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Discussion Starter · #1 ·
I have been toying with the idea of building a small sealed sub to use for desktop audio.


My main concerns were sound quality while keeping cabinet size and cost low...


I recently bought on the modified Foster WF-100K amps from the NHT surplus sale... This one uses a 3rd order HP at 21Hz instead of 29Hz, and has a flatter response than the "stock" model.


eD had their e3.8 drivers discounted over black Fri, so I snagged one of those as well.


My original plan was to go with a Dayton RSS210HF, or, if I could save some money, a CSS SDX-10.


I went with the eD driver since it appears to be reasonable well made... and having one of their A3S-250 subs in my living room, have been pleased with its performance. It uses the 11Kv.2 driver, and the e3.8 is supposed to be an improvement on the 9Kv.2 platform.


More than anything else, I went with the eD driver due to price ($80 shipped) and the fact that it likes a very small box.


I've spent some time modeling it in WinISD, factoring in the 21Hz or 29Hz HP and a 2nd order low pass at 100Hz (built in xover in my desktop amp).


It doesn't model quite as well as the RSS210HF... peak output is higher up, it doesn't go as deep, and max SPL is a little lower... but it also isn't pushed past xmax, even with the HP as 21Hz instead of 29Hz. Max SPL at 1M is around 101dB at 70Hz, and 98dB at 41Hz. Given that ear height is about 4ft away from the sub beside the deak, and factoring in room gain (10x13 office)... I'm hoping it will be plenty of SPL for nearfield desktop music.



Anyway... at this point... I'm looking for advice on final cabinet size and a target system Q.


Provided my calculations are correct, the amp will chew up about 0.118ft^3, and the driver displacement is 0.05ft^3. A single "window pane" brace should take up another 0.02ft^3.


So a 12" cube should NET around 0.48ft^3 before polyfill. Adding in 1/2 pound of acoustastuff, and assuming an effective 10% increase in box size, I'd be looking at 3ft^3. This would give me a Q of 0.668.


Thoughts... should I go larger on the cab? Can I go any smaller? Did I factor in effect of stuffing on box size correctly, or should I have added 10% before taking out the displacement for driver/amp/bracing?


Thanks in advance.
 

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Discussion Starter · #2 ·
BTW, I ordered the dual 2ohm e3.8 driver... voice coils will be wired in series for a 4ohm load.



Here are some WinISD graphs.


Yellow - Dayton RSS210HF in 0.9ft^3 box, 29Hz HP, Q = 0.805, Fsc = 44Hz

(this is about as large a box as I'd tolerate, so Q is higher than I'd like)

Blue - eD e3.8 in 0.53ft^3 box, 29Hz HP, Q = 0.668, Fsc = 48Hz

Pink - eD e3.8 in 0.53ft^3 box, 21Hz HP, Q = 0.668, Fsc = 48Hz


 

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first off, you botched the low pass filter. this is good news for you. use the 4th order linkwitz-riley for the 80hz low pass not the second order butterworth.


here it is from the site:

Foster subwoofer plate amplifier. 115W into 4ohms, 80W into 8ohms. 4th order (24dB/octave) low pass adjustable from 50-200Hz.
http://home.comcast.net/~jhidley/
 

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not sure that i will see this thread again, so here is a 1.0 and 0.5 cubic footer. your driver and amp. 3rd butterworth high pass 21hz, 4th linkwitz riley low pass 80hz, 115watts. red is the 1.0 cubic footer. excursion at 115w is below xmax in both cases. i'd go with the larger sub. you could even go a little larger if you wanted to but you don't really gain anything.


model it up yourself.


you did good homework and you will be happy for what you are trying to do. very happy if i guessed.


 

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Discussion Starter · #5 ·
Thanks... definitely appreciate it.


I figured the larger box is better... but one of the main goals of this project was to keep box size as small as possible, but without killing performance.


Just curious... what did you use for Mms for modeling the e3.8? It's not listed in the specs on eD's site... but I found 80.5g somewhere, and that's what I've been using.
 

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Discussion Starter · #6 ·

Quote:
Originally Posted by LTD02 /forum/post/19579664


first off, you botched the low pass filter. this is good news for you. use the 4th order linkwitz-riley for the 80hz low pass not the second order butterworth.


here it is from the site:

Foster subwoofer plate amplifier. 115W into 4ohms, 80W into 8ohms. 4th order (24dB/octave) low pass adjustable from 50-200Hz.
http://home.comcast.net/~jhidley/

Regarding the low pass...


My desktop amp (Tweak City Audio Gizmo) has a built in 2nd order 100Hz low pass filter.... this is why I used 2nd order BW.


I can fine tune with the sub amp's 4th order... but probably wouldn't set it right at 100Hz since it'll create a very steep slope for the high end rolloff.


As a guess... I'd probably be looking at 2nd order low pass at 100Hz, then throw in the sub's 4th order low pass around 120Hz.
 

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hi again...it has been a little slow around here...


"Just curious... what did you use for Mms for modeling the e3.8? It's not listed in the specs on eD's site... but I found 80.5g somewhere, and that's what I've been using."


winisd calculated 83.6 g for mms for me. 3 g one way or another shouldn't affect the big picture.


"Regarding the low pass..."


oh, i see. are you sure the low pass is second order? typically, it is 2nd order on the high pass (speaker side) and 4th order on the low pass (subwoofer side) because speakers naturally have a 2nd order high pass (roughly speaking) just from the speaker's natural rolloff without any crossover.


"Here's a graph showing the differences with the LP filters..."


for your purpose, any of those will work as they are all within a couple db of each other. since the low pass is variable on the amplifier that you chose, you can experiment with turning it down and increasing the overall volume level or turning it up and lowering the overall volume level. i'm sure you will find a combination that makes it work.


"but one of the main goals of this project was to keep box size as small as possible, but without killing performance."


it's not going to kill performance to go on the small side. just recognize that you are giving up a little sensitivity there. if you built the 0.5 and i built the 1.5 and we both played them a-b, you'd say, "hey, yours sounds a little louder." if you never heard the slightly louder one, then you'd say, "hey, this thing sounds awesome." :)


the idea that i was trying to get at there is that it is important to build what makes you happy...not everyone else. i was just trying to help you get a sense of what the tradeoffs were. if after considering it, you choose to go small......win!


edit: i'd nudge you toward at least the 1 cubic footer though...;-)
 
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