3D printed waveguides

[augerpro]

While I may not be able to measure them anytime soon, new wg's for the Transducer Lab N26C-A, Satori TW29RN, and SBA SB19ST have been sent to the printer!

[Ro808]

All very complex, and as I think you wrote above somewhere, not really all of that useful for DIY.

I wonder how cost effective the methodology is, seems like a considerable investment in hardware, software and human intelligence and endeavor.

Ro808, are you working on guides at present, if so, are you willing to describe these?

For the average diy-er the Harman way of prototyping is beyond reach, with the exception of some stages.
The modeling and simulation in COMSOL could be very useful, as was shown some time ago on diyaudio.

I have access to COMSOL and it's my intention to see if I manage to set up an optimization study based on the availabe application data, adjust the parameters, add (boundary) conditions and (define) design constraints. Comsol includes many nice features and add-ons, i.e. import data from Matlab and interfaces with major CAD applications etc.
The most challenging part is overcoming my lack of engineering skills and limited knowledge of physics .

[Ro808]

While I may not be able to measure them anytime soon, new wg's for the Transducer Lab N26, Satori TW29RN, and SBA SB19ST have been sent to the printer!

These tweeters are all very nice Brendon! I'm very curious about their performance behind your guides.

[TN Allen]

For the average diy-er the Harman way of prototyping is beyond reach, with the exception of some stages.
The modeling and simulation in COMSOL could be very useful, as was shown some time ago on diyaudio.

I have access to COMSOL and it's my intention to see if I manage to set up an optimization study based on the availabe application data, adjust the parameters, add (boundary) conditions and (define) design constraints. Comsol includes many nice features and add-ons, i.e. import data from Matlab and interfaces with major CAD applications etc.
The most challenging part is overcoming my lack of engineering skills and limited knowledge of physics .

Ro808,

Perhaps you can collaborate with Brandon to model and analyze the guide files he has sent for printing. He is using Fusion 360 as I recall, he should be able to send you the designs in a suitable file format.

I looked at Comsol a few years ago, but decided to continue milling, and evaluating by listening to guides, given the cost and the need to invest considerable time and effort learning Comsol.

[augerpro]

Ro808> how does Comsol treat the driver diaphragm? Since that plus its interface to the throat is where it all goes wrong or right, with little predictability for the DIY'er. Maybe try modeling one of my guides with the SB26 and see what happens?

[Ro808]

That's a very good question and exactly what I've been asking myself.
So, I did a little research on this matter and it appears:

- It depends on the subjects and (numerous possible) specifics of your study
- COMSOL contains a library with models of transducers you can add to your study and modify
- One of the available example applications specifically deals with optimization of a tweeter waveguide. According to the documentation:

This model illustrates three independent optimization studies. The hope is that these different studies will illustrate how to set up appropriate objective functions.

The studies are:
- Optimizing the frequency response: Optimizing based on the sound pressure level in a point over a frequency band (study A).
- Optimizing spatial response: Optimizing of the spatial uniformity (over a certain spatial angle) of the speaker response for a fixed frequency compared to a target (study B)
- Directivity index (DI) optimization: Optimizing the directed response of the speaker for a fixed frequency (study C)

The same initial geometry and control parameters are used for all cases.

The models lumped_loudspeaker_driver and tonpilz_piezo_transducer are used from the library.


I have also seen similar studies in which the tweeter is a baseline acoustic-structure interaction FE model of a tweeter built in Comsol.

It might even be possible to completely define the tweeter including electrical properties, material/structural properties, geometry of housing, dome, magnet structure and other parts + (pressure) acoustical behavior, heat dissipation, structural/mechanical behavior etc. etc.

I guess, it's best to keep it simple ;-)

To quote Clint: A man's got to know his limitations.

[augerpro]

Do you know how to build a new model? Or tweak an existing one?

[Ro808]

I think it's next to impossible to build a model that represents a specific tweeter without detailed data from the manufacturer.
A simplified baseline model seems the way to go.

An important issue to be clarified concerns the impact / importance of the "dome area"; i.e. ring radiator or (flattened) dome and the surround within the model.

[augerpro]

Are any of the stock models a dome tweeter? I can get reasonably good measurements, radius would be a bit of guess, but certainly better than using a default model.

[TN Allen]

"To quote Clint: A man's got to know his limitations."

And exceed them.

Why not register for the webinar and send in questions?

Here's the link that I received, though the dot at the end may be superfluous: http://comsol.com/c/6bm8.

And here is where the link leads:

https://event.on24.com/eventRegistration/EventLobbyServlet?target=reg20.jsp&referrer=https%3A%2F%2Fwww.comsol.com%2Fevents%2Fwebinars&eventid=1515799&sessionid=1&key=1D8E718ADC4B3DB14669E4AF9EBACF86&regTag=&sourcepage=register

[Ro808]

I found some data from a PhD Research Thesis including a model of a SEAS 27TFFC that was actually made in cooperation with SEAS.
However, this paper is already quite old (2011) and it seems COMSOL has improved its Acoustics Module ever since.

Moreover, the waveguide prototype looks rather strange. This elevated cone was chosen out of 120 tested geometries in order to meet specific critera. Its shape is based on an old patent from the 1930's .

[Ro808]

I have listened to the first 6 minutes of the webinar and downloaded the pdf. I think it's a general introduction to Comsol's applications in Acoustics. I will have another listen.

[Ro808]

Linear Elastic Model domain (blue). The voice coil has a Boundary Load condition (red) in the upwards direction and a Fixed Constraint on the boundary (green) where the diaphragm is glued to the front plate.

[Ro808]

Reading and comparing different research approaches alone is already very time consuming. Add to this the complexity of the physics involved, the necessary input data, measurement equipment etc. and it becomes obvious that it's not feasible to reinvent the wheel for our purposes. Results from research/studies often leave much to be desired due to practical issues or inconsistencies in the setup and (input) data.


This WG was subject of another thesis:



The conclusions do not yield many new insights.


Guidelines for a successful waveguide design

The design of the prototype waveguide was intentionally far from optimal. Still there
is room for conclusions to be made about designing a waveguide with good
performance. According to the analysis of the results, the following conclusions can
be made for achieving successful waveguide design.
Sharp transitions in geometry should be avoided so as to not excite diffraction. It is
shown that smooth a transition to the enclosure baffle is necessary or a severe
diffraction problem will occur.
At low frequencies the directivity of the waveguide is comparable to the directivity
of a circular piston with a size equal to the mouth of the waveguide. This is
consistent with the horn theory.
When the waveguide mouth circumference is comparable to the wavelength, the
geometry of the waveguide dominates the directivity. In this frequency range the
diffraction problem is worst.
When the circumference of the driver is comparable to the wavelength, the geometry
of the driver starts to dominate the directivity characteristics.
An asymmetrical geometry would reduce the diffraction problem by smearing it to a
broader frequency range.


Outputs of the work

There are two unique outputs of the work for which I did not find references in the
bibliography. One of them is related to the directivity visualization tool created. It
enables an intelligible way to compare the measured and modelled results. One
favourable feature is that directivity plots are published by many loudspeaker
manufacturers. Therefore there is already plenty of material with which to compare
the modelled results without making measurements. The real value of this work is to
have a tool for virtually prototyping waveguides. Directivity plots give instant
feedback about how the change in the design affected the performance. With
compact graphs it is possible to compare the directivity of several prototypes at a
glance, whether they are virtual or real.


CONCLUSIONS

The second unique output is the improved transducer model. The idea is to combine
measurements and modelling. The tweeter output can be also modelled, but as stated
before: a modelled result is always second to the real world. The accuracy of the
modelled result was improved by combining a laser velocity measurement with the
model.

[Ro808]

It's also noteworthy that I have yet to come across FEA optimized waveguides that get close to, let alone outperform, Brendon's best WG's.
Evidently, with the exception of professionally developed WG's for commercial products (HARMAN, Genelec etc.).