so the dual tone horns on the early cars are pretty nice & LOUD, partly because of the combination of frequency's.
They can also be repaired when they stop working.
But after refurbishing them, what do you use to calibrate them back to the 335 & 400Hz tones?

The easiest answer would be a microphone hooked up to an oscilloscope.

440Hz is a musical "A" note. Could potentially use an old school guitar tuner with a built in microphone to get to to A and then tune it by ear to be a little bit flat. There are now a number of clip on guitar tuners that might work too.

Once you have the 400Hz (approxiate) horn set, the other would be tuned by ear to give a pleasant harmony.

Alternatively, if you have a good horn, you can activate that one and tune the other to match. As the frequencies get close they will "beat" at the frequecny difference between them. Example a good horn at 400HZ and the one being tuned at 395Hz will "beat" at 5Hz as a wow - wow - wow kind of noise. When you no longer have the "beat" they are in tune with each other.

Hard to describe in words. Here is a video of it.


https://www.youtube.com/watch?v=rmvDu6EY2lE


Regardless, I'd recommend that you not do it at midnight unless you hate your neighbors.

Frequency app on your phone?
Guitar tuning app?

DOH!

Double DOH!

I just looked and there are some pretty cool audio analyzers available. Way better idea than the guitar tuner. I was way too focused on the items I have easy access to. Nothing is easier than a phone app!