A business card form factor can be quite limiting, but that didn’t stop [Schwimmflugel] from creating CardTunes, an ESP32-based Bluetooth audio speaker that tried something innovative to deliver the output.
What’s very interesting about this design is the speaker itself. [Schwimmflugel] aimed to create a speaker out of two coils made from flexible circuit board material, driving them with opposite polarities to create a thin speaker without the need for a permanent magnet.
The concept is sound, but in practice, performance was poor. One could identify the song being played, but only if holding the speaker up to one’s ear. The output was improved considerably with the addition of a small permanent magnet behind the card, but of course this compromised the original vision.
Even though the concept of making a speaker from two flexible PCB panel coils had only mixed success, we love seeing this kind of effort and there’s a lot to learn from the results. Not to mention that it’s frankly fantastic to even have a Bluetooth speaker on a business card in the first place.
The 2024 Business Card Challenge is over, but judging by all the incredible entries we received, we’re thinking it probably won’t be too long before we come up with another sized-constrained challenge.
I can’t help but think that the issue with the speaker was actually enclosure design.
Having two diaphragms pushing/pulling on a tiny sealed(ish) volume of air just doesn’t leave much room for… Amplitude.
Especially when you consider how the wavelength on the order of PCB thickness maps to human hearing sensitivity.
All this to say- they should have cut some sort of porting. Perhaps a slot out to the edge of the PCB?
I’d expect greatly improved performance while staying true to the original design.
The two membranes pull a vacuum or too little air can move between them, I think that’s causing the low amplitude. A normal speaker has a free moving mass where air is moved equally on both sides. Either make channels to let the air escape and use that as the speaker output or perforate the membrane with tiny holes.
Or make the PCB side solid with a fixed coil and some holes for the air to flow through and add some distance between the kapton coil and the PCB so it won’t hit the PCB.
Or generate high voltage and make a static speaker with thin metal foil membranes. That will limit to how low you can play though.
The quality is probably just a matter of signal processing. Driving the same signal through two opposing coils only allows them to be neutral or push away from each other, which is going to effectively square the signal. You need to either drive DC through one coil and drive the other normally or take the square root of the signal (or the square root of the magnitude for negative swings), drive that signal through one coil and a rectified version of the square root through the other coil.
Doesn’t account for properly sealing the air volume, tiny back volume, very little compliance, and a whole host of other linear distortions, but it would at least address the harmonics generated by just driving the coils in opposite directions.
A collapsible popup silicone cone. Similar to popular drinking glasses came to mind.
Collapse it for transit.
The other idea I had was a Tesla arc speaker. But don’t hold it up to your ear.
Chromii
If “The receiptant” had a wireless charger, and the speaker coil circuitry was printed on a layer of kapton. No idea how to make it work but it will wiggle.
Try a piece of mylar on top of the coil.
Depending on how much space is left on the card and how much arts and crafts you want to do, I can see a 3 layer card with the top being the pcb, and the diaphragm being printed out of that flex rubber and perhaps with a rim similar to the way they do the diaphragms on dynamic mic’s. Only one with the voice coil etched in it, and a very strong rare earth magnet under it, perhaps embedded in the bottom layer. The middle layer would be a labyrinth phase delay and acoustic transformer. It sounds silly, but as a child of the 60’s and growing up playing with 2-3″ dynamic speakers and really getting to know how bad they sounded, it is astounding today both the volume and fidelity that can be got out of a small transducer in a small space. Some cell phones floor me.
Where can I buy this business card is it for sale anywhere?