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-   -   18 Sound 8NMB420 tapped horn (http://www.diyaudio.com/forums/subwoofers/169863-18-sound-8nmb420-tapped-horn.html)

SOSx 7th July 2010 01:02 AM

18 Sound 8NMB420 tapped horn
 
Hi,

At the moment I am designing a small system for friend as a side project.
I wanted a bass cab that was relatively compact, efficient, a width of 290mm and Flat for the octave covering 50-100hz.

I decided after reading a few threads on hear covering smaller tapped success stories I thought i would through a few numbers into horn response and see what came of it.

Firstly the main (full range) cab consists of 1 x 8nmb420 + 1 x 8cx400. I have designed it to be a three way cabinet, actively driven from 3 channels of TK2050 amp boards (amp + active crossovers internally mounted).
This means that there is 1 spare amp channel of around 50w 8r/100w 4r to drive the separate bass Cab (small speakon link out from the main cab).

Well enough digressing!! Here is the tapped horn i have come up with.
I would be interest in feed back/criticism or even alternate driver cab combination's that fit the brief!!

http://i245.photobucket.com/albums/g...ata8NMB420.jpg
http://i245.photobucket.com/albums/g...ta8NMB4202.jpg

SOSx 7th July 2010 01:05 AM

Oh yes I Should note that the design has a 2.2 mh inductor in the design hence the 2.55mh Le.

TundraLTD 7th July 2010 06:33 AM

I'm still trying to catchup with my understanding of the TH but, as far as I understand currently... Raising the inductance [adding an inductor, coil, whatever you want to call it] you are simple adding a 1st order lowpass filter. Since Hornresp doesn't allow ext. components as far as I know. Does this actually have a place in a real-world built design or is it null/pointless when using a real active filter?

SOSx 7th July 2010 09:23 AM

Just read through what i posted last night.
Do excuse the bad grammar and miss pelt words.
It was late!!

I should also add that the screen shot showing excursion is taken at 50w, 8r in half space, as is the spl graph showing 115db output in band.

TundraLTD

I am relatively new to Tapped horn theory myself (had always ruled it out for sound quality purposes). Maybe wrongly so? We will see soon! Also for this design compact efficiency is the name of the game.

As for whether adding the inductor to flatten response could be duplicated with processing is also a question I have been wondering?
Maybe some of the TH veterans will be able to shed light on this?

hm 7th July 2010 04:48 PM

sosx
 
1 Attachment(s)
Hello,
i constructed a double bass horn with this driver,
useful 34-200 Hz and the possibility of a 6 dB crossover,

have a look:

i think your approach will work not better than a simple BR for this driver,
but test it, down to 50 Hz is to much work for the enclosure.
Your max. SPL is a bit optimistic if you see the membran movement.

SOSx 7th July 2010 11:31 PM

Hi hm
 
Hi hm,

Thanks for the reply.
I should point out again that they will only have
a maximum of 50w into 8r realistically. The peak displacement
at this power, peaks at 6.3mm at 57hz. In the real world
the displacement should be less (due to not applying a constant voltagensine wave). I would think very close to 5.5mm xmax. This has been shown both here and in my own reaserch in pro audio designs.

As for a br achieving the same! You could achieve the same output in this volume, but not without using twice the no of drivers and 1.5-2 times the amount of power! Which is not available. Also to achieve the f3 of 43hz means tuning the box well bellow drivers fs, which provides an uneven impedance plot and over extended (peaky) response from 45-70hz.

Your design it is a dual tuned rear loaded horn right?
Shurely this design must be an impedance and phase nightmare?
With the three different radiating surfaces all with different acoustic lengths!
I have personally never liked the sound of rear loaded horns!

hm 8th July 2010 05:06 AM

sosx
 
"Shurely this design must be an impedance and phase nightmare?"

may be you donīt understand my double horns,
take a look on my HP,
and compair it with you Imp and membranmovement.
between 89-92 i worked in a disco the amps never turn over 5 !! W
as i see your simu you will have 5 mm at 1 W between 60-80 Hz,
but if you believe, test it an show us the dynamic compression, good luck.

littlemike 8th July 2010 05:18 AM

Quote:

Originally Posted by TundraLTD (Post 2237082)
I'm still trying to catchup with my understanding of the TH but, as far as I understand currently... Raising the inductance [adding an inductor, coil, whatever you want to call it] you are simple adding a 1st order lowpass filter. Since Hornresp doesn't allow ext. components as far as I know. Does this actually have a place in a real-world built design or is it null/pointless when using a real active filter?

Series inductors do work to flatten a response, I have measured a tapped horn with and without. The added series impedance "puts the brakes" on the resonant peaks in a tapped horn.

http://lh5.ggpht.com/_WXjTASetbPo/S6...%20Hz%20TH.JPG

Red = inductor, Blue = no inductor. The peaks are cut more than the rest, so it is not a simple low-pass filter in this application. Parametric EQ could do the same, no doubt.

No free lunch though - what are you giving up to get that flat response?

Efficiency. There is an increase in series resistance - that's the difference between the red and the blue lines. There may also be transient response issues, but that is beyond what I can measure with my current gear. If I can't measure it, I try not to talk about it.

SOSx 8th July 2010 08:38 PM

Hi hm,
I think I may know where your confusion lies!
As I have stated a couple of times. The peak excursion/membrane
movement hs screenshot is with 50w nominal input. Maybe the way I laid
the screen shots out had confused you?

The excursion at 1 watt is bellow 1mm!!

I must say I am not clear what you are trying to achieve with the double rear
horn designs? I couldn't find any predicted or measured
plots for these cabs, do you have any? I understand horn theory well
and can only see major issues occurring! Maybe I am missing something?
But in the pro world these wouldn't ever get considered as an option!

Tinymike- thanks for the measured plots it is nice to see the theory in operation!

Does any body else on here have any coments/opinions or ideas on improving the design?
Are there any mistakes In the design?

Kind regards

SOSx 8th July 2010 08:43 PM

Strange that last post had all the line/word spacing jumbled up after I pressed post?
Maybe due to how my iPhone browser displayed it?


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