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Old 4th June 2007, 02:36 AM   #1
Marzen is offline Marzen  United States
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Location: Cleveland
Default Need help with 1st horn sub design

I am looking for a horn sub project for home theater that will play loud to 20hz. Size is not much of a factor provided it fits through a 32" (81cm) doorway. Price is more limiting as I only have around $200 USD to spend on the driver & I'm not really picky regarding distortion specs considering its intended usage for loud, low bass in movies.
After finding William Cowan's horn sub page I downloaded the software mentioned and have been trying to have a go at this.
I was hoping someone could walk me through this process of determining if a driver is suitable as well as being a resource during my design process.

I have a 28ft x 40ft basement theater, a Rane PEQ, SoundEasy w/ ECM8000 mic and a complete wood shop for construction.

I'd hoped the Mach5 Audio 18" would work given it's price point of ~ $200.
Power Handling: 800 Watts
SPL (2.83v/1m): 89.2 db
Impedance: 4 Ohm (SVC)
T/S parameters:
Fs = 15.4 Hz
Re = 3.2 Ohms
Qt = 0.36
Qes = 0.39
Qms = 4.87
Mms = 228 grams
Cms = 0.370 mm/N
Vas = 556 liters
Sd= 1029.2 cm2
Xmax = 22 mm
Diameter = 36.2 cm
Le = ~3 mH (reportedly)

Best regards,
Ward
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Old 13th June 2007, 12:04 AM   #2
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Let's see if I understand this.

You want a HORN subwoofer.
You want it to go down to 20hz.
You want to use an 18" woofer.
You want it to fit through a doorway.

That's like saying that you want to marry a woman who's a ten, who cooks and cleans, and who will also support you while you pursue a career in loudspeaker design.

In other words, what you are trying to do is impossible.

The only way you're going to get to 20hz with an eighteen loaded in a horn sub is with something the size of a house.
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Old 13th June 2007, 01:16 AM   #3
djk is offline djk
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You will have no problem with the size of a 20hz tapped horn for HT, but an 18 with a Fs of about 30hz will work much better.
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Old 13th June 2007, 06:58 AM   #4
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Hi Marzen,

I'd like you to build it! But it will be big.

Bill Fitzmaurice designs some smallish bass horns. This one in the link below almost gets to 20Hz and just fits though a door. This could give you some idea of what you could end up with. The response curves show what happens (even for a well designed horn) when you limit the size of a horn mouth.
http://www.billfitzmaurice.com/THT.html

Fire up Hornresp!

Regards Philip
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Old 13th June 2007, 09:20 AM   #5
Marzen is offline Marzen  United States
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Thanks Philip for the link to Bill F's web site. I should have some available time soon for reading up on horn design and perusing some of the web sites I've found.

DJK, I couldn't locate a calculator or any white papers for a tapped horn, only references to Danley's site. Regarding a higher Fs, I do have a pair of 12" subs that are Fs=25hz.

Patrick, don't be such a narcissistic, psycho killer . Normal people just call it a collection of compromises rather than impossible. Worst case scenario - I learn 'WHY' it won't meet my requirements.
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Old 13th June 2007, 10:38 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
think about Tom Danley's Labhorn.
4feet square by 20inches wide.
You need 1.5 of them when corner loaded and three of them when backed up against a wall/floor junction.
That is 700litres times 1.5 (=~1000litres) for 30Hz roll-off into a corner.
Widen out a standard 20inch design to 30inch and you get an impression of the size for 30Hz. To get down to 20Hz you will probably need double the volume (2cub m). But this should be good quality bass and won't need a lot of power to give prodigious SPL.

Bill F's site talks about "average room" gain below 30Hz.
Can anyone give us details of how to arrive at some gain numbers for actual rooms?

Look at it from the mouth end.
To reproduce full bass at 20Hz requires a mouth area of 23sqm (=~250sqft) when in open air loading.
One can reduce this by half for floor loading and half again for floor and wall loading. That comes out at 5.8sqm. If you corner load your mouth you can reduce it again by half giving 2.9sqm. That is bigger than the size of the door you are talking about.
Some designers say you can still get good bass response by truncating the horn to about 60% of those area figures.
On that basis I reckon you should be aiming for a corner loaded mouth area of at least 1.7sqm (~=18sqfeet).

Patrick was not exaggerating.
BTW,
that 36inch THT is 730litres for 30Hz roll-off.
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Old 13th June 2007, 05:59 PM   #7
djk is offline djk
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A quickie from AkAbak, you could probably do better with some time and effort (On floor within 25cm of a wall):

Click the image to open in full size.

Def_Driver '4018LF'
SD=1188cm2 |Piston
fs=32Hz
Re=6.19ohm Le=4.78mH
Qes=.36 Qms=10.38 Vas=238L
Def_Reflector Wall=25cm
HAngle=0 VAngle=-90

System 'S1'

Driver 'D1' Def='4018LF' Node=1=0=20=21

Waveguide 'W1' Node=20=21
STh=1200cm2 SMo=3500cm2
Vf=15L Len=4.3m T=0.9

Horn 'W2' Node=21
STh=3500cm2 SMo=4100cm2
Len=45cm Conical
x=0 y=0 z=0 HAngle=0 VAngle=0
WEdge=55cm HEdge=1.14m reflection
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Old 13th June 2007, 10:07 PM   #8
simon5 is offline simon5  Canada
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You can also "cheat", I mean using a huge ported rear chamber to hit that 20 Hz with more authority. You use the horn for the 30-40 Hz stuff and up, and a peaky huge ported rear chamber for the 20-30-40 Hz stuff.

Like someone said earlier, fire up Hornresp and have fun! This tool does wonders.
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Old 13th June 2007, 10:43 PM   #9
simon5 is offline simon5  Canada
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For example, I did a little experimentation for you...
If you are a creative horn folder, maybe you could manage to fit this through a door... maybe hehe !

Pictures :

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Good luck !

P.S. #1 : As shown, this design can't use more than ~300W of power (~21.7mm Xmax used) or you will go in overexcursion at 18.25 Hz.

P.S. #2: This also a "normal" closed rear chamber, maybe a ported rear chamber would be bigger but you'd end up with a smaller horn I guess.
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Old 14th June 2007, 12:30 AM   #10
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simon5,

good job with hornresp.
Xmax always gets you in the end!

Maybe you could make S1 = 1/2 Sd and get a longer horn path, probably a smaller rear chamber, a smoother response and smaller Xmax. The horn path shown is a little shorter than ideal for 20Hz.

Hi Marzen,
Almost forgot to add this page for tapped horns
http://diy.cowanaudio.com/th.html

Regards Philip
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