USM-2 DIY Speakers (ND91-4 + ND16FA-6)

Discussion in 'DIY' started by ultrabike, Nov 26, 2017.

  1. ultrabike

    ultrabike Measurbator - Admin

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    Anyhow, for reference, here are the crossover analysis stuffs.

    Tweeter:

    Full

    tweeter.jpg

    Zoom - Looks more like a 5 kHz cutoff, though on PSD-Lite it seems more like a 3.5 kHz all things considered. Note 6 kHz to 8 kHz are in the passband, both on analysis and PSD-Lite, which is why I blame the tweeter for the null. As said before, replacing the capacitor by a 5.6 uF part and the inductor by a 0.25 mH part might improve things though.

    tweeter_zoom.png

    On the code below, z and p are the coefficients of the s-transform polynomial ratio (numerator and denominator respectively). This is a second order analog high pass filter.

    Code:
    L1=0.15e-3;
    C1=4e-6;
    R1=2;
    R2=8.2;
    R3=8.2;
    z = [R2*L1*C1 0 0];
    p = [C1*L1*(R1+R2+R3) C1*R3*(R1+R2)+L1 R1+R2];
    w = 0:2*pi*2e4;
    h = freqs(z,p,w);
    figure;
    semilogx(w/2/pi, 20*log10(abs(h)));
    grid on;
    axis tight;
    new_xtick_label = get(gca,'xtick');
    set(gca,'XTickLabel', new_xtick_label);
    title('tweeter crossover');
    xlabel('frequency');
    ylabel('magnitude')
    
    Woofer:

    Full

    woofer.png

    Zoom - Looks more like a 2 kHz cutoff.

    woofer_zoom.png

    This is a second order low pass filter. Tweeters tend to be more efficient than small "full range" woofers. Therefore there is a 6 to 7 dB of attenuation for the tweeter accomplished through an 8.2 ohm resistor.

    Code:
    L1=0.75e-3;
    C1=8e-6;
    R1=2;
    R2=8.2;
    z = [R1*R2*C1 R2];
    p = [C1*L1*(R1+R2) (R1*R2*C1+L1) R2];
    w = 0:2*pi*2e4;
    h = freqs(z,p,w);
    figure;
    semilogx(w/2/pi, 20*log10(abs(h)));
    grid on;
    axis tight;
    new_xtick_label = get(gca,'xtick');
    set(gca,'XTickLabel', new_xtick_label);
    title('woofer crossover');
    xlabel('frequency');
    ylabel('magnitude')
    
     
    Last edited: Dec 21, 2017
  2. iFi audio

    iFi audio MOT iFi Audio

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    Please do continue with this project. Are you going to veneer the whole thing?
     
  3. ultrabike

    ultrabike Measurbator - Admin

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    LOL! Already did. Used cherry wood with cherry stain. Already applied black lacquer finish to the baffle. On the last acrylic coats on the cherry veneers.

    I already fully assembled the crossovers as described above. Need to add some connectors and apply seal gasket to the baffle. In other words, this is almost done.

    Will do further characterization. I believe it will all come out alright. Not necessarily the best micro monitor that ever existed. But hopefully a good project.
     
  4. ultrabike

    ultrabike Measurbator - Admin

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    Alright. Finished one speaker. Though I could add a screw to hold the crossover down instead of hot glue, and give it one more pass with paint. Final assembly of the other one will have to wait till sometime next year.

    Here it is with POS beer to give an idea of dimensions:

    IMG_6194.JPG

    The baffle is fairly flush, but the picture may seem otherwise due to the lacquer light reflection. It does look pretty nice. Maybe later I'll see if I can take better pictures.

    Here is the crossover. Internals are not pretty, but I did what I could to place crossover components efficiently. The crossover board is resting on felt pad feet to avoid resting on the wires. Hot glue for now. Screw hold maybe later early next year.

    IMG_6196.JPG

    And indeed after I clear some path by removing temporary stuff, 70 Hz came back all happy and nice! Here is the frequency response at 0 degrees along with distortion. These are in room. The peak around 300 Hz is furniture close by vibrating a little due to the tone sweep.

    UB1_FR_0deg.png

    At 15 degrees (Things get really flat!!!):

    UB1_FR_15deg.png

    At 30 degrees:

    UB1_FR_30deg.png

    At 45 degrees:

    UB1_FR_45deg.png

    Now vs Mirage Nanosats (Satisfying 70 Hz extension)

    Comparo_w_Mirage_nano.png
     
    Last edited: Dec 22, 2017
  5. ultrabike

    ultrabike Measurbator - Admin

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    Some more pics. The back:

    IMG_6195.JPG

    The port:

    IMG_6197.JPG

    And impulse response at 0 degrees, for those who like to see impulse response:

    UB1_Impulse_0deg.png

    Impressions will have to wait until I finish the second speaker.

    I will also add impedance plots. In theory it should be 3 ohms minimum, but had no problems characterizing this with my $300 Yammy receiver.
     
  6. ultrabike

    ultrabike Measurbator - Admin

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    Also note I did not use any lining inside. This was so to maximize bass output. I could add some felt later if I change my mind.
     
  7. Priidik

    Priidik MOT: Estelon

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    To a point damping should increase 'virtual' volume and extend the bass output a bit lower.
     
  8. Serious

    Serious Inquisitive Frequency Response Plot

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    Are you sure the null isn't caused by diffraction issues from not flush mounting the tweeter? Or maybe it's the port? What does the driver measure like on a huge baffle? I guess you could try to line it with 2-3mm felt, which looks like ass but probably sounds better.

    At least that's my guess since it seems to go away off-axis. It looks pretty bad right now so I'd try to get rid of it. Also the 3rd order distortion from 400Hz to 3kHz looks pretty worrying. I guess that's the limitation of the small driver, but I expected better at that frequency. I probably wouldn't worry as much if it was 2nd order, but 3rd order like that across the whole midrange is bound to have a "low-fi" quality to it.
     
    Last edited: Dec 22, 2017
  9. ultrabike

    ultrabike Measurbator - Admin

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    I'm not 100% sure yet. But most of the models and results I've seen suggest that this is not the case. Lining affects the enclosure Qb. It can reduce peakiness in the bass area (a problem I don't feel this speaker has). But I don't think it can extend it. I tried when I felt the enclosure I selected was too small. It did not work. All I did was reduce bass output which is handy when things are bloated. Things are not bloated in this design.

    A port can extend the bass, but going too far will do so at the expense of bass output (among other things like port resonance and noise). So far, my understanding is that there is no substitute for real enclosure volume.

    I am confident there are no strong diffraction issues. Diffraction IMO is overused as a cause of problems. The tweeter is mounted as it should. It is possible that the response of the tweeter has to do with it's little horn loading there, but that's built in. I cannot remove that.

    As far as a huge baffle, I don't recommend that. For a tweeter, I think it will do nothing. I also don't recommend using felt on the baffle. Not only will it look like ass, I think it will sound like ass.

    As far as distortion, I do not see issues between 400 Hz and 3 kHz at all. That's not the driver. That's a noise floor issue. I measured other speakers and also listened to the tones. I did not hear any issues in that range with this speaker. When I run tone sweeps, I don't just blindly run them. I listen to them. Before I see a problem I can hear it. Again, the range you are concerned about is not a problem as far as I can tell.

    I also do not think that a 3" driver would experience any issues in the 400 Hz to 3 kHz in general. The diameter is sort of optimal for that frequency range.

    Based on the sweeps, this speaker did not sound "lo-fi" IMO. Well, so far. Have not done extensive listening. Still need to finish the other one.
     
    Last edited: Dec 22, 2017
  10. philipmorgan

    philipmorgan Member of the month

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    As the muscle car dudes say, there's no replacement for displacement.
     
  11. ultrabike

    ultrabike Measurbator - Admin

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    @Serious. actually I see what you mean as far as 3rd order distortion, that could be background noise or furniture vibration.

    Like I said, I did not hear obvious issues coming directly from the speaker. But furniture did vibrate a little. Tones where a little loud. We'll see later.

    I had this problem before when measuring the Mackies and others. Depending on where I measured that 3rd order would come and go because things around vibrate. These are not anechoic chamber measurements. They are in room.
     
    Last edited: Dec 22, 2017
  12. Serious

    Serious Inquisitive Frequency Response Plot

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    The 6-8kHz dip:
    I'm mainly just curious to see how infinite baffle results would look like. The specs show no such dip and the fact that it goes away off-axis does point to some cancellation effect to me. Specs are usually measured flush mounted on an infinite baffle. I see that other projects don't flush mount the driver, either, but that gap is going to cause issues. There's a nice FR comparison with a bigger ring radiator here: http://www.troelsgravesen.dk/measurements.htm
    The horn loading probably helps to reduce the effects, but I do think the FR will look better with the tweeter flush mounted.
    It's a bit odd that the dip is at a similar frequency even though the tweeter faceplate is much smaller here. I suggested the felt because you can easily cut felt that's the same thickness as the step here to effectively flush mount the driver. That should usually work. Cutting felt is much easier than cutting the baffle.

    The 3rd order distortion:
    I don't mean spikes that come from vibrations or reflections, I mean the general level of the 3rd order distortion. A very smooth -45db from 400Hz to 2kHz at that level, that's about 0.5% distortion. I take it these were done closer than 1m distance, so the level isn't even 85db at the listening position. Not terrible, but it's still 0.5% D3 (which is worse than 0.5% D2). The bass distortion looks suprisingly low, though. It's not a big driver, so hard to expect much more than that. I don't think my 3" Fostex driver was much better than that and it also had a 2kHz distortion spike.
     
    Last edited: Dec 22, 2017
  13. ultrabike

    ultrabike Measurbator - Admin

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    I see what you mean about the tweeter. It is possible that this is indeed the problem. At this point this is hard to fix since the hole is already cut.

    I would have to add about 1/16" at a 1/16" depth around the existing 1.25" hole. The spec calls for 1.28" hole. But that's not something that can be achieved at 1/16" resolution steps with the usual jig. It means I would have to cut a 1.3125" hole at a 1/16" depth, and inlay a 1.25" hole through. It is what it is. Maybe I'll fix this in another life. I like what I heard so far. But if I was going to do this all over again, you bet I will do the 1.3125" + 1.25" cut deal.

    As far as distortion, you have good points. It is possible that this particular 3" driver trades useful bass output for midrange distortion. Maybe this is a case were we can't have it all. The Mirage Nanosat uses a 2.5" driver and it had higher sensitivity. I would have to re-run distortion measurements on the Nanosat. But the Nanosat design/driver cannot extend that low. I really wanted to get 80 Hz cleanly to properly cross with a sub.

    This is sort of a special 3" driver. There is another one that got me curious that can do similar things as this ND91-4 driver. But it needs two extra components to tame it and in the end one ends up with similar sensitivity (actually 1 or 2 dB less).
     
    Last edited: Dec 22, 2017
  14. ultrabike

    ultrabike Measurbator - Admin

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    BTW, if @Serious is right and the problem at 0 degrees is the difficulty flush mounting the tweeter, then I can say I'm very pleasantly surprised at how well PSD-Lite works to figure out crossovers. Kudos to those dudes!!!
     
  15. ultrabike

    ultrabike Measurbator - Admin

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    LOL! I can pop another baffle sometime next year to test @Serious idea of placing the tweeter completely flush. You know, for science. Originally that was my goal, but did not think about that extra 1/32" lip.

    We'll see. @Serious points are valid and indeed helpful at least to me.

    I also feel I better understand the trade offs about having a small driver with lots of xmax and bass capabilities, assuming that's what is causing the small increase in distortion around the midrange. It sort of makes sense. Maybe there is less control of the woofer around the mids area due to the woofer surround.

    If that is the case, I think it is a good compromise for small home theater monitor applications. Or at least an option. The nanosats are fully ominidirectonal and they measure well as such. But those decisions also impact other performance metrics.

    On the quick tests, that distortion did not bother me much. At least it didn't stand out as much as the 300 Hz vibration issue. We'll see once I get the other speaker done and test with music.

    Anyhow. Thanks guys!
     
    Last edited: Dec 22, 2017
  16. iFi audio

    iFi audio MOT iFi Audio

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    Yes, how does it sound like? That's the most important part now ;)
     
  17. ultrabike

    ultrabike Measurbator - Admin

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    Yeah. I'm just going to finish the pair, screw the crossovers, maybe put some little bit of felt (take it out if it doesn't work), enjoy, and let you guys know how they sound.

    Regarding witty things I can find in the internet:

    It seems surface mounting is more of a problem when the tweeter flange is large in diameter and thick. None of which are the case with the ND16FA-6. Here is some discussion about it:

    http://www.zaphaudio.com/mtg-surface.html

    Furthermore, I'm not doing the flush thing for practical reasons at this point. It may solve the "problems" at 0 degrees (again, not sure about that). But the fact is that things look fairly reasonable from 15 degrees or less to 45 degrees anyway. I roughly know what the optimal placement should be. It's just not worth it at this point IMO.

    So, again, will let you guys know how they sound as soon as I'm done.
     
    Last edited: Dec 23, 2017
  18. Cspirou

    Cspirou They call me Sparky

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  19. Cspirou

    Cspirou They call me Sparky

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    Can you do a felt mod around the tweeter to compensate for not doing a flush mount?

    [​IMG]
     
  20. ultrabike

    ultrabike Measurbator - Admin

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    I already made a jig to do the cut out on the existing baffle.

    I made the cut on one of the speaker, but I probably went 1/16" too wide. Depth is good. Will see next year. Gotta go to visit the grandpas.

    Worst comes to worst I can redo the baffle anyway. It's not a big deal at this point.
     

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