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Discussion Starter #1
I guess my scottish nature must be showing because I actually started thinking about doing something really cheap but way better in performance than my old subs, a pair of the original HSU 10" subs in a sonotube. The unfortunate thing is that they never really quite had enough nads to do the job at anything approaching 115 db. The extension was pretty good, the distortion acceptable but not wonderful, but not enough power handling capacity. They are in rather sad shape now with the surrounds coming loose from the cones. I have glued them back but it really was not that great of a job and I know their performance is not what it once was. SO, having improvised the need for new subs in terms the wife will at least tolerate if not understand, I decided to see if I couldn't do HSU one better and build an upgraded equivalent to the HSUs with mostly stuff I had on hand. I have a couple of old 12" MTX T4124 car subs mounted in a sealed 2 ft ^3 enclosure that came out of a car I no longer have. My thought is to build a couple of large sonotube enclosures with the old 12s in them and retire the HSUs. I ran a plot with Win ISD and came up with some pretty interesting numbers. I'll be using an old adcom 555 II which is capable of 325 wpc into 4 ohms and these are 4 ohm woofers.
Using a 500 liter enclosure with a single port I get the following raw freq response numbers. I'll be crossing over to the subs at about 45 hz, so the subs are only expected to cover the lower two + octaves. This is the curve I came up with...

Number of drivers : 1
Box type : Vented
Box size : 500.6 l
Tuning frequency : 17.00 Hz
Vent : 1 vent(s)
0.211 m length for each
0.140 m round


Relative
Freq Gain SPL
[Hz] [dB] [dB]
10.00 -25.11 89.30
15.00 -4.46 109.96
20.00 -2.34 112.08
25.00 -2.99 111.43
30.00 -2.61 111.81
35.00 -2.11 112.31
40.00 -1.68 112.74
45.00 -1.34 113.07
50.00 -1.09 113.33
55.00 -0.89 113.52
60.00 -0.74 113.67
65.00 -0.63 113.79
70.00 -0.54 113.88
75.00 -0.46 113.96
80.00 -0.40 114.02
85.00 -0.35 114.06
90.00 -0.31 114.11
95.00 -0.28 114.14
100.00 -0.25 114.17

With these kinds of numbers, add in a second sub and I have achieved the target spl of 115 over the audible range and reasonable extension below 20 hz. The tuning frequency of the box is 17 hz.

Anyone know of anything I am overlooking in my approach to this that I should consider? I have access to a router to cut the end caps and to carve the holes for the driver and vent tube and obtaining the sonotube shouldn't be too difficult.

If these sound better than the HSU subs, then I will figure the project to be a success and can think about a driver upgrade down the road to really improve things some more.

ANy thoughts??? Bear in mind I am something of a newby in this arena. Any gotchas I should look out for?

Many thanks in advance,

Hotshoetom
 

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I ran a plot with WinISD and came up with some pretty interesting numbers.....
Any thoughts??? Bear in mind I am something of a newby in this arena. Any gotchas I should look out for?
You don't state what input power your modeling has, if Xmax is exceeded or if a Hi-Pass filter is being used. Could you post the WinISD file for the T4124?
 

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Discussion Starter #3 (Edited)
You don't state what input power your modeling has, if Xmax is exceeded or if a Hi-Pass filter is being used. Could you post the WinISD file for the T4124?
Hi Mike,

Not sure how to answer your question as I am new to WinISD. I reran some numbers and have included the winISD file. I based the plot this generates on applying the rated power of the speaker. Not sure if the attached the Win ISD file has the info you need or not. But if not, I am sure you'll let me know, right? :R


I am not sure if the file I attempted to upload actually 'made it' or not...if not I will try again. I attempted to upload the winisd file and the site won't let me due to it being an invalid file type. Any suggestions on how I get around this?

Tom
 

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Discussion Starter #4
Mike, I will attach a text file of the rerun numbers embedded in this post. I am assuming a crossover frequency of 45 hz as applied by my active electronic subwoofer eq. I am also assuming a maximum input power of 200 W rms which is the rated power for the speaker. Whether or not this causes the speaker to exceed Xmax is a good question that I do not know how to answer.
I have some built-in protection against over excursion below 20 hz due to the filtering in my subwoofer eq. The subsonic filter in that rolls off everything at 18db/octave below 10hz. The curves using the reworked numbers are as follows:

Driver : MTX -T4124 Thunder 4000 12"
Project by : Tom
Project for : Tom
--
Number of drivers : 1
Box type : Vented
Box size : 555.0 l
Tuning frequency : 17.00 Hz
Vent : 1 vent(s)
7.09 in length for each
5.51 in round


Relative
Freq Gain SPL
[Hz] [dB] [dB]
10.00 -24.47 88.34
15.00 -4.11 108.70
20.00 -2.04 110.77
25.00 -2.54 110.27
30.00 -2.15 110.66
35.00 -1.68 111.13
40.00 -1.29 111.52
45.00 -1.00 111.81
50.00 -0.79 112.02
55.00 -0.64 112.18
60.00 -0.52 112.29
65.00 -0.43 112.38
70.00 -0.36 112.45
75.00 -0.31 112.50
80.00 -0.26 112.55
85.00 -0.23 112.58
90.00 -0.20 112.61
95.00 -0.18 112.63
100.00 -0.16 112.65

Thanks for your help in advance....

Tom
 

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Discussion Starter #6 (Edited)
Ok Mike, thanks for that input. Considerably different results than I got, but what you are telling me is that going to a very large enclosure like I tried with my model gave me very different results. What happens when the enclosure size gets very large? Does that 'encourage' the driver to be too sloppy and create more distortion as well as go into overexcursion? Inquiring minds want to know...:bigsmile:

tom

PS - what the is HPF

Is that the Housewife Protection Factor???? :devil: Also, is there any reason why you tuned the enclosure to 21 HZ? Seems like tuning it lower would help the extension. My guess is you have done some calculation that indicates the cone would be driven into overexcursion below 20 hz??? I realize any design is a series of compromises. But I am trying to understand the reasoning behind your choices. many thanks again...
 

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A bigger enclosure produces more low end extension at the cost of overall SPL. The trick is to find the right box size and tuning frequency for the subs capabilities. This sub has an Xmax of under 10 mm, tuning below 20 hz or so isn't worth it. Going bigger the 250 liters means the sub reaches Xmax with less power input, thus less SPL. 250 liters tuned to 21 hz gives the best conbination of low end extension and overall SPL.

HPF = Hi-Pass filter. A Hi-Pass filter blocks the frequencies that are lower then the chosen frequency of the filter, in this case 18 hz. For your application a Hi-Pass filter set at 18 hz will block all frequencies below 18 hz that would cause the sub to exceed its Xmax. The HPF protects the sub from over excursion and bottoming out.
 

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Discussion Starter #8
Thanks for the explanation Mike. I know what a HPF is, but it didn't dawn on me at the time of day I was reading the post that it was an abbreviation for high pass filter...duh! Career electronics tech here...so once I get used to all the abbreviations here I'll ask fewer newby questions. Sounds like building these enclosures yield less than desireable results considering the effort involved. One more question...does the use of a cylindrical enclosure seem to have any bearing on the performance as opposed to a conventional rectangular box? Seems like sonotube enclosures would be the easiest to build and perhaps perform better. Comments?

Many thanks,

Tom
 

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Sounds like building these enclosures yield less than desireable results considering the effort involved.
It depends what your goals are and how much money you want to spend. If your goal is 115 db across the spectrum then consider a different sub.

does the use of a cylindrical enclosure seem to have any bearing on the performance as opposed to a conventional rectangular box?
The only slight disadvantage would be giving up a couple of mm of cone excursion due to the sub being horizontally mounted.
 
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