Horn Design tips for cell phone
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 21st March 2012, 03:46 AM #11 peepsalot   diyAudio Member   Join Date: Oct 2007 Well, I printed out a test horn. It is exponential with a throat diameter of 10mm and mouth diameter of 100mm. The length is 100mm. It definitely amplifies the sound, but mostly the higher frequencies. I am now planning to make some sort of folded design so that I can greatly increase the length of the wavepath, since 100mm is about as tall as I can print. I read from Martin King's paper here about an equation for lower cutoff frequency based on the flare constant m. http://www.quarter-wave.com/Horns/Horn_Physics.pdf He gives these equations m = ln(Sl / S0) / L fc = (m * c) / (4pi) And using the parameters for my printed horn, I calculate a lower cutoff frequency of 1247Hz. But then further down in the same paper, there is a Table 5.2 where the frequencies are calculated completely differently. It says "In Table 5.2, the lower cut-off frequency fc is calculated based on the open end (the horn’s mouth) cross-sectional area." But I don't know what equation is used for that, and it seems to have opposite predictions from the first cutoff frequency equations that he gives for exponential horn. i.e. if you have a constant throat area and length, and you increase the mouth area, the first equations seem to indicate that your cutoff frequency would rise since the constant m would be higher. However, this table 5.2 shows the cutoff frequency dropping as the mouth is widened. Can anyone explain that? Edit: I uploaded a video of it in action http://youtu.be/85x4PlhWHkc Last edited by peepsalot; 21st March 2012 at 04:12 AM. Reason: added video link
 21st March 2012, 07:40 AM #12 picowallspeaker   diyAudio Member     Join Date: Apr 2007 Supernice ! 2 of them , a little shorter,(=Hyperbolic ) Delivery free
 21st March 2012, 07:44 AM #13 chris661   diyAudio Member     Join Date: Feb 2008 Location: Sheffield Hi, try adding blu-tac or similar to the base of the horn, so that its acoustically sealed to the phone. I found this helped the lower mids considerably. V.cool though. Me likes. Chris
 21st March 2012, 04:25 PM #14 w_oswald   diyAudio Member   Join Date: Jan 2010 Location: Manitoba Hi: Very nice effort - thanks for the picture and video. May I suggest including a flange at the base - it would make it easier to balance it on your phone, plus it would assist sealing the joint with blu-tac or such, as mentioned. Sounds like you're on to bigger models - again, flanges at the mating joints should be helpful with alignment and connection of segments (whether the axis is straight or curved). Sorry, I can't help you with the finer points of horn theory w.r.t. various profiles, cutoff frequency, etc. As I said though, the differences may not be hugely significant in your application - I'm sure that little speaker has a very limited low-frequency output, so I don't think any horn size or shape will drastically change that. But hey, have fun experimenting! Is the small end opening exactly matched to the speaker opening? Is the speaker sound emanating from a single opening, or is it an array of tiny holes? As I said, creating a step-like change in cross-section at that juncture would likely be non-optimal. Perhaps you could evaluate the audible effect of that with a couple of tests. Tractrix, iirc, has the characteristic of terminating (at the large end) in a plane that is perpendicular to the axis of expansion. Thus, it would present essentially no discontinuity when mounted in an infinite baffle (such as a wall or very large speaker baffle). As picowallspeaker implied, it would be interesting to include with your future efforts the study of the effectiveness of horns in front of various sorts of tweeter drivers. It looks like 3D printing is the new frontier - what fun! Does your machine use spools of thin plastic rod as the consumable? Along the lines of re-filling ink or toner cartridges: is there a way to make your own by grinding/melting/extruding prior printouts or even recyclable household plastic items? I know this was (is?) common in the plastic injection-molding industry - even the less conscientious record manufacturers (or so I've heard). And finally: thanks for the link to FreeCAD. Wow - from the screen shots, it looks like a very sophisticated program - and it runs on Linux too! I'll definitely have to check that out - maybe a defection from the TurboCAD camp is in my future! Regards, Wilf
 21st March 2012, 06:19 PM #15 peepsalot   diyAudio Member   Join Date: Oct 2007 The phone case has 17 little holes for the speaker in a slightly oblong layout. The horn throat is 10mm across which just covers the longer side of this pattern. I'll try adding something to better seal the end. A flange is definitely a good idea too. As for the input material for the printers, yes it uses a spool of round plastic filament. Currently the two most common print materials are PLA (polylactic acid)which is a plastic made from corn, and ABS (Acrylonitrile butadiene styrene) which is a very common plastic used in all kinds of commercial products, like LEGOs for example. The filament comes in 1.75 or 3mm diameter and various colors. I use 3mm diameter because it is cheaper per weight. It gets heated and extruded through a 0.5mm nozzle. Some people use smaller size nozzles for finer detail, but that also makes the prints take longer to complete. I get my filament from a site called ultimachine.com, but there are many other vendors selling similar product. When bought in 5lb quantities, it comes out to roughly \$1 USD / oz. , including shipping costs. The horn above was printed with a single wall thickness which ends up being about .65mm for my printer. The extruded plastic gets squished down a little bit, which is why it ends up wider than the nozzle. As for recycling, there are a few people working on DIY style plastic recycling. The big important thing is to have a consistent diameter along the filament otherwise your prints will come out lumpy. Here's one that looks promising MiniRecyclebot - YouTube And there's also these guys Filabot: Plastic Filament Maker by Tyler McNaney — Kickstarter
 22nd March 2012, 04:32 AM #16 peepsalot   diyAudio Member   Join Date: Oct 2007 Well, i went ahead and published the script as is. I might add more horn profiles in the future as I learn more about them. Parametric Acoustic Horns by thehans - Thingiverse The sort of folded horn idea I have in my head will probably require a completly different approach to generating the model, so I'll just do that as a separate script if I ever get around to it. The idea is to fold the horn so that the wavepath travels in concentric rings. I would probably try to approximate a exponential expansion where the area would only change on each transition to the next "ring". If I tried to make a continous curve... the math is too complex for me to think about right now. I found at least one example which is kind of what I am thinking, only mine would do more folds, and I would keep the rings centered. "Folded" Sonotube TL Sub Complete! Except that is a transmission line design, and I would be trying to pretend mine is a horn, so I have no idea if that would work at all.
picowallspeaker
diyAudio Member

Join Date: Apr 2007
Quote:
 Sorry, I can't help you with the finer points of horn theory w.r.t. various profiles, cutoff frequency, etc. As I said though, the differences may not be hugely significant in your application - I'm sure that little speaker has a very limited low-frequency output, so I don't think any horn size or shape will drastically change that. But hey, have fun experimenting!....... As picowallspeaker implied, it would be interesting to include with your future efforts the study of the effectiveness of horns in front of various sorts of tweeter drivers.
Yes! That's definitely what I had in mind ,but in a vague mode...Thx W_Osvald !!
Indeed , claiming the frequency/ driver diameter ratio without the power response would be incomplete . For a cell phone speaker , which I think it may be very good for a tweeter , when listening very close , together with the shell of the phone ...as sound transmits even mechanically trough the vibrations ( as we can hear trough our in-bone constitution ),ah ! Ok ! Sorry , we were talking about music ,not speech .
As I said , I think those are good tweeters ; I use two 3/4" 'button' loudspeakers ( the same that are inside notebooks and flat monitors)
with little horns / waveguides .
As your experiment demonstrated ,that when putting some surface near a sound source , some effects arise . The change from direct radiation ( which is not due to the holes in the plastic that put a 'cover' ) to horn expansion is clearly altering the media , changing directivity and amplifying in a certain manner . Speech reproduction may be a good test to make a decision for different horns ( also direct radiation ,, ) and distinguish from kinds
of coloration, echo ,reverb that damage the sonic intelligibility .....

w_oswald
diyAudio Member

Join Date: Jan 2010
Location: Manitoba
Quote:
 Originally Posted by peepsalot The sort of folded horn idea I have in my head (...) is to fold the horn so that the wavepath travels in concentric rings. I would probably try to approximate a exponential expansion where the area would only change on each transition to the next "ring". If I tried to make a continous curve... the math is too complex for me to think about right now. I found at least one example which is kind of what I am thinking, only mine would do more folds, and I would keep the rings centered. (...) Except that is a transmission line design, and I would be trying to pretend mine is a horn, so I have no idea if that would work at all.
Sounds like you're visualizing something like a re-entrant horn, but with cylindrical sections? Re-entrant horns are commonly used in (voice) PA applications, and for car and home alarms. So, by keeping the area of each section constant, it might have some similarity to the horns described in this thread: http://www.diyaudio.com/forums/full-...eneration.html
Interesting idea... Those Nagaoka (if that's the correct designation for them?) horns allegedly manage to extract amazing performance from relatively small drivers. Building a set of those for some Fostex FE103A on hand is another item on my endless project list.
Wilf

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