 465 views)
File Type: jpg StepResponse_Composite_II.JPG (198.9 KB, 40 views)
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Last edited by LineArray; 10th April 2010 at 10:42 AM.
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Old 10th April 2010, 11:15 AM   #27
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Step responses of some conventional fullrange drivers
found on "fullrangedriver.com"
Attached Images
File Type: gif JX92S%20Step%20resp.gif (4.2 KB, 48 views)
File Type: jpg LowtherPM6A-step.jpg (118.4 KB, 44 views)
File Type: gif Pioneer_B20FU20-51FW_Step.gif (3.5 KB, 41 views)
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Old 10th April 2010, 11:47 AM   #28
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Interview with Paul Paddock, developer of the "Airfoil"
loudspeaker ...

Stereophile: Impact Airfoil 5.2 loudspeaker system
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Old 14th April 2010, 07:38 PM   #29
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These large rubanoids are bending wave transducers too:

[Tubebbs]̳Ⱥ - Powered by Discuz! Board
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Old 20th April 2010, 12:21 PM   #30
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White paper describing the Naim "Ovator" which uses
a wideband "Balanced Mode Radiator" (BMR):

http://www.music-line.biz/cms/filead...ovator_bmr.pdf
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