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Helium - a true micromonitor

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  • Originally posted by scottsehlin View Post
    In this larger box, the 8 ohm ND-91 would work better. It can be tuned to 60 Hz using a 1" diameter by 6.5" long port, which gives an f3 of about 56 Hz. For the 8 ohm ND-91, the best option is to use the crossover for the Helium Soundbar, which you can get from the Sehlin Sound Solutions link at the bottom of this post.
    Scott, What the optimum volume/tuning for the nd-91 8ohm? 12mm thick material its enough right? i am assembling 2 heliums 8ohm using de XO for the helium soundbar. will build them within the next weeks and upload pictures. Sent from my ONEPLUS A3000 using Tapatalk


    • Did a little reading of this thread and also did my own Sims of the ND91-4 and -8 using the box modeler in PCD. I was seeing little difference in the response graphs for the two drivers in my larger box, so I must be missing something. Since these would be driven by a mini Panasonic DVD receiver with only 15 watts/channel, I'm thinking going with the 4 ohm version would be a better match because of the low efficiency of the speaker. Like most digital amps, the Pioneer puts out more power into 4 ohms than 8. Even at that, I doubt it could drive the speaker hard enough to cause problems with over excursion. Does this all make sense?

      I also opened up the old speakers I planned to use. They were Standard Radio Corps that come with a matching receiver. Bought them in Japan back about 1970. I was impressed with the build quality. Box is nicely veneered with a brace, the 4 inch woofer has a cast frame and large magnet and the crossover is second order on the woofer, with a cap and two resistors (L-pad) on the tweeter. I was never thrilled with their sound. Probably because it's sealed (not much base) and a cone tweeter. Caps are all NPEs so probably bad or out of spec.


      • Thought I'd share some pics of my paint job. I'll be applying another round of paint and a clear coat. Let me know if you want to know more! More details to follow in the next weeks.


        • Hi

          I want to thank Scott for posting this great build! I am getting ready to make a bluetooth stereo for a Christmas present for a family member using this design. It is perfect given the small enclosure requirements and rave reviews. I am trying to go for a retro look on the finished project, so I would really like to use a slot port. I noticed the SB Accoustics Micro uses a 10 mm slot port, so it seems it can work (

          Using WinISD and changing the port from round to rectangular I can get a manageable slot port that is 0.25"x3"x11"(h x w x l). Basic Parameters: Box 70 cu. in., Ftune = 70 Hz. I am not constrained on the box size since I am making my own enclosure and I can grow it keep the box volume - drivers volume constant, the cross overs are going into a separate enclosure.

          With such a narrow slot I am not confident WinSID is able to give an accurate prediction of tuning or air velocity. The challenge with a slot port for me is that it is not easily adjustable once done. If Scott or anyone else with experience with small slot ports can give me some guidance it would be greatly appreciated!

          Thanks in advance!


          • MY experience w/slot ports is that WinISD does model them accurately. Your biggest "challenge" (with this one) will be setting that 1/4" (smallest) dim. properly. I suggest using some (temporary) "spacers" to set that slot HEIGHT (if you will). If you accidentally end up w/a slot that's only 1/16" higher (making it 0.312"), THEN the length would have to be nearly 3" longer to achieve the same tuning (as your X-sectional area would then be +25% larger). Also, you MIGHT end up w/chuffing unless you can use a roundover bit around the exit opening. Also (since the walls will be so close together) you MIGHT have to have a pretty smooth finish on them (think - "plastic"-like). A lot of ifs.


            • Chris raises some good points. My recommendation is to use the thickness of a sheet of 1/4" material as a spacer or structural piece to define the height of the slot and then measure the actual thickness of the material using a caliper, using the measured height result in your length calculation.

              The effect of the walls being close together can be turbulent flow, which will lower the effective tuning frequency at high port velocities. I don't think this will be a big problem for the Helium, and might actually help the woofer stay composed at higher volumes.
              Keep an open mind, but don't let your brain fall out.

              Sehlin Sound Solutions


              • Just finished a set for my daughter. Thanks Scott! These are awesome! Sent from my Pixel using Tapatalk


                • Finally finished. Sorry for the poor photos. I can upload more if requested.

                  I mounted the woofer underneath the baffle. The port is attached to the back with a removable plate.
                  It was nearly impossible to fit everything in. I chose way too big inductors and I probably should've got 5watt resistors. Lessons learned!


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                    Could anyone explain why my Xsim doesn't match the original post? This is using ZMA/FRD data from Dayton...


                    • There are a couple of key things that using the Dayton data doesn't take into account.

                      1. Effects of the baffle. My understanding is that the Dayton FRD files use an infinite baffle measurement. In that case, the driver is mounted on a large (typically 4' by 8') baffle to eliminate baffle effects. The actual baffle causes a reduction in output at low frequencies and some further variation due to diffraction effects at higher frequencies. My driver measurements were taken in the actual enclosure.

                      2. Time of flight. The sound coming from the woofer is coming from farther behind the baffle than the tweeter. This effects the way the responses of the drivers sum together (frequency response includes a magnitude and a phase). When I measure, I use a fixed microphone position, then measure both drivers from that position so I capture time of flight differences in the phase information.

                      I know there have been a few articles published about how to use manufacturer's data to do a reasonably accurate sim. Paul Carmody did a good writeup at some point that is still on his website the last time I checked. 15 years ago, when I was getting started with designing crossovers, there weren't very many DIY'ers with accurate measurement capabilities, so we used to do a lot of simulations from manufacturer's data. Now, it is quite a bit easier to pull together an affordable and easy to use measurement setup.
                      Keep an open mind, but don't let your brain fall out.

                      Sehlin Sound Solutions


                      • If you "flip" the tweeter's phase (in your sim) it'll probably (at least partially) fill in the divot where the drivers look to be crossing (near 5k)?
                        What appears to be around a +5dB "climb" as you go down in freq. from about 1000Hz down to near 200 is actually the designed in "baffle-step" compensation.
                        Get Tolvan's "Edge" software off the net and set up the Helium's baffle w/drivers. It'll show you how a "flat" system actually loses output going down from 1kHz to about 100Hz due to the bass sounds not projecting forward (like the mids and highs), but actually wrapping around to the back of the enclosure. What you see in this sim is actually the "inverse' of the Edge plot (actually the "compensation" that needed to be in the design to counter the effects of the actual in-cabinet baffle "loss".


                        • Ok thanks for the replies. Is there anyway I can (1) modify the FRD files to account for the baffle (tolvan's edge software displays the pheonomnon, and Bagby's response modeler doesn't work on the newest version of excel) and (2) tweak the FRD to account for "time of flight".

                          Something fishy is going on with the FRD files of the ND16 Dayton has posted. In fact if you inspect the published specs and the posted FRD files they do not match (even with the 1/24 smoothing)! at 4.5Hz its at approx 80db whereas in you look at the PDF at no point in the response does it dip so low.

                          Which spec is correct?

                          For reference I've been trying to use this build as a teaching experience regarding crossovers. Especially since I built them!

                          I managed to find some FRD files for the tweeter from Wolf and Zaph and boy are they all over the place. How does one even design a crossover from theoretical measurements if they vary so wildly from one test setup to the next?

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                          • The "freshest" files/.zips for the 275-025 look like they match each other, to me.
                            YOUR .frd has 2 dips from 3k to 5k that shouldn't be there?


                            • Yes Chris the .zip fro the Dayton website doesn't agree with the posted PDF files. There's a strong dip at 4.5KHz. The on axis FRD file looks very wrong, the off axis measurements look correct, however.


                              • Hi Scott

                                Thank you so much for posting this great build! It was the perfect size for a Bluetooth Speaker I made for a Christmas present. It sound great! Though it was definitely tight getting everything into the compartments!

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                                I am curious if adding some stuffing would tighten up the base a little, just asking because once it get in there it will be some work to get it out.

                                Thanks again!
                                Last edited by helicon; 01-04-2019, 11:42 AM. Reason: I am not good with pictures