The Frequency Cyclists Need (And Already Have)

Mighty algorithm decided I should watch a video on youtube titled:
Škoda DuoBell - The First Bike Bell Designed To Penetrate Noise-Cancelling Headphones
The gist of the video was there is a frequency that noise cancelation headphones have troubles to cancel out. According to Skoda researchers it is 750Hz and they found a way how to construct a bell that rings in that frequency. Pretty cool, but the bell looks to be pretty hefty massive.

I immediately thought hey I made this holder for a speaker, if I can play a sound in that frequency it should work the same way.
And I already had everything I needed.
A while back I built a small Android app called Pucker — it pairs with a Puck.js Bluetooth button and plays a sound when you press it. Originally just a fun experiment with BLE. But sitting on my bike handlebars, that little button suddenly had a real job to do.
The setup is simple:

→ Puck.js button clamped to the handlebar
→ Phone in my pocket running Bike Horn app
→ Bluetooth speaker mounted to the bike via my 3D printed holder (STL available in the shop)

Press the button → Puck.js sends a BLE signal to the phone → Bike Horn app generates a 750 Hz sine wave tone → speaker fires it forward toward whoever is about to walk into me.
The 750 Hz tone is generated programmatically using Android's AudioTrack API — a pure sine wave, no pre-recorded audio file needed. Android handles routing automatically, so as long as the Bluetooth speaker is paired and connected, the tone goes straight there.
Is it as elegant as a precision-machined mechanical bell? No. Does it fit in my pocket and work with hardware I already had? Absolutely.
If you want to build your own, the speaker mount STL files are in the shop and the Bike Horn app source is available at github. Škoda also made their 750 Hz research freely downloadable — worth a read (link).

As a side note, you can play different sounds — not just for people wearing headphones, but also for city dwellers trained to react to the sound of a tram, or people from the countryside who instinctively turn around when a cow greets them from behind.