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peepsalot 16th March 2012 09:03 PM

Horn Design tips for cell phone
Howdy folks. I'm interested in experimenting with 3d printing technology to create fancy horns.

I currently have a Reprap Prusa 3D Printer, with a build area of about 190x190x100mm
The nice thing about the 3d printer is it's ability to handle complex shapes just as easily as very simple shapes. It just handles any 3d model 1 layer at a time, so you can make some pretty wild or intricate designs.

I was thinking a fun thing to try at first, and given my relatively small build area, would be some sort of phone dock with integrated horn/waveguide for amplifying a cell phone speaker. I realize this won't be winning any kind of awards for sound quality, but I'm interested in understanding the concepts, and seeing how much of an effect it can have. And maybe in the future using this knowledge I could scale up the idea for a full range enclosure with a serious driver in it.

So I'm wondering if anyone can provide some tips for how to design a horn profile for maximum efficiency and sound quality. Would I have to know very specific specifications(thiele&small?) of the actual driver used in the phone? I have an HTC EVO which I would be using to test this with. Is there some way I could measure or derive whichever significant parameters I would need for this?

One thing I'm curious about is if there is an optimal total length of the horn, or if you can just keep going larger and larger for more and more effect.

Here is an example of a commercial product produced for the iPhone which is the sort of thing I would be interested in doing:
AirCurve - fits iPhone 3G and original iPhone - Griffin Technology

peepsalot 17th March 2012 03:35 AM

Hrm, maybe this was the wrong section to put this in.

w_oswald 17th March 2012 05:32 AM


Originally Posted by peepsalot (
Hrm, maybe this was the wrong section to put this in.

Hmmm, indeed... I would venture a guess that many if not most folks here aspire to projects on a slightly larger scale. Not to dissuade you - I myself have some tiny (~1") speakers salvaged from a laptop PC just waiting for horns something along the lines of what you're suggesting. I've seen a variety of creative efforts to accomplish this sort of thing. Some of them are relatively crude, cardboard and masking tape affairs, yet they manage some impressive results. Maybe not hi-fi, but still a useful improvement. I've even come across plans for a matchbox sized, kardboard Karlson enklosure!

Design tips... lets see:
First off, don't be intimidated by the "mathematically engineered" design in your linked example. I believe that engineering of interesting speakers - including folded/convoluted horns - can't be distilled down to just mathematics. As for measuring parameters of the phone's speaker driver, that may be of use eventually in refining your design, but why not start with some experimental builds based on the dimensional constraints at hand? Perhaps you could construct some straight (non-convoluted) cardboard/tape mockups to evaluate different horn expansion profiles and mouth areas before consuming a lot of - I imagine - pricey printer ammo. Listening tests at this stage would probably be the most useful indicator of whether the concept has the potential to perform to your expectations.

I'd suggest a throat section that tightly couples to the speaker opening on your phone - try to minimize discontinuities or cavities that would surely lead to troublesome resonances. The (eventually) convoluted section should be of narrow, minimally expanding cross-section with curve radii as large as possible relative to cross-section diameter. Once you get it to the desired direction, then you can do the final expansion to the largest practical mouth opening. Expansion profile seems to elicit a lot of emotional discussion - I guess it can affect things like directivity. Once again, some quick mock-ups should give you some idea of how critical that decision will be, and how to direct your further efforts. A conical expansion is pretty hard to beat for simplicity, and personally, I'd start with that.

Once you've made a start and have some initial impressions (and pictures!) to share, I'm sure more pointed suggestions will follow.
By the way, I'm curious about your CAD software of choice for your printer modeling - care to share?


picowallspeaker 17th March 2012 07:00 AM

Dezeen Blog Archive Change the Record by Paul Cocksedge

freddi 18th March 2012 06:45 AM

re: Karlson-coupler - here's approximately the size for you already done 50 years ago :D

chris661 18th March 2012 01:51 PM


Originally Posted by w_oswald (
Some of them are relatively crude, cardboard and masking tape affairs

You called?


PS - I recently got a new phone, one that actually plays MP3s (had an old Nokia brick before), so stay tuned for another cardboard horn.

peepsalot 19th March 2012 01:28 AM

Thanks for the links and great ideas everyone.


Originally Posted by w_oswald (
By the way, I'm curious about your CAD software of choice for your printer modeling - care to share?

I have been using mostly FreeCAD lately. I am a software developer by trade, and I prefer to create models through scripted CAD. This allows for parametric designs, where you can change a variable and see the output without redesigning an entire part. FreeCAD uses Python for it's scripting, which is one of my more preferred languages.

Another option which is popular with other 3d printing people is OpenSCAD. It has it's own scripting language, which is fairly simple and easy to learn, but not nearly as powerful as Python.

peepsalot 20th March 2012 02:29 AM

Well, I know it was suggested that I make some stuff with cardboard first, but generating 3D models is just too much fun.

I've put together a script for generating a wide variety of horn shapes. It is written in Python, for FreeCAD. It mostly generates the Mesh vertexes independently of the FreeCAD functions, so it could possibly be split out to a standalone script. I'm mainly using FreeCAD for visualization and ability to export to common 3d formats in this case.

The idea is that you can provide mathematical functions for each of:
  • The horn path, which is the line that the center of the horn follows in 3d space, so you can have straight horn or spiral shapes, etc.
  • The radius as a function of distance along the horn path
  • The horn cross section(circular or rectangular for example), as a function of the radius
And from different combinations of these you can generate all kinds of pretty models.

Some examples just for fun:
Radius functoin: Oblate Spheroid
Path Function: Linear
Section: circular
A bunch of exponential radius horns, linear path, circular cross section
exponentinal "radius", circular path along 3/4 turns, Rectangular cross section

I'm still unsure about the way to define certain horn radius equations though. For example exponential horns. I know the simplest form of exponential equation is y = e^x. But this doesn't give any parameters for starting radius, or flare rate(is that the correct term?). Is there a common form of this function used for making horn profiles? I've implemented a sort of exponential function already, just not sure if I did it "properly".

I've also read about tractrix as another common profile, but I'm having trouble finding any euqation to model the radius for that as well.

Plus any other common shapes that I might be missing. Is there any cheatsheet of common horn equations on these boards or elsewhere online?

The script is still a work in progress, but I will be releasing it as open source in the near future when I feel it is more polished.

peepsalot 20th March 2012 05:26 AM

OK, I found this paper about exponential horn which explains the equation in the first page and a half.

Cool, so now I can just specify a start radius, end radius, and length, and it computes the rest.

Still trying to figure out the tractrix, and maybe hyerbolic, or others.

picowallspeaker 20th March 2012 07:43 PM

Also , you may read the introduction of Ajhorn manual
AJHorn manual

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