SEAS 10" semi-DIY sealed sub project

[Guest scumbag]

I purchased a SEAs L26ROY subwoofer on SNA recently. http://www.seas.no/index.php?option=com_content&view=article&id=400:d1001-04-l26roy&catid=94&Itemid=501

It's a brilliant sub. It has a stiff aluminium cone and a really good suspension and magnet system. It has a long stroke (XMax) but most importantly it has a very well designed magnet / voice coil arrangement so the amount of magnetic flux is maintained over a long part of the movement of the voice coil - this document gives some idea - http://www.seas.no/images/stories/design/pdf_datasheet/L26ROY_Application_Note.pdf

 

I bought a Passive Radiator that matches it at the same time. To be honest, I've never done a PR sub before and I didn't feel all that keen to do one now. Call me a chicken. I like sealed subs. Of course there are compromises in bass extension for a given enclosure size but I really like the advantages of a more gradual roll-off, no port noise and the fact that the air acts as a spring to reduce voice coil movement as the frequency drops. 

 

There were a few design goals here. The sub had to have some WAF. In my house that means, not too big. So no hulking wheelie bins for me. I wanted a nice tight enclosure and got some prices to make up a box. CNC work in Brisbane is a rip-off. After searching I came across this enclosure which was pretty much perfect for the SEAs woofer - https://www.parts-express.com/denovo-audio-knock-down-mdf-10-cu-ft-subwoofer-cabinet-for-dayton-audio-10-reference-seri--300-7076 

 

Yes, I know, it's 1 cubic foot which is about 30litres. That's a nice small size but you're thinking, damn, there's not going to be a lot of bass extension out of that. Sure enough, my simulations don't look super promising:

F3=55hz. Hardly sub-bass. I do prefer to look at the F6 of drivers when I evaluate their lower frequency cut off but that's about 37hz so even then it's not getting more than my 3 way main speakers. The good news is that at full volume, the woofer still has lots of excursion left in reserve. Me like.

 

But I have a secret weapon. Well 2 actually. Number 1 is room gain - everyone has it. Some people are aware of it, some not. In my experience, room gain works best with sealed boxes - the natural lift of being near room boundaries seems to blend in well with the 12dB/octave roll off from the sealed box. Well that's my story and I'm sticking to it.

 

2nd secret weapon is this baby - https://www.daytonaudio.com/index.php/spa250dsp-250w-subwoofer-plate-amplifier-with-dsp.html

It's got parametric EQ. And a few other tricks. 250watts RMS, Class D and you can apply up to 6dB+ and -12dB with variable "Q" so you can make the peak / dip gentle (over a few octaves) or very sharp (to deal with sudden dips and peaks). Adjustable high and low crossover points, subsonic filter (say no to that!) and even limiter setting (OK, if you were using this in a commercial environment or at a party that might be good feature). Not cheap but nice.

 

So I put together the subwoofer enclosure. I did get a bit help here to add a small sub-enclosure to house the plate amp. It's only small so it's not cutting into the main enclosure too much. The amount of pressure that will build up in a sealed sub box is going to be enormous and that thin piece of metal on the back of the amplifier will flex. Or at the very least all the internal components are going to get a constant massage. I'd rather not risk pulverizing the amp'.

I didn't take pictures of the build. I mean, it's a flat pack that you glue together with clamps. That's not terribly exciting.

 

I did spend a bit of money and time on the finish. I sanded the hell out of it and used some good automotive filler, then sanded that. Then I used a good primer, sanded that and then applied a metallic paint. I got Supacheap to match the colour of my main speakers - it's a Range Rover colour called Carpathian Grey. I'll take a picture if anyone's interested. I then applied some clear coat. To be honest, the colour is a match but the metallic chip is nowhere near as obvious as it is on my main speakers - they were professionally painted so you'd expect them to look better. The clear coat that I used from Supacheap was not all that good either. I should have stumped up for the really expensive one that had a mirror finish. By the time I'd finished prepping and painting I was a bit over it. It is what it is.

 

Anyway, the fun bit. I fired up the sub and let it run in for a day. I took a near-field measurement of the "raw"output of the box without any lowpass filter, EQ etc.

Since this is near-field, ignore the higher frequencies roll off.

Pretty ragged. But at least the F6 is someway close to the projected one - it is in fact about 31hz which is a bit lower than the simulations suggest. Bonus!

 

Anyway I'll cut a long story short. I applied a low pass filter of 60hz. Then I zoomed in to get a good idea of what was happening in the lower frequencies. I spotted peaks at 41hz and 60hz. So I applied a fairly sharp (Low "Q") dip to flatten those out. Then I applied a bit of boost at 20hz to prop it up. This is the result overlaid on the raw response:

 

Now this is looking good. F6 is 21hz or thereabouts. That's actually subwoofer territory now.

 

So that's all fine and well but I did some listening and it didn't sound right. Boomy and uneven. So I guess the lesson here is, nearfield measurements are sorta useful but you need to test what the speaker is actually doing in the room and for that we need far-field measurements. REW (the package I used to make these measurements) is great for measuring real in-room response.

So I put my measuring mic' at my exact listening location and fired her up. I had the exact same setup as the perfect blue trace above, dips at 41hz, 60hz, boost at 20hz though there was no low pass filter (from memory).

This is what I got:

 

Damn. What went wrong?? Well all my super flat near-field trickery looks pretty crap in room. But it also confirmed what I was hearing at a subjective levell.

There is a nasty peak at 41hz. Even with the EQ lowering it by about 3dB at that frequency, it's still right up there. That would be a room mode.  The peaks at 60hz and 100hz were also making things very uneven. There was an obvious boominess in my room and that 42hz peak was the main culprit.

 

So again, I adjusted the EQ and took a series of measurements. I added the 60hz low pass again but even with that in place the 60hz and 100hz peaks still bothered me. Everything higher up is being taken care of by the lowpass filter. So after fiddling and measuring I ended up using:

-4db Q4 at 90hz
-3db Q6 at 60hz
-10db Q6 at 42hz

 

Due to the gain from the room I was able to remove the boost at 20hz. That's a bonus. Less amp' power and less woofer excursion - everything is less strained and will last longer  . So now we have (blue trace):

 

The final trace seems to suggest this thing is almost flat to 20hz! There is a steep roll off at 45hz - 60hz though. The next step is to do far-field measurements with my main speakers in play. Then I can totally tweak the settings to get the crossover region perfectly blended and to set the gain on the sub to properly match the mains. That will be next week I think. I'm sure there will need to be more adjustment when I put it all together and measure it properly.

 

At this point in time I'm getting low, tight bass. When I dare turn it up I'm getting pulverising pressure waves. The concrete slab that forms the floor of my listening room is vibrating (actually everything is vibrating) and it makes your knees feel like jelly.

I like that.

 

 

 

[Wozza_Lee]

Not bad for a 10", any pics of the actual sub

[Guest scumbag]

Just now, Wozza_Lee said:

Not bad for a 10", any pics of the actual sub

I'll take some tomorrow. It's basically a dark charcoal cube with a silver driver on front though. Not much to look at.....

[jamiebosco]

Great work. looking forward to seeing a picture of the sub as well

Any chance of you uploading a graph showing a bit more detail of the FR?  using the 10hz to 200hz range (click the tab at the bottom right of screen) , and click the "Limits" tab (top right of screen) to change the vertical axis (dB)  to something like 30dB to 80dB to get 5dB increments?

[Guest scumbag]

Since you asked nicely, and I am susceptible to Jedi mind tricks, here you go:

 

[jamiebosco]

haha.cheers. looking good

 

on a sidenote I like the look of REW with the Dark theme. I might start using that!

[Guest scumbag]

3 minutes ago, jamiebosco said:

haha.cheers. looking good

 

on a sidenote I like the look of REW with the Dark theme. I might start using that!

No, don't go to the Dark Side.

[jamiebosco]

11 minutes ago, scumbag said:

No, don't go to the Dark Side.

..oh man I walked straight into that one haha

 

 

[Guest scumbag]

Here you go. Apologies for the quality. It isn't a brilliant paint finish but the photos make it look horrible. It's stuck under a desk so the light is bad and my phone makes dark images look really grainy.

You'll note I put the amp in upside down so that when I lean over it from the front I can read all the text.

I put 4 big rubber "feet" on the bottom that I picked up from Bunnings. They are actually heavy duty door bumps. It's about 360X360X360mm all up. The premade opening on the front has a gap of about 3mm around the outside of the woofer. It is 18mm all round except for the front which is 2 sheets of 18mm. There is some really good internal bracing too. I added a moderate amount of acoustic foam sheet to the inside to reduce reflections off the back panel. I rolled up some lengths and pushed them into the gaps around the bracing.

 

 

 

 

 

 

Edited September 14, 2018 by scumbag

[Ando]

Nice project. I saw those drivers for sale a while ago and thought what a good buy they were.

I agree with Scumbag these drivers are the basis for building an excellent sub. The flatpack cabinets look good too. Hows the shipping cost to Australia ?

Here is a picture ( really nothing to see ! ) of one of my two almost identical ( same driver, same size box ) subs built several years ago but still unfinished. Mine were  driven via mini DSP and Crown XLS 2000 amplifier .

Cheers Mike

 

[Guest scumbag]

1 minute ago, Ando said:

Nice project. I saw those drivers for sale a while ago and thought what a good buy they were.

I agree with Scumbag these drivers are the basis for building an excellent sub. The flatpack cabinets look good too. Hows the shipping cost to Australia ?

Here is a picture ( really nothing to see ! ) of one of my two almost identical ( same driver, same size box ) subs built several years ago but still unfinished. Mine were  driven via mini DSP and Crown XLS 2000 amplifier .

Cheers Mike

 

Very similar indeed. Do you still use the sub?

I got the box here - https://www.ebay.com.au/itm/Knock-Down-MDF-1-25-ftÂ-Subwoofer-Cabinet-for-D/322788102879?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

Not super cheap I guess but it was a simple solution for me as I have zero wood working skills.

 

[Ando]

I havent used the subs in years. I built them to use with my Music and Design NaONotes. When I sold the Notes the miniDSP went with them.  For a few reasons I had pretty much lost interest in DIY audio at that stage so havent  bought a new miniDSP or anything else since. I have had a few thoughts about reviving them lately as I know they work well and would definitely add enjoyment . 

I think the boxes you bought actually are good value even with the shipping . I think people underestimate the time and effort needed to make even a basic box  to reasonable standard. I hide my basic wood woking behind veneer which covers a multitude of sins.

Cheers Mike

Edited September 14, 2018 by Ando

[davewantsmoore]

16 hours ago, scumbag said:

The final trace seems to suggest this thing is almost flat to 20hz! There is a steep roll off at 45hz - 60hz though.

Hello.

I'm a pretty big fan of the whole ~10" subwoofer in a sealed box affair ... as long as they're not pushed too hard.    They have high WAF and ease of positioning and neither of these things should be underestimated.    

 

 

The approach you have used has allowed the "room response" to become part of "designing the filters for the woofer" .... which has caused the "steep roll off at 45hz - 60hz" you noted above.   It can help a lot to keep those issues very seperate - and if nothing else will give you a clearer/confident idea of what is caused by the woofer and what is caused by the room.

 

When you got the "boomy and horrible" sound from your near field filtered woofer .... it was time to move the woofer around to find the best position(as opposed to immediately change the filters).   I'm not sure if you did that or not.

[Guest scumbag]

Dave,

 

In my case, the location of my subwoofer is not flexible so I need to work with what I have. So adjusting the EQ was the only option I had. I am confident that I can adjust the steep roll off when I combine the sub with the mains next week,

 

Mark

[Guest scumbag]

Some more measurements from today. I moved the sub slightly based on some simulations in REW.

Then I measured with just a 80hz low pass as below:

 

I fudged around for a bit and eventually came up with the green trace:

 

 

That looks a lot smoother but the real test will be to see how well in it conforms to a 2nd order roll off:

 

Pretty close. I ran out of EQ on some of the bumps i.e. I was right the way down to -12dB on some of them. It's only 5dB down at 20hz with NO boost at low frequencies which is good.

One thing I have noticed though is all the minus EQ is lowering the overall output of the sub (surprise) so the gain is down which means it might struggle to match the output levels of my main speakers. Bummer. So there may have to be a further round of EQ on this thing.