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Old 28th August 2017, 04:08 AM   #1
Brian Steele is offline Brian Steele  Grenada
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Default The Boom Unit...

Introduction:
For years I've owned a Logitech X-503 multimedia system that provides the sound for my main PC. At the time I purchased it, it was around US$70, which was quite a good price, considering that the system consisted of 10" small 1.7" drivers (two in each of the satellites) and a 4" driver in a gnat-sized vented box to provide bass duties. The satellites aren't too bad (nothing that can't be fixed with a bit of EQ), but the bass unit was definitely a study in compromises. The driver has a relatively high Q and the box is tuned to around 80 Hz, so the end result is a big peak around that frequency that makes everything sound "robust", until you notice that a lot of the low notes are actually missing. Interestingly enough, the output from the bass unit's amplifier is not rolled off (significantly) below 80 Hz, which means that the driver does end up getting signal below Fb. The amplifier must incorporate some sort of dynamic EQ though, because even when turned up, there's no sounds of significant distress from the driver when it's fed very low frequencies.

Anyway, with no very low frequencies to speak off, the "bass unit' had to go, and I decided to replace it with a subwoofer design based around the Dayton DCS205 8" subwoofer driver. For this particular design, I wanted a solution that had an Fb around 32 Hz, as my intent was to use EQ to address the low-frequency performance in-room, and the lowest band on the simple EQ included with the PC was centered at 32 Hz.

So, I went to work on a new Excel workbook to describe the design, and I eventually came up with an MLTL-looking design that was around 26.6 litres net and 45 litres gross, primarily due to the use of 21mm MDF for the build. The corresponding Hornresp sim showed a response that gently slopes down from 100 Hz to just above 30 Hz, just right for what I wanted. The workbook is also able to export a data file in the required format for easy importing into Hornresp, which made the whole simulation process a lot easer. Note that in Hornresp I opted to include in the model the effect of puttling some polyester fiberfill in the first section of the alignment. According to the sim, this would drop the resonance frequency (Fb) a bit, and smooth the response in the passband. Real-world measurements of the built subwoofer confirm that the added filling achieved these goals. Note also that the predicted frequency response in Hornresp is slightly optimistic, as Hornresp does not account for the impact of box losses. The rolloff from 100 Hz to 40 Hz is slightly steeper than predicted, and the "corner" at around 32 Hz is a bit shallower.

Results
After building the box, the first thing I did was perform an impedance measurement, which suggested that Fb, which turned out to be around 33 Hz, was pretty close to that predicted by the Hornresp simularion I also performed some linearity tests, to see if subwoofer maintained its frequency response at higher input power levels (which would happen if port compression was low). Hornresp predicted that Xmax would start to be exceeded at 24.3V (the equivalent of just under 150W), and I tested at up to 25.4V (the equivalent of just over 160W). The subwoofer maintained its composure up to about -6dB, and then vent noise started to become audible. Still, it reached around -3dB from the peak input, or basically 80W, before the response at lower frequencies changed noticeably (according to the measured response), As this subwoofer is not going to see anything more than 25W in actual use (it's going to be powered by the Logitech X530's amplifier, after all), it looks like I'm never going to hear that vent noise - a good thing because this subwoofer is going to be located about three feet away from me, right behind my desk. Peak THD is also pretty low at 5.22% at about 20W input, most of which is 2nd order, so a lot less noticeable and objectionable. This suggests that the subwoofer is going to sound "clean" in actual use, and I can confirm that it does.

Conclusion
After messing around with tapped horns for awhile, it was nice to return to a relatively simple alignment, and this little subwoofer based around an 8" driver delivers all that I need for my multimedia system. If I was building this for another purpose, I might likely opt to flare the port's exit a bit, to reduce vent noise and compression effects at high volumes even further.
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Last edited by Brian Steele; 28th August 2017 at 11:07 AM.
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Old 28th August 2017, 04:20 AM   #2
GM is offline GM  United States
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Old 28th August 2017, 04:41 AM   #3
freddi is offline freddi  United States
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The Boom Unit...
Way CooL !
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Old 10th September 2017, 11:31 PM   #4
jay53 is offline jay53  India
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Default MLTL Worksheet

Hi Brian,

I am trying to understand S2. Can you pl. show it on the sketch ? Is it possible to get the Worksheet that you have used ?

Thanks !

Jayant
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Old 11th September 2017, 01:06 AM   #5
Brian Steele is offline Brian Steele  Grenada
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Quote:
Originally Posted by jay53 View Post
I am trying to understand S2. Can you pl. show it on the sketch ? Is it possible to get the Worksheet that you have used ?
S2 is where the driver is located. As it's around a bend, some guesstimation is done in the spreadsheet to "derive" S2 when you tell it how far down the baffle the driver is located.

The spreadsheet is available in the thread that I use to cover all of those spreadsheets for describing folded horns - Spreadsheet for Folded Horn Layouts...
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Old 11th September 2017, 01:31 AM   #6
CharlieLaub is offline CharlieLaub  United States
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It's just a ported box, no?
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Old 11th September 2017, 01:36 AM   #7
Brian Steele is offline Brian Steele  Grenada
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Quote:
Originally Posted by CharlieLaub View Post
It's just a ported box, no?
Functionally, yes, just modeled as an MLTL. My intent here was to smooth out the 100Hz~200Hz region, which can (according to the sim) be affected by the location chosen for the driver on the baffle due to the size of the vent. I expect that the internal panel does load the vent a bit, but this is taken into consideration in the model.
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Old 11th September 2017, 10:59 AM   #8
USRFobiwan is offline USRFobiwan
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The Boom Unit...
Can i downscale this to be used in a dual TB W5-1138SM design ?
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Old 11th September 2017, 11:33 AM   #9
Brian Steele is offline Brian Steele  Grenada
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Quote:
Originally Posted by USRFobiwan View Post
Can i downscale this to be used in a dual TB W5-1138SM design ?
I don't see why not. You can adjust the box dimensions in the spreadsheet until you get something that's interesting, then export the results to Hornresp for analysis.
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Old 11th September 2017, 12:53 PM   #10
LORDSANSUI is offline LORDSANSUI  Brazil
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Nice job. grats.

I'm still messing around with tapped horns
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