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Boominator MINI (development thread)

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Old Croc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote all bass Quote  Post ReplyReply Direct Link To This Post Posted: 19 November 2014 at 1:12pm
Originally posted by Saturnus Saturnus wrote:

The reason why the cut out diameter is 96mm and not the about 99mm of the driver is that a tube resonance is inevitable. If it's 99mm then that tube resonance base frequency would be 1742hz which is extremely close to (within 1%) of 1760hz which is a harmonic of the note A on which most western music is based. It is important to avoid resonances that coincide with harmonics of musical notes, and especially harmonics of A should be avoided, and doubly so in the 2khz range where the ear is most sensitive. By having the cut out at 96mm the resonance is 1797hz which falls between A and B-flat which is much less of an issue. It could also have been 102mm which would see the resonance fall between A-flat and A but I choose the smaller radii. You can try 102mm cut out if you like. I have not.

Now you might think it's not that important and surely a speaker should play and treat all frequencies the same but that's completely wrong. Special care must be taken to eliminate resonances at harmonics of musical notes. Musical notes aren't arbitrarily chosen. They are those frequencies because those "sound" the best to our brains. Anyone can test this by tuning your guitar to A=452hz (right between A and A-flat). It will sound awful and give you a head ache after only a few minutes. 
I am truly baffled by the amount of thought you put into this design Saturnus. My hat's off to you sir!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Aterren Quote  Post ReplyReply Direct Link To This Post Posted: 19 November 2014 at 1:29pm
@saturnus - Wow!  The education continues.  If you are ever in NYC I would love to buy you dinner.

Incidentally, the after sanding and processing my speaker cutouts were >96 mm which may well be contributing to the problem.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote lutkeveld Quote  Post ReplyReply Direct Link To This Post Posted: 19 November 2014 at 1:56pm
Everytime I think I know every aspect of the Boominator design Saturnus comes and proves me wrong ;p


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Cole Quote  Post ReplyReply Direct Link To This Post Posted: 11 December 2014 at 4:25pm
Dont know if this is any use to anyone but I have just assembled my first attempt at a boominator mini using 2 pairs of Mission 731 speakers and a TA2020 picked up cheap off ebay. Very pleasantly surprised by the sound and battery life using two 7aH lead acid battery's. I don't know how it compares with ones built with the Aura's or Monacors as I don't know any one who has built one. I thought it seemed a little underpowered at first but the addition of a bluetooth receiver instead of direct connection from smartphone seems to have sorted that out . I am happy to say it has certainly put a smile on the faces of people I have demo'd it to.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Wrecks Quote  Post ReplyReply Direct Link To This Post Posted: 03 January 2015 at 12:15am
I started building these last year and put the project aside until I had a break over the Holiday.  I just finished up my mini (Version 1) with the Aura 525s and the filter and amp as Saturnus recommended and before sealing everything away for good I gave it a test drive.  While it sounds crystal clear and crisp, I would say it is lacking in the bass department.  I don't yet have any acoustic wool in the cabinet and while I had them closed for the test it certainly wan't an air tight seal as it will be when I glue and staple the bottom pieces in place.  Any thoughts or suggestions on how to boost the base?

Rex  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Palomino Quote  Post ReplyReply Direct Link To This Post Posted: 03 January 2015 at 12:51am
I would play it 80 hours or more and see if you feel the same way.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Saturnus Quote  Post ReplyReply Direct Link To This Post Posted: 03 January 2015 at 1:11am
Big difference in sealing it up with the acoustic wool in place and just have the cabinet loosely fitted. To get an approximation very securely clamp the lid on or put weights equaling about 150-200kg on top. It'll still not be completely air tight but it'll give a reasonably good idea or how it's going to sound. 

Edited by Saturnus - 03 January 2015 at 1:15am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Wrecks Quote  Post ReplyReply Direct Link To This Post Posted: 05 January 2015 at 7:20pm
Thanks, I will probably use pillow stuffing from the local craft store instead of "acoustic wool" as I saw that somewhere in this thread Saturnus suggested it was about the same.

Now that I have this V1 of the mini finally wrapped up I'm thinking of diving into the updated and slightly smaller V2.  Maybe I will give this first version to a friend after I enjoy it for a bit! 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote lutkeveld Quote  Post ReplyReply Direct Link To This Post Posted: 05 January 2015 at 8:06pm
After my experience with improvised stuffing I would just spend the extra couple of bucks and use 'real' stuffing.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Aterren Quote  Post ReplyReply Direct Link To This Post Posted: 20 January 2015 at 12:44am
Saturnus -  I am quite mindful that any changes would result in this no longer being a Boominator.   I would be happy to move this to another thread if you prefer. 
-------------------
Some of you may recall a previous post on my listening observations.   In summary I found the mid-range strained and tiring.   The sound was better if I attenuated the 1.5 to 3kHz range.    Well, to better understand was happening I decided to I picked a Dayton OmniMic and DAT2 system and do a bit of playing.   Below are my results to date.   I am open to any advice folks want to offer on how to best furthering my learning. 

Summary observations:
1) There is a ~10db difference in output across between 800 and 3,500 hertz.  This is consistent with my listening observations.
2) Adding clay, or foam to my incorrectly constructed chamfer result in a measurable change but not a significant one.
3) Indoor and outdoor measurements were very similar.  Future testing will be in the environment with nicer weather.  
4) Measured parameters on the SP60 varied from published specifications (nominal/measured).  10 drivers were tested.
    fs:  65hz / 86hz
    Vas: 6l / 2.8l

    Testing geometry and notes:
    1) Boominator and microphone both 83cm above the ground at a distance of 1 meter.   Additional distances and elevations are planned for the future.
    2) Cross over values were verified on 1 side of the boominator (I cut the x/o out.    Testing on a per driver basis is from this side.  Wires to the drivers now exit the side of the Boominator.   The bottom is 12mm ply that has been hot melt glued to the the Boominator.  Responses between the cut/uncut sides are quite consistent.

    This is a typical frequency response:
    - note the hump at 1.5kHz

    Here are the curves for each driver.   Sorry for the mixed scales and smoothing in these snapshots.  

    There seems to be an opportunity to improve the cross over.   I am playing with BoxSim and I am starting to appreciate how hard a simple cross over is.  That said, I am willing to give up some efficiency, and incur cost and complexity, to smooth things out a bit.  Below are the results predicted by BoxSim.   The shape is similar but frequencies and magnitude of deviations are not.  


    And so my journey begins.  Any guidance from the more experienced folks here would be much appreciated.  
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    Young Croc
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    Post Options Post Options   Thanks (0) Thanks(0)   Quote lutkeveld Quote  Post ReplyReply Direct Link To This Post Posted: 20 January 2015 at 2:00pm
    I'll get my mic out asap so we can compare. The frequency response indeed leaves much to be desidered. It's tested in the MINI enclosure right? The Thiele Smalle parameters are also not so close to spec. Monacor seems to have a not-so-great quality control, I've seen several people reporting large deviations in the TSPs. 
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    Post Options Post Options   Thanks (0) Thanks(0)   Quote Aterren Quote  Post ReplyReply Direct Link To This Post Posted: 20 January 2015 at 4:14pm
    Yes, the enclosure is stock build. Crossover is built with the choke not the resistor.    Regarding the T/S parameters,, when I get a chance I will type up and post complete measurements.

    Aditionally, the results from each of the 4 driver pairs was consistent. I do not think it is a case of having 1 bad driver.

    Edited by Aterren - 20 January 2015 at 4:16pm
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