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250-4k horn

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Category: General
Forum Name: Advanced Discussion
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URL: https://forum.speakerplans.com/forum_posts.asp?TID=106172
Printed Date: 02 March 2024 at 10:27pm
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Topic: 250-4k horn
Posted By: Augusts
Subject: 250-4k horn
Date Posted: 21 March 2021 at 4:28pm
Hello

I am planning to construct a mid freq horn for an 8" Oberton 8NMB250v4

What do you think?  I am using an exponential horn flare. Its depth is 350mm

I will try to do a phase plug next








Replies:
Posted By: JulianDA
Date Posted: 21 March 2021 at 11:28pm
250-4k is, to say the least, pretty challenging if you want to have an even dispersion in that range. Did you try to simulate it in akabak or a similar software? I even struggle to get good directivity from 200-2k with a phaseplug in front of the driver...
That said, your horn looks rather decent with a nice curved geometry, but i would try to eliminate the sharp edges at the mouth, since they will be the cause for diffraction und reflection of soundwaves back into the horn. This will result in peaks and dips in frequency response and the onset of resonances over the length of the horn, which in turn will have a bad impact in the time domain (i dont know the rigth word in the english language, but you will see it in the burst decay and that can lead to an unpleasant sound)


Posted By: Augusts
Date Posted: 21 March 2021 at 11:30pm
Well said saiar!

Thank you for commenting


Posted By: VECTORDJ
Date Posted: 22 March 2021 at 2:23am
The hard part is 2k-4k. Please show Us the Phase plug design and real life testing. Thanx....




Posted By: Augusts
Date Posted: 22 March 2021 at 8:09pm
Julian, do you mean the sharp edge of the perimeter at the very end of the mouth or the not so sharp corners towards the end of the mouth?

I simulated the horn in hornresp, as far as it could be simulated. Have not got my head around akabak




Posted By: Augusts
Date Posted: 22 March 2021 at 8:25pm
The phase plug is probably going to be just a round cilinder with a cd horn at the end. I still have a long way to go until testing. Ill be routering this baby out of polystyrene for testing, then adjust, then another routering to make a negative. then fiberglass 


Posted By: Elliot Thompson
Date Posted: 22 March 2021 at 10:31pm
How big is the Horn's mouth? It seems you are basing this design on Quarter-space. This may be fine for bass horns. However, for midrange horns, if the mouth is not big enough to play 250 Hertz at a substantial level, you are looking at the bulk of your SPL will hover around 500 Hz, with a box offering a mere 350 mm in depth.

I might add with the loudspeaker you are considering offering around -3 dB @ 5 kHz (all courtesy of the Le) you may want to consider finding a driver that reaches to around 7 kHz if the goal is  4 kHz.


Best Regards,


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Elliot Thompson


Posted By: Augusts
Date Posted: 22 March 2021 at 10:37pm
Thanks for commenting.

The mouth is around 3000sq/cm

Hornresp shows promising results


Posted By: Elliot Thompson
Date Posted: 22 March 2021 at 10:53pm
The radius of the mouth nears 250 Hz full-space, which is good. You will definitely be relying on equalisation once, you go below 500 Hz due to the limited depth of the horn.

Best Regards,


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Elliot Thompson


Posted By: Augusts
Date Posted: 22 March 2021 at 11:05pm

This is a sim on axis. I think the phase plug could sort things at 2-4 kHz


Posted By: Elliot Thompson
Date Posted: 22 March 2021 at 11:21pm
Originally posted by Augusts Augusts wrote:


This is a sim on axis. I think the phase plug could sort things at 2-4 kHz



The graph correlates to what I mentioned in regards to the limitation of the horn's length. Remember, -3dB is half the power. So anything below 500 Hz will rely heavily on equalisation. Also, the loudspeaker in question is not suited for the application for, you should not have such a drastic decline from 1 kHz - 2 kHz. The driver's Le is the problem.

Try a different loudspeaker.

Best Regards,


-------------
Elliot Thompson


Posted By: citizensc
Date Posted: 22 March 2021 at 11:37pm
Your goals of a coaxial mid/HF horn are the dream, if I could design a phase plugged mid horn that could play from 250-4k with a compression driver in the phase plug I would.

The issue is that getting a mid horn to play from 250-4k is a serious challenge, not impossible but seriously difficult. As VectorDJ said above, the 2-4k is the hardest part. Simply putting a phaseplug looking object in the centre of the horn will not get you there. 

I had a go at designing a mid horn but gave up when I realised how involved it was going to be. The process of tweaking a cad model over and over, then simulating it over and over was soul crushing.


I suggest you read this thread  https://forum.speakerplans.com/xtro-flare-redevelopement-thread_topic98310_page1.html" rel="nofollow - https://forum.speakerplans.com/xtro-flare-redevelopement-thread_topic98310_page1.html

A lot of the images are missing which is a shame but there is some really good information there. After all that work, all those AKABAK 3 simulations, they ended up with a horn that could play from ~300-2k.


Take these tips with a grain of salt as I have never actually successfully designed a mid horn but this is what I would do:

-There are worse choices than the 8NMB250v4, but there are also better ones, as I understand it, more EBP is better in mid horns, a popular driver in the past has been the 8PE21 but I would use a closed back driver like the 8NSM64

-I would use AKABAK 3 to simulate the horn and the phaseplug, hornresp tells you pretty much nothing when it comes to these kinds of horns, they are very sensitive to small changes in geometry, which you can not simulate in a 1d simulation program like hornresp.

-While your goals are the dream, I would target a much lower xover point and use 1.4 inch compression driver to meet it. This is just far more achievable. 

Keep working at it, I think its a cool project! If you keep posting updates here, I'm sure people will help you out and will enjoy following your progress.


Posted By: JulianDA
Date Posted: 23 March 2021 at 4:35pm
Originally posted by Augusts Augusts wrote:

Julian, do you mean the sharp edge of the perimeter at the very end of the mouth or the not so sharp corners towards the end of the mouth?

I simulated the horn in hornresp, as far as it could be simulated. Have not got my head around akabak



Yes i mean the perimeter on the hornmouth. You have to keep in mind that all sudden changes in acoustical impedance (i.e. flarerate) will produce diffraction effects which can be interpreted as new point-like "sound-sources" which then will radiate omnidirectional and therefore also back into your horn, causing resonances.
Another thing to keep in mind is the big spike in groupdelay on the tuning frequency of a horn. In an optimal world you dont want to have this spike in the used range of the horn. At lower bass frequencies the groupdelay is less noticeable, but in the mid-range it can be problematic. So a really good midrange horn should be tuned ~1 Octave lower than the desired crossover frequency. But this leads to really big horns...there is a compromise to be found somewhere inbetween big enough and to small 

But on another note: you can listen to the advice of citizensc, he knows his stuff regarding horns and akabak ;)

 


Posted By: Augusts
Date Posted: 23 March 2021 at 5:56pm
Thank you for the insights

I am now learning Akabak

Do you know why it could be that tha phase plug is absent in the BEM section but visible in the General- Mesh section?







Posted By: Augusts
Date Posted: 23 March 2021 at 6:16pm
This is what I got without the phase plug. First try






Posted By: JulianDA
Date Posted: 23 March 2021 at 8:26pm
Originally posted by Augusts Augusts wrote:

Thank you for the insights

I am now learning Akabak

Do you know why it could be that tha phase plug is absent in the BEM section but visible in the General- Mesh section?

Sorry i cant answer that question on the fly since i am not that deep into akabak at the moment (there is alot of other stuff to be done in my life :D ), but maybe citizensc has encountered a similar problem?


But your directivity plots dont look to bad :) For easier comparison of plots i would advise you to normalize your plots. There should be the option "normalize to x-axis" in VACS-Viewer under "processing" (or "postprocessing"). After that you can choose your dB-steps as indicated by the different colours to show the -6dB Point better. 



Posted By: toastyghost
Date Posted: 23 March 2021 at 9:01pm
Take a look here for tips on BEM simulation for horns. They’re using ABEC3 but it’s very similar to the new AKABAK, so the same concepts apply.

https://sphericalhorns.net/2019/10/20/bem-simulation-for-a-freestanding-horn/" rel="nofollow - https://sphericalhorns.net/2019/10/20/bem-simulation-for-a-freestanding-horn/
https://sphericalhorns.net/2019/10/22/drba-calculator-and-bem-simulation/" rel="nofollow - https://sphericalhorns.net/2019/10/22/drba-calculator-and-bem-simulation/


Posted By: citizensc
Date Posted: 24 March 2021 at 9:17am
Good work getting up and running with AKABAK 3 so quickly! it is not the most intuative program when you first start using it.

Can you please post a top down view like this and include the horn itsself 



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https://www.facebook.com/voyager.system

@voyager_soundsystem


Posted By: Augusts
Date Posted: 24 March 2021 at 10:48am
I found a very good guide on youtube on how to use AKABAK, without it I would not be able to work my way this far.

I do not know how to do a simulation like that with the horn. Could you give me some keywords?LOL


Posted By: doller
Date Posted: 25 March 2021 at 9:08am
Have a look at the fane speaker book page 50 w type bass bin. They claim 60 to 4500hz. There is a plot included. Just saying. I have made some and they are surprisingly OK psy trance sounds just sweet through them. I struggle to get them to 1.8hz with a pd 121. 
That said look at the pm 60 90 streach the fane design a bit and squeeze it a bit to fit that 18 sound horn and two drivers. Quite similar. Not saying that is was the inspiration for the pm in anyway shape or form. But fane say in the book stick a comp in the middle. If you want.


Posted By: citizensc
Date Posted: 25 March 2021 at 12:19pm
Originally posted by Augusts Augusts wrote:

I do not know how to do a simulation like that with the horn. Could you give me some keywords?LOL


In the general tab define a new node with 4 points eg. x=2y=2, x=2y=-2, x=-2y=-2, x=-2y=2 ,this example is for a 4x4m square but you can make it any dimensions you like. - This step defines the plane you want the measurement to be taken from and displayed. 

Then go to the observation tab, create a 'fields (nodes)' then under planes tab inside the 'fields (nodes)' add the node you just created. From memory, it should just work now if you re run the simulation. If not, PM me!


-------------
https://www.facebook.com/voyager.system

@voyager_soundsystem


Posted By: Danny
Date Posted: 18 April 2021 at 8:30pm
Hello,

For a 3-way, 15"+8"+CD I'm thinking of using the Oberton 8NMB250v4,
the specs look very good.
If you have purchased them, how do you judge them ?

Regards,
Danny



Posted By: Augusts
Date Posted: 19 April 2021 at 8:39am
Hi

They are excellent drivers, the response is smooth up to 5kHz, I would compare them to BMS.


Posted By: Danny
Date Posted: 19 April 2021 at 9:50am
Thanks,
I will put them in my basket :-)



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