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Helium - a true micromonitor

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  • scottsehlin
    started a topic Helium - a true micromonitor

    Helium - a true micromonitor

    Objective:

    When I designed the Lithium, the goal was to create something small enough to go almost anywhere, without giving up much in the way of sound quality (even bass). One of the things I have learned from the feedback (particularly non-audio enthusiasts) is that what I or other folks who frequent this forum may consider small isn't really small in the eyes of the larger population.

    The laws of Physics won't really let you go much smaller than designs like the Lithium or Overnight Sensation and remain nearly full range (for music anyway), so I had to set my expectations accordingly. For the Helium, the goal was to create something the size of a typical computer speaker or HTIB satellite that could either provide a reasonable music experience on a desktop or legitimately reach down to a desirable subwoofer crossover point in a small room home theater application (80 Hz or so).

    Enclosure:

    The Parts Express 0.04 cu. ft. flat pack (Part Number 300-7060) has external dimensions of 6.5" x 4.5" x 5", which is just about perfect for what I have in mind. While (as my better half would attest), I could easily find enough scrap laying around the garage to put together something like this, the flat pack is a real time and sawdust saver, and an easy choice at the current price of $19/pair.

    Driver Selection:

    The Dayton ND91-4 (290-224) is a pretty impressive little driver with 4.6 mm xmax and a claimed 25 mm of peak excursion. According to simulations, this driver will be quite happy in 0.03 to 0.04 cu. ft., reaching an f3 of around 70 Hz in the selected enclosure. It's not the most economical small driver in the world, but my experience with the ND105 along with the specs gives me confidence that the ND91 can take the beating that trying to reach below 70 Hz as a 3" driver will dish out.

    Once the ND91 is in that box, there isn't enough room on the baffle for a lot of tweeters. Fortunately, the ND91 should be able to play pretty high in frequency, which should open the door to a small tweeter. The Dayton ND16 (275-025) has been used to good effect in a number of designs and is very small and under $10, so it is the obvious choice.

    Crossover Design:

    The ND91 does pretty well up to 8-10 kHz, while the ND16 can extend down to 3.5 kHz, so there is a fairly broad window to work with. I decided to target something around 5 kHz, so I would have some flexibility with respect to crossover slope. One consideration is that there is not a lot of room in the box for a crossover (the floor of the box is only 4" x 3.5" and some of that space will be constrained by the woofer frame.) This means that to some extent, the crossover point and slope needs to be determined by finding a point where the drivers will integrate with a small number of components. After a number of iterations, I settled on a 6 component crossover network that yielded approximately 4th order acoustic slopes with a 5 kHz crossover point.

    The final crossover requires a 1 mH inductor, which will definitely be a challenge to fit into this box. Recently a thread on PETT regarding small, inexpensive crossover components included the suggestion of dynavox.com. Sure enough, they have a 1 mH laminate core inductor that is about the size of a sand cast resistor (for $0.84) - so I used it. You can also get the 4 uF capacitor and a terminal cup from Dynavox. You might even get away with their 2.2 uF capacitor for the specified 2 uF. For the 0.2 mH inductor, I used a surplus inductor from some old Klipsch crossovers I bought from Ken McCullough. The closest match I have been able to find is a 23 ga Visaton air core sold by MCM Electronics. The 20 ga Jantzen air core from Parts Express will also work fine. Since there is a 2 ohm resistor in series with that inductor, the small change in DCR of the inductor will have no significant impact. The 2 uF capacitor I used was a surplus mylar from Madisound (still available at $0.35), but the Dayton met poly (027-414) will work.

    Impressions:

    I can attest that the original design goals were met. Those who were at the evening hotel session Saturday night at DIY Iowa 2014 may have more to offer from a review perspective, as I am probably too biased to provide anything useful.
    Attached Files
    Last edited by scottsehlin; 05-14-2015, 08:47 PM. Reason: Added 39 ohm resistor to schematic

  • scottsehlin
    replied
    I've used the sound bar as a portable music system at a couple of small gatherings, and it has held up well. The internal volume isn't that much larger than the original Helium, but enough to get another 5-10 Hz on the low end.

    Leave a comment:


  • mga2009
    replied
    Originally posted by Chris Roemer View Post
    How big ARE those (internal). Tuning? These are ND91-8s?
    Yes, I have ND91-8. The design is based on the Helium Soundbar (same crossover and same internal volume).

    In post #351 Scott Sehlin said the 8 ohm version required an slightly larger optimal cabinet, and as a result can play deeper.

    Also said the soundbar has isolated chambers for the left and right channel that would be 4" wide by 4.75" deep by 9" tall as stand-alone speakers, and that I could flip that around and make them 4" deep by 4.75" wide by 9" tall. Material is 12mm MDF, no bracing. net internal volume is around 1.5 liters I think.

    Regarding the tuning it's the same as the soundbar: "3/4" schedule 40 PVC, 4 inches long" with the slight difference the tube I have is 20mm (vs 19,50mm)

    Originally posted by Jake View Post

    Have you tested the nd91-8 in that larger volume? You may not be happy.
    Why is that? Any suggestions?

    Leave a comment:


  • Chris Roemer
    replied
    How big ARE those (internal). Tuning? These are ND91-8s?

    Leave a comment:


  • Jake
    replied
    Originally posted by mga2009 View Post
    Hi,

    these are my Helium 8ohm for surround duty... still a WIP. Within the next days will veneer them and install the drivers.

    As you see they are taller and shallower... and bigger total volume.



    Have you tested the nd91-8 in that larger volume? You may not be happy.

    Leave a comment:


  • mga2009
    replied
    Hi,

    these are my Helium 8ohm for surround duty... still a WIP. Within the next days will veneer them and install the drivers.

    As you see they are taller and shallower... and bigger total volume.




    Leave a comment:


  • garychelette
    replied
    Originally posted by Deadlytoaster View Post
    You guys think there's enough room in the enclosure for a TPA3116 amp board and 3-4 18650 batteries for it to not impact the sound from the host enclosure?
    I don't think so. When I built the 8 ohm version, I barely had enough room to put the Xover! The box is very small!
    G

    Leave a comment:


  • Deadlytoaster
    replied
    You guys think there's enough room in the enclosure for a TPA3116 amp board and 3-4 18650 batteries for it to not impact the sound from the host enclosure?

    Leave a comment:


  • mrmky
    replied
    Shocked at how small they are and how big they sound. First of 5 in a bedroom theater. First attempt at veneer as well

    Leave a comment:


  • Scarface1
    replied
    Scott and Jeff thanks, I will look at post #236😀

    Leave a comment:


  • Jeff F.
    replied
    Originally posted by Scarface1 View Post
    Hi Scott, I also want to build the heliums the question, what is the impedance, can I hook it up to a 8 ohm amp.
    The original design is 4 ohms. If you want to build the 8 ohm version see post #236. It has a revised crossover.

    Leave a comment:


  • scottsehlin
    replied
    There is an impedance plot on page 1. The minimum impedance is around 5 ohms, so most manufacturers would consider it a 6 or even 8 ohm speaker. I wouldn't anticipate any issues with an 8 ohm amp.

    Leave a comment:


  • Scarface1
    replied
    Hi Scott, I also want to build the heliums the question, what is the impedance, can I hook it up to a 8 ohm amp.

    Leave a comment:


  • dkalsi
    replied
    Originally posted by scottsehlin View Post
    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.
    Scott,

    Thanks for posting these comments. I'm in the midst of building your Lithium speakers. I'll build the crossover and, as you did when you were designing, take measurements before gluing everything up since one only gets one chance at the crossover before its permanently locked up inside the cabinet.

    I recently built Jeff Bagby's Quarks. Unfortunately, my measurements didn't quite match up with Jeff's :-(.

    With the Lithium, I plan on following the procedures you indicated above.

    Thanks,
    D

    Leave a comment:


  • helicon
    replied
    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!

    Click image for larger version

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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

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