New speaker (concrete sub) project

edited March 7 in DIY
This is to go with my current main DIY speakers.
My intention was to add a dedicated bass speaker to each of the stereo speakers, crossing over somewhere below 100Hz. I had a look at cross over value options and the simplest I could work out crossed over at 75Hz. I bought the necessary capacitors and inductors for each.
But I'm so happy with the DIY Monacors as they are I don't think that an additional bass section for each is necessary, or desirable, in my lounge.
However, as I say, I'd already bought some of the components for the crossover. And who doesn't want the option of occasional extra bass...?

So I have decided to have a complete experiment and build a single subwoofer.

I chose a driver with a double voice coil so that it can be fed directly (though via suitable crossovers) by both channels of my One4 amp.
Also, as the pricey caps and inductors I already had were chosen on the basis of my using 8 ohm drivers, my options were a little limited (most dual voice coil subs are 4 ohms per coil). To the rescue Dayton Audio's SD315A-88 12 inch DVC subwoofer.


I'm currently awaiting its delivery from the US.

In the meantime on with the crossovers.
Check out those enormous inductors! (Ignore the shabby MDF cases. Shameful.)



The red and black cables will feed the sub. The other sets will go to the inputs of the 2 stereo Monacor speakers.

Also, have got busy with the cabinet.
Now I'm hoping this will be interesting. Possibly laughable.

There was a thread on here recently about using a sandwich approach to cabinet walls. Two 9mm sheets of ply separated by a layer of some sort of squishy glue. I wanted to try that using the workshop available at art college. But, due to the necessary covid restrictions there I have deferred my 2nd year. I don't have the tools at home for complicated carpentry.
I am able to build a basic (but carefully calculated) cab using MDF. Which I have done.
But I liked the idea of trying sandwich cabinet walls.
And, I was happy with the effects of painting the inside walls of my DIY Monacor cabs with bitumen paint.
So a new idea has been hatched...
I am pouring a 2mm depth of bitumen paint on to the inside walls of the cabinet as the sandwich filling. Easy enough. (I knew that old turntable spirit level would come in handy!) It is taking ages to dry. But I'm in no rush.
I'll need to fix the front baffle before adding the bitumen to any of the other inner surfaces.



Of course, once dry I am going to need to add the second, inner piece of bread of the sandwich.
Not ply or MDF though. Oh no.
Instead, I am going to pour in 3mm of floor self-levelling compound over the bitumen to act as the inner bread.
I've used floor leveller before in its more conventional application. It's e easy to use, very good at self-levelling, fast setting and extremely durable (down to 3mm depth). This may be quite a heavy piece!
«1

Comments

  • Bonkers! Or so it seems to the minimally DIY-inclined Dave  :)
    I'll be glued to developments (pun intended).
  • edited January 3
    May be slow progress, but obviously will delight in keeping you posted!
    I'm interested to see how the sensitivity of the sub driver will play out. Specs say it's a few dBs down on the Monacors. But the Moncaors are about 70cm off the floor whereas the Dayton sub driver will be only a few inches off the ground, so will be reinforced there.
    Only one way to find out!
  • Any thoughts of building in an adjustable crossover point for fine tuning and maybe another pot to adjust volume.

    I always found having the crossover point as low as possible and the the volume set to discrete i.e. less than the main, always helped with integration.

    I know it makes things more complicated but bound to give more desireable results.

    Orrrrrrrrrr, replace the dust cap with an SDS bit, whack up the amp and dig up the kitchen floor....
  • cj66 said:
    Any thoughts of building in an adjustable crossover point for fine tuning and maybe another pot to adjust volume.

    I always found having the crossover point as low as possible and the the volume set to discrete i.e. less than the main, always helped with integration.

    I know it makes things more complicated but bound to give more desireable results.

    Orrrrrrrrrr, replace the dust cap with an SDS bit, whack up the amp and dig up the kitchen floor....
    All good thoughts!
    May well try these things CJ!
    I will get the basic build done and see how it sounds.
    Should be easy enough to add and tweak things accordingly. :-)
  • edited January 4
    Hi Ben
    I have bought some of this kit to maybe try inside my speaker cabinets.
    https://www.deadening.co.uk/collections/dynamat - used mainly for car stereos 
    More expensive than your bitumen but not so nasty to use I suspect. 
    Still mulling over whether to apply it as it is apparently a bugger to get off and may actually overdampen the acoustics.

  • But Merv, why would I want to use something less messy...?!

    It's a tricky one if a tweak isn't easily reversed isn't it?
    Personally I have never introduced a brace or some dampening to any or my diy efforts that has made things sound worse. But maybe I just haven't gone far enough!
  • I'm always fascinated by speaker modding/building threads.
    It's the one area where I have barely DIY'd.
    My limit is a xover converted to biwire, another converted to biamp and my pinnacle was making completely new, higher specified, xovers for another.

    Bit of a black art for me, mucho respeto a todos que pueden!
  • cj66 said:
    I'm always fascinated by speaker modding/building threads.
    It's the one area where I have barely DIY'd.
    My limit is a xover converted to biwire, another converted to biamp and my pinnacle was making completely new, higher specified, xovers for another.
    Hardly meagre achievements, Sir!
  • edited January 12
    Update...
    Bitumen has dried on the first internal face to be coated. It took about a week to be stable enough.
    The driver has arrived from the States.
    But I'm still waiting on the oversized aeroport (10cm diameter, approx 30cm length) that I ordered from Germany at the end of last year.



    I've contacted the seller and have been informed that Brexit is causing significant delays to surface deliveries to the UK. Approx 2-3 weeks. Bloody Brexit...

    The wait has complicated things a bit as to pour the bitumen on to any of the remaining 5 internal faces I need the final panel to be glued and screwed into place to contain the bitumen, and I can't do that easily until I've fixed in the aeroport inside the cabinet.

    However, I'm impatient so have constructed a bit of a work-around to allow me to crack on with bitumening the next internal face. I decided to fit a small mdf lip (about 5x5mm) along the open edge of one of the internal faces. This acts as a dam (in-lieu of the currently missing panel) to hold back the bitumen.



    I could have made a better job of fixing the damn in place. As can be seen, there was some seepage, but nothing drastic.

    I'm hoping that by the weekend the aeroport will have arrived so that I can fit it before sealing the cabinet with the front panel and move on to bitumening the next internal face. (Obviously I'll still be able to access the inside of the cabinet via the hole for the driver.)

  • edited January 13
    The aeroport has finally made it through the Brexit backlog!
    Chose this one as it's big and has flaring at both ends.
    The wider the diameter the openings of a port the less prone it is to chuffing, but also the longer the port needs to be (hence the benefits of flared ends).
    To achieve the right frequency for the speaker - cabinet combo, this port will need to be 30.2cm. So I'll be trimming down the central tube to achieve that.



    I have made a bit of compromise in the design.
    I've read that ideally the port's opening should be the same distance away from all walls as the port's own diameter. For various reasons i decided on a placement that does not achieve that internally (the internal port opening will be <10cm away from the cab's top panel, and there's a brace <10cm below it), but I'm hoping it doesn't make an enormous distance. Fingers crossed!




  • You'll be doing this for money soon, Ben!
  • Dave, what wouldn't I...?
  • Ben fancy building a evil LF power amp to drive it msg me
  • Thanks Colin.
    Give me a month or so, and I'll give you a shout!
  • OMG! Evacuate Glawster!!!  :s  :s  :s
  • Just one face to go with the bituman. (They take about 10 days to dry).
    Then the floor leveller. That sets in a day.
    So 3-4 more weeks and we should be ready...
  • Don't mess about with that floor leveller stuff, just get it slapped together, connected and VOILA!

    HOUSE LEVELLER    >:)
  • Docfoster said:
    Just one face to go with the bituman. (They take about 10 days to dry).
    Then the floor leveller. That sets in a day.
    So 3-4 more weeks and we should be ready...

    Take your time, Ben ;)
  • cj66 said:
    Don't mess about with that floor leveller stuff, just get it slapped together, connected and VOILA!

    HOUSE LEVELLER    >:)
    I barely have permission for this project as it is.
    Your suggested outcome may tip the balance against me...
  • uglymusic said:
    Docfoster said:
    Just one face to go with the bituman. (They take about 10 days to dry).
    Then the floor leveller. That sets in a day.
    So 3-4 more weeks and we should be ready...

    Take your time, Ben ;)
    I'm surprised how patient I'm being!
  • What's happening?
  • I am waiting for the last section of bitumen to dry.
    It seems to be taking a lot longer than the other ones. As it is the same depth, I can only imagine that it is because as the cabinet is lying on its front (ie with the cut out for the driver facing downwards) there is less ventilation through the cabinet.
    Hopefully will be dry enough to start with the floor leveller over the weekend. That dries overnight so all 6 inner faces should be done in a week.
  • Amazing, Ben. Just amazing.
  • You're too easily please Dave! :-)

    I'm pleased to say that today (at last) the final face of bitumen had become more solid than liquid, so I was able to move on to the floor leveller.
    I started with the first face to which I applied the bitumen.
    Thought I'd document it (badly)...


  • The giant egg whisks are great for getting a boring job done fast and properly, mine is a drill attachment.

    Yesterday made up loads of water proof grout and ceiling plaster. Grout for a walk in shower that had leaked to the floor below and the plaster for the room below! Had to replace a 1.5 metre sq. panel of plasterboard style ceiling and make good.

    Remembered to cut the hole for a recessed light but forgot the 15" driver hole and port  ;)

    Can't wait to see your report on the finished sound....What amp will be driving the wee beastie?
  • edited March 1
    Good work sir.
    Bloody leaks.
    Water is the spring of life, but in a wrong and unwanted place it is such a pain in the arse!

    Good question about the amp.
    Honest answer is I’m not sure where that might end up.
    There are 3 plans that I will probably work through in order:
    A. Use the sub passively, from the SECA. I’ve built a pair of 75hz crossovers to split the signal between the sub and my main DIY monacor speakers. My fear is the subwoofer will be insufficiently sensitive relative to the main speakers. Possible I could correct this with L-pads if the difference is not too great.
    B. Use the subwoofer plate amp I have lying about in in my shed.
    C. Take up Colin’s kind offer of his sending me plans for his subwoofer amp design.

    I’m a pretty busy at the moment, so not sure how long all this will take.
    But Plan A should be completed by the weekend, so I should have something to report by then.

    BTW just completed floor levelling the 3rd internal face of the cab.
  • 5 of the 6 faces now done.
    And I have just enough of the binding posts sticking out! :-)
    A bit of bitumen to clean off that bottom left one. That bloomin' black stuff gets everywhere!  

  • Finished today.
    Early listening very promising.
    Fears of lack of sensitivity unwarranted. If anything the sub is slightly too sensitive (approx +1.5dB). Will play around with some L-pads tomorrow.
  • edited March 6
    OK, here it is...
    (Along with its accompanying grinning loon.)
     B) 


  • edited March 6

    As mentioned in last night’s post, if anything the sub has proved too sensitive.

    My initial thought was to stick an L-pad on it. But having measured its resistance this morning, it's 1ohm lower than the main speakers (5.9 v 6.9). This may mean that my calculated crossover point of 75Hz is higher for the sub than it is for the main speakers. So, as a first step I’ve simply stuck a 1 ohm resistor on both channels to the sub. These will be having the effect of both attenuating the signal and reducing the effective crossover frequency to the sub. Definitely sounds more in balance with these in place. Although a frequency sweep subjectively suggests the sub is still too loud. Oh what a shame.

    As you can see, to help it "blend in" (my wife's one condition) I covered it in cream felt and put a pine top on it so that it can be used as a side table. In fact, I'd be surprised if you can see it at all in these photos.




  • Hehey!

    The huge grin tells us everything we need to know.

    I was a little concerned that you only have one rear firing "bulb super tweeter" in a directional cowl  >:)
  • cj66 said:
    Hehey!

    The huge grin tells us everything we need to know.

    I was a little concerned that you only have one rear firing "bulb super tweeter" in a directional cowl  >:)
    I dialled up the Heath Robinson to 11.
  • edited March 7
    The 1 ohm in-series resistor improved things. But the deep bass was still a bit overblown; announcing that "There is sub woofer present".
    Increasing the resistance to a 1.5ohm integrated the deep bass further, but ironically seemed to thin out voices. Possibly that was a gap opening up around 75Hz as the crossover frequency to the sub dropped to low.
    Not an enormous issue. But enough for me to want to try to get it righter.
    So next step is to let an L-pad take on that extra 0.5ohm to maintain the impedance seen in the sub load by the sub crossover. I've ordered the appropriate resistors to create this. See how that goes. Will be a couple of weeks as the best and cheapest suppliers are, of course, in China.
  • edited March 7
    BTW the rigidity of the cabinet is very impressive. (Bloomin' heavy too. I weighed the finished cabinet on its own, and it was 30kg. So with the driver and the top on, it probably weighs about 35kg.)
    Even at high volumes the walls are not vibrating to the touch. And the various things sitting on the table-top screwed into the top of the sub are perfectly quiet. (The cabs of the main speakers are the same 25mm MDF with a couple of coats of bitumen painted on the inside, and I can feel those vibrating slightly, certainly more so than the sub, when music is playing.)
    Impossible to know how the sub sounds compared to a non-bitumened-and-concreted version (unless I build one), but it's certainly something I'll repeat in future builds to try to explore further. Time consuming, but not difficult or especially expensive.
  • edited March 9
    Once the grin wore off the realisation dawned that I don't really want to be running a dub sound system all the time, so maybe a more nuanced approach is required.
    Had a couple hours this morning to tweak the sub's level. I want the sub to provide bass weight, but I don't want it to be overblown, and I certainly don't want it interfering with overall clarity.

    Fortunately, as the crossovers are external, tweaking levels is a lot easier than normal (I don't need to remove the driver and fish around inside the cab every time I want to make an adjustment)!

    I've ended up with 2.5ohm in series housed on the crossovers.


    And 25ohm in parallel, simply attached via banana plugs to the binding posts.


    Together, this is probably achieving just over 3dB of attenuation, whilst keeping the overall resistance at a level that holds the crossover point at 75Hz.
    As I'm not sure exactly what the impedance of the sub is at the crossover point, this is part science but a lot of art and listening!
  • edited March 19
    I think I've got to a point I'm happy with things.
    L-pads tweaked within an inch of their life!

    I've remembered that the origins of this project were not a subwoofer at all, but a 3-way speaker.
    I don't really want my system to *sound* like it has a "subwoofer". I want the bass to be well blended and in balance. This means carefully judging the overall level of the bass driver and the crossover between the bass and the "mid" drivers. That's really what the L-pad tweaking has been all about. And I think I'm there now. The frequency response is as I want.

    Also, perhaps because the sub is running off the same amp and is in close proximity to the main speakers I seem to be avoiding the timing issues that I have struggled with when using subs.

    I have not yet compared the system with the sub/bass driver to the system without, but I think that the the sub is robbing some "air" and delicacy from the system. Maybe this is unavoidable given the introduction of additional cables and crossover components. But it will be interesting to hear the system with just the main Monacor speakers, after my becoming accustomed to the sound of the system with the bass driver in place. Which will I prefer overall...?
  • Amazing stuff, Ben.

    And now you're a YouTube star as well  :)
  • edited March 20
    Had some fun this morning doing some measurements.
    I never use measurements when I'm designing / tinkering. Always done by ear.
    But when I've finished, I enjoy doing them.

    But I only do them simply:
    1. Downloaded a free dB meter on to my iphone.
    2. Used an online frequency generator: https://www.szynalski.com/tone-generator/
    3. Recorded dB levels in 10Hz increments from 10 - 200Hz. One time with the sub crossover and sub connected, and one time with the amp feeding the main speakers directly.
    4. Recorded the dB levels in Excel.

    This time  some unexpected results.

    With the sub:


    Without the sub:

    Very similar!

    With a few differences...
    • First thing I notice is that with both sets of measurements there's a significant dip at 60Hz (more pronounced with the sub). So possibly the room, or my room treatments, is doing something there. It's possible my tinkering with the L-pad to the sub caused the greater dip with the sub. I.e. the sub output may start falling away before the main speakers start kicking in, and that my ears like a dip there (and so I liked the effect of the room treatment for similar reasons). Subjectively the dip may add extra clarity for me...?
    • With the sub, the levels were about 3dB higher at 20 and 30dB. I suppose some sort of lift there is what I was hoping for. Though it's quite remarkable that that just 3dB at 20 and 30 can make so much difference. (And take so much effort!).
    • With the sub, things are flatter from 70dB (in particular avoiding a dip at 100Hz without the sub). Not sure why. I suppose the sub remains somewhat, if decreasingly, audible above the crossover frequency.
    Obviously measurements are no substitute for listening, but I find them fun, and possibly interesting.


  • edited March 20
    Edit...

    Just had a thought re. investigating the 60Hz dip, using the sub...
    When tweaking the L-pads the final tweaks were made to the parallel resistance.
    Easy to adjust as I had the parallel resistor inserted into the binding posts of the sub:



    I narrowed things down to somewhere between 12 and 16ohm(*). 
    12ohm or below and voices sounded to thick for me (dip at 60Hz reducing...?). 16ohm and above to thin (dip at 60Hz increasing...?).
    I rigged up 13, 14 and 15ohm resistances to switch in and out.
    The ones pictured above are the ones I preferred in the end: 13ohms (made up of a 12ohm and a 1ohm resistor on each channel. (Yes! Each channel: It's double voice coil sub remember!)

    But I still have the 14 and 15ohm resistances...



    On the left, in orange is the 10+4ohm resistors and on the right, in green the straight 15ohm resistors.
    I think my suggested hypothesis is that if I increase the parallel resistance using these, the 60Hz dip will become more pronounced. And, if I don't have any resistance in parallel the resistance will be as much as it can be (without increasing the series resistor) so the 60Hz dip should be most pronounced.

    At some point I may try remeasuring to investigate this... 


    (*My rudimentary measurements, together with Dayton Audio's frequency curve for the sub driver suggest that its impedance at the lower end of its range is lower than its advertised "8ohm", possibly as low as 4ohms, so when combined in paralled with a resistance in the 12 - 16 ohm range the resulting impedance is about 3 ohms. I found, by listening, that I liked a 5 ohm resistor in series, which makes sense given my calculated crossover capacitor and inductor values were based on an 8 ohm load.)
  • And the depressing thing about messing about with that online sine wave generator is that I can her bugger all over 14kHz. And things trail off pretty quickly after 12kHz!
  • Oh, and finally I couldn't resist doing this. Obviously.


  • edited March 21
    Went back today to do those measurements investigating the dip at 60Hz.

    I redid the sweep as is, with the 13ohm parallel resistor of the L-pad in place. If the driver is presenting about 4ohms, this means that together they are presenting +3ohms. Together with the 5 ohm series resistor that makes 8ohm.



    So like yesterday, there's a large dip at 60Hz.

    Next, I removed the 13 parallel ohm resistor altogether. So with the 5 ohm series resistor, and the 4ohm driver now on its own, the crossover was seeing 9ohms (1ohm greater than today's first sweep).

    I repeated did the sweep again...



    And...the dip at 60Hz has *reduced* by about 50%.

    Completely the opposite of what my hypothesis predicted. :-D
    So I know nothing!

    In fact, the whole curve looks flatter. The first one has greater variance and a generally upward lift.
    When, after measuring the second graph, I listened to music, vocals sounded a bit sticky to me. Less clear. So for whatever reason my ears do seem to prefer the more undulating curve of the top graph. Apparently I don't like 60Hz, or whatever it is that the second graph indicates!
  • Ben what value are the caps, I may a few polycarbs hidden in a box.

    Cal
  • AntiCrap said:
    Ben what value are the caps, I may a few polycarbs hidden in a box.

    Cal
    Hi Colin.
    The caps value is 133uF.

    I'm currently using:
    • Mundorf ECap AC 100uf 100V Bipolar Electrolytic Capacitor - RAW
    • Mundorf ECap AC 33uf 70V Bipolar Electrolytic Capacitor - PLAIN
  • edited March 26
    I've been thinking about my most recent posts.
    It concerned me how tricky it was to avoid a thin sound by tweaking the parallel resistor in the L-pad.
    I've just built a huge subwoofer. "Thin" shouldn't be on the cards without really trying very hard indeed!
    So I decided to drop the series resistor from 5ohm to 4ohm.
    Things sound much better.

    And, given that the sub seems to be operating at 4ohm at the 75Hz crossover frequency, there's a bonus.
    I based my crossover calculations on the advertised impedance of 8ohm.
    So the parallel resistor in my L-pad has been combining with the (effectively) 4ohm driver to produce whatever value is required to sum with the series resistor to produce 8ohm overall (4ohm driver + 13ohm parallel resistor = 3.06ohm, combing with 5 ohm series resistor to produce 8ohm).
    So, with a 4 ohm resistor in series, one doesn't need an online parallel resistance calculator to work out that to achieve 8 ohm with the (effectively) 4ohm driver requires no parallel resistor at all!
  • Just to illustrate the point, here's what the curve for the system looks like now with 4ohms on the sub... 

    Compared with how it looked last week with greater attenuation on the sub...


    Clearly more bass energy in the top one.
    And the 60Hz dip is greatly reduced. And it sounds better.
    So I'm going to give up making any interpretations from the graphs.

    I may try a 3ohm or 3.3ohm resistor now.
    See how that sounds.
  • Had my covid vaccine yesterday.
    I had quite a strong reaction...fever, a bit delirious (only for about 18 hours).
    As I slumped on the sofa in my altered state, it occurred to me that sticking the 4ohm resistor in the sub cabinet was a better idea. This will set the effective impedance at 8ohm and I can fiddle with L-pads, outside / before the 8ohm cab.
     
  • edited April 5
    OK.
    All done I think.
    A lot of listening. A lot of fiddling.
    Listening told me I needed 6.6dB attenuation on the sub.
    Studying the driver's impedance curve together with listening told me that the sub's effective impedance in this application was certainly lower than the advertised 8ohm, probably around 4. Half of what my prematurely build crossovers were expecting. 

    So...

    I ended up with this arrangement.



    The 4R resistor attenuates the signal by about 6dB and raises the effective impedance of the cab to 8ohm, which is what my crossover was built to expect to see at 75Hz.

    The 0.5R and 120R resistors form an L-pad that provides an additional 0.6dB attenuation whilst holding the impedance seen by the crossover at 8ohm.

    The diagram does not show me enjoying the music, but rather me sleeping after all the endless soldering and fiddling.

  • OMG. I haven't got a clue what you're on about!

    Maybe it's time for me to post about computer networking?
Sign In or Register to comment.