Horn Design - Sanity Check

[deiksac]

I am about to build a midbass PA horn - the intended frequency rande is about 150-500 Hz and have two rather exotic 15" drivers from Meyer Sound. Had not seen anything like this - see T/S parameters:
Diameter 15"
Fs 142 Hz
Re 3,06 ohm
Res 4,23 ohm
Qms 0,87
Qes 0,63
Qts 0,37
L1 0,31 mH
L2 1,06 mH
RMSE-free 0,41 Ohm
RMSE-load 0,2 ohm
Vas 11,94
Mms 107,55 g
Cms 12 um/N
BL 21,57 T-m
Splref 99,2 dB
Sd 855 cm2

The throat area according to the formula at the
Single Driver website result in some 115 cm2 which feels rather small to the 850 cm2 Sd - What do you think?

The final enclosure will be pretty huge - 102 cm (W) x 51 cm (H) front area since two midtop turbosound enclosures will be sitting atop this one. Another question stems here - the mouth area corresponding to the 150 Hz Cutoff is pretty large - half size horn would need (again according to the formula on the abovementioned page) to be some 2 m2 whereas I only have about 0,5 m2 available

 

[BHTX]

Fs 142 Hz

142 Hz?? Very unlikely. More like 42 Hz maybe.

 

[deiksac]

not at all, exactly as I have written, measured on MLSSA

 

[MaVo]

you could use hornresp. the function "system design, with driver" will design a horn for you. after that, you can play with the numbers.

 

[BHTX]

not at all, exactly as I have written, measured on MLSSA

Ah..apologies then. And you're right, I've never seen anything like that either! Only suggestion I have is what MaVo said. Unfortunately, I don't possess much knowledge about horns at all. Hopefully you'll be able to figure something out. Must be pretty exotic indeed. Goodluck.

 

[deiksac]

sofar I did not become any smarter so I decided to go experiMENTAL
since the saw guys at my plywood distributor are selling me their cutoffs (oops - 6 sqm of 22 or 26 mm mdf, 2 sqm of 4 mm plywood plus some smaller pieces for rack and other speakers for some 50 USD) I will just play and see what will be the result
the prototypes will be 102x51x74 cm big, horn lenght some 54 cm

forgot to add that these speakers should handle some 1200W each

 

[Frode]

As I've said before, I'm no expert. But here's a couple of screenshots from Hornresp.

Frode

[IMGDEAD]https://i32.photobucket.com/albums/d40/frodeh/Audio/Meyer1.gif[/IMGDEAD]

[IMGDEAD]https://i32.photobucket.com/albums/d40/frodeh/Audio/Meyer2.gif[/IMGDEAD]

 

[deiksac]

Thanks Frode, seems promising

 

[GM]

The throat area........What do you think?

Greets!

Yes, the small throats the various formulas predict will require a phase plug to work well out to 500 Hz, which in turn will require a larger throat to accomdate it, ergo a larger mouth than the flare frequency implies. For a prosound app, will it actually have true half space loading? Horns with this high a cut-off usually are in closer to full space. Anyway, this horn's HF corner is ~450.8 Hz, so the throat starts rapidly constricting above this point.

As Frode showed, you can manipulate the design to hit a target mouth area and using Prof. Leach's math yields a slightly larger throat, lower 'M' for a little less distortion at high power, but I don't see it being this smooth nor handling much power for long due to the high compression ratio (CR).

Really, if this driver has the typical stepped HF BW of a horn driver, then we can truncate it with a larger throat, making it shorter and maybe have no need for a phase plug.

BTW, for my records, which model is this and is there any plots available?

GM

 

[GM]

plot:

 

[deiksac]

Re: Re: Horn Design - Sanity Check

Thanks GM, this finally brought some insight as advices like "use hornresp" only give me more theory like the one on melhuish.org - only automated (already seen some guy downplaying hornresp (Why I hate hornresp by Romy the Cat)

So bigger throat seems the way, I was thinking about selecting say 200 sqm and start there. As for loading - the situation is that the box will sit atop bass boxes with mid/hi horns on the top of everything so it will be more like 3/4 space.

I kinda like tractrix expansion - shortest and for what I researched I liked the quotes on tractrix horn the best.

BTW, for my records, which model is this and is there any plots available?

GM

What model are you refering to? I just used the tractrix horn calculator at melhuish.org to get to the 54 cm horn lenght (restricted by box shape). When the prototype gets finished, I will have it measured and will post the data.

d

 

[Scottmoose]

200 square meter throat? You sir, I respect. This will be a HUGE horn. Led Zep don't have anything that good...

Sorry, couldn't resist.

Hornresp isn't that bad, though I personally find the interface a bit clunky. It's more a case of it'll give you a decent idea of the technical side, but you have to know how to interpret the data & also know it's limitations. And no software on the planet will ever be able to tell you exactly what it'll sound like in practice, just the technical side of the design. It's a tool, no more. Useful, but not the be-all & end-all.

Tractrix is good for midbass etc. Could be onto a winner there.

 

[AndrewT]

Hi,
a throat giving a maximum compression ratio (Sd/Ath) of about 2 is often quoted for driver/horn combinations.
For two drivers this would indicate a minimum throat area of about 800 sq cm. But even going up to 1600sq cm would be acceptable.

However, your T/S parameters are extremely suspicious.
VAS =12litres seems very low but does seem to correspond to that high Fs value.

Would it be worth checking the free air resonance frequency manually, rather than through the computer and it's software/drivers?

 

[Frode]

Re: Re: Re: Horn Design - Sanity Check

What model are you refering to? I just used the tractrix horn calculator at melhuish.org to get to the 54 cm horn lenght (restricted by box shape). When the prototype gets finished, I will have it measured and will post the data.

d

I guess GM is thinking of the model number of the driver so he can save and archive the simulations.

Frode

 

[MaVo]

Re: Re: Re: Horn Design - Sanity Check

Thanks GM, this finally brought some insight as advices like "use hornresp" only give me more theory like the one on melhuish.org - only automated (already seen some guy downplaying hornresp (Why I hate hornresp by Romy the Cat)
d

That linked post was hilarious! Thanks for posting

some quotes...

"It is purely magical software, right? Well, not really as it has absolutely no awareness about sound and therefore Hornresp can predict the result with the same accuracy as observing of whether the earth’s minerals content are capable to predict the taste/smell of wine."

"Horns are odd and one of the oddities of horn is in their “extrovertness”."

There speaks the true audiophile.


But apart from the fun part, use Hornresp! Since you can see what happens to the peformance of the horn, if you change a parameter, it is a lovely learning tool, much more time efficient than building boxes yourself. The wisdom gained is abstract, but abstract is good, since its quite a science, not art.

 

[GM]

Re: Re: Horn Design - Sanity Check

Greets!

Apparently I was more tired than I thought when I posted! This should read:

.............this driver's HF corner is ~450.8 Hz..............

It didn't occur to me you might be using the tractrix calculator on JM's site since it's not normally used in prosound apps, though IIRC Klipsch uses (used?) it in its cinema series. If you use a full size mouth for 150 Hz for a given throat area, then the horn will unload at ~1.4142*150, so if you want it to load all the way to 150 Hz, then the mouth needs to have a ~0.707*150 mouth area, which of course will make it longer. Now if we cut off (truncate) this oversize mouth to keep its size reasonable, then much of the low distortion benefits of using tractrix is lost, leaving a ~hyperbolic horn with its excessive beaming due to the narrow wall angle, not what you normally want in a prosound app.

Also, even though you said two drivers I assumed you would be making two horns. If you're using two drivers on one horn with the already too high CRs so far presented, then any throat distortion, HF BW cut-off due to cancellation just got a lot worse.

Assuming you meant 200 cm^2, then this is still too small for even one driver as a general rule for point source drivers, with ~3:1 CR considered being the practical limit for prosound apps, but this driver's specs implies it might can handle higher ones if a phase plug is used to correct the HF path-length error cancellations.

Frode guessed right , it's the driver's model number and any other available data on it I'd like to have for my records.

Anyway, looking forward to the build details/performance report.

GM

 

[Frode]

Re: Re: Re: Horn Design - Sanity Check

Greets!

with ~3:1 CR considered being the practical limit for prosound apps,

GM

How about home/hifi applications, is there a limit there too?

Frode

 

[GM]

Re: Re: Re: Re: Horn Design - Sanity Check

........much more time efficient than building boxes yourself.

Agreed, having spent an intense ~seven months worth of my 'quality' time back in the '60s building all manner of proof-of-concepts and measuring them with a CBS test record spinning on a cheap TT and SLM, then plotting the results on log paper to get enough trends to see the 'big picture' WRT TL, horn design..........and now I just found out yesterday all I have to do is click a mouse button a couple of times to get a BW optimized design, though as this thread points out, without the experience/knowledge I gained doing it the hard way (and more over time) to properly interpret the results is a recipe for wasted $$/effort building it when it doesn't perform as predicted.

GM

 

[GM]

Re: Re: Re: Re: Horn Design - Sanity Check

How about home/hifi applications, is there a limit there too?

Of course, though it varies depending on the horn alignment. Not having the math skills to design using electro-mechanical theory like the pioneers of audio, I did what I figured was the next best thing and used their early ~no compromise designs as guides, starting with the ~10:1 CR they used for their compression drivers and worked my way backwards to 1:1, and later, even lower CRs. Anyway, I found that the practical limit was dictated by the rear chamber Vb, so I experimented with its two physical extremes of BLH (no rear chamber) and sealing up the basket to find the driver's new Fs, which gives you the minimum horn Fc to work it out from there, with ~10:1 CR being the highest I ever tried.

The 'dumpster diving' little TV, portable radio drivers I used were only tested with a 'free' low power table radio SET amp, so assume they never 'felt' > 1 W, but they played loud enough to amuse/confound everyone who auditioned them and certainly enough for the limited dynamics available back then. The real attention 'getter' was the BLH though. Using an old RCA large format driver without its rear cover, I quit adding sections to it when it got to be impractically large, but it was still going strong down into the 50s, so while horn size matters to play loud/low, driver size not so much .

I destroyed plenty of them too, learning about stiffening, mass loading them along the way, but I'm sure someone with the math skills could work out the details to at least figure out a driver's conservative electro-mechanical limits for a given CR before it begins seriously overheating and/or going into cone and/or suspension non-linearity.

GM

 

[Tim Moorman]

A Suggestion

Hi all,

I ran a couple of simulations in HR and my results are nothing like yours.

I would first start with a more reasonable throat size (and cr) of 400 cm2, and a rear chamber large enough to actually enclose the driver, since it is a 15". You will need at least 40 cm of rear chamber length, so start with 35 liters as your volume, and then try the "system design" tool to get up and running, and adjust as needed. At one time the program should have cautioned against a too small rear chamber (see vrc red warnings).

You will have to reset parameters (Bl, cms, rms, mmd, vrc) for the new system design model, so just use the program to arrive at these values again by double clicking each when indicated (highlighted).

I used 140 Hz to 650 Hz for desired bandwidth in the system design queries, and came up with a more practical result.

Give it a try.

Tim