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BART Legacy Inverters

I've grown to be especially nostalgic for the inverters from the old Bay Area Rapid Transit (BART) trains. Unfortunately they have all been withdrawn from service this year (2024) to be scrapped, but luckily I was able to record the sounds with both an inductive recording and a regular microphone.

In theory this means that since we know what the waveform of the inverter looks like, we can write software that will replicate it, so the sounds are never truly lost.

Recording Setup

The recording setup for this is super simple - the only expensive part was the handheld recording device, which you can probably find a cheaper one if needed.

To record these, I got a Zoom H1n recorder off of Amazon, and a 100uH inductor. Simply solder the inductor onto a headphone jack, and optionally wrap it in heat-shrink tubing and then connect it.

It's that simple.

Annotated Video

Here's a video with the ambient regular audio overlaid onto the inductor recording audio.

Note

If you're wondering why the inverter sounds almost like it's shifting gears, it's because older power transistors that are used to generate the sinewave passed to the motor have limitations on how fast they can switch. Every time one of these power transistors is switched, it burns power, which means it heats up the transistor. To avoid the entire inverter heating up too much, they limit how fast they switch these transistors on and off - which can be heard as a sudden drop in frequency.

Data

Here are links to all of the source files.

File Description
Ambient Loop Ambient loop of the Legacy Fleet's HVAC system kicking on and off
Doors Closing Sound of a Fleet of the Future train closing the doors from a distance
Milbrae To SFO Lossless version of the audio in the video.
Ardour Project File Full project file including the above audio and much more