LCM Waveguide Speaker - 3D Printed Near-field Studio Monitor With DSP and Bluetooth

This is a near field mini-monitor speaker, designed for excellent bass response, and intended for use at a desk or mixing console. This is not the cheapest project to build, but it is a scientifically designed HiFi grade loudspeaker which will easily out-perform commercial speakers at the same cost. (The last section gives some video comparisons.) This Instructable will go through the design process, the prototyping and testing cycle and the finished results.

These speakers are suitable for desktop, shelf, or wall mounting and optimized for small to medium sized rooms. The 55cm waveguide port is tuned to 44Hz and provides bass down to 40Hz in a small 14cm x 19cm x 17cm package (5.5 x 7.5 x 6.7 inch). The recomended 4 channel DSP amplifier has bluetooth and 3.5mm input and functions as the crossover to split the signal between woofer and tweeter at 200Hz. (It will be possible to use other amplifiers but you will need to have a crossover of some form.) The speaker prints in 2 halves which glue together with a tongue and groove joint. I also made a version for wooden side panels. This will cut the amount of plastic needed and the print time. I've included desk stands with 5 and 10 degrees tilt.

Drivers:

Woofer: https://www.parts-express.com/Tang-Band-W3-1876S-...

Tweeter: https://www.parts-express.com/Tang-Band-W3-1876S-...

Bill of materials, fully printed version, per speaker:

• Tang Band W3 1876S 3" Subwoofer driver
• Tectonic TEBM35C10-4 2" BMR driver
• Glue
• Speaker wire
• Polyfill speaker stuffing, or similar, enough for 0.4 liters
• 4x M2 x 8mm screws for tweeter
• 6x M3 x 10mm screws for woofer
• Bluetack or foam speaker gasket material

Bill of materials, wood panel version, per speaker:

• Tang Band W3 1876S 3" Subwoofer driver
• Tectonic TEBM35C10-4 2" BMR driver
• Glue
• Speaker wire
• Polyfill speaker stuffing, or similar, enough for 0.4 liters
• 4x M2 x 8mm screws for tweeter
• 6x M3 x 10mm screws for woofer
• Bluetack or foam speaker gasket material
• 2x Wood or MDF panels 19cm x 17.2cm (approx 7.5 x 6.7 inch)
• (Optional) 20x M3 x 10mm screws for side panels

Links to example amps you can use (see amplification section for more info):
https://www.parts-express.com/Dayton-Audio-DSPB-25...

paired with:

https://www.parts-express.com/Dayton-Audio-DSPB-25...

Or the same series of Amps branded by Sure:

https://store.sure-electronics.com/product/AA-JA3...

My objective was to make a compact speaker suitable for use at a work desk, without compromising on bass response. This meant small size suitable for use on a shelf or wall bracket, with a wide dispersion pattern so you don't need to keep your head pinned in place to get the optimum frequency response. Since they are for listening up close in a medium sized room, it was not important for them to go super loud.

I wanted to find how wide a frequency range it was possible to cover in a small economical bookshelf speaker, by applying the best of modern technology to the problem, and this is what I ended up with, 40Hz - 20kHz in a ~2 liter speaker. Historically, this wouldn't have been possible at reasonable cost, certainly not for the hobbyist. The enabling technology is economical Class D amplifiers and easily programable DSP's. The high output of a Class D amp allows us to use low efficiency drivers with excellent acoustic characteristics. The DSP let's us produce a digital crossover at a low frequency, avoiding the need to spend lot's of money on audio grade passive crossover components.

The low frequency section, Tang Band W3 1876S:

This 3" driver is a legend, the star of many compact subwoofer designs and youtube videos, nothing else can match it in small enclosures. I started modeling a few different design options with this, I looked at both bass reflex and passive radiator options. I found a reflex design would get me a significantly lower cut off, 40Hz instead of 45Hz, and have significantly better transient response. The trade off (there's always a trade-off) is the long port required, which could be prone to port resonances, more on that in the next section. However, I know I could get around the port resonance problem if I could select a high frequency (HF) driver capable of playing down below the port resonance frequency, enter the remarkable Tectonic BMR driver technology...

The high frequency section, Tectonic TEBM35C10-4:

Tectonics BMR technology is a remarkable invention which allows excellent angular dispersion (off axis response) across a wide frequency range by exploiting balanced resonant modes in the disc of honeycomb material which takes the place of the cone or dome in a normal driver. There is a great description of the technology here:

https://www.parts-express.com/pedocs/more-info/tec...

BMR technology has received loads of positive response from the HiFi industry, including this very detailed review of this exact driver by an industry legend at Erin's Audio corner:

https://www.erinsaudiocorner.com/driveunits/tecton...

I'll lead you quickly through my modeling process in WinISD, which I used to design the bass enclosure. This isn't intended to be a tutorial for WinISD, there are plenty of those, fore example:

https://midwestaudio.club/resources/winisd-a-begin...

This is just my workflow in WinISD, I have found it to be effective for comparing different candidate designs. First, I model the responses of different candidate designs using the transfer function magnitude graph, in the example graph, I compare the LCM design to two different passive radiator designs with the same driver. Next, I move to the cone excursion graph and adjust the Input Power in the Signal tab for each design, to take the cone excursion to x-max at the peak above the resonant frequency (second graph). After that, I go to the sound pressure level (SPL graph) to compare what maximum output the different designs are capable of. In this case, the third graph shows a clear win for the bass reflex design over the passive radiator alternatives.

Designing a project like this, it is always best not the bank on success first time. Sure enough, there were some very unexpected results, and a bit of detective work required to find the cause. I expected I might need to iterate to adjust the tuning if the bass port, but fortunately the WinISD model was spot on, and the tuning was precisely where I wanted it. Checked that by measuring the impedance with my Dayton Audio DAT's, a useful piece of kit for any DIY speaker enthusiast, but at $130, I wouldn't recommend it for someone just wanting to build some speakers. It is possible the make impedance measurements using just a resistor and your soundcard, if you are doing any speaker customization it is certainly worth spending the hour or so it will take to get to grips with this method:

https://www.roomeqwizard.com/help/help_en-GB/html/...

I measured the bass response using my UMIK calibrated microphone and the near field method described here:

This matched the modeled behavior, with a -3dB point (called F3) at just below 40Hz, and a big port resonance at 315Hz caused by half wave resonance in the port. The port resonance is not a problem because we are going to avoid putting those frequencies anywhere near the port.

Background on half wave resonance:

https://techblog.ctgclean.com/2011/10/ultrasonics-...

The problem appeared when I measured the response of the tweeter, some really nasty resonances just below 1kHz. This was a complete surprise because I'd used the tweeter before in the same size enclosure with great performance. The graph shows the performance in this enclosure vs my previous design which I've labeled "reference".

After some head-scratching and talking to a few experts in the field, we eventually had a theory, the HF enclosure was mechanically exciting resonance in the port. When I designed the HF enclosure I chose a wedge shape for its good performance at preventing standing waves. Unfortunately, I'd taken the HF enclosure all the way back to the wall of the port and this allowed flexure in the wall of the HF enclosure to couple into the port, setting off resonances between the apex of the enclosure and the open end of the port. I updated the design with a curved wall between the sections, keeping the HF enclosure well away from the port. The root cause of the problem, and the solution was just a theory until built and tested. So, another few lb of PLA and I was ready to perform another test. Fortunately, the theory proved to be correct and the new HF enclosure performed beautifully, so I promoted this design from Prototype 2 to Mk 1.

A speaker's high frequency response can be affected by various design factors, including the "baffle step", where the tweeter goes from radiating sound in 360° at low frequencies to radiating in the front 180° at higher frequencies. As a result, it is always good to base your EQ design on measurements in the finished speaker, rather than going driver spec sheets. I used a UMIK-2 calibrated USB microphone to perform the measurements and create this EQ profile. It's generally accepted that is is better to use a few EQ parameters to get an acceptably flat response, rather than use a load of parameters to hammer the response into a ruler flat line. The output of the HF section proved to be very well behaved and I was able to use a single high shelf filter with +2.5dB gain to give level response to out to 20KHz, a remarkably good result. You can see the EQ parameters in the attached screenshot.

The crossover I chose is a Linkwitz-Riley 48dB/octave at 200Hz, with -6dB gain on the tweeter, this is shown in the attached screenshot.

This crossover and EQ profile are contained in the DSP project file linked in the next section.

Printing:

Use at least 3 perimeters, I recommend at least 30% infill. PLA of PETG materials will both work fine. Other materials should also work, provided you can avoid warping problems.

After printing:

Note: for good bass performance it is important to make sure all glue joints are airtight, as well as the gaskets for the drivers and the cable exits.

1. Clear any stringing from the port.
2. If making the half-wood version: Glue on the side panels, making sure to get a good seal all around. Use some glue between the wall of the tweeter chamber and the pannel to prevent buzzing. It's probably a good idea to apply more sealant after the glue has set.
3. Put masking tape on the outside around the perimeter of the glue joint, to avoid dribbles making a mess of the outside.
4. Glue the left and right half together using plenty of glue, I suggest clear gorilla glue or slow cure epoxy. Clamp the the two halves together untill fully dry.
5. Push the speaker wire through the holes on the back.
6. Use bluetack or foam to make gaskets for the speakers.
7. Stuff the tweeter chamber with polyfill or teddy bear stuffing. (The bass chamber should not have any stuffing.)
8. Connect the wires to the drivers, you can solder or use spade terminals
9. Screw the drivers in place, with the M3 screws for the bass driver, and M2 screws for the tweeter
10. Seal the holes for the wires from behind with bluetack or silicone

Recommended Amplification:

I suggest using the Dayton Audio or Sure range of DSP amplifiers, along with the Sigma Studio DSP project I created specifically for these speakers:

The DSP file is attached here:

https://www.thingiverse.com/thing:4980197

The amplifier enclosure 3D model and full setup instructions can be found here:

https://www.thingiverse.com/thing:4761687

My tutorial on sigmaStudio for DSP design can be found here:

https://www.instructables.com/DSP-Speaker-Developm...

Alternatives:

It is possible to roll your own in so far as an amplifier and crossover solution, this may be of interest if you already have a DSP and amplifier solution, in which case I expect you know how to use it.

It would be possible to build an analog crossover for this speaker, but it has to be around 200Hz at must be at least 24dB/octave. Audio grade components will be quite large, and will need to be outside the enclosure, and you will probably need a couple of hundred dollars worth of components. This really is a project best suited to a DSP!

I think this project achieves its goal of being a small speaker offering good bass response and wide dispersion, for use at a work desk or in a small room. I recored a couple of sound demos, the first is a comparison to a professional studio monitor (Tannoy System 10 DMT), and the second is a comparison to a popular 3D printed speaker design, the BL2. I just recorded these videos with my iPhone, but if you listen with headphones, they give a good impression of the relative performance of the speakers.

In my opinion the LCM gives a very solid performance compared the 10 inch Tannoy studio monitors, actually reaching a lower frequency response, 40Hz vs 44Hz. It should be said, in this demo, the LCM's are playing at a good fraction of their maximum volume, where the Tannoy's are barely ticking over, and could play much louder. The LCM's will perform admirably in home offices, dorm rooms, and small living rooms, they are not going fill huge spaces. Horses for corses, as they say in the UK.

Why call them LCM? It's short for Little Californian Monitor, a nod and a wink to AudioSmile's Little British Monitor, brainchild of AudioSimon, my compatriot and author of:

https://www.instructables.com/Mr-Speaker-3D-Printe...

Thank you for reading. If you build these speakers, I hope they serve you well, and if so please spare the time to post a make here.

Luke