Multiple Entry Waveguide Bluetooth Build
Need/Use Case
I’ve always wanted a decent bluetooth speaker that I could carry when traveling/hiking/camping/etc. but could never find one that really provided a quality listening experience.
In addition to this, I’ve been looking for a no-frills fill box that I can throw in a corner and feed a signal to during shows without having to worry about it, so the appeal of a battery powered “set it and go” box is high for me.
Background
I’ve had a fascination with multiple entry horns over the last year or so having prototyped a few larger format MEHs at this point. If you know anything about MEHs, there are some considerations regarding the size, flare rate, and horn profile to achieve what they do. Going small with a MEH means you do lose out on some of these benefits if you plan to do a full-range build.
However, the primary guiding reason of this speaker’s arrangement is not necessarily to benefit from hornloading the 6.5” woofers, but to offer a highly compact arrangement that wouldn’t be possible with a flat baffle. As an additional benefit this places the acoustic centers of the drivers very closely and provides a good directivity match between the woofers and compression driver, despite the steep crossover. What it doesn’t do is give a substantial sensitivity boost/impedance match to the woofers that you would get from a larger conical MEH.
Build Requirements
I set out to design a speaker that would attempt to hit these metrics:
Full range with at least 100dB output down to 50Hz
Wide 60x90 pattern with well controlled directivity in both horizontal and vertical axes
Highly portable (<15 LBS and <25 liters)
At least 8 hours battery life
The Design
Acknowledging that I wasn't constrained by a straight wall conical horn, I opted to utilize Ath to design a waveguide that would provide a more uniform wavefront than would be possible with a pyramid shaped horn. For this reason, I’m hesitant to call it a horn and lean more towards a waveguide (I’m loosely defining this based on my understanding of Geddes’ classification).
For components, I opted towards using the new B&C DH350 compression driver, two 6NDL38 6.5” woofers, and two Scanspeak SB16PFCR-00 passive radiators (ports were out of the question due to the small enclosure size). This blend of components lends itself nicely to a crossover point somewhere in the 1.0-1.2kHz region, so that is what I targeted in Ath/Hornresp.
To achieve that, there was a good deal of fine tuning/tweaking the models in Ath/Hornresp. The basic idea was to really nail the directivity/impedance curve in Ath, then (loosely) translate that profile into Hornresp so that I could leverage the MEH wizard to integrate the woofers and find the point at which they could meet the CD.
Knowing that my crossover would be somewhere in the 1-1.2kHz area, I then sized out and placed the woofer taps in Fusion along the corners of the horn where the centroid of the taps is about ¼ wavelength @XO from the CD exit. I then modeled a flange for mounting the woofers leaving a flat area open so that I could come in later on and add cone fillers if needed.
My 3D printer bed isn’t large, so the model is split into 3 pieces - a left and right half with a throat adapter. I used 2 part clear epoxy to join the halves together. The woofer mounting nuts are heat-set M4 inserts.
The enclosure is a simple hexagonal shaped box made from 12MM birch plywood with about a 15 liter volume after accounting for the displacement of the horn/drivers.
Unfortunately I misjudged the amount of clearance I'd need for the passive radiators, so they do butt up against the rear of the woofers when installed. If I do another box, I’m going to add a couple extra mm clearance there. The good news is that there is a hole in the basket of the passive radiators that lines up perfectly with the 6NDL38’s pole vent, so I don’t anticipate any overheating issues there.
During the initial testing I taped the front of the waveguide to the enclosure. This went about as expected, but I was at least able to get the data I needed. I use an outdoor ground plane measurement setup according to this guide from Scott Hinson. Measuring with a Dayton audio UMM-6 with a 20 cycle FDW applied at 1 meter (2 meters for off-axis). 30°C at 72% humidity.
For this first prototype, I wanted to validate the Abec/Hornresp model and check physical fitment - wasn’t going for a final rendition. The thought was to build the initial prototype and gradually add cone filler until I achieved the desired woofer response. For the most part, this was a success, but for whatever reason there was a secondary peak in the woofer response that I hadn’t anticipated. I’m going to consider my failure to anticipate this a happy accident, because it meant that the woofers conveniently worked out perfectly to meet the compression driver on the first try. If you have any insight/theories into why that secondary peak might exist, I’d love to hear.
Barring that, the unfiltered responses of both drivers overlaid looks like this.
Given these results, I decided to go ahead and finish out this prototype. There’s some details in the physical construction of the box I might make in a future version, but overall I was quite happy with the performance. I chose a Dayton Audio KABD-4100 amplifier paired with one of their 6 cell 18650 BMS boards, mostly due to availability and balance of features. One of the outputs was left unplugged, so it's not the most efficient setup but I found it works quite well for the price. The sigmastudio file is provided in the zip file.
Regarding the enclosure, whatever you build make certain it's well sealed. You may want to modify the flange on the waveguide for a cleaner mount. These little drivers move a lot of air and I spent the better part of an afternoon working out caulking/gaskets to fully take care of air leaks.
Specs
Frequency Response: 60-16kHz (+-2dB)
Coverage: 75Hx60V
Maximum SPL: 106dB (about where the 6NDL38s and PRs reach xmax - can reach higher if you HPF at 80Hz.)
Battery Life: Subjectively well over 8 hours, but I’ll be doing an M-noise stress test later at some point to get a real validated runtime number.
Weight: 14.9lbs (6.75kg)
Outer Dimensions: 18.5” wide x 10.2” deep x 7.9” tall (47x26x20cm)
Components:
2x B&C 6NDL38 Woofers
1x B&C DH350 Compression Driver
1x Dayton Audio KABD-4100
1x Dayton Audio LBB-6S
6x Samsung 35E 18650 cells
2x Scanspeak SB16PFCR-00
Download the .STLs and SigmaStudio file here.
Compromises: These are the areas that I’ve identified as weaknesses in this build or areas for improvement.
Coverage is a bit narrower than I would have liked it to be. I think I need to refine my Ath skills a bit more since this didn’t necessarily match the polars I generated in ABEC.
The steep crossover slopes may not be ideal for some listeners and introduce a small amount of group delay (about 1.3ms at 1200Hz.) This could probably be corrected with FIR.
The DH350 breaks up at around 16kHz, so don’t expect full extension up to 20kHz
Distortion is higher than I’d like it to be on the low end - no doubt due to nonlinearities with the passive radiators. They’re cheap - buy better ones if you can. Also - you should be able to achieve much better bass performance by increasing the box size by about 25%, so if space isn’t an issue that should be considered.
Subjectively, the DH350 lacks a sort of upper midrange clarity that I’ve heard with other (larger) compression drivers. I’d wager this is due to some diaphragm resonances, but haven’t measured it. I’d recommend buying one to prototype with other horns/waveguides to assess this yourself before determining if this build is for you.