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lower-mid horn design

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JD01 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JD01 Quote  Post ReplyReply Direct Link To This Post Posted: 25 September 2004 at 12:40am

@ RayVaughn: How do you cover the range from 100-1000Hz now? And what tops are these, going only from 1000Hz up? Seems there is already a misunderstanding or misconception......

I agree with Pinheiro that you won't get this spectrum out of one box in a good way. Around 200-300Hz there is kind of a boundary for what speakers/horns can do, so you can design a horn that will be very good below 200Hz or you can do one that will be good above 200Hz. This is, of course, NOT a limitation coming from the diameter of the speaker, but the type of the horn. If you want real output around 150Hz and below you need to have the horn folded for getting it into a transportable shape. Above 200Hz you need a straight horn, because the mids won't play well in a folded design. Of course a folded horn will go even lower then 100Hz.... and a good 10" horn with phase plug will go above 1kHz...... but there is no good way to do it with one speaker in one box.....

As I said the output capabilities have nothing to do with the drivers diameter. The most amazing sub horns use 12" speakers.... Why? Simply because bigger drivers won't take the stress on the cone (bigger cone plus proportionally higher BL --> more stress). We have bass horns with 4 10" drivers per horn. They can do 40Hz (in a stack of four) to 250Hz real flat and really good sounding (flat doesn't mean that the timeline looks good too!) and they get very loud, so you see, the size doesn't matter in this case! Of course these are 10" drivers made for bass, not for mids.

15" drivers in horns should not be operated above 700Hz and even that doesn't sound really good.... and of course a horn of that type won't go below 130 or 150Hz (because it has to be straight and has to have a reasonable size)! So....

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Post Options Post Options   Thanks (0) Thanks(0)   Quote RayVaughn Quote  Post ReplyReply Direct Link To This Post Posted: 25 September 2004 at 9:51am
Originally posted by JD01 JD01 wrote:

@ RayVaughn: How do you cover the range from 100-1000Hz now? And what tops are these, going only from 1000Hz up? Seems there is already a misunderstanding or misconception......

It's a 4 way system (per stack)
6 or 8 Fane 18" in short horns (a la TSW 118)  currently up to 175Hz
Array of 16 direct radiating 7" Kevlar mid-bass drivers 175Hz - 1KHz
4 direct radiating 6" surroundless Kevlars 1kHz - 7Khz
4 EV DH3 compression drivers on CD horns 7kHz up

Don't ask me about the kevlars, they're not available to the public - they're a favour from a friend. The 6" are most unusual as the effective radiating area of the diaphragm narrows with frequency as a result of having no surround. Nothing else sounds quite like them.

The  6"  are  fairly sensitive in the region we use them (100dB/w)  but will be the limiting factor  when i upgrade the lower mid. Thats ok. There's a price to pay for effortless clarity in vocals etc.

The 7" are not very sensitive and  despite the largish array  don't cut the mustard.  I could just replace them with a light coned 2" voice coil driver and throw more power in but a horn is clearly a better way to go.

Originally posted by JD01 JD01 wrote:

I agree with Pinheiro that you won't get this spectrum out of one box in a good way. Around 200-300Hz there is kind of a boundary for what speakers/horns can do, so you can design a horn that will be very good below 200Hz or you can do one that will be good above 200Hz. This is, of course, NOT a limitation coming from the diameter of the speaker, but the type of the horn. If you want real output around 150Hz and below you need to have the horn folded for getting it into a transportable shape. Above 200Hz you need a straight horn, because the mids won't play well in a folded design. Of course a folded horn will go even lower then 100Hz.... and a good 10" horn with phase plug will go above 1kHz...... but there is no good way to do it with one speaker in one box.....

This makes sense. Maybe I need to be realistic and design for a 150Hz to 500Hz range.
I'm just not sure whether to try and design something where the roll-off is somewhat outside the range that will be used or match it exactly.

Originally posted by JD01 JD01 wrote:

As I said the output capabilities have nothing to do with the drivers diameter. The most amazing sub horns use 12" speakers.... Why? Simply because bigger drivers won't take the stress on the cone (bigger cone plus proportionally higher BL --> more stress). We have bass horns with 4 10" drivers per horn. They can do 40Hz (in a stack of four) to 250Hz real flat and really good sounding (flat doesn't mean that the timeline looks good too!) and they get very loud, so you see, the size doesn't matter in this case! Of course these are 10" drivers made for bass, not for mids.

Agreed.

Originally posted by JD01 JD01 wrote:

15" drivers in horns should not be operated above 700Hz and even that doesn't sound really good.... and of course a horn of that type won't go below 130 or 150Hz (because it has to be straight and has to have a reasonable size)! So....


Also true.

So...
I'm going to try and design for a 100Hz to 800Hz range with a view to using 150Hz - 500Hz
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RayVaughn Quote  Post ReplyReply Direct Link To This Post Posted: 25 September 2004 at 10:27am
Originally posted by Timber_MG Timber_MG wrote:

To get that sort of bandwidth a phase plug is probably on the cards. I have been doing some planning myself for a 130Hz tactrix horn with the P.Audio SN-12B which models fantastically. The thing is that if you want to get low frequencies out of a Mid-horn, you need to move air and this means increasing the distance between the phasing plug and the driver cone. In my case Rog’s phase plug equation results in an upper cutoff of 1.6kHz for a 5mm clearance and I assume that this is an ideal case scenario for a multi-slit annular phase plug (I’d probably stay with one slit for my first try at a bass-mid horn)

 

A good phase plug evens out the arrival time of different parts of the cone to the horn throat (one wants a planar wavefront at the horn throat) and the directivity is governed by the horn mouth at lower frequencies and progressively by sections nearer to the throat as frequency increases. Could anyone post the speed of sound is through a paper cone? Rog?

 

My only concern with neo drivers is the small rear chamber which might cause the magnet to de-magnetize at the high temperatures, so perhaps an aluminium access plate is a good idea (the models favour around 3l rear chambers if one wants to keep excursion in check around fc)

 


I've been experimenting in McBeans program with various drivers and horn flares. The best results have been with the TAD 1201 driver but I'm unhappy with it's low frequency performance.
The SN-12B has a relatively high moving mass which seems to diminish high frequency performance greatly.
I've also had a look at the Beyma 122ND which has a high BL and a very low moving mass.
I can only assume that these very light cone/coils will be more susceptible to break-up modes...

I too had assumed that a phase plug would be necessary but don't really know what I'm doing! Still, I'll have a look at the phase plug equations of Rogs that you're talking about.

Currently the models I've done in McBean are only exhibiting 2mm excursions even at 100Hz...

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Post Options Post Options   Thanks (0) Thanks(0)   Quote JD01 Quote  Post ReplyReply Direct Link To This Post Posted: 25 September 2004 at 1:55pm

I see. So you want to go down to 500Hz with the 6" speakers then? Will they 'like' that?

How do you crossover? All active or some ways passive?

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Post Options Post Options   Thanks (0) Thanks(0)   Quote RayVaughn Quote  Post ReplyReply Direct Link To This Post Posted: 25 September 2004 at 3:44pm
Originally posted by JD01 JD01 wrote:

I see. So you want to go down to 500Hz with the 6" speakers then? Will they 'like' that?

How do you crossover? All active or some ways passive?



4-way active using BSS Omnidrive, 24dB L-R slopes, per band limiters.
Only eq is a 6dB/octave ramp from 10kHz up to compensate for the roll-off of the tweeters.

I tried running the 6" down to 750Hz last weekend and it sounded good. I'd say slightly better than crossing over at 1K but It's subtle. According to the designer of the drivers 500Hz is ok. I'm planning to add another 4 of these per side anyway and angle them down by 20 degrees becuase the top end of the range is quite directional.
They should add well below 1K if they're close enough together so I'll get another 3dB at the bottom of their range.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JD01 Quote  Post ReplyReply Direct Link To This Post Posted: 25 September 2004 at 7:56pm

Adding up on the 6" seems to be a good idea...

 

Have you considered designing a passive crossover for the 6" to tweeter cross? So you could really do two horns... it's because I don't think that your existing basshorns could do better if crossed over at 100Hz or even slightly lower. I mean the rest of your system seems to be very audiophil, so why getting the 'slow' and heavy 18" above 100Hz?

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Post Options Post Options   Thanks (0) Thanks(0)   Quote RayVaughn Quote  Post ReplyReply Direct Link To This Post Posted: 25 September 2004 at 8:44pm
Originally posted by JD01 JD01 wrote:

Have you considered designing a passive crossover for the 6" to tweeter cross? So you could really do two horns... it's because I don't think that your existing basshorns could do better if crossed over at 100Hz or even slightly lower. I mean the rest of your system seems to be very audiophil, so why getting the 'slow' and heavy 18" above 100Hz?


It's a fair point - maybe...
I'm reluctant to split the mid any further - more sources, more x-over trouble ...
On the other hand maybe the lower mid horn can extend down a bit.
Here's the SPL plot from McBean with the TAD TM-1201 on a cosh horn


I know its a pie in the sky plot here but maybe 120Hz is on the cards ;)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Timber_MG Quote  Post ReplyReply Direct Link To This Post Posted: 25 September 2004 at 9:00pm

My sims show the SN-12B to be very good up to around 800Hz regarding power response and if one looks at 0 to 30 degree performance,  I seem to be able to get the horn up to 1kHz and a little higher or there abouts until I get -6dB @ 30 degrees off axis (designing for a 60 degree cab). I presume the -6dB @ the angle I want to cross over in a horn top to be the highest I can push the horn before I get lobing, right? A 40 degree solution might allow one to cross higher.

The problem with McBean for HF horn design is that it doesn't accurately model the behaviour of a horn at high frequencies (David says so himself). Methinks this is where AJHorn, Speak, Akabak and what not come into their own, but Akabak is a tad too much for a student's budget. This is the bit where one just keeps the side-walls conical as far as possible and lets the phase plug make the horn contour and hopes for the best. The other approach is to make a phasing plug like a big compression driver and a couple it to a “normal” = “without nose” horn. The reverse mounting the driver seems a little risky, though it allows the horn to be shorter physically and lowers power compression.

 

For simplicity there is nothing like buying a working solution though.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Timber_MG Quote  Post ReplyReply Direct Link To This Post Posted: 25 September 2004 at 9:18pm
And you should model into 4x pi, way too little ripple in that plot for it to be a reasonably sized horn (except if you plan on tops much wider than 50cm). That is some extreme bandwidth you have there, but TAD is very expensive and IIRC has around 100W power handling. I generally use Tractrix flares as they end up shorter and show less ripple than hyperbolic/expo horns of the same mouth area (though I may be doing something wrong)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote roborg Quote  Post ReplyReply Direct Link To This Post Posted: 26 September 2004 at 7:48pm
Originally posted by Timber_MG Timber_MG wrote:



<p ="Msonormal">To get that sort of bandwidth a phase plug is probably on
the cards. I have been doing some planning myself for a 130Hz tactrix horn with
the P.Audio SN-12B which models fantastically. The thing is that if you want to
get low frequencies out of a Mid-horn, you need to move air and this means increasing
the distance between the phasing plug and the driver cone. In my case Rog’s
phase plug equation results in an upper cutoff of 1.6kHz for a 5mm clearance
and I assume that this is an ideal case scenario for a multi-slit annular phase
plug (I’d probably stay with one slit for my first try at a bass-mid horn)




<p ="Msonormal"><o:p> </o:p>




<p ="Msonormal">A good phase plug evens out the arrival time of different
parts of the cone to the horn throat (one wants a planar wavefront at the horn
throat) and the directivity is governed by the horn mouth at lower frequencies
and progressively by sections nearer to the throat as frequency increases. Could
anyone post the speed of sound is through a paper cone? Rog?




<p ="Msonormal"><o:p> </o:p>




<p ="Msonormal">My only concern with neo drivers is the small rear chamber
which might cause the magnet to de-magnetize at the high temperatures, so
perhaps an aluminium access plate is a good idea (the models favour around 3l
rear chambers if one wants to keep excursion in check around fc)






Yeah i suspected a mid-horn cab i've built was getting seriously hot (2x300w 10"s driven with about 1Kw.) I also have a tiny rear volume (smaller than mt102) & really thick walls, so heat dissipation was only thru the speaker cone (which was getting damn hot after a full power test (so were the neighbours tempers    )
So i fitted a 100mm plastic tube through the rear of the box to each speaker vent (sealed with silicone sealant) & now it's all peachy. It sounds as good as before only without the insane heat buildup & prolly much less power compresssion, i didn't notice any P-C before but then i was getting bad hearing compression
The prob with an aluminium access panel is that for the cooling to work you don't want to put damping material over it & u really need to line all the walls of a mid-cab with a good heavy layer of absorbent stuff.

Cheers,
Rob.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote roborg Quote  Post ReplyReply Direct Link To This Post Posted: 26 September 2004 at 10:29pm
Here's a pic of the mid-cab cooling butchery the holes are now filled with 100mm plastic tubing. The drivers get hottish to the touch now, but no more & that's after a real thrashing.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Timber_MG Quote  Post ReplyReply Direct Link To This Post Posted: 27 September 2004 at 12:36am
Interresting. I take it the driver doesn't have a vented pole piece, as otherwize you'd have a very leaky box. Come to think of it the SN-12B doesn't seem to have an opening on the back plate either, but I think in the long run leaks might creep in at some stage.

The SN-12B doesn't have a traditional backplate to which one could seal easily, but bringing the driver flush to the outside of the box and sealing off the gap around it with some suitable sealant seems like it might help the driver dissipate some of the heat and shorten the cabinet a little more. For access reasons this would have to be incorporated into the acces panel.

Servodrive for instance circulate external air through their motor and achieve compression levels unheard of in traditional motor designs (like 1.5dB long term @ 2 times rated power)
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