(1)
On Museum Hill in Santa Fe, there is a labyrinth paved with green
and tan pavers, surrounded by a low circular wall. Of course,
this wall produces nice echos when you stand in the center.
I noticed that when you clap your hands, the echo contains a clear note
(in fact it is a chirped note). I suspected that it had to do with the
size of the pavers, so I recorded a few claps with my cellphone.
Play the sound file:
On the right is the frequency spectrum (of the first echo),
made with
Audacity.
There are three clear peaks in the spectrum, the biggest one at 2.9 kHz.
Multiply that by the speed of sound, and you get a wavelength.
The corrugations in the brick pavement favor frequencies that match their spacing.
5 brick take up 59 cm, for 11.8 cm per brick.
Perfect match!
(2)
In 2023, we were in Yucatan for the
annular solar eclipse, and
we visited the great pyramid of Chitzen Itza.
We noticed people standing at the center line facing the steps, a fair
distance away, and clapping to produce a nice echo from the stairs,
which had a clear note in it.
You are not allowed to climb these stairs.
A video was taken by our fellow travelers Jim and Elias, I used
Shotcut (a free video editor)
to split off the audio, an used
Audaciy (a free
audio editor) to make the spectrum.
The peak is at 575 Hz, which gives 59.1 cm for the step width.
Correcting for
the temperature (~30°C) and the humidity (~80%) increases
this by a few percent.
The higher frequencies are multiples (×2 and ×3) of the
base frequency
The next day, on the backside of a pyramid Uxmal, there were some
impossibly steep stairs. Here we could come
close enough to take a measurement. I measured a step width (my hand spans 22cm),
so the step is 19.3 cm.
Links:
xxx
Sep 2022 and Oct 2023
Internal combustion
A soda bottle, 1/2 teaspoon of rubbing alcohol, and
a hand-held Tesla coil
Click to play the movie →
Links:
xxx
added Jun 2024
Camera Obscura
I had done a unit on color and vision in Nadine Potterfield's art class,
and she asked about about doing a 'camera obscura'. To do this, you
have to completely darken the classroom. The window frames were plain
rectangular frames, with the glass set back almost 2 inches. Rather than
black cloth (it is hard to find black cloth that is completely opaque),
or big cardboard sheets and lots of duck tape, I made form-fitted panels.
These are the exact size of the openings (minus 1/4"), with 2" flanges.
These push into place in seconds, so I can set up a classroom quickly,
and re-use them. I also discovered that all classrooms in
the school have
the same windows.
The screen is a rolling whiteboard, placed a few meters from the window.
A ~5cm hole produced an image that was bright enough, but too blurry to
be of interest. Making the hole smaller made the image a less blurry, but
too dim. Time to buy a lens with a few meter focal length, a strength in
diopters of say 0.25 for a screen distance of ~4m. Cheap reading glasses
don't go below 1.0. My scientific supply houses did not have anything close
to what I needed. I found some chinese 0.25 reading glasses, but then saw
an article in
Make magazine Vol. 91 mentioning 'lens blanks'.
Got some 0.25's on Ebay, diameter 65mm. Work great.
