Engineer's Speedo [Updated]

"Snuze: Needs another Swede" (markg)
04/22/2020 at 10:46 • Filed to: shitpost

I created a new speedometer overlay for anyone who wants to use it. If you need a higher range, let me know.

“But officer, I was only going 4 0...”

[Update]: Thanks to JimZ for pointing out that speed is an amplitude quantity and should be represented as 20*log and not 10*log, which is for power quantities.

DISCUSSION (25)

Rusty Vandura - www.tinyurl.com/keepoppo > Snuze: Needs another Swede
04/22/2020 at 10:54

As in decibels? Is this submariner wizardry? Legal trickery? Please explain!

MasterMario - Keeper of the V8s > Snuze: Needs another Swede
04/22/2020 at 10:56

How do I wear that?

Darkbrador > Rusty Vandura - www.tinyurl.com/keepoppo
04/22/2020 at 10:56

Something something about dB being on a logarythmic scale ...

Just Jeepin' > MasterMario - Keeper of the V8s
04/22/2020 at 11:00

Sewn on the front, or I suppose you could implant it in your chest like Stark.

Kinja'd!!!

MasterMario - Keeper of the V8s > Rusty Vandura - www.tinyurl.com/keepoppo
04/22/2020 at 11:00

https://knops.co/magazine/if-a-cars-speedo-was-expressed-in-decibels-youd-be-cruising-at-10000-mph/

The concept is right, but the details are wrong in this article I found

Edit: Correcting the details

1 MPH is the reference for 0 dB

10 dB = 1 x 10 = 10mph

20 dB = 1 x 10 x 10 = 100mph

Highlander-Datsuns are Forever > Snuze: Needs another Swede
04/22/2020 at 11:06

It should start at 0, not a -infinity, otherwise you would need a -scale too.

jimz > Just Jeepin'
04/22/2020 at 11:07

er, something you want to tell us?

jimz > Snuze: Needs another Swede
04/22/2020 at 11:10

speed/velocity is an amplitude quantity, so shouldn’t you be using 20log(V2/V1)?

Snuze: Needs another Swede > Highlander-Datsuns are Forever
04/22/2020 at 11:13

Nope.

10* log10 (0) = 1

10* log10 (-inf) = 0

Snuze: Needs another Swede > Rusty Vandura - www.tinyurl.com/keepoppo
04/22/2020 at 11:15

It’s in decibels. 10* log10 (V /Vr) where Vr is the reference speed (1 MPH). I work in acoustics and we use decibels a lot.

Ash78, voting early and often > Snuze: Needs another Swede
04/22/2020 at 11:17

“Can I get one?”

“Sorry, we only have one. The German subcontractor who printed it shuttered the factory after the prototype was done and we don’t have another $3B to purchase new equipment to start over.”

“Didn’t you have any financial or forecasting plans ?”

“BEAN COUNTERS RUIN ALL THE FUN!”

Snuze: Needs another Swede > jimz
04/22/2020 at 11:23

I believe you are c orrect. I actually did give this some thought and briefly looked for an answer. I couldn’t find anything decisive (in my 2 minute search) so I went with 10*log. But some further research suggests it should be 20*log.

Thanks for the tech review!

jimz > Snuze: Needs another Swede
04/22/2020 at 11:27

10*log is for power quantities. SPL is a power quantity. Voltage is an amplitude quantity and would be 20*log.

Turbineguy: Nom de Zoom > Snuze: Needs another Swede
04/22/2020 at 11:27

20? Hell, my car shimmies at 10...

Snuze: Needs another Swede > jimz
04/22/2020 at 11:32

Fixed! Thanks again for pointing that out.

Everything I do is SPL so I’m always using 10*log. I vaugely remember in an intro course using 20*log to solve intensities and convert to SPLs or something, but I’ve obviously brain-dumped all that since I don’t ever actually do it in the real world.