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To take a pro 18" down to 20Hz
To take a pro 18" down to 20Hz
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Old 20th October 2015, 10:49 AM   #1
technofreak is offline technofreak  Estonia
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Default To take a pro 18" down to 20Hz

So I bought one of these: the box Speaker 18-500/8-A - Thomann UK

Obviously not something that is described as a true subwoofer in home theatre circles -- not enough Xmax, flimsy paper cone, small power handling, etc. True, won't work in a small sealed cab with aggressive EQ and a 3 kW amp. But in a large vented cab tuned low? Maybe; I shall find out in next few days. Starting to build the cabinet tomorrow.

360 litres tuned to 18 Hz looks promising. I'm trading off lots of SPL for extension, no need for 124 dB in home setting (arguable, of course). But I should still get as much output as with Eminence LAB15 with less power -- in a box twice as big.

To be continued...
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File Type: png sub_plot.png (45.5 KB, 827 views)
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Old 20th October 2015, 02:43 PM   #2
Inductor is offline Inductor  Portugal
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Your sim doesn't look right.
The best I can get with THE BOX SPEAKERS 18-500/8-A is -8dB's@20Hz for a BR, with an extended alignment of VB = 360.0 L, FB = 22.0 Hz.
https://images.static-thomann.de/pic..._uk_screen.pdf
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Old 20th October 2015, 03:05 PM   #3
technofreak is offline technofreak  Estonia
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Hi Inductor,

This plot is with all the processing involved - like I said, I trade HUGE amount of efficiency/SPL for extension, EQ is necessary to flatten the response. This is also the case with modern small sealed subs.

Basically, 4th order L-R HP filter at 45 Hz flattens the response and gives F6 at around 60 Hz. Then, if room gain doesn't take care of the rest, small amount (1...1,5 dB) of boost around 20 Hz should do the trick.

Of course, one could choose a driver with heavy cone, low Fs and sensitivity and yield the same result, but these kind of drivers are expensive (e.g RSS460HO-4 - Dayton audio reference 18" Reference HO Subwoofer - Europe audio or Eminence LAB 15 - Thomann UK), need a powerful, more expensive amp and I'm on a budget here

Here is the raw response (looks awful, right? But I still have 86 dB of sensitivity at 20 Hz):
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File Type: png sub_response_raw.png (63.2 KB, 784 views)
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Old 20th October 2015, 04:39 PM   #4
Djim is offline Djim  Netherlands
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Hi technofreak,

You can find the measured data of the 18-500/8-A here

Fs = 40,1 Hz
Vas = 208,3L
Re = 5,5 Ohms
Qms = 7,61
Qes = 0,450
Qts = 0,425
Sd = 1313 cm^2
Mms = 183,849 grams
Mmd = 156,651 grams
Cms = 0,086 mm/N
BL = 23,794
Xmax = 5,4 mm

Cheers,
Djim
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Old 20th October 2015, 04:48 PM   #5
technofreak is offline technofreak  Estonia
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Thanks, Djim - these seem to be agreeable with the data sheet downloadable from Thomann (and also supplied with the driver).
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Old 20th October 2015, 05:48 PM   #6
tb46 is offline tb46  United States
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Hi technofreak,

How about a T-TQWT? Enter the simulation below into Hornresp, and play around with it.

Regards,
Attached Images
File Type: jpg the_box_18_500_8A_TTQWT_Input.jpg (34.8 KB, 741 views)
File Type: jpg the_box_18_500_8A_TTQWT_SPL.jpg (30.1 KB, 738 views)
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File Type: txt techno_1.txt (974 Bytes, 10 views)
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Old 20th October 2015, 05:57 PM   #7
Djim is offline Djim  Netherlands
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Hi technofreak,

Indeed those specs are close.

Ps have you looked at two Thomann 12-280/8-W. You have twice the the motor power, 30% more moving mass, just a little lower Fs, will need less EQ and a smaller box and can be arranged in a PPSL configuration. They will also work nice in TH's.

Cheers,
Djim

Last edited by Djim; 20th October 2015 at 06:00 PM.
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Old 20th October 2015, 08:24 PM   #8
technofreak is offline technofreak  Estonia
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Hi, Djim,

Yes, I've contemplated almost every cheap(ish) driver out there, even wacky stuff like eight 8" drivers in one box. That 12" sure is a looker (Qms 20! Nice basket, too), but I already have the 18" one.

On T-TQWT note, my sanity check shows no real advantage in a similar total volume box. Funny thing, Hornresp shows around 3 dB more output for vented box at excursion limit (but Hornresp isn't be-all-end-all program. No simulator is). A tapped horn is on my bucket list, though, maybe with some 10...12" drivers. As for now I'm also running a parallel build of some 3-way speakers using Scan-speak, ACR and Seas metal cone and dome drivers, so my hands are pretty full
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File Type: png TH_vs_Vented.png (22.4 KB, 713 views)
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Old 20th October 2015, 08:27 PM   #9
BP1Fanatic is offline BP1Fanatic  United States
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I used the cutout diameter as the Sd since I didn't see it on the data sheet. I let HR calculate the rest of the speaker parameters. This is a Tapped Quarter Wave Pipe (TQWP) or straight flare tapped horn.
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File Type: jpg Speaker 18-500_8-A1.jpg (549.9 KB, 172 views)
File Type: jpg Speaker 18-500_8-A2.jpg (231.8 KB, 157 views)
File Type: jpg Speaker 18-500_8-A3.jpg (214.6 KB, 96 views)
File Type: jpg Speaker 18-500_8-A4.jpg (207.7 KB, 94 views)
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Old 20th October 2015, 08:39 PM   #10
esl 63 is offline esl 63
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You can easy tweak a subwoofer element to reach lower fs by cutting out strips in the spider. Take a razor blade and cut stripes radially from the center to the primeter of the spider. Equally wide and at least 3. I have some JBL 2226H where i cut out 8 stripes approx 10mm wide at the perimeter and 3mm close to the coil former.
I also replaced the surrounding coated cloth, to 2235 foam surronding.
So i went from fs 41Hz to 22Hz without adding mass. I will cut out slighly more of the spider until i reach 18Hz. Works wery fine! But on your own risk. I take no responsibility if you s**** things up. If you place the driver vertically, the spider could have been veak and you may experience som sagging making the coil touch the pole piece. Do not fear! You just add a thin synthetic thread radial and vertical out from the center of the voice coil, letting it take some force from the gravity.. the length should be approx 13cm on a JBL 2226 as an example.
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