Newbie bought a cheapo 15" sub, help me with the box - diyAudio
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Old 13th July 2016, 01:57 AM   #1
goa604 is offline goa604  Croatia
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Default Newbie bought a cheapo 15" sub, help me with the box

Hello folks, been registered here for a while but now need some help.
Just bought a cheap Magnat Xpress 15. I know its not the best sub there is but this IS diyaudio and i like doing things on the low budget side so i think this is the best place on the internet to seek help.
Anyway i would like if you guys could help and guide me with box design and parameters. Here are the screenshots of subwoofers specifications to make it easier for you guys.

Click the image to open in full size.

I like music with really low bass sometimes hitting 30hz and lower so whatever you guys think would be the smartest thing to do please let me know.
Id like to mention that i have a box that is 146x56x31 cm (57.5x22x12.1 inches) measured from inside so that gives about 253 litre of volume, made from good quality wood and im ready to repurpose it whole just for this enclosure. It would be preferred if i could leave the box as is and just port it or put a horn or something.
I would really like it to go as deep as possible and im not expecting miracles from that sub but it would be nice to hear a boost in that part of the bass feels nice instead of that cheap crap that only rumbles in your ears.

Anyways guys, be it closed, ported, horned or whatever things exist that i havent even heard of please give me ideas, links, plans, opinions or whatever else, im all ears and im very excited to read your posts tomorrow!

Im probably in a different time zone than a huge majority of you guys and going to sleep so i cant wait to see your responses in 15ish hours.

Thank you guys!

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Old 13th July 2016, 02:18 AM   #2
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Well it turns out sealed 250 litre has Q=0.707. F10 <20 Hz.

If you heavily stuff it that will further push the Q down so that you have a better match to room gain.

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Old 13th July 2016, 02:22 AM   #3
goa604 is offline goa604  Croatia
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Planet10 Thank you for the quick response!
Could you perhaps point me towards wiki of sorts so i can understand what you guys actually mean when you answer? hah sorry!
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Old 13th July 2016, 03:07 AM   #4
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This is not a very good driver, it has limited usefulness.

Simulated here in both ported and sealed 250 liter boxes. Hornresp says qtc and f10 are both a bit higher than predicted by P10 in the previous post. I had to guess at Sd and Le. And I'm not sure if the xmax is rated for one way or peak to peak, so I've assumed one way which is the best case scenario.

Shown at 125 watts (which is enough to drive the sealed box several mm past xmax, while the ported box stays within excursion limits as long as a 16 hz 4th order high pass filter is used).

The sealed response is not bad and will blend ok with room gain but it's not going to be loud at all even at it's limits. The ported box will be pretty boomy in room unless you eq since the room gain will boost the already boosted lows. It would require a MUCH larger box with lower tuning than shown to get a better response with a ported box, but for a cheap driver like this a box like that is a waste of money and space. If you had 20 or 40 of them and used them in infinite baffle that would be another story ... that could work really well.

This kind of driver is meant for car audio infinite baffle, or seriously boosted response in smaller boxes. There's a 100 liter ported box recommendation in the manual for the driver, as you can see it's got a seriously boosted low end that doesn't go much lower than 30 hz.

Here's the manual - http://www.magnat.de/media/image/art..._15_Manual.pdf

Here's the sim -

Click the image to open in full size.
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Old 13th July 2016, 03:11 AM   #5
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Quote:
Originally Posted by goa604 View Post
Planet10 Thank you for the quick response!
Could you perhaps point me towards wiki of sorts so i can understand what you guys actually mean when you answer? hah sorry!
If you don't understand his response your best bet is probably to put the driver in the 250 liter box you have without a port and try it out - also put lots of fiberglass or polyfill or pillows in the box.

If it doesn't have enough low bass for you, you can try to add a port. The port shown in the sim above is 100 sq cm and 25.1 cm long. This will give fairly high port velocity, so if you are not using a high pass filter you might want a bigger port.
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Old 14th July 2016, 12:26 AM   #6
Ron E is offline Ron E  United States
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Nothing wrong with the big box response. If I had my druthers, I'd put it in a ~80 liter sealed box or the smallest cube you can fit a driver on the face. Maybe experiment with stuffed and unstuffed (or even better a parametric EQ or pole shifter). F3 will be in high 30's.
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Last edited by Ron E; 14th July 2016 at 12:31 AM.
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Old 14th July 2016, 03:31 AM   #7
goa604 is offline goa604  Croatia
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I listen to music mostly on my headphones connected to a usb DAC and AMP, with that setup i can easily hear 24 Hz sine but the speaker system im using isnt even audible under 50. Those have really nice mids and tweeters do great job as well but i just have to literally shake some things up in order to properly enjoy it.
I apreciate every each one of your responses guys, id just like it a lot if i could learn what i "Q=0.707", "F10 <20 Hz." or "P10" actually meant.

Thank you just a guy for the simulation, I did see their box plans in the PDF but i figured you guys would know
what to do to get it to play lower notes than that crappy 50hz car boxes i assumed they put there.
I want to mention that im not that scared of going a bit over Xmax.
One more question, would it be okay to mount the woofer facing forwards, roughly at the height of my head sitting in the chair?

Click the image to open in full size.

Kinda like this.

Still not sure if its smarter to keep it sealed and be quiet or port it low but play with equalizer and crossover to kill it slowly over 50hz?

Thanks a bunch guys.
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Old 14th July 2016, 04:03 AM   #8
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Q = .707 - this means the calculated sealed box qtc with that driver in that box. You can study up on sealed box qtc if you like but here's the quick breakdown. .7 qtc gives you the flattest response to the lowest frequency that is possible with a given driver. .5 qtc is usually called "critically damped" and will usually blend nicely with room gain to give a flat(ish) in room response. Qtc higher than .7 will give a humped response above fs.

Here's a quick graph I made a few years ago. It shows 4 theoretical drivers that are all the same except they have different values of qts. They are simulated in infinite baffle, so driver qts = sealed box qtc. As you can see, different values of qtc give wildly different response shapes. Most people like to shoot for .5 - .7 qtc when designing a sealed box, sometimes that isn't possible since the box has to be too large, and you can't ever have qtc less than driver qts.

Click the image to open in full size.

F10 < 20 hz - this means the frequency at which response is down 10 db relative to the mid frequencies is under 20 hz. Check the sim I posted. For the sealed box in my sim the F10 frequency is about 17 hz (I was mistaken when I said f10 was a bit higher than P10 claimed, I should have looked a bit closer).

P10 = Planet 10 = the author of post #2.

If you put this driver in a sealed box of any size it won't go very loud and it won't play low notes with any authority at all. If you put it in a ported box you might need a bit of eq to bring down the low frequencies or it might sound boomy (or you might like the low frequency boom, a lot of people do like it). This is a cheap driver and it won't perform very well regardless of what you do with it. As I mentioned, the best bet would be 20 or 40 of them in infinite baffle.
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Old 14th July 2016, 04:44 AM   #9
goa604 is offline goa604  Croatia
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just a guy, got it! Looks like i have nothing to lose to try it closed first to see how i like it, if i do not like it if i understood correctly i could try building a port in it of 100cm2 area that is 25.1cm long. (hope dimensions dont have to be THAT precise) It might be worth the mention that the total RMS of the rest of my system is 100W total and from the specs i looked at they actually go 48Hz at -16db. Unusable for music with low notes so ill be optimistic with this build.
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Old 14th July 2016, 05:23 AM   #10
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That is the ABSOLUTE MINIMUM port size I'd consider, and even then I'd only use it if I had a proper high pass filter included and roundovers on both port ends, otherwise velocity would be way too high. You might want to consider a larger port (which would have to be longer to maintain the same tuning) and you might even want to consider tuning lower (which would require a longer port). If you tune higher you'll get a bump in response above tuning like the design in the driver manual. But it's up to you. Try it sealed and if you want more low bass put a port in it.
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