Reading driver displacement with radar!

[davygrvy]

I've read about using Doppler radar to read loudspeaker displacement before. There was an article in Audio Amateur once. So I tried myself using a Mattel Hot Wheels Radar Gun (J2358) and dismantled it for the waveguide and tried it myself.

It works nice. This is 30Hz being pushed too far into excursion-based distortion on one of my folded horns with an Eminence HL10c


Please excuse the sawdust. This is the waveguide in my fabbed holder with a weighted base. The radar is passing through 2 layers of plywood.


Limiter control circuit coming today. I'll add a thermal monitor as well for a complete protection system.


See you all at Burning Amp this Sunday!

[littlemike]

Interesting!

So - you can measure driver displacement at power in the cabinet with this.

What does the technique require (other than the radar and the O-scope)?

Sawdust - no worries - glad someone out there is making some...cause I'm not.

[davygrvy]

Hey dude. LTNS. The only trick appears to be a little inverse square compensation needed. As the driver moves closer to the waveguide, the output level of the mixer signal increases causing a visible 2nd H distortion. A little magic with a 4 quadrant multiplier and the receive signal looks right again. The pre-amp does need about 70dB of gain though.

What's interesting to me is that while the wavelength of 10.525 GHz is only 28.2mm, the displacement I'm measuring is just under that. So even though a frequency to voltage conversion is normally used for determining velocity with the doppler technique (like a car travelling @ 55MPH with police radar), I just get a phase output as I'm not crossing a wavelength boundary. And the phase output is directly displacement without the need of any fancy conversions.

Placement of the waveguide is critical.

[littlemike]

Afraid that's a bit above the electronics kung fu I currently possess. My skills pretty much end with knowing which end of the soldering iron to hold.... Though I understand the basic concept of doppler radar and the issue at hand (measuring a back and forth motion), I sincerely doubt I could make it work. Very cool work. I want one.

Been so crazy busy with work and other projects all summer I've not had much time for speakers. I've not done anything cool with subs since the sub for my brother in March. As usual - more ideas than money or time.

[davygrvy]

I know the feeling! I'm only doing this project now cause I got laid-off. This idea for a realistics-based limiter for subs has been floating in my head for at least six months.

The idea is to push EQ for depth, not taking into account excursion. Once the driver limits are known for excursion, I can setup my ol' six-pack and let nature be my limiter. Maybe I'll be lucky enough to setup properly corner loaded one day, then 1/2 space the next. So EQ for good depth below my corner at reduced volume, and let the limiter manage what the coupling I get from the space.

I haven't put the thermal circuit together yet, but being able to monitor coil temperature in real time is just the sweetest thing ever. Imagine a readout at the FOH desk showing you power compression!

[kenpeter]

You going toward a radar based servo loop?

[davygrvy]

yup

[davygrvy]

all this 30Hz exposure is making me a bit sick today. I feel woozy in my stomach and have a headache. There is a brown note, I'm sure of it.

[kenpeter]

You could probably read subwoofer displacement
and/or Doppler by ultrasonic reflections too.

[davygrvy]

Can you buy that as a $35 toy to dismantle? 2 years ago these Hot Wheels Radar Guns went for $9.99 at CircuitCity