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8" MID HORN (development thread)

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luthier View Drop Down
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    Posted: 11 February 2019 at 1:22am
So I'm planning on having a go at making a 8" mid horn, the design criteria is: 

 - 350hz to 2khz (and above if possible)
 - highest sensitvity I can achieve, hopefully 104 - 105db (+/- 3db in pass band) at 4pi
 - Maximum depth of 412mm driver + horn
 - Maximum width 498mm

So far I've been modeling on hornresp based around the B&C 8pe21, I have started with this driver due to it's massive EBP and reputation in horn loaded designs.  

My issues at the moment are:

 - Getting the horn to play above 2khz

 - Controlling the dispersion/measuring the dispersion

 - Lack of sensitivity 

 - Length of the horn

 - Is a 14:1 compression ratio too high for an 8" driver?

Also, I'm planning on trying to get hold of a trial of COMSOL futher down the line, to model the horn accuratley, has anyone on here any experience with it or similar?  






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Post Options Post Options   Thanks (0) Thanks(0)   Quote MattStolton Quote  Post ReplyReply Direct Link To This Post Posted: 11 February 2019 at 10:20am
My only comment would be that, without a phase plug, you may get some cancellations in the mouth of the horn, as upper frequency waves bounce from side to side, forming standing waves in the mouth.

2KHz wavelength is approx. 17cm, so any dimension of this order of magnitude will lead to some issues.


To get low frequency control, your mouth will be big, so top end will be affected.
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Mircea Bartic View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mircea Bartic Quote  Post ReplyReply Direct Link To This Post Posted: 11 February 2019 at 12:51pm
looks like you want to build something like this:
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Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 11 February 2019 at 2:21pm
14:1 does seem like a lot - most 8" horns I've seen are only about 2 or 3:1

You could  try modelling an existing horn and see what the graphs look like? Because I would doubt Hornresp's accuracy in the upper midrange. I think it tends to show more of a roll-off than will actually happen IRL. And I don't think it models phase plugs at all well.

For example I have some phase-plugged horns here, designed for the 8PE21.

(very approximate values from quick measuring)
S1 = 77cm2 (annular slot around plug) so about 2.85:1 ratio
S2 = 838cm2 (mouth)
L12 = 27cm 
Exponential profile
Throat chamber approx 264 cm3 (driver cone volume minus rear part of plug)
Rear chamber volume approx 1.25 litres (2.5litre back bowl, assuming driver takes up about 50% of space)
Rear ch depth 10cm

See how you get on....These horns do actually work quite well in real life. The plots don't necessarily suggest that!
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Post Options Post Options   Thanks (1) Thanks(1)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 11 February 2019 at 4:43pm
you aren't modelling the directivity. model a single section horn and use the directivity tool to get an idea of how much directivity you gain at different frequencies. then go back to modelling a multi-section horn and aim for a power slope that decreases with frequency (approx -3dB per octave).

making your first throat section much shorter will give you this slope and help the very high end. make it just 2cm long and the flare cutoff is about 2kHz. should give a flat on axis sensivity to about 3kHz. this is often called 'rubber throat'.

the throat you have will also cause a lot of distortion at high levels. faster flare rate where the throat is small reduces this problem too.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ceharden Quote  Post ReplyReply Direct Link To This Post Posted: 11 February 2019 at 7:28pm
14:1 ratio is practical in any shape or form.  Even HF drivers with complex phase plugs don't go that high.

For a cone driver I would say 3:1 is probably a realistic maximum.

You have to also take into account path length matching.  All areas of the cone need to have a similar path length to the exit of the horn and a small circular throat is probably about the worst for that.  It would also be very hard to avoid a large front chamber without a phase plug.



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Post Options Post Options   Thanks (0) Thanks(0)   Quote luthier Quote  Post ReplyReply Direct Link To This Post Posted: 11 February 2019 at 9:16pm
Originally posted by MattStolton MattStolton wrote:

My only comment would be that, without a phase plug, you may get some cancellations in the mouth of the horn, as upper frequency waves bounce from side to side, forming standing waves in the mouth.

2KHz wavelength is approx. 17cm, so any dimension of this order of magnitude will lead to some issues.


To get low frequency control, your mouth will be big, so top end will be affected.

Completely understand, the plan has always been to design a horn flare with a phase plug for the very reasons you mention.  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote luthier Quote  Post ReplyReply Direct Link To This Post Posted: 11 February 2019 at 9:19pm
I've actually got two sitting in my workshop as a temporary solution and a base line. I plan on A/B testing against them. 

My issue with them is the low end extension and big dip just after 2khz. Also I just want to learn more about horn design.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote luthier Quote  Post ReplyReply Direct Link To This Post Posted: 11 February 2019 at 9:20pm
Originally posted by Mircea Bartic Mircea Bartic wrote:

looks like you want to build something like this:

I've actually got two sitting in my workshop as a temporary solution and a base line. I plan on A/B testing against them. 

My issue with them is the low end extension and big dip just after 2khz. Also I just want to learn more about horn design.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote luthier Quote  Post ReplyReply Direct Link To This Post Posted: 11 February 2019 at 9:25pm
Originally posted by studio45 studio45 wrote:

14:1 does seem like a lot - most 8" horns I've seen are only about 2 or 3:1

You could  try modelling an existing horn and see what the graphs look like? Because I would doubt Hornresp's accuracy in the upper midrange. I think it tends to show more of a roll-off than will actually happen IRL. And I don't think it models phase plugs at all well.

For example I have some phase-plugged horns here, designed for the 8PE21.

(very approximate values from quick measuring)
S1 = 77cm2 (annular slot around plug) so about 2.85:1 ratio
S2 = 838cm2 (mouth)
L12 = 27cm 
Exponential profile
Throat chamber approx 264 cm3 (driver cone volume minus rear part of plug)
Rear chamber volume approx 1.25 litres (2.5litre back bowl, assuming driver takes up about 50% of space)
Rear ch depth 10cm

See how you get on....These horns do actually work quite well in real life. The plots don't necessarily suggest that!

Well that is incredibly useful to know. Thank you for such a detailed reply! 

Is there any other software you know of that might model this type of horn more accurately? I'm currently looking into LEAP and COMSOL.




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Post Options Post Options   Thanks (0) Thanks(0)   Quote luthier Quote  Post ReplyReply Direct Link To This Post Posted: 11 February 2019 at 9:26pm
Originally posted by snowflake snowflake wrote:

you aren't modelling the directivity. model a single section horn and use the directivity tool to get an idea of how much directivity you gain at different frequencies. then go back to modelling a multi-section horn and aim for a power slope that decreases with frequency (approx -3dB per octave).

making your first throat section much shorter will give you this slope and help the very high end. make it just 2cm long and the flare cutoff is about 2kHz. should give a flat on axis sensivity to about 3kHz. this is often called 'rubber throat'.

the throat you have will also cause a lot of distortion at high levels. faster flare rate where the throat is small reduces this problem too.

Got it, cheers! I'll bare it in mind.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote luthier Quote  Post ReplyReply Direct Link To This Post Posted: 11 February 2019 at 9:28pm
Right, that's matches up to what I've been reading.

Interestingly I've been on that website quite a lot recently, I'll have to take another look.
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