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Sealed Subwoofer Design & Build
Sealed Subwoofer Design & Build
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Old 5th November 2017, 10:01 PM   #1
undefinedza is offline undefinedza  Europe
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Default Sealed Subwoofer Design & Build

Hi all,

Always been passionate about audio and DIY, but living in London has limited my options.

I have a selection of tools for electronics work, but very little space or tools to do woodworking and am without a workshop.

I currently run a DIY dual mono DUG TPA3116 integrated amplifier (LM4562 preamp stage) with a pair of Royd Sintra II speakers.

As part of a wider project, I'm looking to build a few components to upgrade the system, including a new amplifier, preamplifier and subwoofer.

This thread is for cataloging the design and build process of the subwoofer. My aim is a pragmatic balance between high quality sound and cost. I expect this build will take some months as I don't have lots of time to commit to the project.

Here's the plan so far:

50L internal volume sealed MDF enclosure with a 12" driver and DSP-capable plate amplifier. WinISD says this results in a Qtc of 0.732. With the MiniDSP, there's also the potential to apply parametric EQ to obtain a flatter response lower.

Amplifier

MiniDSP PWR-ICE125 in bridge mode to deliver 450w peak into 4 ohms
DSP capability allows for parametric EQ and room equalisation

Plate Amplifiers : PWR-ICE125

Click the image to open in full size.

Driver

Scan-Speak Discovery 30W/4558T00
12" with 17Hz fs and +/-12.5mm of linear excursion

http://www.scan-speak.dk/datasheet/pdf/30w-4558t00.pdf

Click the image to open in full size.

The thin blue line is the modelled response of the driver, untouched. The thick blue line is with 3.2db of gain applied at a centre frequency of 35Hz and a Q of 2.

Screen Shot 2017-11-05 at 22.47.22.png

Without the space to build the enclosure, I'll need to have this made. I've made early contact with some furniture manufacturers to see if any will take on the work. If anyone knows someone who could help with this and a decent finish (veneer would be ideal, but otherwise spray paint).

Any feedback at this point would be appreciated. Does anyone think I should seriously consider another driver?

Best,
Chris
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Old 5th November 2017, 11:53 PM   #2
DiscoJones is offline DiscoJones  United States
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Have you modeled the excursion at high SPL yet? Adding boost to a sealed system requires a fair amount of excursion and 12.5mm is only about average with today's higher end subs. It would be good to see what kind of SPL you can achieve with that combo and see if it will meet your needs.

Dan
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Old 6th November 2017, 10:04 AM   #3
chris661 is offline chris661  United Kingdom
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Hi Chris,

Might be worth talking to Colin (Toppsy) here - MY own OB speaker project - audio-talk
He does commision builds on occasion. Did a pair of speakers for me a little while ago, and they came out great. They won't be particularly cheap, but he uses good wood, gets it CNC cut to spec, and knows all the tricks for veneers.

Depending on how loud you want to go, you can probably push a lot lower than 35Hz. Remember this, though - the mechanical limits are always in place. That driver in a sealed box will do 96.3dB at 20Hz (groundplane, 1m), and that's that. You can either apply EQ so that the entire range will be flat down to 20Hz with the knowledge that if you're listening louder than 96dB, if the music has any 20Hz content the sub will make some bad noises. You could aim for 30Hz instead, in which case it'll do 103.3dB.

The maximum SPL of a sealed sub is a function of cone area, excursion, and frequency. If you use a really big box, the low-end efficiency improves (Qtc goes down), meaning it takes less power to move the cone. A tiny box might need 1000w to hit the excursion limits, while an infinite baffle arrangement might need 100w. They will both produce the same mechanically-limited SPL.

When it comes to choosing the cabinet size, don't worry too much about Qtc. Ensure that you can use all of the mechanical travel of the driver with the power you have. That way, you're getting everything you can from the driver while using the smallest box you can.

I expect a single sealed 12" to be plenty for a UK living room, unless you're a home-theatre nutter and want to feel your body move at 5Hz. Check out AVS for that sort of thing.

FWIW, I'd seriously consider investing in some kind of measurement setup, and learn how to use it. Best audio-related money I ever spent - rooms do some nasty things to carefully-tuned LF alignments. One I worked in for a while had a 10dB spike at 40Hz. Made small speakers sound big, and big speakers sound ugly.

Chris
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Old 6th November 2017, 09:57 PM   #4
Djim is offline Djim  Netherlands
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Hi undefinedza,

The Scan-Speak 30W/4558T00 is a very nice driver that also does a great job for music. I wouldn't worry too much about excursion, just keep an eye on the rms power. Like Klippel explained/showed in his paper: Nonlinear Modeling of the Heat Transfer in Loudspeakers

Click the image to open in full size.


Regards,
Djim
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Old 7th November 2017, 02:27 PM   #5
undefinedza is offline undefinedza  Europe
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Join Date: Sep 2011
Thank you all for your feedback!

@DiscoJones

I don't intend to go extremely low or loud with this subwoofer build, and I'm looking for a musically capable subwoofer more than anything else.

Excursion graph attached (using the params in my original post, 50L sealed + EQ). The driver's Xlim is 28mm, so there's some room before damage.

Screen Shot 2017-11-07 at 15.15.43.png

This is at 100w input power. SPL is 106db at 40Hz and 103db at 30Hz.

@chris661

You're spot on, as you can see 30Hz (f3) at 103dB at max excursion. The amplifier is specified to do 450w mono BTL, but I'm sure this is rated with a music crest factor. I think the amp and woofer are probably well matched power wise, in terms of the excursion limits.

Thanks for the tip on the measurement setup. Do you have any opinion of the MiniDSP UMIK-1, and more generally, their DRC tech?

I'll reach out to Toppsy.

@Djim

Thanks for this graph. The driver has an RMS Pe of 150w, which is a bit higher than the 100w that brings it to max excursion in the WinISD model, so hopefully the reality is there's a bit of power and excursion headroom.

Keep it coming!

Best,
Chris
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Old 7th November 2017, 09:50 PM   #6
chris661 is offline chris661  United Kingdom
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No opinion on the MiniDSP UMIK or the rest of their products. The word on the grapevine is that it's all pretty solid stuff, but I haven't used them myself.

I use a Beyerdynamic MM1 for measurements - cheapest I could find that comes with an individual calibration curve.

Chris
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Old 7th November 2017, 10:27 PM   #7
MemX is offline MemX  United Kingdom
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Quote:
Originally Posted by Djim View Post
Hi undefinedza,

The Scan-Speak 30W/4558T00 is a very nice driver that also does a great job for music. I wouldn't worry too much about excursion, just keep an eye on the rms power. Like Klippel explained/showed in his paper: Nonlinear Modeling of the Heat Transfer in Loudspeakers

Click the image to open in full size.


Regards,
Djim
Hi Djim,

I'm afraid this post completely confuses my poor old brain!


I was aware that small voltage modelling is not completely accurate, but is this graph and text suggesting that all modelling software is junk below, say, 50-60Hz?

Or that we will need to plan into our system the need to supply X times the modelled power/voltage to achieve anywhere near the modelled/desired response curve?

Or that we need to build a subwoofer system twice as large as the modelling would suggest for our desired output??


Was that graph derived from just one driver being tested?

Surely the graph would be markedly different for different drivers with different materials and/or construction?

Is it likely that the soft-suspension drivers with low Fs, designed specifically for home theatre use, will not suffer the issues illustrated in the graph?

Was the 12.5mm linear excursion derived from the manufacturer's stated T/S parameters (therefore potentially 'optimistic') or actual measured parameters (therefore realistic)? If the former, would using a graph of the latter result in a better match between the lines?

I just find it hard to believe that all drivers would (under-)perform the same!
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Old 8th November 2017, 04:00 AM   #8
hollowboy is offline hollowboy  Australia
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Quote:
Originally Posted by MemX View Post
I just find it hard to believe that all drivers would (under-)perform the same!
Maybe see the measurements on the Data Bass website.

For example, this 12" system was measured as having about 7dB LF loss when comparing high and low power - but the output doesn't begin to collapse until <30Hz.

Data-Bass

This 12" system doesn't have any LF loss when comparing high and low power.

Data-Bass

Quote:
Originally Posted by chris661 View Post
No opinion on the MiniDSP UMIK or the rest of their products. The word on the grapevine is that it's all pretty solid stuff, but I haven't used them myself.
I have one and it was easy to set up, works well. I've dropped it a couple of times without harming it.
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Old 11th November 2017, 04:58 PM   #9
PJ303 is offline PJ303  United Kingdom
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Hi Undefinedza,

I'm similarly in London and looking to do a sealed sub design, though my design criteria are probably a bit more "getto" - I'm looking to do something cheaper, maybe hacking some car-audio sub and box (placing a single sub in a double sub box but sealing the other opening - so lowering the response Q).

I checked you box/speaker parameters in this online calculator, I'm not sure on accuracy but it suggests 0.711 system Q.

The only thing that jumped out at me from your proposal is that peaking filter with a Q of 2 - is that in the MiniDSP? Given the high Q and low frequency that's going to possibly "ring" on for some time. Does the software allow you to view the time / step response and phase response - possibly you could be undoing all the benefits of the sealed design.

If it gives you the option it might be better to use a low-shelf filter topology, with a 2-pole low-shelf, setting the gain to +6dB and cutoff to 40 Hz (your -3dB point, assuming the filter is specified at the mid-point of the shelf slope) then the amplitude response would effectively be like shifting the light blue line in your graph to the left, so you still extend the 'flat' range but the response at 10Hz would be -18 not -24dB, and the phase / step response should be smoother.

Last edited by PJ303; 11th November 2017 at 05:26 PM.
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Old 11th November 2017, 05:53 PM   #10
bentoronto is offline bentoronto  Canada
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Sealed Subwoofer Design &amp; Build
Sad to take a driver with a wonderfully low resonance (ummm, somewhat different on the spec versus the impedance curve) and stick it in a box barely bigger than itself (1.77 cu feet)... with the result that it poops out below 35 Hz. Weird: where do these recommendations for tiny boxes come from? No room for bigger box?

With modern lumber yards cutting panels to your specification and with good glues that expand to fill your errors, not hard to make boxes. Or to buy something from Ikea roughly to your size. Even if you need to buy a saw or two (and sell on eBay later), you are still on budget.

BTW, for music to sound good, you want an upward trending boost to your bass, not a flat response. Ask anybody.

B.
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Last edited by bentoronto; 11th November 2017 at 05:56 PM.
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