Talk:Acoustic Tape Measure

Some ideas

Measure velocity using doppler?

For something like the "dance revolution graph" activity - maybe you could take some shortcuts.

idea 1: Have one computer broadcast an intermittent square wave at a given frequency. I understand that the result would lack some high harmonics necessary for a true square wave - but I bet it would be good enough to make the calculation of when the thing starts pretty simple. Certainly less robust than full-spectrum white noise, but probably good enough after an initial calibration against white noise.

idea 2: Have one computer broadcast a continuous square wave, and the other one just count beats (using some reasonable assumptions about possible accelerations to throw away bad data). This would tend to get progressively out-of-sync, but it would be fully real-time. Maybe have an intermittent transient to resync...

The point is, doing a whole deconvolve or whatever to match a known sound is computationally much heavier than just binning against a known frequency - sine or square, either way it's even easier than an FFT.

I actually imagine that maybe you could find some good algorithms in use in GPS recievers for electromagnetic stuff - the problem is similar.

(I know - everyone can kibbutz, few will code.)

--Homunq 16:46, 5 October 2007 (EDT)

ps. speed of sound depends on altitude as well as temperature. I think that asking people to guess the temperature is more accurate AND educational than just guessing it using time/location.

That was my first thought as well, but it is wrong. Except to the extent that air is not an ideal gas, the speed of sound does not depend on altitude. We often say that it does, but that's mostly because high-altitude air is cold. Humidity has a bigger effect, but you can ignore that too. Temperature is what matters. See wikipedia for the numbers and math. AlbertCahalan 22:40, 6 October 2007 (EDT)

OK, my bad. But about my other points:

1. I was thinking further about ideal waveforms - you want something with well-understood shortcut algorithms to recognize it in spite of noise, and the algorithms should be very specific in the time domain. FFT and friends are good algorithms, but they are particularly bad at time distinctions. It's been ages since I "studied" this stuff (and I never understood it any better than I had to to make my HS math teacher believe I was doing my independent study), but, have you thought about wavelets?

2. I still think that a human guess at temperature - that is, if you don't have a dongle for use with the "measure" activity - is the best, and most educational, answer. Using the stereo speakers, aren't you going to have problems because of the speed of sound in the laptop plastic? (problems that are avoided if the laptops are more-or-less facing each other head-on). Homunq 08:01, 13 October 2007 (EDT)

More ideas

You have 2 speakers. That can pretty much let you determine angle, given a few minor assumptions about a flat world and similar. Maybe you place the laptops on hyperbolas.

The speakers are a known distance from each other. With stereo, you can get the speaker-to-speaker distance. (they share the same start-up lag) From the error, compute temperature.

AlbertCahalan 04:31, 6 October 2007 (EDT)